invited paper recent advances toward optical devices in...

INV ITEDP A P E R

Recent Advances TowardOptical Devices inSemiconductor-BasedPhotonic CrystalsPhotonic crystals are being developed to perform fast optical switching, filtering and

routing, and optical frequency conversion, and to form spectrometers-on-a-chip

and several types of lasers.

By Henri Benisty, Jean-Michel Lourtioz, Senior Member IEEE, Alexei Chelnokov,

Sylvain Combrie, and Xavier Checoury

ABSTRACT | Photonic crystals, artificial, wavelength-scale

multidimensional periodic structures, have given birth to a

number of realizations in semiconductors. Photonic integrated

circuits, especially around new integrated lasers, are challeng-

ing directions of research for miniaturization and new func-

tions in optical telecommunications. We review the basic

physics behind such applications and underline the current

status of this very active research field worldwide.

KEYWORDS | Demultiplexers; lasers; optoelectronic; photonic

crystals (PhCs); photonic integrated circuits (PICs)

I . INTRODUCTION

A. Brief History and Basic Concepts of PhotonicCrystals (PhCs)

This introductory part will give an overview of PhC

concepts and the challenges of miniature photonic circuits

before detailing the outline of the rest of this paper. We

assume that the reader is familiar with basic Bragg

reflection physics, as occurs in fiber Bragg gratings (FBGs),

distributed Bragg reflectors (DBRs) and distributedfeedback (DFB) devices.

Light–matter interaction is the heart of optoelectronic

devices. A core aspect of this is refraction, the fact that

materials react with their dielectric constant. When

properly used, e.g., in DFB lasers under the form of a

one-dimensional (1-D) grating, it profoundly shapes the

device emission.

PhCs are essentially a multidimensional generalizationof periodic structures. Two notable features distinguish

this field from the previous studies of optical gratings.

First, most of the physics based on PhCs reveals its

interest for large index contrast. Second, PhCs represent

an unprecedented link between solid-state physics and

optics. From the former, they borrow most notably the

bandgap that makes them optical analogues to semicon-

ductors. From the latter, they borrow the Bopticalphysics[ background from interference to beam shaping,

thereby bridging the gap between lasers and ultimate

sources exploiting quantum electrodynamic effects. They

are indeed one of the preferred options toward the

ultimate control of light, down to spontaneous emission

itself.

Yablonovitch [1] and John [2] in 1987 marked the

emergence of the field. John’s idea, simplified, was thatlight scattering in a disordered assembly of very strong

scatterers could turn to localization of light, trapped in

subparts of the medium. Yablonovitch’s idea was that the

Manuscript received May 26, 2005; revised September 15, 2005. This work was

supported in part by the FUNFOX FP6-IST 04582 European project and in part by the

CRISTEL French project.

H. Benisty is with the Laboratoire Charles Fabry de l’Institut d’Optique, Orsay Cedex

91403, France.

J.-M. Lourtioz and X. Checoury are with the Institut d’Electronique Fondamentale,

UMR 8622 du CNRS, Orsay Cedex 91405, France (e-mail: [email protected]).

S. Combrie is with Thales Research and Technology TRT France, Domaine de

Corbeville, Orsay Cedex 91404, France.

A. Chelnokov is with CEA CEA-LETI, Grenoble 38054, France.

Digital Object Identifier: 10.1109/JPROC.2006.873441

Vol. 94, No. 5, May 2006 | Proceedings of the IEEE 9970018-9219/$20.00 �2006 IEEE

three-dimensional (3-D) propagative states of light couldbe separated by bandgaps forbidding propagation, just as

for electron in semiconductors (Si, Ge, GaAs, InP, GaN,

etc.). Spontaneous emission would then be inhibited for an

emitter having its natural spectrum within this photonic

bandgap (PBG).

Furthering this idea, the ideal electrooptic converter

was some kind of thresholdless laser whereby light

emission was forbidden in all but the one desired mode.However, the route from the concept to the device was

long and inhibition was not the whole story. First of all, in

order to achieve such ultimate devices, a very small mode

volume is necessary. The use of dielectric and semicon-

ductors is to be preferred to metals because of their much

lower losses at optical wavelengths. Despite the recent

emergence of Bplasmonic[ devices at subwavelength scales

[2], semiconductors are indeed a sound choice, thanks totheir mastered emission properties. Today, spontaneous

emission control is best achieved in semiconductor-based

microcavities that can be mostly viewed as engineered

defects in a periodic structure (with the clear exception of

microdisks). A resonant mode is thus created at a specific

frequency in the structure bandgap.

The opening of an omnidirectional bandgap is still a

touchstone of PhC studies. Two-dimensional (2-D) and3-D structures are the salt of the story. Multidimensional

periodic media were familiar in optics, as, for instance,

holograms, but the index contrast was not strong enough

to provide a full bandgap. Only a directional gap arose.

Actually, the bandgap condition is akin to the standard

Bragg condition of X-rays; the path difference in the

scattering by two planes/rows in succession should be an

integer multiple of the wavelength. Fig. 1(a) representsthe quasi-normal incidence situation of a wave impinging

on a stack of alternate materials. This stack is nothing

but a high-reflection coating. If it has a small index

contrast, the band opening and the reflection stopband

are narrow [Fig. 1(c) and (d)]. Upon tilting the angle �of incidence, the peak frequency evolves with a classical

cosð�Þ factor in simple cases.

Hence, the initial quest was to find whether dielectricscould be structured enough to create very wide bandgaps.

If wide enough, a shift with direction will still leave a

sizable overlap between all the directional bandgaps. As

illustrated in Fig. 1(e) and (f), the goal is to find periodic

structures, e.g., arrays of cylinders that possess an

omnidirectional gap in 2-D or 3-D.

In 1987, everything was to be proven in 3-D. The use of

the vector Maxwell equations in 3-D soon revealed that thesimpler close-packed structures could not have a true PBG.

A desired band splitting was forbidden by symmetry at the

W point of the fcc lattice’s first Brillouin zone. This

prompted a thorough understanding of the nature of the

bands and their associated electromagnetic field distribu-

tion. Two variants of the diamond lattice emerged from

this. On the theoretical side, a diamond-like array of high-

index spheres was proved to be the simplest system. Aclever version imagined by Yablonovitch consisted in

drilling three sets of holes that mimic the (111) oriented

Bgalleries[ of silicon as displayed in crystallography

textbooks. This indeed resulted in the first demonstration

of a bandgap at microwave frequencies. Another clever

structure imagined by the team at Iowa State University is

the Bwoodpile[ or Blayer-by-layer[ stack [4]. PhCs became

increasingly popular after these successes.Large investigations of these novel structures toward

optical frequencies started around 1995. Simultaneously,

with the prospect of mass production, self-assembly was

sought to obtain large arrays of spheres. However, despite

impressive fabrication results, only some degrees of

functionality toward optical devices have been demon-

strated yet, and the field of 3-D PhCs in 2005 appears as

essentially academic to the telecommunication andoptoelectronic community.

Conversely, 2-D PhCs have been easier to explore using

existing planar technology. In order to achieve fine

structures with 200–800-nm periods and mark-space

ratios often different from unity, several tools were avail-

able, such as e-beam lithography and, to a lesser extent,

the deep-UV lithography stepper projectors of main-

stream microelectronics. The first convincing demonstra-tion of 2-D near-infrared PhCs was made in 1996–1997

[5], [6].

The subsequent years rapidly brought to light more

complex structures of interest for telecommunications

such as microcavities, waveguides, and their various

combinations. This paper is focused on these structures,

as they lead to consider PhCs as promising candidates

either for future miniature photonic integrated circuits(PICs) or for ultimate sources based on the extreme

confinement of light. Actually, there is hardly a

competitor to PhCs in the nanophotonic arena, display-

ing such a fascinating physics.

The basic physics of 2-D bandgap is summarized in

Fig. 1(e), (g), and (h). Fig. 1(e) represents the main di-

rections along which a wave can impinge on a triangular

array of holes drilled in a high-index matrix ðn � 3:36Þ.The hexagonal Brillouin zone has six equivalent K and Mpoints. The band structure in Fig. 1(g) represents the

frequencies of allowed photon modes in such a lattice.

Only the TE modes with the magnetic field H along the

hole axis are shown here for simplicity. The allowed

modes are also called Bloch modes because the Bloch

theorem used for solid-state crystals also applies to PhCs.

The absence of resemblance with more familiar guidedwave diagram is a consequence of Bfolding.[ Each mode

is indeed composed of interrelated Fourier components of

wavevectors k; k þ G1; k þ G2 . . . , where fG1;G2;G3; . . .gis the discrete set of 2-D vectors of the reciprocal lattice.

Although the gap along �K direction lies at higher

frequencies than the one in the �M direction (K is further

away from �), they have a sizable overlap: the full photonic

Benisty et al. : Recent Advances Toward Optical Devices in Semiconductor-Based Photonic Crystals

998 Proceedings of the IEEE | Vol. 94, No. 5, May 2006

band gap. Fig. 1(h) shows the evolution of the full PBG

versus the air-filling factor f of the crystal for both TE and

TM polarizations, the latter being far more delicate to

obtain (especially due to the tiny veins between the holes

at high f values). Conversely, for the TE gap, hitting a

predetermined frequency range is well achieved by state-

of-the-art nanofabrication, whose first-order accuracy on

the f value is about 5%.

Fig. 1. Basics of PhCs. (a) The 1-D case, in quasi-normal incidence, with Bragg-type reflection. (b) Wavevector representation. (c) Dispersion

relation of waves in the periodic stack. (d) Reflection stopband. (e) Left: omnidirectional reflection on a piece of 2-D PhC with a hexagonal lattice.

Right: first Brillouin zone of the lattice. (f) Inhibition of emission in a 3-D PhC. (g) TE band structure of a 2-D triangular lattice of holes in a matrix of

dielectric constant 11.3, with a 30% air filling factor. (h) Gap map for the two polarizations.


Vol. 94, No. 5, May 2006 | Proceedings of the IEEE 999

B. The PIC ConceptIn the area of Bconventional[ integrated optics, con-

finement is obtained by using a core material of higher

index than the surrounding medium. The index step is a

crucial parameter, determining losses at circuit bends,

propagation losses in waveguides and coupling losses to

fibers. These quantities are expressed in dB/90�, dB/cm

and pure dB, respectively. The photonic lightwave circuit

(PLC) platform refers to doped silica on silica, with minuteindex steps of �n ¼ 0:01. The minimum bend radii are

about R ¼ 10–40 mm to keep the losses below 1 dB/90�.

Meanwhile, the mode profile is very close to that of fibers,

as the index step �n and the core size (� ¼ 5–10 �m) are

comparable. Scattering losses at the interfaces are also very

weak. Their basic scaling law can be derived from

perturbation theory. The polarization induced by the

unperturbed field E in a microbump of volume V is�P ¼ ð��EÞ ¼ ð�"EÞ. As the power radiated into a

homogeneous environment goes like the square of the

induced dipole p ¼ ðV�PÞ, incoherent scattering losses

then scale like ð�"Þ2 � ðn�nÞ2 if �" 2n�n is used.

Increasing the index step thanks to, e.g, silicon nitride,

or semiconductors, bend radii can be drastically reduced,

but tighter mode profiles are obtained, hence a more

delicate coupling to fibers and a higher sensitivity tosurface roughness. The almost extreme case is that of

silicon-on-insulator (SoI) with large �n (2–2.5), the value

of which depends on the cladding used on both sides of the

silicon guides.

The use of such high-index steps allows not only the

fabrication of sharp bends, but also that of compact splitters

(Y junctions) and compact frequency selective devices

(demux). For example, the area needed to implement aphasar (or AWG) on SoI can be as small as 100 100 m2

instead of several square centimeters on silica [7]. Compact

single-frequency devices such as cavities closed by two

distributed Bragg reflectors can be designed as well. Con-

tinued efforts are devoted to optimize the mirror perfor-

mances in confined wave geometries.

Regarding active devices that accomplish functions

such as wavelength conversion, reconfigurable add–dropfiltering, signal regeneration and clock recovery in PICs, it

might be believed that a minimum physical interaction

length is needed to operate them in proper conditions. For

instance, a length of 300 �m is typical of edge-emitting

lasers. This being, if optimally designed in-plane mirrors

are used, one can produce edge-emitting, integrated lasers

as short as a vertical cavity surface-emitting laser (VCSEL)

with the same material gain. More generally, themastering of the in-plane confinement of light opens large

opportunities to revisit active and passive devices as well as

to explore new phenomena as mentioned in Section I-A.

Thus, it has been progressively recognized that

innovative solutions could stem from the ultimate control

of light by wavelength-scale structures. In this review, we

shall give a flavor of several novel effects that go well

beyond the simple index-step scaling, tackling, forinstance, the management of group velocity and disper-

sion, or the generation of single photons on demand.

Fig. 2 presents some basic aspects of PhC building

blocks, along with a futuristic PIC. Fig. 2(a) and (b) show

a localized defect consisting of: (a) one missing hole and

(b) three missing holes. Holes in the neighborhood of the

defect are of reduced size in (a) as compared to the rest of

the crystal, while in (b) two side holes are shifted (seelater). Such defects support tightly localized modes,

quickly decaying in the cladding (see Section II-C).

Fig. 2(c) and (d) show two uses of a line-defect in a

PhC. In (c), the defect is used as a waveguide along the line

of missing holes. In (d), it is used as an in-plane Fabry–

Perot (FP) cavity for a wave that propagates transversally.

The essential difference with the classical guiding

mechanism based on total internal reflection is pictured in(e)–(f). PhC waveguiding actually occurs because all the

elementary waves scattered by Bphotonic atoms[ interfere

constructively in the defect region, and destructively in the

outer PBG regions. The difference between the two types

of guiding is obvious in the Bdielectric rod[ configuration,

whereby PhC guiding takes place in the low index medium.

Conversely, for the Bair hole[ situation in a high-index

matrix, the PhC represents a medium of average index wellbelow the core index. Hence, index guiding and photonic

bandgap effects can be combined. (See Section II-A).

Having substantiated some simple PhC building blocks,

it is possible to envision the advent of ultracompact PhC-

based circuits [Fig. 2(g)]. Most elements are those of a

general PIC except for in/out functions (ideally wave-

length-scale tapers) needed to couple to external fibers. In

the present example, photodiodes are placed near theBbar[ channel on top in order to monitor specific

wavelengths, namely, those of wavelength division multi-

plexing (WDM) or coarse WDM (CWDM) channels (see

Section III for more detail). Bends and couplers are used to

derive a fraction of the input signal toward a switching

system. Selected wavelengths are mixed with those from

integrated lasers. Next, they are amplified in a slow wave

amplifier specially designed to achieve high gain in aminiature footprint. Note that in order to combine active

and passive functions on a chip, the electronic bandgap

requires adjustment, or very ingenious arrangement, in

order to overcome the intrinsic absorption of unpumped

active material.

C. PhC Versus Previous BreakthroughsThe development of semiconductor heterostructures,

one of the pioneering advances in the 1960s, was prompted

by the need of a first level of confinement for photons and

electrons in optoelectronic devices. A second decisive

progress in the 80–90s has been the development of

quantum well and quantum dot structures with a much

radical improvement of the carrier confinement also

leading to a much wider exploration of optoelectronic



materials. However, contrary to electronic integration,

optical integration has not developed at the fast pace of

Moore’s law: no more than two or three functions can

currently be cascaded on sixteen channels, for example. In

this context, the concept of PhC arises considerableexpectations. They indeed lead to envision the possibility

of realizing optical circuits to channel, analyze and

combine an increasing number of optical signals in a

more compact form than previous solutions of integrated

optics. Even if catching up Moore’s law remains beyond

reach, the breakthrough of full wave control would lead to

a decisive acceleration of integration. The practical impact

of PhCs in the domain of optical telecommunications is

already recognized with the development of PhC fibers,

which have definitely reached the market place. In the

field of semiconductors, going from concept to actualapplications has taken more time than had been expected.

Fortunately enough, most of the technological difficulties

are in the process of being progressively solved thanks to

the advances, which are taking place in the fields of

microtechnology and nanotechnology. This is the main

purpose of this paper to survey the progress that has

Fig. 2. (a) Defect forming a microcavity (note the smaller holes in the immediate neighborhood of the defect). (b) An elongated microcavity design

(here an SEM view is used) proposed by Noda et al., note the deliberate shift of the arrowed hole. (c) Line defect as a waveguide. (d) As an FP cavity.

(e) Classical index guiding. (f) PBG guiding, by construction of all interferences. (g) A futuristic PIC with many PhC elements: in/out coupling,

couplers, bends, add–drop filters, dichroic splitters, slow-wave active devices, tunable lasers.



recently been accomplished in the development ofsemiconductor-based PhC components and circuitries.

D. Description of the Paper ContentsThe rest of this paper is divided into four sections and a

conclusion. Section II deals with building blocks of

semiconductor-based photonic circuits such as wave-

guides, bends, and microcavities. State-of-the-art perfor-

mances are reported for these elements with a special

attention to waveguide propagation losses and microcavityQ factors. Sections III and IV deal with systems closer to

components. Passive components are first considered in

Section III. It is shown how the use of bandgap and

dispersion properties of PhCs can help us to take up some

of the great challenges of optical communications, namely,

the control of group velocity dispersion at small length

scales and the design of miniature filters and spectro-

meters for wavelength selection. Active componentsincluding lasers and nonlinear devices are considered in

Section IV. It is shown that 2-D PhCs are potentially

applicable to a large variety of laser systems from ultimate

photon sources to VCSELs and efficient single-mode

miniature lasers. New opportunities are also open for

nonlinear devices and low-threshold all-optical switches

thanks to the tight confinement of light in PhCs. All these

device perspectives are illustrated through recent works.Section V briefly presents the recent advances in the field

of 3-D PhCs. Section VI concludes the paper.

II . SEMICONDUCTOR-BASED PHCCIRCUITRY

A. Vertical Confinement of Light in 2-D PhCsWhereas 2-D PhCs are ideally adapted to planar

semiconductor technology for an in-plane confinement

of light, an obvious question arises: what is the confine-

ment of light in the third direction? In the simple case of

an unstructured slab cladded by media with index nclad, the

guided modes possess propagation constants, which are

necessarily larger than nclad!=c. The dispersion curves ofthese modes lie outside the light cone k ¼ nclad!=c. In the

case of a periodically structured slab, band folding occurs,

and dispersion branches that previously were outside the

light cone may now partly or completely lie inside it.

Correspondingly, the guided modes may become leaky

modes when the periodic modulation (lattice vector G)

can generate a Fourier component at a k value smaller than

nclad!=c [Fig. 3(a)]. This leaky component is, in turn,eliminated using a smaller value of nclad [Fig. 3(b)]. As

could be expected, a high-index contrast between the slab

waveguide and the claddings is more desirable to provide

lossless guided modes over a wide range of frequencies.

In practice, there are two main approaches of 2-D PhCs

in planar optics. The most standard approach for III–V

optoelectronics is the substrate approach where periodic

holes are etched through multilayer waveguides with a lowor moderate index contrast between the core and cladding

layers [Fig. 3(c)]. A deep etch process is required to reduce

the diffraction losses at the PhC holes. Because of the high

index of the cladding layer, the light line typically inter-

sects the Brillouin zone edges below the gap [Fig. 3(e)]. In

other words, defect modes created in gap will be leaky

modes. The second approach relies on a strong vertical

confinement as provided by a III–V semiconductor mem-brane in air [Fig. 3(d)] or by the SoI technology as well.

The etch process is much less demanding on account of the

limited depth. Truly lossless defect modes can exist in the

gap [Fig. 3(e)]. In contrast, the pure membrane approach

is penalized by an increased influence of heating effects

and a difficult implementation of electrical excitation.

If we now consider the linear waveguide of Fig. 2(d)

inserted in a slab [Fig. 3(g)], the issue of the light linearises again, but only one dimension ðkin�planeÞ is

concerned. In a generic case and focusing on a single

dispersion branch of the waveguide, the situation will be

that of Fig. 3(f). A mode in the waveguide manifests itself

as a dispersion curve in the diagram, while the bands of

the ideally infinite PhC surrounding it, once projected,

take the form of a continuum. Let us consider, for in-

stance, the membrane case with an air cladding. There aremany different regions along the frequency axis. Region 1

still has no mode at all in the gap (below the light line),

region 2 is only part of the waveguide mode dispersion

curve below the light line. In region 3, the guided mode is

intrinsically leaky, and radiates in the air cladding.

Finally, in region 4, the mode has the same characteristic

as region 3, but modes of the PhC continuum are present

at the same frequency, so that the Blight insulating[properties are lost. Light injected at the guide entrance

may then spread into the surrounding PhC rather than to

couple only to the desired mode.

It is thus clear that the vertical confinement has a

strong interplay with guiding properties. Nevertheless,

when losses are acceptably low, the behavior of the system

can be largely thought as 2-D.

B. Waveguides

1) Waveguide Dispersion Playground: Fig. 3(f) shows that

PhCs present various kinds of dispersion characteristics.

There is actually a vast playground for waveguide

dispersion in these systems, much more than in nonper-

iodic photonic wires.

A first-order design, that of canonical structures, isobtained by not drilling one or several rows of holes. These

waveguides are commonly termed BWn[ for n missing

rows ðn ¼ 1; 2; . . .Þ along the �K axis, i.e., the dense rows

of period a.

The dispersion relation of a W1 waveguide in a 2-D PhC

is shown in Fig. 4(a), with the same general features as

Fig. 3(f) (projected bands, etc.). Even in this narrow



waveguide, two guided modes exist in gap. One presents aneven field pattern at various points of the dispersion curve,

whereas the other possesses an odd symmetry of the

magnetic field distribution. Let us concentrate on single-

mode domains. The single-mode domain of low frequency/

high k is actually the only one extending outside the light

cone, thus in the Blossless[ region. A large dispersion

arises in this domain, as the slope of the dispersion curve,

the group velocity of light, goes rapidly to zero. For 1.5-�mapplications, there are sizable frequency ranges, say tens of

gigahertz, within which the group velocity can be of the

order of c/100 to c/1000. It is believed that this per-

formance, low group velocity on sizable spectral span, is

inseparable from the large index contrast approach. In the

single-mode domain, the group velocity dispersion (GVD)reaches values larger than 100 ps/nm/mm, i.e., values 107

times larger than those of optical fibers. This is simply

because the same dispersive retardation, say 1 ps/nm, is

attained within one tenth of a mm (100 �m, or even less)

in the PhC instead of � 100 m in the optical fiber. The

price to pay for this rather exceptional guiding behavior is a

strong modification of the mode profile with a large

extension into the surrounding PhC [Fig. 4(c)], making itmore sensitive to structural imperfections.

Birefringence is another aspect of PhC waveguides.

Due to the very nature of the bandgap, the two polar-

izations have drastically different behaviors. The TM gap is

much smaller than the TE one [Fig. 1(h)]. On the other

Fig. 3. (a) A guided mode and the effect of periodicity G in k space, inducing leakage. (b) Not inducing leakage for small index cladding. (c) The

substrate approach. (d) The membrane approach. (e) Light line superimposed on the 2-D PhC dispersion relation. (f) Dispersion relation of a

line-defect, and the various frequency region defined by the light line and the gap. (g) A line defect defined into a semiconductor slab.



hand, guiding in both polarizations can be achieved in theabsence of gap thanks to index guiding of the TM mode

while the TE mode is guided by a bandgap effect.

Birefringence can then be huge, with beat length of a

few micrometers. For more complex structures, with

slanted hole walls, polarizations can mix, and the picture is

more complex. The exploitation of such effects is sought

for polarization conversion and polarization diversity.

Periodicity may induce specific gaps on the dispersionrelation. This is apparent in Fig. 4(b), for the case of a W3

waveguide. The several branches in the gap are associated

to modes that are the various intermediates between those

of Fig. 2(c) and (d). These modes possess different

propagation constants along the guide. Different signs of

slopes arise from the folding at the Brillouin zone edge. As

a result, branches tend to cross. However, as shown by an

arrow in Fig. 4(b), anticrossing may occur instead, leadingto a small Bminigap[ or Bmini-stopband[ in the dispersion

relation of the dispersive mode, e.g., the fundamental

mode of W3 with the largest negative slope on this

diagram. At such specific frequency and wavevector

values, energy is exchanged between the fundamental

mode and a higher order mode, as these modes share the

same frequency and the same momentum modulo a

reciprocal wavevector shift of �k ¼ 2=a. The energylaunched in the fundamental waveguide mode can then be

Btransferred[ into the laterally oscillating mode. This is anovel game, not feasible in classical devices based on Bragg

reflection. We will report later on its exploitation in

microspectrometer chips (see Section III-D).

The periodic modulation of the waveguide width can be

an asset if one wishes to play with dispersion. Transitions

and tapers are thus welcome. One such example is given in

Fig. 4(d) for a practical realization based on the

introduction of a few holes of variable diameter [8].Once more, it is the beauty of strong index contrast

systems that such a short taper (a few micrometers) may

work almost perfectly in a sizable frequency range of tens

of nanometers around � � 1500 nm.

Practitioners in the domain are thus facing a delicate

task in theoretically exploring the various PhC structures.

Let us give here a brief list of the available tools. The plane-

wave expansion method is certainly the most familiar toolfor calculating the dispersion relations. A free version

exists from the Massachusetts Institute of Technology

(MIT), Cambridge1 [9]. This method has been used, for

instance, to calculate the dispersion relations of Figs. 1(g)

and 3(e). It amounts to write the field’s scalar component,

say the vertical component of the magnetic field Hz, as a

1[Online]. Available: http://www.opticsexpress.org/abstract.cfm?URI= OPEX-8-3-173

Fig. 4. (a) Typical dispersion relation of a W1 waveguide (matrix index� 3, air filling factor� 30%), with projected bands shaded in gray. (b) Same

for a W3 waveguide. (c) Magnetic field maps of three selected modes along the slow branch fraction of the W1: note the spreading in the

surrounding crystal as the continuum band is approached. (d) Micrograph of a taper between the two kinds of waveguides (see [8]).



sum of plane waves on the basis {G} with adequatecoefficients. A 2-D supercell has to be used to periodize the

nonperiodic dimension of a waveguide, provided the

guided modes do not couple each other through the PhC

Bwalls[ of the supercell. The use of a 3-D supercell is more

cumbersome for 2-D waveguide applications, since the

fictitious periodicity along the Bvertical[ axis normal to the

waveguide induces many artifacts and parasitic mode

couplings. In any case, the plane wave method is limited tofinding mode frequencies and patterns of an infinite

system (in the periodic directions), but is not adapted to

describe the fate of a particular external wave impinging

onto a finite structure.

A number of methods exist to treat finite problems

while remaining in the frequency domain. Many of them

can be collectively assigned as scattering matrix methods

[10], resting on various basis functions such as Besselfunctions for spherical scatterers and Fourier components

in most of the other systems. These treatments are also

tightly related to advanced diffractive optic treatments and

grating theory [11]. Alternatively, readers familiar with

solid-state physics may find interesting to look at an

application of the Fermi golden rule to efficiently calculate

scattering losses from a guided mode to a continuum [12].

The eigenmode expansion freely available at Ghent2 is alsocommendable. Finally, the finite-element method and

associated tools such as the popular BHFSS[ software of

microwave practitioners are also of current use.

In the time domain, the finite-difference time-domain

(FDTD) method is extremely popular. Freewares are

available.3 FDTD calculations can indeed produce snap-

shots of the strange field behavior in these novel materials.

Precautions with boundary conditions must be taken toavoid spurious reflections, by using, for instance, the so-

called perfectly matched layers. The FDTD method is,

however, a modest design tool for the early step of

conception, as it completely ignores the modal picture.

2) Waveguide Propagation Lossesa) Predictions: Losses in PhC waveguides are of

course a crucial question. The light line story only tells thebasis, i.e., whether radiation losses are possible or not for a

guide of infinite length. In practice, the guide is of finite

length with possible transitions to other photonic devices.

It has also imperfections such as irregularities in hole

diameter and position, nanometric roughness of the walls,

etc. Hence, even outside the light cone, losses may be

induced by these imperfections.

Three-dimensional modeling incurs a large computa-tion penalty. Accounting for the third dimension in a

fictitious manner in 2-D simulations is thus of great

interest. This has been done through an imaginary part of

the dielectric constant in the air holes [13], [14] andapplied to several structures. Within this approach,

scattering is described as a mere dissipation and, therefore,

all coherent effects, including the light line, are ignored.

This approach is thus most appropriate for PhC structures

carved in substrates [Fig. 3(c)]. Practical loss figures are

known for state-of-the-art deep-etching as explored in the

PCIC� project.4 The imaginary part of the dielectric

constant in the air holes may be as low as "00 ¼ 0:02. For ahomogeneous loss dielectric medium, the field amplitude

would decay as expð�n00ð!=cÞrÞ, and the power attenua-

tion would be given by A "00 4:34 ð!=cÞ=n0 (deci-

bels per unit length) where n0 is the modal index and

"00 ¼ 2n0n00. However, for waveguides, the modal overlap �of the fundamental mode with the PhC holes may be tiny.

This alleviates the penalty, since we should now take

A � "00 4:34 ð!=cÞ=n0. Thus , the va lue of"00 ¼ 0:02 translates into losses of 10–20 dB/cm for

W3 waveguides ð� � 0:01Þ, and about 10–20 dB/mm for

W1 waveguides (larger �). Hence, propagation through

100 periods (40 �m) of a W1 waveguide still leaves, in

principle, a quite reasonable signal (0.5-dB loss).

As for membranes, theory predicts that losses for the

mode of Fig. 4(a) and (c) are huge above the light line (say

1 dB for ten periods!), and zero below. Imperfections ofthe structure may give rise to scattering. There is still a

debate whether the roughness, for instance, will lead to

losses comparable toVor smaller thanVthose of photonic

wires with a similar degree of lateral confinement: the

average medium being highly structured around the PhC

waveguide, hand-waving arguments are delicate. Optimis-

tically, one may assume that spontaneous emission

inhibition implies that an irregularity only gives rise to alossless evanescent wave.

Systems with intermediate index contrast such as SoI

and reported membranes pose different problems. Not

only is there a slightly smaller room below the light line

than in the pure membrane case, but also there is no

definite mode symmetry and polarization, since the top

and bottom claddings now differ. This couples previously

uncoupled TE and TM branches at their numerouscrossings.

b) Measurements: Measurements of PhCs and related

waveguides recourse to various methods. A generic

approach is the Bpseudocutback[ method, whereby the

transmissions of guides of different lengths L are compared

under the same excitation and assuming the same

conditions of fabrication. The results of measurements

are fitted to an exponential law expð��LÞ from which themodal loss � is deduced.

Let us consider the excitation by an external laser

focused onto the cleaved facet of the studied sample [end-

fire technique of Fig. 5(a)]. In practice, ridge access

2[Online]. Available: http://camfr.sourceforge.net/3E.g., at KTH, Sweden. [Online]. Available: http://www.imit.kth.se/

info/FOFU/PC/F2P/ 4[Online]. Available: http://www.ist-optimist.org/proj.asp



waveguides have to be implemented to accommodate the

very different lengths involved in the sample: a few tens of

micrometers for the PhC waveguide, and about 1 mm for

the cleaved chip itself. A careful fabrication is then

needed, since proximity effects in electron-beam lithogra-

phy jeopardize fabrication uniformity. A reproduciblecoupling to external fibers is also needed to perform the

pseudocutback method with a good accuracy.

Another method relies on Btop view[ measurements

[Fig. 5(b)]. The uniformity of the light scattering

mechanism is built-in for leaky mode (unlike roughness-

induced scattering). The light scattered by the PhC is

measured along the guide to be studied. SoI structures

have been investigated by this method, and Bloch modeswere evidenced through their standing wave patterns in a

top view arrangement [15].

Alternatively, the reflections on the uncoated cleaved

facets can be advantageously exploited. FP cavities are

indeed formed between the cleaved facets and the

ridge/PhC transitions. The analysis of the FP fringes con-

trast gives access to the cavity losses and the corresponding

exponential factor expð��LÞ as well.The use of an internal light source instead of external

injection may offer an additional flexibility for character-

izing either a large number of structures or structures of

high complexity. Quantum well or quantum dot layers

embedded in the waveguide core can be photoexcited for

this purpose. This technique holds mostly for III–V

structures (GaAs, InP), and Ge islands on Si. Fig. 5(c)

shows the schematic arrangement exploiting the collectionof light through the cleaved edge. It also shows how

microcavities can be probed by measuring the front

photoluminescence diffracted at the cavity edges. The

main drawbacks of this method are the limited sensitivity

(weak photon fluxes, 1 nW-1 pW) and the restricted

spectral range (5%–20% in relative units). The latter

impediment can be circumvented by probing structures of

different lattice constants a, and then stitching togetherthe spectra plotted versus the dimensionless frequency

a=� Fig. 5(d) shows the experimental results for 15 rows of

crystal probed along �K.

Waveguides losses are a benchmark of choice. The

lowest values measured in the substrate approach for a

W3 guide are 15 dB/cm [16]. They fall to 2 dB/cm in the

broader W7 structure. In turn, the lowest values in W1

are around 100–200 dB/cm. Deep-etching of high-aspect-ratio PhC holes, crucial to the obtainment of low

losses, is achievable using the inductively coupled plasma

Fig. 5. (a) Measurement of a PhC device by the end-fire method, using ridge access waveguides. (b) Measurement of locally scattered light

intensity , and logarithmic plot. (c) Internal light source technique, based on embedded quantum wells or quantum dots in the heterostructure.

(d) Corresponding results for 15 rows of PhC along the �K direction. Data from various periods are stitched together on the normalized

frequency axis.



(ICP) technique, the electron cyclotron resonance (ECR)

reactive ion etching (RIE), as well as chemically assisted

ion beam etching (CAIBE).

Waveguide losses in the membrane approach have

been extensively measured for the canonical W1 system.

They are around 1000 dB/cm for points above the light

line. In contrast, recent efforts from teams in the United

States and Japan [17], [18] have resulted in losses as low as1–8 dB/cm below the light line. This is indeed a tremen-

dous achievement, reaching the regime where surface

roughness associated to the etching process becomes the

only limiting factor, with negligible structural errors.

3) Waveguide Bends: Strategy and Results: Sharp bends at

the wavelength scale are one of the major expectations

from photonic-crystal-based PICs. Bends break the trans-lational symmetry of waveguides. Even in the absence of

in-plane losses due to bandgap, bends can lead to in-plane

reflection as well as to scattering in the third direction A

near-unity transmission was predicted in 1996 by an MIT

group for a sharp bend in a quasi-single-mode waveguide

[19], but it was in the case of a lattice of dielectric rods in

air. The effective wavelength in the guide core was large

enough to ignore wall corrugations and out-of-plane

diffraction. The situation is less favorable for the practical

case of a lattice of air holes in a dielectric.

Moreover, in multimode waveguides, many in-plane

modes are excited resulting in a transmitted beam of poor

quality in general. One solution is to Bsmooth[ the

waveguide bend over a few micrometers [Fig. 6(a) and

(b)]. The excitation of multiple modes is still the general

case but there is a Bquiet[ region quite immune to thephenomenon where the transmission is maximized.

Another elegant solution consists in using a taper to a

single-mode W1 ðWi ! W1Þ at the bend entrance while

using the inverse mode converter ðW1 ! WiÞ at the bend

output [Fig. 6(e) and (f)].

The optimization of bends in W1 waveguides has been

investigated in detail. Such a bend can be seen as a short

PhC structure whose modes have to be matched to themodes of the connected waveguide arms. More simply,

this can also be seen as an impedance matching problem.

Finally, different strategies rely on a general shape

optimization, devoid of physical assumption, using some

figure of merit such as reflection and bandwidth. The

results of Fig. 6(c) and (d) are not intuitive. A high

transmission of the bend is predicted on a sizable

bandwidth, which is a delicate requirement [20].

Fig. 6. (a) Modified bend in a W3 geometry: calculated transmission (bold line) and measured data (gray line). (b) Same for a more modified bend.

(c) Optimized bend in a W1 geometry: snapshot of the magnetic field. (d) SEM of a corresponding fabricated structure. (e) Use of a W3 to W1 taper

to implement a bend in a monomode section of the waveguide. (f) SEM of a corresponding realization.



4) PhC Waveguides and Their Competitors: All in all,issues of PhC waveguiding are similar to those met for

photonic wires in the SoI technology, and the compaci-

ty/bandwidth compromise may be not much better. Deeply

etched ridge waveguides or photonic wires can indeed

provide an almost perfect guiding, although there is no

gap to stop the in-plane irregularities. Typical losses of

� 500-nm-wide silicon wires [21], [22] are close to those

of W1 membrane PhC waveguides. Bent photonic wireswith small curvature radius of a few micrometers have

also been realized with loss inferior to 0.1 dB over a

bandwidth of � 250 nm. This performance is hard to

achieve in PhC systems whose TE gap width is typically

of 250 nm [Fig. 1(h)]. In turn, the PhC technology is

certainly the best adapted to the fabrication of ultra-

narrow waveguides (e.g., Wx with x G 1) with an

extreme confinement of light. PhC waveguides also pre-sent a smoother surface topology, which is more adapted

to large-scale integration: only one level of metallization

is required a priori for the integration of active

components.

C. Microcavities

1) Microcavity Modology: Any defect surrounded by aPhC with a band gap defines a cavity. The modes of this

cavity, however, have no simple properties as compared to

those of, e.g., rectangular boxes, and they do not have

either, in general, a simple relation with the modes of the

surrounding crystals. One exception is that of shallow

modes, which arise for slight modifications of the PhC

(chirping the lattice [23], [24], etc.). As for dopants in

semiconductors, these modes are of Bdonor[ or Bacceptor[type, with a field pattern closely similar to that of the

surrounding crystal.

The number of modes per unit frequency, or within the

bandgap interval, can be roughly predicted from the

density of states (DOS) of the defect Bmaterial,[ which is

simply bulk dielectric in many instances. In triangular

lattices, the simplest mode symmetry is of the Bmonopole[type, invariant by sixfold rotations. Fig. 7(a) depicts amode pattern in the case of a H2 cavity, with a high but

distinct symmetry, close to a microdisk-type mode.

Among the various cavities, it should be noted that the

B1-D[ cavities of Fig. 2(b) are still a good tool to quantify

reflectivities, a delicate task otherwise. In the substrate

approach, this latter is now around R ¼ 0:97, which

qualifies it for a number of in-plane confinement

applications in lasers notably. In membranes, microcav-ities are often different in nature, but modal reflectivities

at the closed end of W1 waveguides can apparently reach

99.99% in extremely optimized cases, where the Q factor

is of several tens of thousands. The Kyoto team of Noda has

realized several breakthroughs recently using specific

Bclosed W1[ designs [L3 cavity in Fig. 7(a)] [25] or even

more subtle Bheterostructured W1[ designs [26] for

ultrahigh Q. We discuss below the physics underlying theobtainment of very high quality factors Q.

2) Microcavity Q Factor: As can be seen from Fig. 7(a),

there has been a sustained progress in PhC microcavity Qfactors over the last years. The main strategy of

improvement has been to expel the in-plane Fourier

transform of the resonant mode outside the light cone.

However, even if this task is performed, the strength ofthe radiation is not only connected to the mode Fourier

components, the problem being rather akin to that of an

arrayed antenna with interacting elements, a still contro-

versial topic. Breaking the PhC symmetries is another

aspect, achieved either by modifying a few holes in the

crystal or by deliberately using an elongated form of

cavity, a most successful approach to date. The latter has

allowed Noda’s team to Bsteadily[ gain three decadessince 2000 [25], [26], while other pioneering groups,

notably at Cal’tech, have also proposed successful

alternative designs [23], [27]. These results show that Qfactors of PhC microcavities can be increased to become

comparable to- (and even higher than-) those obtained for

micropillars [Fig. 7(b), typical Qs around 103–104] and

microdisks [Fig. 7(c), typical Qs around 104–105]. The

insertion of emitters is thus on the verge of being fullyexploited yet.

3) PhC Microcavities Versus Other Microcavitiesa) Q=V Versus Q or Q2=V Challenges for Funda-

mental and Practical Investigations: The enhancement of

the peak DOS at the resonant mode gives rise to the

Purcell effect [28], thus accelerating spontaneous emission

rate. The relevant factor of merit is Q=V, where V is themode volume. In a planar FP, this quantity is basically the

finesse of the cavity. For 3-D microcavities of more

complex geometry, recent results provide a clearer vision

of the compromise to be made between Q and V for

optimizing the factor Q=V. It is likely that Purcell effects

(essentially a measure of peak DOS enhancement) of over

� 100 could be reached soon, opening a way to make

future LEDs as Bfast[ as lasers.The strong-coupling regime is more concerned with

the strength of the modal field itself, which is related to

1=ffiffiffi

Vp

rather than to the volume [29]. The factor to be

maximized is then Q2=V, making the high Q still more a

challenge than the small volume. However, it is not yet

clear how strong coupling, e.g., with quantum dot

emitters, could be put to use in devices.

b) Fabrication Accuracy and Roughness Limits: Evenwith a perfect design, fabrication has various limits. The

very position of interfaces, at the level 0.1–10 nm, may be a

trouble for ultrahigh Q structures. The role of the lateral

scale of roughness has yet to be established. Some

Bsmoothing[ techniques exist (oxidation of Si, electro-

chemical wet etching, etc.) to reduce small-scale roughness,

but they have to respect the overall interface positions.



The learning curves of both design and technology, given

the current trends, are equally concerned, making further

progress avenues both challenging and exciting.

III . PASSIVE COMPONENTS INSEMICONDUCTOR-BASED 2-D PHCS

A. Full PhCs as Dispersive Elements

1) Superprisms: When used in transmission bands

instead of optical bandgaps, a piece of 2-D PhC may

readily serve as a dispersive device, much like a glass

prism or a grating. However, the 2-D nature of the

arrangement and the strong index contrast between air

and semiconductor result in very nontrivial dispersion

relations of Bloch modes, opening in turn the way to more

sophisticated devices. Fig. 8(a) shows the isofrequencycontours of a heuristic 2-D PhC with square geometry:

! ¼ !ðkx; kyÞ. A wave of frequency !0 impinging on the

crystal will be transmitted in such a way as the kcomponent along the interface (here ky for the superprism

input) will be conserved modulo G ¼ 2=a. The trans-mitted Bloch wave group velocity in the PhC is then

normal to the isofrequency curve !ðkx; kyÞ ¼ !0 at the

conserved ordinate k ¼ ky.

A giant dispersion of the group velocity direction is

obtained when the incident light is coupled to those bands

singular around the Brillouin zone edge kx ¼ 2=a[Fig. 8(b)]. Such a Bsuperprism[ effect [Fig. 8(c)] has

been reported for the first time by Kosaka et al. in 1999[30]. Dispersions as high as 5�=nm were demonstrated.

Recent experiments have been conducted in integrated

optics [Fig. 8(d)] [31], [32] leading, for instance, to dis-

persions of �1:3�=nm in the SoI system. However, two

main difficulties subsist. First, a very accurate crystal fab-

rication is needed to operate near a singular point of the

isofrequency curve. Second, the collimated input beam

Fig. 7. (a) Trend of Q factor increase over last seven years, with pictures of cavities as inset ; Q=V follows about the same trend, although a factor

of � 5 exists between the various PhC cavity volumes displayed here. (b) A micropillar type microcavity, where a mode resonates like in a VCSEL.

(c) A microdisk type microcavity, with the mode Btrajectory[ at the disk periphery sketched.



must be wide enough to limit the ky dispersion at the

entrance. This may lead in turn to bulky PhC devices with

poor transmission figures and poor crosstalk performances.

The future of this approach is thus unclear.

2) Supercollimators and Negative Refraction: The con-verse operation, Bsupercollimation,[ is obtained when

employing a particularly flat portion of the dispersion

relation [33] [Fig. 8(d)]. Then, a bunch of rays diverging at

the PhC entrance may display group velocity Bfocusing[along the specific normal to the flat dispersion contour.

Notomi [34] has proposed illuminating discussions of this

effect as well as of the superprism effect.

Negative refraction is yet another interesting case; itarises when the isofrequency curves are nearly circles, and

the group velocity, normal to the curve, is oriented toward

its interior [Fig. 8(f)]. The crystal then refracts as a

homogeneous medium with a negative effective index

would do: a bunch of rays diverging at the PhC entrance is

focused within the crystal and also beyond it [Fig. 8(f)].

One application of negative refraction is to focus light

outside a chip without using any integrated lens, but ratherjust by inserting a piece of PhC between the guide output

and the chip facet [35]. Let us stress that the concept of

negative refraction may carry a misleading analogy as the

same phenomenon occurs in the so-called left-handed

materials. These latter are a special class of metamaterials,

where " and � are both negative, thanks to specific

resonances in LC-type resonators [36].

B. Optical Delay LinesThe interest of slow modes with low group velocity in

regions near gaps has already been mentioned for

waveguides (Section II-B1). To date, convincing results

of slowing down light in full PhCs are scarce. It can be

reasonably admitted that the losses incurred in slow

regions are intrinsically large, being naively proportional

to the wave Bdwell time.[ Waveguides can, in turn, slowdown waves without interacting too much with them. This

is illustrated, for instance, in the case of elongated

microcavities, which are but closed W1 waveguides

(Section II-C2). Very low losses are obtained for these

cavities whose mode corresponds to a slow mode of the

open W1 waveguide. Further exploration is needed to fully

exploit this fascinating effect into real-world devices.

One application of slow PhC waveguide modes is toembody an optical memory. Optically storing bits of

information is clearly of great interest to manage

Fig. 8. (a) Isofrequency curves for a heuristic dispersion relation of a square lattice crystals, with singularities and flat regions, for three close

frequencies. The conservation of the tangential k component is represented in dashed lines. (b) Zoom on the singular zone edge. (c) Real space

picture: beams at different frequencies have a large walk-off. (d) Supercollimator whose input waves are indicated by gray arrows in (a). All

k-components have nearly the same group velocity inside the crystal: an output beam from a narrow waveguide will experience almost no

diffraction upon traversing the supercollimator. (e) Generic implementation, showing the conserved k component (results from T. F. Krauss,

St Andrews University). (f) Negative refraction at a bulk/PhC interface and its use in a Bflat lens[ for imaging a point source inside and

outside the PhC.



transparent networks. The typical time constant needed is

25 ps per bit for a 40-GHz operation. A time delay of

100 ps in a 30-�m-long device represents a velocity

c/1000, a value that seems attainable at a single frequency.

An interesting way to go to slow modes is the

coupling of a string of cavities. This new waveguide type

proposed by Stefanou in 1998 [37] was further termed

Bcoupled resonator optical waveguide[ (CROW) by Yarivin 1999 [38]. PhCs represent indeed a wonderful oppor-

tunity to implement this concept, as was demonstrated

soon after in [39]. Other configurations of interest are

those based on the use of chirped PhCs [40]. These latter

could also be used as laser mirrors for pulse compression

in mode-locked semiconductor lasers (Section IV-A2c).

The ability to manage dispersion on a large wavelength

span ð�� 10–100 nm) makes chirped PhCs attractive

components compared either to Bragg gratings in fibers

or to multilayer mirror stacks that are tedious to growaccurately. Chirped PhCs can also be applied to phase-

matching in nonlinear optics (Section IV-B1).

Fig. 9. (a) Mode spectrum measured at the output of a W3 waveguide when photoexciting an H7 cavity on it side. The cavity is separated

from the guide by either two or three PhC rows. (b) Results for an elongated cavity on the side of a W1 waveguide in the membrane

approach [25]: dropped signal intensities for different shifts of the holes at the cavity ends. (c) Q variation versus shifted hole coordinate.

(d) Principle of coupling between guides based on the transfer to a higher order mode. (e) Predicted results for a 11-�m-long device showing

the large loss tolerance of the device. (f) Device based on contradirectional coupling between a W0.8 and a W1 waveguide [46]. (g) Predicted

and measured results for the dropped spectrum of the first prototype with �� ¼ 8 nm resolution (see [45]).



C. Add–Drop FiltersAdd–drop filtering is a much desired function in PICs,

above all if reconfigurable. One needs a bus and a selective

element. Moreover, the selective element should support

two orthogonal quasi-degenerate modes to ensure direc-tionality in the add/drop line. In microdisks, this is

naturally the case of the two counterpropagating modes.

For basic single-mode microcavities, one may use a pair of

them. The MIT team proposed theoretical implementa-

tions of this concept [41], but none that would translate

into feasible semiconductor systems at 1.5 �m. In the

current status of the topic, the roads offered by PhCs can

be divided into two kinds. First configurations are based onguide-cavity coupling, whereas the second ones use a

direct coupling between guided modes.

1) PhC Guide-Cavity Coupling: The cavity should lie on

the side of the input/(output) guide(s) to let all the

unaffected frequencies propagate along the device;

Fig. 9(a) and (b) exemplifies pioneering realizations. The

optical coupling of the cavity to free space (verticalradiation loss) and the optical couplings between the cavity

and the waveguide(s) can be cast into the effective Qfactors Qv, Qin (and Qout), respectively. Optimization of the

Q values actually depends on the filter design and its

application.

In 2000, Noda’s team achieved impressive results by

using straight waveguide coupled to a point defect H1

microcavity [42]. The dropped wavelength was extractedvertically (no output guide), and equal values were

chosen for Qv and Qin in such a way as half the photons

flowing in the guide were emitted vertically. The overall

Q reached about 500. In 2004, the same team used a

different configuration where the elongated cavity shown

in Figs. 2(b) and 7 was coupled to W1 input/output guides

[Fig. 9(b) and (c)] [25], [43]. The overall Q reached

30 000 (and even the ultrahigh value of 600 000 in anew design [26]). The achievement of a high Qv is crucial

for the use of such a device in dense WDM. Recent

progress in integration and performances have been

reported by the NTT team [44]. In the substrate ap-

proach, early results on nonultimate cavities [Fig. 9(a)]

[45] showed a clear potential, furthered by alternativeclever approaches. One of these approaches is depicted in

Fig. 9(f) and (g), while it actually pertains to the case of

coupling between guides (here W0.8 and W1) [46] as

discussed in the next section.

2) PhC Guided Mode Coupling: In a family of proposals,

the coupling between two guides in PhC was used to

implement selective functions [47], [48]. Due to theperiodicity, PhC waveguide couplers are natural grating-

assisted waveguide couplers, whereas the bandgap effect

suppresses in-plane radiation loss. A short beat length

between two codirectionally coupled waveguides is a

typical feature attainable with proper design.

Another recent scheme exploits multimode sections to

convert the fundamental guided mode into a specific high-

order mode [see Fig. 4(b)] that can easily tunnel throughthe PhC barrier between guides [49] [Fig. 9(d) and (e)].

This configuration indeed constitutes a fault-tolerant and

loss-tolerant filter. The illustrations of the two waveguide-

coupling based devices [Fig. 9(d)–(g)] suggests that this is

a privileged road to be further exploited.

D. Spectrometer-on-ChipMode coupling in a PhC multimode waveguide can also

be used to build up an integrated spectrometer-on-a-chip.

Fig. 10(a) illustrates the principle while Fig. 10(b) shows a

first prototype fabricated in a collaboration of one of the

authors with HHI and Alcatel [50].

A wedged waveguide provides the mode-coupling

situation at different places along the guide for the

different wavelengths. Light injected at a given wavelength

into the fundamental mode of the guide is redirected into aspecific lateral channel after conversion to a higher order

Fig. 10. Spectrometer-on-chip (wavelength monitor) based on a wedged PhC waveguide. (a) Principle: in each section, the fundamental mode at

a specific wavelength is converted into a higher order mode that tunnels through the thinned waveguide barrier. (b) Micrograph of a

realization at HHI (Berlin, courtesy of K. Janiak), with superimposed light paths toward photodiodes.



mode, whose strong leakage is favored by its deeppenetration into the thin PhC walls. Integrated photo-

diodes can be placed to monitor the various signals. PhC

waveguides offer such a unique situation where light

confinement and grating-type action can coexist to provide

a mini-stopband at specific values of k and !. Other

Bselective[ structures are selective either in k (a simple

corrugated waveguide acts on all frequencies) or in ! (a

point cavity acts for all directions of impinging waves). Qfactors of � 300 have been obtained for the spectrometer

prototype. Q factors of � 1000 are expected from

optimized versions. Such performances are of interest for

coarse WDM networks, where the wavelength spacing is

�� ¼ 20 nm.

IV. ACTIVE COMPONENTS INSEMICONDUCTOR-BASED 2-D PHCS

A. Lasers

1) Surface-Emitting Lasersa) PhC Microcavity Lasers: The perspective of

achieving a better control of spontaneous emission in

optoelectronic devices has been a major impetus toresearch on PhCs in its early development [1]. Ideally,

the full control of spontaneous emission could lead to

thresholdless lasers where all injected electrons would be

converted to photons emitted in a single cavity mode [51].

More realistically, the increase of the spontaneous

emission factor into a given mode and the simultaneous

reduction of the active layer volume can provide ultralow-

threshold lasers with original properties including lownoise and fast dynamics [10], [52], [53]. A reasonable

approach consists in creating a small defect in a 2-D

photonic-bandgap slab. Because the k-spectrum of the

confined in-plane field possesses sizable components

down to k ¼ 0 within the light cone, there is a certain

leakage of light along the third (vertical) direction, which

actually constitutes the useful laser output. The tradeoff is

then to achieve a high-Q cavity and a low laser thresholdwhile keeping a sufficient level of vertical emission and an

acceptable beam shape.

The studies of surface-emitting defect mode lasers have

closely accompanied those of PhC defect microcavities

(Section II-C). The first PhC laser emission near

� ¼ 1:5 �m was reported in 1999 by the Caltech group

[54]. A modified H1 microcavity was used where two holes

among the six ones surrounding the defect were designedwith a larger diameter to lift the intrinsic mode

degeneracy. The laser was operated under pulsed optical

pumping with an estimated mode volume of � 0.09 �m3.

A recent and decisive progress has been achieved by the

Kaist group in Korea, demonstrating an electrically driven

single-cell PhC laser at room temperature [24]. As seen

from Fig. 11(a), the introduction of a central post under the

PhC slab allowed the current injection into the activeregion while it did not notably degrade the quality factor Qof the H1 defect microcavity. The latter was optimized by

using a chirped pattern size around the defect (Fig. 11(a),

inset), thereby leading to Q 2500 and a mode volume of

� 0:68ð�=nÞ3 ¼ 0:058 �m3. A record value of � 0.25

was estimated from the light–current characteristics. In

contrast, the threshold current ð� 260 �AÞ was rather

Fig. 11. Surface-emitting PhC lasers. (a) Cross-sectional SEM image of the electrically driven single-cell PhC laser demonstrated in [24]. Inset

shows the chirped pattern size in top view. Regions from I to V correspond to PhC holes of increasing diameter. (b) Schematic view of the band-

edge PhC laser reported in [65] : an InP-based perforated membrane with four active InAsP quantum wells is bond onto silica on silicon. The inset

shows a magnified part of the PhC structure.



larger than the previously reported � 36-�A record in avertical-cavity surface emitting laser (VCSEL) [55].

Identification of several current leakage paths in the

fabricated structure actually shows that there is a wide

room for further improvements.

Clearly, the above results represent meaningful steps

toward ultimate photon sources. The use of a single

quantum dot emitter instead of quantum wells in PhC

microcavities offers additional opportunities, especiallytoward on-demand single photon sources for quantum

cryptography applications. Another challenge would be to

funnel most of the emitted photons into a neighboring low-

loss PhC waveguide instead of extracting them vertically. A

first result by Noda’s team [43] on a passive membrane

structure will be certainly soon translated to active

structures in III–V materials.

b) 2-D PhCs for VCSELs: The configuration of defectmode lasers as seen in the former section is essentially

based on an optimized PhC slab whose thickness optimizes

in-plane guiding. A somewhat more complex structure

consists in adding a vertical confinement of light with

Bragg multilayers for optimizing the vertical extraction as

well as the laser output mode profile. This structure is only

a VCSEL with a lateral PhC.

Previous studies have shown that the efficiency ofsurface-emitting LEDs can be increased beyond the limit

values reached in planar cavity by using 2-D PhCs at the

periphery of the emitting area [56]. The recycling of

photons guided in the active layers and/or their extraction

by vertical diffraction mainly explained the improvement

in that case. A similar use of 2-D PhCs was also proposed in

resonant cavity LEDs (RCLEDs) where the resonant ver-

tical cavity is formed by a pair of multilayer Bragg mirrors[57]. The situation for VCSELs equipped with highly

reflective Bragg mirrors is different due to the dynamics of

the lasing mode(s) and of the laser populations.

If the lateral 2-D PhC is used within its gap, the VCSEL

mode(s) is (are) indeed confined both in plane and

vertically . The mode properties will obviously depend on

the respective strengths of the confinements in the

different directions. In the case of broad emitting(unstructured) areas, lasing would rather start in the

horizontally guided modes thus losing most of the

advantages of VCSELs. In contrast, for narrow emitting

areas, 3-D calculations are needed for analyzing the modal

properties of the cavity, but bigger potential can be fore-

seen for applications. This latter configuration represents

indeed an intermediate step toward lasers with a 3-D

confinement of light. The practical fabrication of suchstructures is still restrained by the requirement of high-

aspect-ratio dry etch of submicrometer patterns through a

multilayer stack of typically 5-�m thickness. A somewhat

thinner structure has recently been proposed where the

top mirror consisted of a 1-D PhC membrane [58], but

this configuration has not been experimentally demon-

strated yet.

From a simpler and more practical viewpoint, 2-DPhCs can rather be used to laterally modulate the

refractive index of VCSELs than to create a 2-D photonic

bandgap. In this case, the PhC VCSEL Bmimics[ a slice of

microstructured fiber with all the benefits associated to the

transverse mode control. For a sufficiently large emitting

area, the essential modal properties of the PhC VCSEL can

be understood by the decoupling of the vertical and lateral

confinements. Guided photons are vertically confined bythe distributed Bragg reflection, but the transverse mode

profile is determined by the lateral modulation of the

refractive index. In the same way as microstructured fibers

can be single-mode even with a core of much larger

diameter than the wavelength, large-emitting-area PhC

VCSEL can be designed to support the propagation of only

the fundamental mode. Typical periodicity of the structure

can be of the order of several micrometers as inmicrostructured fibers, while the etched patterns should

not necessarily traverse the whole thickness of the VCSEL

mirrors [59], [60]. Such PhC VCSELs are thus of great

promise for the achievement of single-mode operation

even at high injection currents. The control of spontaneous

emission and light polarization can also be strongly

enhanced as compared to ordinary VCSELs. Single-mode

powers in the 1–10-mW range have recently been obtainedfrom PhC-VCSELs [61] and VCSELs with holey structure

in the GaAs/AlGaAs system [62]. These very encouraging

results should be further extended to other material

systems.

c) Band-Edge Lasers: The simplest surface-emitting

PhC laser structure is that of Bloch mode lasers, also called

band-edge lasers, where the PhC is used as a whole instead

of being located at the periphery of the active region.Actually, such a structure makes an intentional use of the

radiative losses of modes, which propagate in the 2-D PhC.

Lasing preferably occurs on slow modes, notably at the �,

K, and M points of the band diagram where light strongly

interacts with the active medium [63]. To some extent,

band-edge lasers can then be viewed as a 2-D extension of

popular DFB lasers. However, it has to be noticed that the

1-D gratings of DFB lasers are always buried andfurthermore seldom etched to the very active layer.

To date, most of the band-edge lasers have been

fabricated within the membrane approach. The K and Mpoints are then below the light line, and the vertical

extraction of light only results from the finite PhC size

and/or the presence of fabrication errors in that case. By

contrast, the vertical emission is more naturally favored at

the � point. Low lasing thresholds have been reported byseveral authors for InP-based structures of small emitting

area [64]–[66]. For instance, the effective threshold pump

power was estimated to be below 50 �W for a graphite-

like PhC structure of � 7 �m2 [66]. One important

improvement in these experiments stems from the

transfer of the thin multiquantum-well InP-based hetero-

structure onto a silicon host wafer [Fig. 11(b)], thus



providing a more efficient cooling of the active region thanin the free standing membrane case.

2) Edge- (or In-Plane) Emitting Lasers: Edge-emitting

lasers are workhorses of optical telecommunications.

They easily deliver single-mode output powers in the 10–

100-mW range. They can also be integrated with other

optoelectronic components within planar technology

appraoches. Regarding this point, the development ofPhC waveguide lasers with an in-plane emission opens

unprecedented perspectives for the large-scale integration

of lasers with miniature guided-optical devices [Fig. 2(f)].

The simplest geometry of a PhC waveguide laser is that of

a canonical PhC waveguide formed by one or several

rows of missing holes in a triangular lattice PhC

[Fig. 12(a)]. Another interesting geometry is that of a

CROW [Fig. 12(b)] first proposed by Stefanou [37]. Inany case, one advantage of the PhC laser structures

compared to those of more traditional ridge waveguide

lasers stems from the fact that the laser fabrication does

not require any regrowth step, which is a critical fab-

rication step. Another advantage of PhC-based integrated

optics around laser sources results from the fact they

operate in a single polarization (TE) mode. This relaxes

the constraints on the behavior of the optical chain in theTM polarization, in a sharp contrast with the require-

ments imposed on a receiving system.

a) Canonical Waveguide Lasers: Despite the apparent

simplicity of canonical waveguide lasers, different situa-

tions may occur depending on the waveguide width (one,

three, or more missing rows; see Section II-B), on the

crystal direction ð�K;�M; . . .Þ and on the vertical

structure (membrane or substrate). In any case, lasingpreferably occurs at low-group-velocity points (band

edges, � point) where light-matter interactions are

enhanced [63]. The in-gap situation is also preferable,

as in-plane scattering is inhibited. However, an in-gapsituation does not necessarily ensure low 3-D radiation

losses, especially in the substrate approach where the

guided modes essentially lie above the light line.

Moreover, carrier recombination at the sidewalls can

also impact a lot on the mode selection, since it penalizes

modes extending in the crystal regions. This effect

obviously favors the emission on the fundamental mode,

which is the best confined in the guide as in the case ofclassical waveguide lasers.

i) Ultracompact waveguide lasers (W1) An ultranarrow

waveguide formed by one row of missing holes is

but a very elongated microcavity. To date, lasing of

a triangular lattice PhC W1 waveguide has only

been achieved by photopumping and using the

membrane approach [67]. The emitting laser area

was � 0.5 13.6 �m2 [Fig. 12], and the pumpthreshold was estimated to be � 690 �W. The laser

emission was considered to occur on the mode of

lower energy in the gap near the K point [see the

band diagram of Fig. 4(a)]. The dispersion curve of

the fundamental mode actually intersects the

Brillouin zone edge at a lower energy below the

gap. Lasing on the fundamental mode of a narrow

W1 waveguide has been obtained in the case of asquare lattice [68]. The PhC waveguide was

fabricated on InP using the substrate approach

[69]. Experiments were conducted under optical

pumping. Despite the absence of a complete gap, a

low-loss situation occurs at the second folding of

the fundamental mode (� point), where the laser

emission is intrinsically single-mode [Fig. 13(a)].

The selection mechanism based on band-edgedependent losses is actually similar to that reported

earlier for a second-order DFB laser by Henry and

Kazarinov [70]. Here, only one of the two band-

edge DFB modes is well confined in the guide core.

This illustrated in Fig. 13(b), which shows the

calculated field pattern of the confined mode.

Unlike the lasing mode of the W1 triangular lattice

waveguide [Fig. 13(c)], the field does not spread inthe crystal region.

ii) Medium size waveguide lasers (from W3 to W5) For a

�K orientation of the guide in the triangular lattice

and a standard air-filling factor (� 30%), the

fundamental mode does not fold in the gap

whatever the width of the guide is. In other words,

the fundamental mode is not a slow mode in the

gap region. Other slow modes exist in this region[Fig. 4(a) and (b)], but their fields penetrate more

deeply in the PhC claddings [e.g., Fig. 4(c)]. If the

substrate approach is used with a modest vertical

confinement of light, all these modes also suffer

out-of-plane losses, and the in-gap situation is not

so advantageous. The laser action rather takes place

out-of-gap at the folding points of the fundamental

Fig. 12. SEM images of in-plane emitting PhC waveguides. (a) W1 defect

waveguide laser demonstrated in [67]. (b) Coupled resonator optical

waveguide laser demonstrated in [73].



mode. This is indeed observed for the W3 laser

[Fig. 14(a) and (b)], whose emission preferably

occurs above gap at the second folding of thefundamental mode (� point). The laser was

experimentally found to be single mode with a

side-mode-suppression-ratio larger than 40 dB [71].

This single-mode behavior is analogous to the one

previously described for the square lattice W1 laser

(Fig. 13).

iii) The same behavior should be obtained, in princi-

ple, for wider waveguides such as W5 and W7.However, a more direct solution for selecting the

fundamental mode is to create a periodic modula-

tion of the guide in such a way as to fold the

dispersion curve of this mode into the gap

[Fig. 14(e)]. Such a laser has been demonstrated

where the basic W5 guide was constricted to a W3

geometry every six periods of the PhC matrix [72].

A continuous-wave (cw) single-mode emission hasbeen obtained under electrical pumping with a

wavelength selectivity better than 25 dB [Fig. 14(f)]

and an external efficiency over 0.15 W/A

[Fig. 14(g)]. As a major result, this work has

demonstrated the absence of fundamental impair-

ments toward a large efficiency from PhC wave-

guide lasers in cw.

iv) The periodic modulation of the waveguide width issomehow built-in for guides oriented in the �Mdirection. For instance, the width of the W2–3

waveguide is alternately determined by two and

three missing holes. For a 30% air filling factor of

the lattice, the third folding of the fundamental

mode now occurs in gap [Fig. 14(c)], thereby

allowing a genuine DFB laser emission at the M-

point [71]. Typical output spectra measured for the

W2–3 laser either with or without anti-reflecting

coating are shown in Fig. 14(d). The large

separation between the two DFB components ofthe laser emission reveals an equivalent � coeffi-

cient as high as � 400 cm�1.

b) CROW Lasers: W2–3 structures can be seen as

CROWs, albeit with low-Q resonators. PhCs actually allow

exploring a lot of CROW systems with various types of

resonators and various types of coupling between resona-

tors. A familiar system is that formed by coupled hexagonal

cavities [38]. Such a device has been fabricated at theUniversity of Wurzburg [Fig. 12(b)] [73]. Stable single-

mode lasing has been obtained at � � 1:53 �m with a side-

mode suppression ratio greater than 40 dB. The laser

emitted up to 2.6 mW under cw operation at room

temperature.

One formal interest of the CROW concept is that it

leads to dispersion relations, which are both simple and

remarkable for the guided modes. The coupling of theindividual resonator modes creates minibands within the

photonic gap, each of them being almost centered on a

mode of the isolated resonator. Adjusting the coupling

strength (e.g., the number of rows between resonators)

allows varying the group velocity of guided modes. In

standard PhC waveguides, there does not exist such a

simple guideline for the resolution of certain inverse

problems, especially that of finding shapes and frequenciesthat slow down the group velocity of propagating waves.

One possible drawback of CROW lasers stems, in turn,

from the fact that inescapable vertical losses may arise.

c) PhC Laser Systems and Applications: Another

interest of PhC mirrors/guides lies in the possibility of

revisiting earlier single-mode laser configurations, like the

so-called C3 laser, or cleaved coupled cavity laser [74].

First investigated by Happ et al. [75], the PhC versions

Fig. 13. (a) Calculated band diagram of a square lattice W1 waveguide with an air filling factor of 26% and a refractive index of 3.21. Inset zooms of

the second folding of the fundamental mode. (b) Calculated H-field pattern of the fundamental mode at the second folding in the Brillouin zone.

(c) Calculated H-field pattern of the lasing mode of the triangular lattice W1 waveguide laser [67].



have been recently used in an elaborate system of a two-

channel tunable PhC laser diode developed at the Uni-

versity of Wurzburg [Fig. 15(a), (b), and (c)] [76]. Each

laser source is based on the contradirectional coupling of

two independently contacted PhC waveguide segments ofslightly different lengths. The waveguide sections are

oriented in the �M direction of the PhC, and are separated

by an intermediate PhC mirror of one lattice period

(Fig. 15(b), left). The frequency of each laser can be tuned

by separately adjusting the injection currents in the two

laser sections, respectively [Fig. 15(c)]. The outputs of the

two tunable lasers are coupled into a single waveguide

using a PhC Y-coupler structure oriented in the �K direc-

tion (Fig. 15(b), right). Quasi-continuous tuning has thus

been achieved in a � 30-nm window with 36 WDMchannels spaced 0.8 nm apart (ITU grid). The simplicity

of fabrication as well as the promising output charac-

teristics should indeed make this tunable laser design

an interesting source for monolithic integration into

highly integrated photonic circuits.

Fig. 14. Calculated band diagrams of medium-size PhC waveguides and measured output spectra of the corresponding waveguide lasers.

(a)–(b) W3 laser. (c)–(d) W2–3 laser. (e)–(g) W5 laser with a periodic constriction of the waveguide width [72]. In each case, the lasing frequency is

indicated by an arrow. The zig-zag plot in (e) schematizes the fundamental mode folding due to the larger periodicity introduced in the W5 laser.

The light–current characteristic of this laser is shown in (g), with a 0.15-A/W efficiency.



Two-dimensional PhCs can also be used for the

fabrication of laser subcomponents in the more traditionalsemiconductor laser technology. This is illustrated in

Fig. 15(d), which shows the two-section ridge waveguide

master oscillator power amplifier (MOPA) system devel-

oped in [77]. The use of a 2-D PhC side reflector allows

operating the MOPA on a narrowband while simulta-

neously isolating the two cleaved facets. The use of a PhC

reflector at one cavity end provides a high reflectivity in

the desired band while it is compatible with the integrationof a power monitoring photodiode. Two-dimensional

structures are preferable to 1-D grating because they are

less dependent on the roughness and details of etched

profiles and they offer more flexibility for integration. The

PhC-MOPA system fabricated in [77] was capable of

delivering 0.6 W in a narrow spectral band of 2 nm. The

wavelength shift was found to be less than 10 nm from

threshold to 3-A injection current.

The potential of 2-D PhCs for confining light with a

simultaneous control of the group velocity dispersion finallyopens new perspectives for high-speed waveguide lasers.

The achievement of very high power densities in small laser

structures can lead, for instance, to fast laser dynamics and

wideband modulation [78]. The control of the lasing mode

group velocity dispersion can be exploited for short pulse

compression and short pulse generation as well [40], [79].

B. Toward Nonlinear PhC Devices

1) Frequency Conversion: The two conditions for an

efficient frequency conversion process in a nonlinear

optical material are the existence of a high value of the

nonlinear susceptibility and the possibility of phase

matching between the interacting waves. The first

condition is quite fulfilled in bulk III–V materials, but

phase matching is not allowed due to their cubic

Fig. 15. (a) Schematics of the two-channel tunable PhC laser diode developed in [76]. (b) Aggregated SEM images of the device. Left: PhC coupled-

cavity laser source with waveguides oriented in the �M direction. Right: PhC Y coupler in �K orientation. (c) Output spectra under simultaneous

operation. Laser 1 is tuned while the wavelength of laser 2 is fixed. (d) Schematics of the MOPA system developed in [77] with an SEM view of PhC

subcomponents.



symmetry. Phase matching is, in turn, obtainable when

structuring the refractive index of III–V materials.

Actually, microstructured materials such as PhCs notonly allow a genuine engineering of the refractive index,

but also provide the simultaneous control of the group

velocity dispersion at the different wavelengths. This is

schematized for second harmonic generation (SHG) via

the unfolded dispersion diagram of Fig. 16. Practically, the

phase matching condition is obtained by appropriately

designing the band structure of the 2-D PhC and using its

birefringence properties: the energy carried by a TE waveat ! and k! can be transferred to the harmonic TM wave at

2! and k2! ¼ 2k! in the example of Fig. 16. The existence

of slow PhC modes at both frequencies reinforces the

mutual interactions of the two waves with the nonlinear

material. Thus, the SH conversion efficiency achieved at

� 1500 nm in a 1-D PhC composed of alternate

AlGaAs/Alox layers has been found to increase almost as

N6, where N was the number of layers [80]. This result isof great promise for very compact frequency converters in

semiconductor-based PhCs. The extension from a 1-D PhC

to W1 waveguides in 2-D PhCs has been discussed in [81],

and application to full 2-D PhCs has been reported in [82].

More generally, all semiconductor sources that use a

nonlinearly stimulated process can benefit from the in-

trinsic properties of 2-D PhC waveguides and resonators.

For a given optical power injected in a resonant device ofvolume V and quality factor Q, the circulating intensity is

proportional to Q=V. Thanks to the recent performances of

PhC resonators, intensities larger than 100 MW/cm2 are

achievable at pump powers less than 1 mW. This can be

exploited in third-order nonlinear processes including, for

instance, stimulated Raman emission and four-wave

mixing. Self-phase modulation in narrow PhC waveguides

could also be used for short-pulse broadband emission,following previous demonstrations in PhC fibers [83].

2) Fast Optical Switching in Reconfigurable PhCs: Among

third-order nonlinear processes, refractive index nonlinea-

rities (Kerr effects) are known as a powerful means ofcontrolling light by light in semiconductors. Very low

thresholds can be expected from PhCs structures that

provide a strong confinement of light. This opens the way

toward miniature versions of fast all-optical switches and

routers in integrated optics. The progress in this field is

illustrated by the recent work of Raineri et al. [84], where a

narrow (0.4 nm) reflection band of a 2-D PhC was

blueshifted by more than 8 nm under optical pumping inthe 0.1–1-mW range. Fig. 17 shows a schematic view of the

sample used in these experiments (left) along with the

results of spectral measurements (right). The InP-based

PhC slab included four InGaAsP quantum wells whose

refractive index was varied with pump intensity. Pump and

probe beams were incident perpendicularly to the slab

surface. The 8-nm blueshift of the slab reflectance was

obtained at a rather constant level of reflectivity, whereas astrong amplification of the vertical probe was observed for

pump intensities above � 4 kW/cm2 [85]. All-optical PhC

switches as the one of Fig. 17 can also be seen as

reconfigurable or tunable PhC wavelength filters. Of

particular interest for the domain of optical telecommu-

nications is the possibility of achieving in-plane versions of

these devices. All-optical bistable switches using 2-D-PhC

nanocavities fabricated on SoI have recently been demon-strated in the thermal regime [86]. Extension of this work

to III–V devices employing fast optical nonlinearity can be

expected soon.

V. 3-D PHOTONIC CRYSTALS

The original dream of being able to manipulate the flow of

light in all three dimensions, and ultimately to control thespontaneous emission from a single emitter with a

complete 3-D photonic bandgap, is still very much alive

despite the difficulties in fabricating 3-D structures.

Recent results obtained on the association of light emitters

with 3-D crystals are encouraging in that sense [87], [88].

Three-dimensional PhC microcavities represent indeed an

ideal configuration for thresholdless lasers and single-

photon sources (Section IV-A1).Several approaches exist for the fabrication of 3-D PhCs

in semiconductors. One approach consists in creating a

3-D template in a low-index dielectric, and then in-filling

the template with semiconductor. Either artificial opals

made of small silica spheres [89] or 3-D structures fab-

ricated by X-ray lithography in a polymer [90] can be used

as templates. Infilling of artificial opals has been success-

fully achieved with various semiconductors, although notof epitaxial quality [91]–[93]. However, the template

approach does not allow an easy incorporation of guides

and microcavities into the 3-D periodic structures.

Another approach based on thin-film technologies consists

in constructing 3-D PhCs layer by layer much in the same

way as one stacks a woodpile. This other approach has been

successfully demonstrated on silicon [94] and III–Vs [95].

Fig. 16. Schematic representation of a SHG process in a PhC (unfolded

dispersion diagram). Phase matching creates a coupling between TE

and TM slow modes at ! and 2!, respectively.



The insertion of microcavities and waveguides is straight-

forward in its principle, but still delicate in its realization.

Thanks to the continuous progress in planar microtechnol-

ogies, there is no doubt that sophisticated 3-D structures

will be realized soon within this technique. For small-scale

fabrication in laboratory, other technologies were also de-

monstrated such as chemically assisted ion beam etching

[96], focused ion beam etching [97], and autocloning [98].

VI. CONCLUSION

We have shown that the understanding of the physics of

semiconductor-based PhCs and the required technologies

are now all making rapid progress. At the beginning of

1998, nobody could imagine an ultrahigh Q cavity

(Q ¼ 600 000 in [26]) or single-cell electrically injectedlasers on a membrane [24]. Although these are futuristic

devices for real-world applications, there are many other

entrance points for functional devices such as PhC-VCSELS

and various lasers, or spectrometers on a chip, for which

the market place becomes a reality more than a dream.

With proper design, semiconductor based 2-D PhCs can

indeed bring new functionalities to optoelectronic devices,

as, for instance, the dispersion control for which abreakthrough is not unlikely in the forthcoming decade.

Perhaps still more important than their application to

new isolated devices is the fact that semiconductor-based

PhCs and, more generally, semiconductor-based high-

index-contrast structures are a real opportunity to bring

the large scale level of integration into the world of

photonics. PhC components take advantage of the

nanostructuring technologies developed for microelec-

tronics. From this point of view, the successful techno-

logical approach of III–V nanostructures for active devices,

on one hand, and that of SoI for low losses and high

confinement, on the other hand, should be pushed further

for future developments. An additional step of optimiza-tion would be to combine these approaches more

systematically as it has already been done, for instance,

in the report of III–V membranes on SoI. Ideally, an

ultimate solution for 2-D integrated optics would be to

have access to a III–V semiconductor-on-insulator tech-

nology equivalent to the SoI one. Telecommunications

remain the most relevant field of application of PhC

structures. As such, telecommunications also prompt thestudies of PhC themselves, their limits and the underlying

physics. This wonderful synergy is, fortunately, likely to

continue for a long time. h

Acknowledgment

The authors would like to acknowledge the discussions

with the members of the FUNFOX European project andthose of the former PCIC and PICCO FP-IST European

projects. The authors would like to thank the members of

the French RNRT project CRISTEL for their helpful

collaboration, with a special mention to the technological

support of Thales-Alcatel.

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ABOUT T HE AUTHO RS

Henri Benisty was born in Casablanca, Morocco,

in 1963. He received the Ph.D. degree in accumu-

lation layers at Si interfaces from Ecole Normale

Superieure, Paris, France, in 1989.

He was with the University of Versailles until

2002 and is now with Laboratoire Charles Fabry

de l’Institut d’Optique, Orsay, France. Since re-

ceiving his Ph.D. degree, his research topics have

been first nanostructure growth and physics

(Thales, then Thomson, Orsay) and lamellar III–VI

compounds (Paris 6 University). Since 1994, he has been involved in

research on planar cavities (mainly for LEDs) and in photonic crystals in

two dimensions on III–V, with both experimental and theoretical

approaches at the Laboratoire de la Physique de la Matiere Condensee

in Ecole Polytechnique, Palaiseau. He currently investigates applications

of photonic crystals to LEDs, biophotonics (he cocreated the start-up

Genewave in 2001), and miniature photonic integrated circuits.

Jean-Michel Lourtioz (Senior Member, IEEE) was

born in Lens, France, in 1948. He graduated from

Ecole Centrale, Paris, France, in 1971 and received

the equivalent of the M.S. degree in physics and

the Ph.D. degree from the University of Paris in

1975 and 1981, respectively.

Since 1976, he has been with CNRS and has

worked at the Institut d’Electronique Fondamen-

tale (IEF), University of Paris-Sud, France. He is

currently Directeur de Recherche at CNRS and is

the head of IEF, which includes 125 permanent researchers, technicians,

and administratives and 80 Ph.D. students and postdoctorals. From 1996

to 2001, he coordinated in France the research studies on photonic

crystals and microcavities. Since 2002, he has coordinated the French

network on nanophotonics. His current research interests include optical

and fast electronic devices, semiconductor nanostructures, photonic

crystals, and microcavities.

Alexei Chelnokov was born in St. Petersburg,

Russia, in 1965. He received the B.S. degree from

St. Petersburg Polytechnical Institute in 1988 and

the Ph.D. degree for work on dynamics of high-

power semiconductor lasers and on planar erbium

doped amplifiers from the Institut d’Electronique

Fondamentale (IEF), University of Paris-Sud,

France, in 1995.

He worked for three years as a member of

research staff at A.F. Ioffe Physico-Technical

Institute, St. Petersburg, on short pulse generation from semiconductor

lasers. From 1996 to 2001, he spent five years as a CNRS researcher at

IEF, working on microwave and optical photonic crystals. In 2001, he

joined Fontainebleau Research Center, Corning SA, France to develop

semiconductor optical amplifiers, rapid photodetectors, and photonic

crystals. Since 2003 he has been at the CEA Leti laboratories, Grenoble,

France. He is author and coauthor of over 40 scientific papers and over

40 conference presentations. His research interests include integrated

optics, III/V optoelectronic devices, and nanooptics.

Sylvain Combrie received the Diploma degree in

engineering from the Ecole Nationale des Tele-

communications in Paris and the M.S degree from

the University Pierre et Marie Curie, Paris, France,

in 2002. He is currently working toward the Ph.D.

degree in nanooptics devices at Thales Research

and Technology in Orsay. His research interests

include design, fabrication (e-beam lithography,

ICP plasma process), and characterization of

high-Q cavities in photonic crystals.

Xavier Checoury was born in Boulogne-Billan-

court, France, in 1974. He received the engineering

degree from the Ecole Nationale Superieure des

Telecommunications in 1998. He is currently

working toward the Ph.D. degree at the Institut

d’Electronique Fondamentale (IEF), University of

Paris-Sud, France.

From 1999 to 2002, he worked as an R&D

engineer at EADS Telecom (formerly Matra Nortel

Communications). His research interests include

photonic crystals, semiconductor lasers, and numerical modeling.



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