n00014-01-1-0803 annual report 2004xlab.me.berkeley.edu/muri/kickoff/muri annual progress... ·...

Annual MURI report for 2003-2004 – Scalable &Reconfigurable Metamaterials, UCLA

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Office of Naval Research

MURI Annual Progress Report (Jun. 2003 – Aug. 2004)

For Dr. Michele Anderson and Dr. V. Browning on

GRANT NUMBER: N00014-01-1-0803

DOD/ONR MURI

Scalable and Reconfigurable Metamaterials

Principle Investigator: Xiang Zhang

Co-PIs: G. Chen, T. Itoh, E. Yablonovitch, J. D. Joannopoulos, J. Pendry, D. Smith, and S. Schultz

University of California in Los Angeles

Engineering IV, 420 Westwood Plaza

Los Angeles, CA 90095-1597


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Contract Information Contract Number N00014-01-1-0803 Title of Research Scalable and Reconfigurable Metamaterials Principal Investigator Xiang Zhang Organization University of California in Los Angeles

Technical Section

Technical Objectives The goals of this project are to develop new synthesis technologies for fabrication of 3D scalable and reconfigurable meta-materials, to explore the new physics and simulation methods of meta-materials, to experimentally characterize the physical properties of meta-materials, to demonstrate prototype meta-material-based devices for novel electromagnetic wave applications and to transfer the developed technology to defense industries and to develop an interdisciplinary educational program that trains a new generation of graduate students and post-docs in the science and technology of meta-materials. Major Accomplishments during This Period (3rd year) Considerable progresses have been made during the third year on metamaterial synthesis, novel physics study, and metamaterial devices and metamaterial characterizations. (1) Metamaterial synthesis

• Sub-100nm plasmonic photolithography demonstration using Al hole array: Zhang • Sub-100nm plasmonic photolithography demonstration using Ag hole array: Zhang • Self-assembling 3-D roll-up structures: Chen

(2) Metamaterial physics

• Study of field radiated by an array of point sources in a composite comprising alternating layers of positive and negative refracting index material: Pendry

• Evanescent wave enhancement in photonic crystals: Joannopoulos • Design and demonstration of subwavelength focusing and imaging in all angle negative refraction

photonic crystals: Joannopoulos/Pendry/Chen/Smith/Schultz • Study of superprism effect in photonic crystal system: Joannopoulos • Demonstration of highly aberrant focusing in indefinite media: Smith • Surface plasmon imaging and lithography design: Yablonovitch • Thermoelectric energy conversion using surface plasmon polaritons: Chen

(3) Metamaterials characterization and metamaterials devices • Bianisotropic characteristics of THz magnetic metamaterials: Zhang • Angular resolved surface plasmon scattering from silver film: Zhang • Study of generalized surface plasmon in microwave: Itoh • Electronically-Scanned (1D) Leaky-Wave Antenna and extension to 2D: Itoh • Demonstration of Fixed-frequency arbitrary-angle electronically-tuned directive reflector: Itoh • Prototype of negative ε and negative µ pair CRLH-TL resonator: Itoh • Prototype of electronically-controlled leaky-wave (LW) antenna: Itoh • Nanoscale optical waveguides with negative dielectric claddings: Chen

Detailed Description of Accomplishments and Results


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Metameterials synthesis:

(1) Plasmonic Lithography using Hole-array (Zhang, UCLA) An extraordinary transmission of light through sub-wavelength hole arrays has been reported. The observed transmission light is several orders of magnitude of the transmission predicted by conventional optics. The periodic hole arrays can couple the surface plasma between metal and dielectric layer interface into the propagation wave. This phenomenon implies the possibility of plasmonic nanolithography. Zhang’s group have investigated the novel plasmonic nanolithography by exposing a photoresist layer through a plasmonic mask, which is an opaque metal film with subwavelength hole arrays in it. The hole arrays of various diameters are fabricated by using focused ion beam (FIB). The far-field transmission spectrum properties have been studied. Fig. 1. Left: A plasmonic mask fabricated by FIB. The diameter of the holes is 160nm, and the lattice constant is 320 nm. Right: The far-field transmission spectrum using different excitation wavelength. The schematic drawing of plasmonic lithography setup is illustrated in Fig. 2. Through the lithography, the hole array patterns are transferred into negative photoresist. As a result, high contrast dot arrays with the smallest diameter of 120 nm, equivalent to ~λ/3, are observed by atomic force microscope (AFM).

Fig. 2. Left: The schematic setup for plasmonic lithography. The light source we used is i-line of mercury lamp, and the photo resist is SU-8. Right: AFM images of the transfrred dot array pattern. The crossectional view along the white line shown right-bottom corner. The lattice constant is 500nm, the spacing layer thickness is150nm, and the exposure time is 25 sec at power of 8 mW/cm2.

By optimizing the exposure procedures, the resolution has been improved drastically. The exposure result of ~90 nm dot array pattern has been observed at 170 nm period of aluminum hole-array mask which is shown in figure 3. Further extension of the optical limit (high resolution and high density) has been explored. Through the considering of the surface plamons property of good conductive metals (Ag, Al, Au and Cu), silver (Ag) has significant largest surface plamon wave vector among others at the designed exposure wavelength of 364 nm (i-line).

350 400 450 500 550 600 650 700 7500

5

10

15

20

25

Tran

smis

sion

inte

nsity

(%)

Wavelength (nm)

320 nm 500 nm

resist

spacing layer

aluminum

quartz

UV illumination

resist

spacing layer

aluminum

quartz

UV illumination

mask

500nm


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Alternative scheme of plasmonic lithography using structured silver film has also been demonstrated. This technique utilizes near UV light to excite ultra-short wavelengths of SPs on silver thin film to achieve high-resolution lithography. The mask consists of silver thin film perforated with 2-D hole arrays exhibiting superior confinement due to SPs with a wavelength equivalent to ¼ of that of the illuminating light (365 nm). This short wavelength of SPs can confine the field on an area much smaller compared to the excitation light wavelength, leading to the higher resolution lithography than conventional photolithography methods. Due to the strong SPs coupling between up and bottom surfaces of the silver thin film, the strong electric field will be well confined above the silver area rather than the sub-wavelength aperture area. Feature size as small as 100 nm on a 200 nm period with superior confinement has been successfully obtained using an exposure radiation of 365 nm wavelength. Furthermore, our finite-difference time-domain (FDTD) simulation shows significantly enhanced electric field and tight confinement of near field profile from silver plasmonic masks, where features as small as 30 nm can be potentially resolved.

Fig. 4.. Left: surface plasmon wave vector of good conductive metals (Ag, Al, Au, and Cu) at the glass interface. Right: AFM images of exposure patterns on 200 nm period, where the hole size of 100 nm, the silver film thickness of 50 nm, the spacer layer thickness of 30 nm, and the exposure dose of 80 mJ/cm2. (2) Microscale 3-D Roll-up Structure (Chen, MIT) A self-assembling tubular structure with diameter on the order of microns is investigated in Chen’s group. The goal is to produce self-assembling swiss-roll structures and validate predictions that their electrical properties will lead to a negative effective magnetic permeability for a narrow frequency band in the mid- to far-infrared spectrum. If successful, the structures could contribute a vital component (µeff<0) necessary for developing a LHM (left-handed material). The fabrication technique consists of depositing a series of two or more thin films on a substrate such that the topmost films have the greatest tensile stresses, patterning the films by photolithography into rectangles with a large aspect ratio, and releasing the multi-layer structures by removing an underlying sacrificial layer (see Fig. 5 at left below). Structures were also demonstrated by using Cr in place of Ni as the layer with high tensile residual stress, Si in place of SiO2 as the insulating layer, photoresist in place of Cu as the sacrificial layer, Pt as an intermediate layer to

Fig. 3. AFM image of the transferred dot array pattern. The lattice constant is 170 nm, the aperture diameter is 40 nm, the spacer layer thickness is 30 nm, and the exposure time is 9 sec at power of 8 mW/cm2


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improve stiffness and conductivity, and considerable other combinations that met with comparable or lesser degrees of success.

Fig. 5. Left: The residual stresses cause the rectangles to roll up into tubular structures (often double-tubes, or scroll structures). Right: A tube diameter of 3.4 µm was measured using a SEM for 30 nm of Ni on 35 nm of SiO2 with Cu as the sacrificial layer.

The best materials system for producing consistent nanoscroll structures has been (from the bottom up) silicon dioxide/ chrome/ gold / chrome. Silicon dioxide serves as a relatively stress-free insulating layer. The first (very thin) chrome layer serves as an adhesion layer for the gold. The gold provides high conductivity (2.2 µΩ-cm resistivity) and tends to make the structures more resistant to breakage when rolling up. The second chrome layer is highly tensile and provides the force required for rollup. In addition, all of these materials have strong chemical resistance to nitric acid, which is used to dissolve an underlying sacrificial layer of copper. Using this system, nanoscrolls have been fabricated with 4 µm diameter tubes when the sacrificial layer is copper, and with 8 µm diameter tubes when the sacrificial layer is photoresist. In both cases, sample uniformity is excellent when the structures are completely released from the substrate, with only a few defects observed over a thousand structures. For partial release from the substrate, the uniformity is poor since some structures (especially those near the edges) completely release from the substrate before others have completely rolled up. Several different samples were assembled from nanoscrolls released from the substrate and tested on the FTIR. They hoped to find a narrow peak in the reflectance spectrum to verify our calculation of the effective magnetic permeability of the structures. No such peak has been found yet. This may be due to poor quality of the samples tested, or the actual resonant frequency of the nanoscrolls may be out of the testable range of the FTIR.


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New Physics in Metamaterials

(1) Field radiated by an array of point sources in a composite comprising alternating layers of positive and negative refracting index material I (Pendry, IC)

Let us consider the geometry of a sample of one-dimensional meta-material as depicted in the figure below. Though seemingly a canonical problem, to the best of our knowledge, this has not as yet been addressed by the physics community. Still, it can be considered as a first step towards the complete solution of the corner reflector that was discussed in our spring report: one can indeed map our stack of slabs into a checkerboard geometry (one-to-one correspondence thanks to a co-ordinate transformation). Of course, the multi-layered structure is worth a study on its own since it provides new and exciting extension of previous studies on PBG materials.

Our approach amounts to solving the full vector Maxwell system by taking full account of the invariance of the structure in 1x and 2x (using a Fourier Transform) and its periodicity in 3x (by means of Floquet-Bloch decomposition). We only assume that the electromagnetic field satisfies the classical energy criterion of square integrability. We achieve a clean mathematical derivation which leads to an analytical expression for the field. Before moving on to the mathematical sketch of the method, we shall maybe point out few possible extensions of this work. Of course, the primary interest lies in the apparent similarity with the Negative Corners via a coordinate transformation, but a whole class of lenses can also be modelled in this simple way, as shown in (J.B. Pendry, Opt. Express, Vol. 11, 755-760 2003) and (J.B. Pendry and S.A. Ramakrishna, J. Phys. Condens. Matter, Vol. 15, 6345-6364 2003). For instance, if one is to build a magnifying lens, then one may think of cylindrical and spherical lenses. Our current one-dimensional problem, which contains within it most of the physics at work in these upper dimensional problems, will be the bottom neck of further investigation in these directions. Our algorithm not only tackles perfect lenses, but we can also treat doubly periodic arrays of meta-materials with point (or line) current sources. In this way, the study of planar meta-waveguides such as periodically loaded transmission lines with negative refractive index seems to be within an easy reach.

A. Description of the generalized transfer matrix algorithm

We consider the multi-layered periodic structure displayed in the Figure above. A point current source sitting within a first layer of thickness 1h consisting of air, is radiating an electromagnetic field ( ),E H bathed in a succession of N layers, possibly made out of negative index material. These N layers of total thickness 1 Nh h h= +L repeat periodically in the 3x direction.

First of all, we make the obvious remark that the structure is invariant along the 1x and 2x directions, therefore the Maxwell system will simplify dramatically when expressed in the Fourier space. For this we introduce the partial Fourier transform as

( ) ( ) 1 1 2 2

21 2 3 1 2 1 2 3

1, , , ,2

ik x ik xG k k x dx dx G x x x e e− −=π ∫

R

(1)

We now consider the four tangential components 1 2 1 2, ; ,E E H H of the electromagnetic field, since they are continuous when they cross the interface between two successive homogeneous layers. Of course, these

Fig. 6. Geometry of the problem


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components will suffer a jump induced by the vector 0 1 1 2 2 3 3P P P= + +P e e e located at the point source. We would like to use a transfer matrix approach. For this, we introduce the field

( ) [ ]3 2 1 2 1, , , , 1 Tx E H H E=F so that

( ) ( )413 3

14

010 11

jj TF x h F x⎡ ⎤+ ⎡ ⎤ ⎡ ⎤=⎢ ⎥ ⎢ ⎥ ⎢ ⎥

⎣ ⎦⎢ ⎥ ⎣ ⎦⎣ ⎦

(2)

where hj stands for the thickness of the layer j, 041 denotes a null matrix with 4 rows and 1 column and 014

its transpose (denoted by T throughout the text). Also, jT is the 4 by 4 transfer matrix of the

homogeneous layer j (see for instance J.B. Pendry, Jour. Mod. Opt., Vol 41, No. 2, 209-229 1994). The somehow mysterious introduction of a fifth component to the vector F becomes clear when we compute the field in the neighbourhood of the point source

( ) ( )40 0

14

0 00 11 1IF h F h∆+ −⎡ ⎤ ⎡ ⎤⎡ ⎤

=⎢ ⎥ ⎢ ⎥⎢ ⎥⎣ ⎦⎣ ⎦ ⎣ ⎦

(3)

where 4I is the 4 by 4 identity matrix and ∆ is the 1 by 4 matrix which provides us with the jump in F induced by the singularity 0P . From the Maxwell system we derive that

( ) ( )2 3 0 2 1 1 3 0Tik P h i P i P ik P h∆ = − ε − ω + ω − ε⎡ ⎤⎣ ⎦ (4)

We note that the generalised transfer matrices satisfy the prerequisite property of a unitary determinant. We can now pile the successive layers.

B. Particular case of interest for double corner

If we consider for instance 4 homogeneous layers, with the point source sitting in the first one, we should consider 6 transfer matrices (three of them for the first layer). The matrices associated to homogeneous layers can be written as

41

141,3,4,5,6

0,

0 1j

iT

i⎡ ⎤

Π = =⎢ ⎥⎣ ⎦

(5)

whereas the matrix which induces the discontinuity in the electromagnetic field in x3=h0 takes the following form

42

140 1I ∆⎡ ⎤

Π = ⎢ ⎥⎣ ⎦

(6)

The matrix associated to the whole four layers is therefore

6 5 114

....0 1T ∆⎡ ⎤

Π = Π Π Π = ⎢ ⎥⎣ ⎦

(7)

where 6 5 4 3 2 1T T T T T T T= is the classical transfer matrix associated to four homogeneous layers

( 1 3T T being the usual transfer matrix for the first layer without point source). Also, the discontinuity is sitting in 3 2T∆ = ∆ .

We therefore obtain the set of solutions to the Maxwell system which are square integrable in 1x and 2x and bounded on every compact interval in 3x . In order to ensure the uniqueness of the solution of our problem, we need an additional condition, that we call Floquet-Bloch condition in 3x ,


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( ) ( ) ( ) 1 13 3 2 2exp 0 , 2 , ,F h i F k k ⎡ ⎤= Φ Φ = π ∈ − +⎣ ⎦

. (8)

From the expression of Π we obtain

( ) ( )0F h TF= + ∆ (9)

We end up with

( ) ( ) 14 expF h I i T −

= Φ − ∆⎡ ⎤⎣ ⎦ (10)

It can be shown that ( )4 expI i TΦ −⎡ ⎤⎣ ⎦ has an inverse for all real frequencies provided that we introduce

some loss whenever we consider a layer with negative refractive index.

(2) Field radiated by an array of point sources in a composite comprising alternating layers of positive and negative refracting index material II (Pendry, IC)

We describe a new formulation of imaging by negative media above. It is realised that two adjacent slabs of materials, of equal thickness, optically annihilate one another provided that

,ε µ in the one is the negative mirror image of ,ε µ in the other. Although this result is quite plausible where rays follow a simple distorted trajectory in each medium, in some instances the theorem has startling consequences. Consider figure 7, a system drawn to my attention by David Smith: the mirror theorem applies but a ray construction contradicts the theorem. Applying the laws of refraction to ray 2 implies that the ray is rejected by the system instead of being transmitted through to the other side and a dark shadow behind the cylinders is predicted by the ray picture. In fact a full solution of Maxwell’s equations shows that ray 2 is transmitted and emerges through the system just like ray 1 and no shadow is formed. The apparent paradox is resolved by recognising that a series of resonances form on the surfaces of the cylinders and these resonances enable radiation to tunnel across the gap between the cylinders. A clue to the nature of these resonances is given by the closed loop of dotted rays in the centre of the figure which indicates the presence of a state which traps radiation i.e. we have a resonance.

One important restriction of the new lenses as currently formulated is that they produce images of exactly the same size as the objects. To do otherwise we must introduce curved surfaces and this is most easily done through coordinate transformations and, again in a previous report, we described one way in which a spherical lens may be constructed. Using the new theorem we have now realised that yet another formulation of the spherical can be made, this time with even more novel properties than the previous one. The following is the new prescription for a spherical lens:

11

−→−→

µε

11

+=+=

µε

1

2

Fig. 7. The left and right media in this 2D system are negative mirror images and therefore optically annihilate one another. However a ray construction appears to contradict this result. Nevertheless the theorem is correct and the ray construction erroneous. Note the closed loop of rays indicating the presence of resonances.


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Fig. 8. It is possible to design a spherical annulus of negative material lying between 2r and 3r that acts like a

magnifying glass. To the outside world the contents of the sphere radius 3r appear to fill the larger sphere radius 1r with proportionate magnification.

Outside radius 2r the structure is empty space, between 2r and 3r lies the negatively refracting material, though now more structured than before, and finally inside the annulus is a material of constant high permittivity, high permeability. This structure has the unusual property that viewed from beyond a radius,

32

21 / rrr = , the contents of the inner sphere (any electrical or magnetic sources of radiation) appear to be

expanded to fill a sphere radius 1r filled with 1ε = µ = material i.e. magnified by a factor 2 22 3r r . The

region of space between 1r and 3r has vanished and is not visible.

Viewed from inside radius 3r the world outside radius 1r appears to be shrunken by a factor 2 23 2r r and

now reaches to radius 3r and appears to have 2 22 3r rε = µ = i.e. the same as the filling of the internal

sphere. Again the region of space between 1r and 3r has vanished and is not visible.

This result is remarkable because the region between 1r and 2r is simply empty space, and yet according to our theory appears to be filled! We plan to elaborate on this result in the next phase of our work.

(2) Photonic Crystals as Alternates for Metamaterial System (Joannapoulos, MIT) We are continuing our studies of photonic crystals as alternates for metamaterial systems at optical frequencies. In this regard we have been studying the intriguing possibility of subwavelength imaging, or what has come to be known as superlensing. Members of this MURI have described how a slab of uniform “left-handed material” with permittivity ε = -1 and permeability µ = -1 is capable of capturing both the propagating and evanescent waves emitted by a point source placed in front of the slab, and refocusing them into a perfect point image behind the slab. While the focusing effect of propagating waves can be appreciated from a familiar picture of ray optics, it is amazing that perfect recovery of evanescent waves may also be achieved via amplified transmission through the negative-index slab. Some of the discussion of this effect relies on an effective-medium model that assigns a negative ε and a negative µ to a periodic array of positive-index materials, i.e. a photonic crystal. Such an effective-medium model holds for large-scale phenomena involving propagating waves, provided that the lattice constant a is only a small fraction of the free-space wavelength in the frequency range of operation. However, in the phenomenon of superlensing, in which the subwavelength features themselves are of central interest, an effective medium model places severe constraints on the lattice constant a: it must be smaller than the subwavelength details we are seeking to resolve. The question of whether and to what extent superlensing would occur in the more general case of photonic crystals still remains unclear. Thus, we have investigated the possibility of photonic-crystal superlensing in detail by studying the transmission of evanescent waves through a slab of such a photonic crystal. It is important to note that the

3r

2r

1r

r13r

2r

1r

22

323

22

3 22

2

, 0

,

1,

, ,

x y z

x y z

x y z

x x y y z z

r r rr

r r r rr

r r

ε = ε = ε = + < <

ε = ε = ε → − < <

ε = ε = ε = + < < ∞

µ = ε µ = ε µ = ε


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transmission considered here differs fundamentally from its conventional implication of energy transport, since evanescent waves need not carry energy in their decaying directions. Thus, it is possible to obtain transmission amplitudes for evanescent waves greatly exceeding unity without violating energy conservation. Here, we have introduced two mechanisms linking amplification of evanescent waves to the existence of bound slab photon states. These bound states are decoupled from the continuum of propagating waves, thus our findings are distinct from the effect of Fano resonances in electromagnetism, which were recently studied in the context of patterned periodic structures and surface-plasmon assisted energy transmission. As for the problem of negative refraction, we have found that the concept of superlensing does not in general require a negative refractive index. Moreover, we find that the surface Brillouin zones of both photonic crystals and periodic effective media provide a natural upper cutoff to the transverse wavevector of evanescent waves that can be amplified, and thus no divergences exist at large transverse wavevectors in photonic crystals or in effective media. As with effective media, the lattice constant of photonic crystals must always be smaller than the details to be imaged. We have derived the ultimate limit of superlens resolution in terms of the photonic-crystal surface periodicity, and infer that resolution arbitrarily smaller than the wavelength should be possible in principle, provided that sufficiently high dielectric contrast can be obtained. We performed realistic studies of the bound photon states in carefully designed 2D and three-dimensional (3D) slabs of AANR photonic crystals. In Fig. 9. We present the results of a comprehensive numerical study of a 2D superlensing structure. A subtle and very important interplay between propagating waves and evanescent waves on image formation is revealed, which makes the appearance of the image of a superlens substantially different from that of a real image behind a conventional lens. These numerical results confirm our qualitative understanding and can be readily compared to experimental data.

Fig. 9. Numerical results of the imaging for various frequencies throughout the first photonic band for a 2D square lattice of air crosses in dielectric. (a) Intensity distribution along the transverse direction, commonly measured at z = 0.5as for several frequencies shown as insets. This z value is chosen to exhibit large near-field effects at certain frequencies (e.g. ω = 0.145(2πc/a)). The transverse intensity distribution at larger z values has a similar-shaped background but weaker near-field modulations. (b) Intensity distribution along the z axis for the shown frequencies. In both panels the inset numbers are the frequencies corresponding to each curve, in units of (2πc/a). (c) Calculated transverse intensity distribution for imaging with lossy photonic crystals. Each inset number correspond to the permittivity of the dielectric host for the curve of the same color. In general, appreciable material losses will impose severe limitations to the transmission coefficient of evanescent waves, in a manner similar to that of the intrinsic energy leakage rate of a crystal mode above the light line, which in turn reduces the superlensing effect. However, it is also expected that, in the limit of extremely small material loss, in the sense implied by the original proposal of a perfect lens, their findings about the image of a superlens will remain valid. As an example, the calculated focusing effect in a series of slightly lossy photonic crystals is shown in Fig. 9(c). The losses are modeled as a positive imaginary part on the permittivity of the dielectric host. As the losses increase, the strength of the transmitted near-fields is attenuated, and the subwavelength features in the central image peak gradually

(c)


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disappear. The effects of surface imperfections on subwavelength imaging can also be qualitatively analyzed. We consider these defects to occur only on a length scale that is smaller than a lattice constant, and thus much smaller than the operating wavelength, with correspondingly little influence on propagating waves. Since the transmission of evanescent waves depends sensitively on the bound surface photon states, which in turn depend sensitively on the surface structure, imperfections are expected to be most influential on the crystal surface. Their effects may thus be minimized by improving the surface quality. These considerations suggest that the predictions of this work should be observable in realistic situations. Thus an ongoing effort involving future work is the close collaboration with Schultz/Smith and Chen groups to explore and verify these predictions experimentally.

(3) Superprism effects in Photonic Crystals System (Joannapoulos, MIT) We are also continuing our studies of superprism effects in photonic crystal systems. Our primary focus is in exploring the possibility of a superprism effect based on phase-velocity dispersion, i.e., an effect that will induce large changes in Bloch wave vector k with respect to small changes in the incident parameters. Recently, we succeeded in demonstrating computationally that photonic crystals can be used to realize a magnitude of phase-velocity dispersion much larger than that of classical gratings in their grazing angle limit and thus comparable to that achieved with group-velocity dispersion effects. In Fig 10. we show how a photonic crystal superprism can be designed such that a one-degree incident angle change can lead to a transmitted angle change of twenty-degrees.

Fig. 10. Time-domain simulation snapshots of the magnetic field perpendicular to the plane for the incident angles shown. The arrows in the outgoing waves indicate the directions of the peak Poynting vectors. Red, white, and blue correspond to positive, zero, and negative field values. Note that an incident ∆ϕ = 10 leads to transmitted ∆ϕ = 200. (4) Superlensing using photonic crystals (Joannapoulos, MIT; Chen, MIT; Schultz/Smith, UCSD) In a collaborative effort among MIT and UCSD, they have designed a high-index dielectric 2d periodic photonic crystal structure that can exhibit single-beam negative refraction in for all incoming angles in a regime of positive effective index of refraction. The frequency range is chosen so that for all incident angles, one obtains a single negative-refracted beam. This structure has been fabricated by Chen group and tested by the Schultz/Smith group.


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A 2D PBG sample designed to demonstrate the near field imaging of a "line" source places at sub-wavelength distances from the incident surface of the sample. The measurement result is shown in Fig. 11. Sub-wavelength interesting near field imaging occurred at specific frequencies but they were also associated with far-field patterns not found in the theoretical simulations. The MIT theory group has suggested that this may be due to the presence of higher order modes emanating from the source antenna which are not sufficiently attenuated by the choice of the measuring chamber height. Experiments with much thinner "2D" samples are under consideration to cure this problem.

(5) Focusing in Indefinite Media (UCSD) There are distinct parallels between the far-field focusing associated with the AANR photonic crystal, and certain types of indefinite media—media for which not all elements of the permittivity and permeability tensors have the same sign. UCSD has demonstrated experimentally the type of highly aberrant focusing that occurs when using a slab of indefinite medium. The sample used consisted of a planar metamaterial slab composed of split ring resonators designed to exhibit a permeability equal to -1 along the longitudinal (propagation) axis. Such a slab redirects s-polarized electromagnetic waves from a nearby source to a partial focus. This work was published in Applied Physics Letters, and was also featured on the cover. The summary composite image, showing experimental data, simulation and ray tracing, is shown below.

Fig. 12. A spatial map of the magnitude of the electric field. Left: The simulated slab, indicated by the solid lines, has ε=1 and a diagonal permeability tensor for which the longitudinal component µz=-1 and µx=µy=+1. The slab is 16 cm long, with a line source placed 2 cm from the slab. The slab thickness is 4 cm. Inset: a ray-tracing diagram showing the manner in which the trajectories of rays emanating from a point source are refocused by a indefinite medium slab. Right: Experimentally obtained spatial map of the electric field in the 4 cm region to the right of the slab.

(6) Surface Plasmon Imaging and Lithography (Yablonovitch, UCLA) Surface plasmons promise the ability to achieve X-ray wavelengths at optical frequencies. Exploiting this unique attribute for photo-lithography allows for features on the order of 10nm written by visible laser sources. Working toward this goal, we have made considerable progress in the computational modeling, design, and analysis of tapered structures for planar plasmonic imaging and demagnification. We have also carried out trade studies on various material systems and analyzed novel layered structures.

A. Numerical Analysis of Plasmon Parameters

Fig. 11. Left: AANR sample fabricated by Chen group in MIT. ε ~ 38, Q factor ~ 3300, Frequency range ~ 6GHz. Right: Experimental data measured by UCSD. A sub-wavelength near-field imaging can be clearly seen at 6.2475 GHz.


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Our proposed plasmonic imaging device operates by using a grating coupler to convert laser light into a surface plasmon. This plasmon then propagates down the taper, its wavelength reducing as it proceeds across the adiabatically tapered silver film. When the line image has been focused, it is then outcoupled into the near field, transferring the image onto the photoresist. Design and operation of this imaging system requires very accurate knowledge of the plasmon wavelength and attenuation at all points in the system. We have carried out numerical analyses of the dispersion equations for the air-silver-sapphire system at various silver film thicknesses and demonstrated solutions which yield plasmon wavelengths of less than 10nm at visible frequencies.

Fig. 13. Left: Numerical results of dispersion relations for the air-silver-sapphire plasmon geometry at various silver film thicknesses. The inset illustrates the general function of the plasmonic imaging device, showing the adiabatic reduction in wavelength through the taper region. Right: Decay length of surface plasmons at various frequencies in the air-silver-sapphire geometry.

Our results show that Joule heating will cause non-negligible attenuation of the plasmon wave as the silver film becomes very thin and the plasmon approaches X-ray wavelengths. This will limit the number of pixels in the field of view.

B. Two-Metal Geometry

AirAgAl

0

1

2

3

1 2 3 4 5 k (eV/c)

w (e

V)

100nm5nm

10nm25nm

Plasmon

Fig. 14. Two-Metal Geometry shows a decreasing plasmon wavelength with increasing silver film thickness. The inset shows preliminary dispersion curves at various silver film thicknesses.

As shown in the Fig. 13, achieving very short plasmon wavelengths (<50nm) requires very fine control over film thickness and high attenuation due to Joule heating. To alleviate these design and fabrication constraints, we have devised a two-metal geometry. This design consists of a substrate metal with a high

t=1nm

t=20nm t=5nmt=2nm

0 0.1 0.2 0.3 0.4 0.50

1

2

3

4 200 100 50 20 153040 10

0.6

Plasmon Wavelength in nm

Plasmon Wave-Vector (2π/wavelength in nm)

Plas

mon

Ener

gyin

eV

hν

k

t=thickness of metal film

SapphireAg

Air

t=1nm

t=20nm t=5nmt=2nm

0 0.1 0.2 0.3 0.4 0.50

1

2

3

4 200 100 50 20 153040 10

0.6

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gyin

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t=thickness of metal film

SapphireAg

Air

2.5 eV

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1.5 eV


page 14 of 22

plasmon frequency coated with a thin film metal taper of a lower plasmon frequency. In this scheme, the plasmon wavelength decreases with increasing film thickness. Preliminary numerical analyses of an air-silver-aluminum geometry are shown below. By initially coupling the incident radiation to a plasmon on the air-aluminum interface, attenuation by Joule heating is greatly reduced, thus relaxing design constraints and allowing rough focusing to be carried out in the plasmon.

C. Plasmons on Dielectric Waveguides Deposited on Silver

Our original plan for plasmonic focusing involved a tapered silver film with the film thickness decreasing as the plasmons approach the line image. The waning film thickness causes a decrease in plasmon wavelength, allowing the increase in resolution near the output coupler. This geometry, however, does present various fabrication difficulties. Not least of which is producing an adiabatic taper while keeping < 5nm smoothness on both sides of the film. An alternative approach has been investigated in which a thin sapphire layer is deposited on a silver substrate (see dispersion relations below). Most of the focusing could then be done in the low loss/high group velocity region of the dispersion curve. Near the line focus, a coupler would then couple the plasmon to a negative plasmon wave-vector with positive group velocity and small wavelength for the final focusing. As illustrated below, this could alleviate some of the fabrication concerns.

D. Path to Large Scale Lithography using Plasmonic Imaging

Recently we have put forth preliminary designs and calculations which will yield a path from theory to full scale production for plasmonic lithography. By leveraging off long established computer hard disk technology, we have been able to show that it is theoretically possible to write over the surface of a 12 inch wafer in under one hour. We are fortunate that modern hard drives work on an air bearing slider which floats 30nm ± 0.3nm (with technology rapidly moving to 5nm spacing), which is excellent for exposing photoresist using the near field. For our application, motion and dimensions must be controlled to ~10nm accuracy. To solve this, we propose using separate heads reading servo code, as well as several interferometers measuring position at all times, it would be possible to scan the write head over the substrate platter spinning at several thousand rpm.

E. Validation of optical constants of silver

sapphire slider

slider suspension

image from

modulator array

photoresist

silicon

plasmon lens

SIDE VIEW

Fig. 16. Geometry for scanning plasmon lens using a slider analogous to conventional hard disks scanning over a spinning substrate

Fig. 15. Numerical results of dispersion relations for the silver-sapphire-air plasmon geometry at various sapphire film thicknesses. The thick black arrow indicates a coupling from a large wavelength positive wave-vector to a short wavelength negative wave-vector, both with the same sapphire film thickness.


page 15 of 22

The crux of the engineering aspect of our device, capable of giving a resolution on the nano-metric scale, is the profile of the adiabatic taper in a metal film. The absorption and scattering losses in a metal determine the profile of the taper. These losses, in turn, depend on the dielectric constants of metal, which is silver in our case. Our first experiment was aimed at validating the values of dielectric constants, which were used for the design of the taper. We coupled light into surface plasmons and made reflectivity measurements from a prism coated with a thin silver film, to determine the optical constants of silver at various wavelengths of interest, in the visible region. F. Fabrication of a grating and a taper Surface plasmons at a metal-dielectric interface have a larger wave vector than the electromagnetic light wave propagating in the dielectric. For efficient coupling of energy from photons to plasmons, we need a periodic structure, which provides the additional momentum. A grating with a precise period and duty cycle is needed. We have developed a process for making a surface grating coupler on a glass substrate. The recipe involves a series of steps --- making alignment marks to define boundary of gratings, evaporating metal onto substrate, using electron beam lithography to write the grating pattern onto PMMA, transferring the pattern onto the metal underneath and finally etching the substrate to form the periodic grating structure. The fabrication of the accurate profile of a three dimensional taper of submicron dimensions, poses a significant challenge. As a first step, a focused ion beam was used to mill the silver film. A three dimensional taper is obtained by gradual increase of the milling depth across the length of the taper. An AFM scan of a milled region gave qualitative information on roughness induced during the process of ion milling.

(7) Thermoelectric energy conversion using surface plasmon polaritons (Chen, MIT) A theoretical model of thermoelectric energy conversion using enhanced energy transport due to surface plasmon polaritons and non-equilibrium between electrons and phonons in the thermoelectric device has been proposed by Chen group in MIT. One of the main requirements of such a device is that the electron-phonon coupling factor should have a comparatively low value (≈ 1010 Wm-3K-1), which can be achieved at low temperatures. This gives us reason to believe that such a device can perform better as a refrigerator rather than a power generator.

As film thickness reduces, it is known that diffuse scattering from the film boundary results in a decrease in the thermal conductivity compared to the bulk value. Since the energy density due to surface polaritons in the vicinity of a surface can be higher than the free space value by many orders of magnitude we have

Fig. 17 A SEM image of grating structure in aluminum. Aluminum is deposited on glass substrate.

Fig. 18 An AFM line scan of a linear taper in a 50nm silver film and the glass substrate. Note vertical scale is in nm and horizontal scale is in µm.


page 16 of 22

investigated theoretically the in-plane energy transport due to surface polaritons because of a temperature gradient along a film of polar material. The effective “thermal conductivity” of this energy transport is shown to be higher than the thermal conductivity of thin films due to phonon contribution alone.

Metamaterials Characterization and Metamaterials Devices (1) THz magnetic response from artificial materials: (Zhang, UCLA)

Last year Zhang’s lab (in corporation with UCSD and Imperial College) reported the experimental demonstration of an artificial magnetic metamaterial at THz frequencies. The further effort towards thorough physical understandings of this artificial magnetism is ongoing in Zhang’s lab. For example, our preliminary FTIR experiments on the MMR samples (Fig. 19 a) reveal a distinct bianisotropic characteristics: an electric polarization as a response to an applied magnetic field and vice versa. We have simulated the dispersion diagram of the split ring resonators, which indeed display a remarkable anisopotry of the fundamental bands (Fig. 19 b) and an analytical model is sought to quantify the bianisotropic coefficient from the geometric factors.

Fig. 19. (a) Orientation issue – upper panel shows when the applied fields are perpendicular to the symmetric axis of split ring structures (i.e., asymmetric cases), we observe the reflection peaks in FTIR oblique angle

measurement. On the other hand, when the applied fields are parallel to the symmetric axis of split ring structures (i.e., symmetric cases), the reflection peaks aforementioned vanish.

Fig. 19 (b). The computed dispersion diagram of sample D1, for TE polarization. On the ΓX and ΓY direction, we can find an discrepancy of resonance ~140GHz.

(2) Angular resolved SP scattering from silver film (Zhang, UCLA) In previous report, Zhang group in has demonstrated evanescent wave enhancement in silver film by measuring surface plasmon scattering intensity in far-field under normal illumination. They further investigated the enhanced surface plasmon scattering at different incident angles. Besides the enhancement effect, they found that the backward scattering of the surface plasma is stronger than the forward scattering. The underneath figure shows their experimental data carried out by reversed attenuated total reflection (RATR) setup. It shows good agreement with surface scattering model they used in their previous APL paper. A direct observation from this figure indicates that a bigger portion of energy is emitted backward when the dielectric constant of silver attain more negative values (|εAg|>>1) while the partition is not as dramatic with the illumination photon approaches surface plasmon energy (|εAg|~1). It can be explained by the fact that more longitudinal components are excited at shorter wavelengths of surface plasmon, and it causes more symmetrical radiation in the backward and forward directions. It is also shown that a richness of surface roughness spectrum can be deduced from scattering intensity measurement by

150GHz 150GHz

0.6 0.9 1.2 1.5 1.8 2.1 2.4 0.0

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lect

ion

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page 17 of 22

scanning the incident angle. For example, a maximum k-vector, 0kksp + , can be achieved and the spatial resolution given by 364nm can be as high as 155nm.

Fig. 20. The intensity ratio between backward scattering and for forward scattering for both measurement and calculation. (a) λ=633nm, parameters used in calculation are: 2ε = 18.31 0.49i− + and 3ε =1.5151. (b) λ=364nm,

parameters used in calculation are: 2ε = 2.54 0.25i− + and 3ε =1.5364.

(3) Novel MIM Implementation of the CRLH Structure (Itoh, UCLA)

So far, we have mainly used an interdigital-C implementation of Composite Right/Left-Handed (CRLH) structures. However, the per-unit-length capacitance of the interdigital capacitor is intrinsically small, because it is provided by the very thin edge of the metal patterns and by relatively weak fringing fields. In contrast, a Metal Insulator Metal (MIM) capacitor has two metal plates facing each other and exhibits therefore a much higher capacitance per-unit-length. Consequently, by replacing the interdigital capacitors in our existing applications could lead to significantly more compact structures.

(4) Two-Dimensional Leaky-Wave Antenna (Itoh, UCLA) We have been trying to extend our 1D backfire-to-endfire leaky-wave antenna to a 2D antenna capable of scanning any point of space. Several structures are being investigated. One of them is the mushroom structure, which we had previously demonstrated to exhibit LHness if the series capacitive loading were strong enough. Full-wave computed dispersion diagrams have revealed the possibility of obtained designs with LH-backward and RH-forward radiating modes. Such a 2D antenna is a promising cheap and efficient potential substitute to conventional arrays for scanning and millimeter-wave imaging applications. (5) Electronically-Scanned (1D) Leaky-Wave Antenna (Itoh, UCLA) The new CRLH antenna we have developed is frequency-scanned. For some application, single frequency operation is required. Therefore, we have been developing a variant of the antenna including varactor diodes, the bias voltage of which changes the capacitive loading of the unit cell, and consequently the scanning angle. Thus, we have a new type of device, which is electronically instead of frequency scanned. It is the first time that continuous electronic scanning is obtained in a planar antenna, in particular both in the backward and forward ranges. Fig. 21 shows a picture of an initial prototype along with measurements providing the proof of concept.

Incident angle (Deg) Incident angle (Deg)

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experiment data theoretical value

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page 18 of 22

-5

-4

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0

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Fig. 21. Electronically scanned CRLH leaky-wave antenna and measured radiation patterns.

(6) Generalized surface plasmons (SP) (Itoh, UCLA)

We have worked out the first full-wave demonstration of microwave SP between arbitrary metamaterials, and in particular at the interface between a RH and a LH media, as illustrated in Fig. 22, using the following simulation scheme. The wave is incident at the left arm of the L-shaped structure and hits the oblique surface with the triangular shaped part filled with air at an angle large than the critical angle. Consequently, only evanescent waves reach the RH/LH horizontal interface, which excite the surface plasmon.

The characteristics of this RH/LH-interface SP were studied rigorously and a transmission line mushroom-structure implementation was proposed. Several original effects have been discovered: 1) this SP can cross the air line (never the case in the conventional SP); 2) in that case, this “radiative” SP, since it is associated with the Brewster angle phenomenon of zero-reflection, is totally refracted; 3) depending on the metamaterials parameters, two SP resonances can exist, and very complex and unusual SP diagrams can appear. These new microwave SP have great potential in miniaturized microwave antennas, components and beam-formers.

(7) Parabolically-shaped metamaterial interface (Itoh, UCLA) We have introduced the novel idea of a refractive parabolic interface as opposed to the conventional reflective parabolic structure. This idea and effective-medium proof of concept are shown in Fig. 23. As for the SP, the transmission line mushroom CRLH structure excited in its LH range can be used for practical implementations. In the case of a partially reflective interface, a beam splitter with source at focus is obtained. This structure has potential for planar rectennas and various plane wave to cylindrical wave transformers.

Fig. 22. Full-wave simulated RH/LH-interface SP(electric field magnitude) in parallel-plate waveguide structure loaded by 3 different materials. (a) Large wavelength. (b) Smaller wavelength. (c) Intersection with the air line. (d) Above the air line (simple negative refraction) In this last case, we have only two media (RH and LH), because coupling occurs to a propagating incident wave.


page 19 of 22

Fig. 23. Parabolic refractive RH/LH interface. Principle and full-wave simulated magnitude/phase in effective medium approach.

(8) Fixed-frequency arbitrary-angle electronically-tuned directive reflector (Itoh, UCLA) This reflector is a new extremely simple and flexible reflecto-directive system, illustrated in Fig. 24. It uses our recently developed fixed-frequency electronically-scanned CRLH leaky-wave antenna. The frequency of operation is fixed. An incident signal is received from a given direction by a quasi-omnidirectional patch-antenna antenna, amplified by an LNA (power boosting), filtered by a band-pass filter (noise reduction), and re-radiated by the antenna. By varying the bias voltage of the varactors integrated in this antenna, the dispersion characteristic is altered and consequently the direction of radiation is changed. This system was studied rigorously from the point of view of transmission lines and dispersion diagrams, and demonstrated experimentally.

Rx : Patch Antenna

,in inf θ∀

LNA

, ( )o u t o u tf Vθ

Tx : Electronically-Scanned LWA Fig. 24. Schematic of fixed-frequency arbitrary-angle electronically-tuned reflector.

(9) An ENG-MNG Pair CRLH-TL Resonator (Itoh, UCLA)

We have recently demonstrated the concept of single negative materials (SNG) through a physical implementation of composite right/left-handed handed transmission lines. A SNG can simply be interpreted as an unbalanced composite transmission line resulting in a stopband between the shunt and series resonant frequencies. When the series resonant frequency is greater than the shunt resonant frequency, the stopband contains a magnetic gap and the structure is µ-negative (MNG). When the shunt resonant frequency is greater than the series resonant frequency contains an electric gap and the structure is ε-negative (ENG). An eight-cell network composed of four MNG CRLH-TL unit cells connected to four ENG CRLH-TL unit cells is designed and fabricated using microstrip. It is observed that such a structure behaves as a resonator in which the resonance can be optimized by simply increasing the number of unit cells and adjusting the equivalent lumped-element parameters. At the resonant frequency, µ1 = - µ1 and ε2 = - ε2 must be resulting in the resonance conditions 2 2

11

R LL

R

C LCL

= , 2 21

1

R RR

R

C LCL

= , 2 11

2

L RL

R

C LLC

= , where LR1 can be

chosen arbitrarily.


page 20 of 22

Fig. 25. a) Implementation of an ENG-MNG Pair CRLH-TL resonator consisting of 4 ENG cells and 4 MNG cells. b) Simulated and measured S-parameters of resonator.

The circuit implementation of the resonator and the simulated and experimental results are shown in Fig. 25. The measured data shows that resonance occurs at 3.18of GHz= , while the simulated data shows resonance at 3.13of GHz= .

(10) Electronically- Controlled Leaky-Wave (LW) Antenna (Itoh, UCLA)

A novel electronically-controlled TL structure was designed based on the CRLH concept. This TL structure is analyzed in terms of equivalent circuit models and dispersion curves, and applied to a LW antenna capable of tuning both radiation angle and beamwidth. A voltage-controlled operation is achieved by incorporating varactors in the TL, while operating at a constant frequency. This antenna provides two functions. First, it can continuously control the radiation angle when the varactors are uniformly biased. Additionally, when the varactors are non-uniformly biased, beamwidth can be continuously controlled. The proposed principles are experimentally demonstrated for antenna applications. At 3.33 GHz, the continuous controllability of the radiation angle and beamwidth is demonstrated by changing the reverse bias voltages. The effects of the varactors are examined in a two-tone test and a modulation test. Harmonics generated are shown to be negligible for antenna applications and a BPSK modulated signal is successfully recovered at a 10 Mbps rate. Therefore, this antenna may be applicable to several digital wireless applications requiring efficient channelization, such as WLAN. Furthermore, since the antenna is implemented with low cost Si varactors and microstrip technology, it is a good candidate for integrated millimeter wave systems due to its ease of fabrication, low cost, and low-profile.

ctorShunt vara

varactorsSeries

-+

ViaFeed) (DCInductor

)(V GND, b −

Via

- ++

)(V bias DC b +

LZinP

38.34 (5.87 )stripeffcm µλ


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ctorShunt vara

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)(V GND, b −

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page 21 of 22

Fig. 26: (a) 30-cell prototype of the voltage-controlled TL structure with a magnified view of the unit cell and (b) the measured relationship of scanning angle versus reverse bias voltage.

10V9V9V9V8V8V8V7V7V6V6V6V5V5V5V 10V9V9V9V8V8V8V7V7V6V6V6V5V5V5V


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(a) (b)

Fig. 27: Measured and theoretical radiation patterns in the case of non-uniform biasing distribution of (a) 5 V to 15 V and (b) 0 V to 10 V with reference patterns at uniformly biased voltages. The number in each nth array cell indicates an applied reverse bias voltage.

(11) Nanoscale Optical Waveguides with Negative Dielectric Claddings (Chen, MIT)

The dispersion properties of waveguides with negative dielectric cladding were investigated. It was found that TM1 modes can exist in the waveguides with the thickness of the guiding layer being ten times smaller than the working wavelength, i.e., the guiding layers could be in nanometer scale. The band-passing, wavelength-response, and transmission loss properties were analyzed. Artificial metal-dielectric composite materials, especially Prof. Pendry’s metal-wire arrays with pre-controllable plasma frequency and low transmission losses, can be used for the claddings. The TM1 modes have a passing band, and hence the waveguides can also work as band-pass filters. The waveguides will not likely be subject to bending loss because the negative dielectric claddings theoretically cause total reflection for all incident angles. The waveguides are expected to find application in integrated optical devices, optical lithography, sub-wavelength optical imaging, and nonlinear optics due to the high energy density in an ultra-thin guiding layer.

Synergy and Interactions During this year, there have been many formal and informal interactions among members of our MURI team. At whole project level, three teleconferences have been arranged to facility the interaction among teams. Especially, the teleconferences offer the opportunities for students and postdocs to present their results. The collaborative efforts among the team members have been summarized in the following table.


page 22 of 22

Participants Activities UCLA interactions Zhang/Smith/Basov THz metamaterials characterization Zhang/Chen Electromagnetic response for 3-D roll-up structures Zhang/Pendry/Smith/Schultz Numerous and discussions and telephone meeting magnetic

metamaterials and superlensing Yablonovitch/Zhang Design, verify and characterize plasmonic photolithography MIT Interactions Joannopoulos/Chen/Schultz/ Smith

Design, fabrication, and characterization of AANR

Joannopoulos/Pendry/ Schultz/Smith

Joint publication on AANR

Joannopoulos/Zhang Povenilli visit Zhang’s lab and discuss about magnetic dipole radiation in PBG

Chen/Zhang Zhang’s student gave a seminar in Chen’s group Four physical meeting between Chen and Zhang and frequent discussion

Imperial College Interactions Imperial college and UCSD Discussion and collaboration. Visits by Pendry and Ramakrishna to

UCSD 2 month sabbatical by Pendry at UCSD Three publications

Imperial college and MIT Discussions and collaboration Three joint publications

Imperial college and UCLA Visits by Pendry and Ramakrishna – Semninars and discussions on the perfect lens and other topics

UCSD Interactions Smith/Starr/Itoh/Caloz Three visit to UCLA and on visit by Dr. Caloz to UCSD

Visit the group of Itoh to understand relationship of the transmission line approach to metamaterials versus the materials approach.

Schultz/Smith/Pendry Host Pendry for sabbatical Mock/Schultz/MIT Coolaborative effort on experimental measurement of AANR Smith/Schurig/Pendry Discussion with Pendry and Ramakrishna regarding practical

implementations of the “perfect lens”. Two visit by Pendry and one visit by Ramakrishna.

Schultz/Smith/Padilla/Zhang Teleconference, email and visits with members from Zhang’s group. Members from the UCSD group have made several visit to UCLA, and many members from Zhang’s group have come to work at UCSD facilities. The primary focus of this interaction has been the design, fabrication and characterization of metamaterial structures designed for THz frequency

Schultz/Basov/Zhang Introduce Prof. Basov work and experimental capability to Zhang. Made a fund transfer to aid Prof. Basov join in THz frequency effort.

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