New organic infiltrants for 2-D and 3-D photonic crystalsRobert A. Norwood,* Hiroshi Sumimura, Savaş Tay, Konstantin Yamnitsky,

Alexander Kropachev, Jayan Thomas, and Nasser PeyghambarianCollege of Optical Sciences, University of Arizona, Tucson, AZ 85721

J. H. Moon and Shu YangDepartment of Materials Science and Engineering, University of Pennsylvania,

Philadelphia, PA 19104

Terje SkotheimIntex Corporation, Tucson, AZ 85721

ABSTRACT

Photonic crystals have now started to make the transition from basic to applied research, with new materials systems and device results being published on a frequent basis. While a number of photonic crystals have been made using organic materials, the lack of high index organic materials has impeded their development. We have investigated several novel high index organic systems for use in both 2-D and 3-D photonic crystals. 2-D photonic crystal templates were made by a rapid multibeam interference method in the photoresist SU-8, using 532nm laser radiation. These samples, typically on glass, were then infiltrated by a number of methods including from solution and melt, as well through chemical vapor deposition. Solutions of a titanium precursor with a cured refractive index of 2.1 at 633nm were infiltrated and cured in the SU-8 structure, with the infiltrant deposited by both by spin coating and casting. The resulting structure was shown to preserve the six-fold symmetry of the initial photonic crystal and subsequent firing at high temperature effectivelyremoved the SU-8 template. We have also explored the infiltration of nanoamorphous carbon into the photonic crystals using chemical vapor deposition. This material, which is essentially a carbon-silicon ceramic, has exceptional infrared optical properties with a refractive index > 2 for wavelengths beyond 2 m. The SU-8 polymer template has been shown to survive the CVD deposition process and the resulting infiltrated structure also preserves the initial PC symmetry. A series of metal-like PCs with a full range of properties is enabled by the ability to dope the nanoamorphous carbon with metals that possess exceptional refractive indices in the infrared regions of interest. We have also investigated the potential for nonlinear optical devices based upon azobenzene copolymer infiltrated silicon PCs and demonstrate the excellent properties of this material with respect to all-optical effects.

Keywords: photonic crystals, high index materials, infiltration, titanium dioxide, nanoamorphous carbon,

azobenzene

* Electronic mail: rnorwood@optics.arizona.edu

1. IntroductionSince their initial conception in the late 1980’s, 1 two-dimensional and three-dimensional photonic

crystals (PC) have experienced rapid development for a wide array of applications including key optical communications components such filters,2 switches, couplers,3 dynamic filters, and light sources,4 among other devices. Initial work relied upon naturally occurring structures such as opals5 which were useful for helping to demonstrate concepts but cannot be used for practical devices. Interest then shifted to conventional semiconductor materials such as silicon6 and gallium arsenide, since extensive experience in two-dimensional patterning of these materials could be leveraged for the fabrication of photonic band gap devices and waveguide devices, through the exploitation of defects.7 While these materials are well-suited to two-dimensional PC fabrication, three-dimensional fabrication has proven to be quite difficult. This has lead to the development, over the last five years, of polymer-based fabrication technologies that rely upon the principles of photolithography and photoresist chemistry to realize three dimensional structures of virtually unlimited complexity. 8,9 Two-photon lithography has been used to make three-dimensional photonic band gap structures that incorporate defects useful for waveguiding. However, a key drawback to using polymers for these devices is that their refractive indices are relatively low, in the range of 1.5-1.7, while it is well-known that to open robust polarization independent band gaps in two and three-dimensional PCs index contrasts of 2 or greater are needed, where the index contrast is ratio of the index of the PC lattice material to that of the material in the interstices. In the case of an air-filled PC, the contrast is simple the index of the lattice material.

More recently, work has begun in earnest on PC devices that exploit electro-optic and nonlinear optical properties of either the lattice material or of an infiltrant The majority of the work to date has used liquid crystals as the infiltrant10,11 since they are readily available and exhibit large quadratic electro-optic effects under modest applied field. This raises the general question of infiltrants for both linear and nonlinear photonic crystals. Clearly, in the case of a polymer-based PC, a high index infiltrant (with n ≈ 3 or above) could directly result in a robust PC device. Alternatively, infiltration with a material with n > 2 and subsequent removal of the initial polymer lattice through thermal decomposition can result in a new high index contrast PC based upon the infiltrant; this process is known as templating. To extend the work on nonlinear devices, it is important to consider some newly developed electro-optic polymers12 and all-optical materials13 as infiltrants to both conventional photonic band gap devices but also in more general microstructured materials structures such as superprisms. 14

In this paper we will discuss several new materials for photonic crystal applications, with the purpose of extending the materials and process tool kit available to PC engineers for both linear and nonlinear devices. We consider in turn a solution processed amorphous titanium dioxide developed by Brewer Science, a chemical vapor deposition (CVD) based nanoamorphous carbon technology developed together with Intex Corporation and suitable for attractive mid-infrared applications and a newly developed azobenzene copolymer with very large optically induced index changes (n > 0.2) that can provide an initial testing ground for all-optical PC device concepts.

2. Solution-based titanium dioxide for high index templatingIt is well-known that metal-based sol-gels (rather than silicon-based) can exhibit high refractive indices

and other properties typical of dielectrics or certain specialty glasses. However, these materials are traditionally very difficult to process since they generally undergo very rapid sol-gel chemistry in either acid-or base-catalyzed environments resulting in immediate gelation and precipitation when the precursors are brought together. Recently, Brewer Science has developed a new approach to this problem aimed primarily at the development of high dielectric constant materials for integrated circuit applications.15

Figure 1 illustrates the precursor materials that are mixed to form a shelf-stable solution in 1-methoxy-2-propanol, also known as PGME and a low toxicity semiconductor industry standard. The titanium compound is mixed with the styrene/vinyl alcohol copolymer and subsequent soft and hard bake

steps, with the hard bake at 300C for ten minutes, result in densification of the titanium based component and elimination of much of the organic character of the film. Resulting refractive indices are as high as 2.1 in the visible and greater than 1.9 at communications wavelengths. Even with a refractive index of 1.9 a number of photonic band gap devices can be considered, although they may be confined to operating at one polarization or under a limited wavelength range. While significant shrinkage occurs during the curing process, multilayer films have been demonstrated with no visible interface developing such that films approaching a micron in thickness are feasible, which is suitable for two-dimensional PC’s; it is also expected that templating with some three-dimensional PC structures will be possible, depending on the mechanics of the shrinking process.

Figure 1. Precursors developed by Brewer Science for a shelf-stable, high index material.

To investigate the potential of the Brewer titania for PC infiltration we fabricated SU-8 templates using the multibeam interference (MBI) process.16 532nm laser beams were interfered to create regular two-dimensional arrays of SU-8 material with approximately 0.5 m feature size on a glass substrate. The resulting hexagonal lattice would exhibit a two-dimensional photonic band gap if a high enough index contrast could be achieved, and, in fact, can exhibit a weak two-dimensional band gap for TM polarized light according to standard calculations. The titania precursor solution was both spin coated and cast onto the SU-8 PC arrays and subsequently cured according to the Brewer standard process. Figure 2(a) illustrates that the SU-8 structure survived the curing process with its periodicity intact, as evidenced by the diffraction pattern of a 632.8nm laser beam directed normally at the partially infiltrated structure which acts like a phase grating in this geometry. The evident six-fold symmetry is expected for a triangular lattice. There is considerable scattering apparent as well, which we believe is related to the partial filling of the structure with titania, resulting in numerous and irregular interfaces between the amorphous titania, the SU-8, and air. Figure 2(b) is an optical micrograph that shows a cross sectional view of the filled structure, where the brightly scattering portions are high index titania, surrounded by SU-8 polymer. Work is now underway to completely fill the polymer structure so that the SU-8 matrix can be burned off to obtain the goal of a high index contrast PC, using only a conventional low temperature semiconductor processes.

3. Nanoamorphous carbon for infrared PC applicationsNanoamorphous carbon (NAC) compounds in both undoped and metal-doped forms are a new class of

optoelectronic materials with electrical and optical properties that can be tuned over a wide range.17 The IR refractive index of NAC can be varied by doping with metals. The concentration of the metals can be > 50 at.%. By varying the type and concentration of the metal dopant we can tune the index of refraction over a wide range. NAC compounds in both undoped and metal-doped forms are currently used as thermoresistive

elements in blackbody emitters, resulting in broadband emission from 3 to 12 m. We have found that the chemical vapor deposition process used for these compounds is well-suited to deposition into polymer-basedPC templates formed by conventional lithography or imprint/stamping techniques. The templates can be burned away to leave a patterned NAC film.

Figure 2. a) Helium-neon diffraction from partially infiltrated titania-SU8 matrix exhibiting the six-fold symmetry of the initial SU8 lattice; b) Optical micrograph exhibiting infiltration - -the bright portions are the infiltrated titania.

Intex has developed a new class of multi-functional electronic materials, nanoamorphous carbon (NAC) coatings, with conductivity that can be varied from dielectric to metallic. The films are obtained by plasma enhanced chemical vapor deposition (PECVD). The PECVD-produced film is an amorphous dielectric with a composition consisting of a substantially sp3-bonded carbon network and an interpenetrating SiOx

network. The films are grown at ambient temperatures. An electroactive NAC film is made by co-depositing (e.g., sputtering) additional elements during the

growth of the carbon:silicon-oxide networks. A wide range of metals and compounds has been incorporated, both insulating and conducting. The incorporated material is in an amorphous phase, even at high concentrations. The resistivity (or electroactivity) of the film is controlled by controlling the composition during film growth. The conductivity of metal-containing NAC films reaches ~103 S/cm. Dielectric films (without conducting additives) have conductivities in the ~10-10 S/cm range. This is a larger range of conductivity than has been observed with any other known material. The films can be deposited with good adhesion on metals, ceramics, semiconductors and some plastics. Concentrations of additives can be up to ~50 at.%. At low concentrations, the additives appear to be in the form of randomly dispersed atoms. Figure 3 shows the conductivity as a function of the concentration of tungsten (W) incorporated into the film.

Glass

Polymer matrix

Titania

Glass

Polymer matrix

Titania

(a)(b)

Figure 3. Conductivity of doped nanoamorphous carbon as a function of metal fraction (tungsten).

Many of the film properties are similar to those of diamond-like carbon (DLC) materials, but the properties significantly exceed those of DLC in important areas. The main characteristics of the films are the following: high hardness (6-20 GPa); high wear resistance; high elastic modulus (100 – 400 GPa); low friction coefficient (0.04 – 0.15); high chemical stability; high temperature stability; wide range of conductivity (10-10 to 103 S/cm); wide range of infrared transparency (1-14 m); high dielectric strength (3 106 V/cm); excellent hermetic sealing properties; low internal stress; and biocompatibility. Films with thickness up to 10 m can be made routinely, which is not possible for DLC due to high internal stress. The deposition process is compatible with standard silicon processing and films can be patterned with ion-beam etching.

NAC films with metal additives are significantly more stable at high temperatures than typical DLC materials. Metal-containing films used as thermoresistors can operate in air at temperatures up to 750C. Normal DLC in contrast, oxidizes at ~350C. This technology forms the basis for a MEMS-based pulsedinfrared light source of very high brightness and capable of operating at frequencies greater than 100 Hz. The main application is as a light source in infrared gas sensors.

It has previously been shown that photonic band gap structures in silicon coated with a metal can demonstrate narrow-band infrared emission by coupling the light- excluding property of photonic band gaps with the resonant emission of surface plasmons. 18 In this device, a 2-D photonic band gap structure is made in Si by lithographic techniques after which a 150nm of gold is deposited on the top surface. Infrared black-body emission in the silicon is coupled to surface plasmon modes in the gold film, and the resonant frequency is defined by the periodicity of the silicon lattice and the width of the spectrum is determined by the refractive index contrast between the silicon and the air holes. Thus, it is possible to “channel” the blackbody radiation into surface plasmon modes such that the out-of-band emission is greatly reduced while the in-band emission is at or near the theoretical limit.

For silicon in the infrared the refractive index is approximately 3.5 and the index contrast, defined as nlattice/nhole is identical for air holes with an index of 1. While the peak wavelength of the emitter is determined by the periodicity of the structure, so long as there is a sufficient surface plasmon resonance, the breadth of the resonance is determined by the index contrast. As shown in Figure 4, the refractive index of

10-11

10-9

10-7

10-5

10-3

10-1

101

103

0 0.1 0.2 0.3 0.4 0.5

Metal Concentration (atomic fraction)

Co

nd

uc

tivi

ty (

S/c

m)

the undoped nanoamorphous carbon is approximately 2.2 in the near infrared, a lower contrast than silicon. This will result in a narrow photonic band gap, and, hence, a broader emission spectrum. More importantly, by selectively doping with appropriate metals, it is possible to vary the index of the doped NAC from 2.2 up to a maximum level determined by the metal and metal fraction used.

Figure 4. Absorption coefficient/refractive index of undoped NAC in the near IR. The reasonably high index makes the material an excellent candidate for 2-D and 3-D PCs.

We have fabricated some initial photonic crystal structures in NAC by direct CVD onto a polymer (SU-8) photonic crystal template that was produced by multi-beam interference lithography. The lattice consists of approximately 0.5 micron rods separated by 1.5 m in a triangular lattice Figure 5 illustrates diffraction from the NAC PC using 543nm, and the form of the diffraction pattern is exactly as would be expected from a triangular matrix, demonstrating that the polymer template has survived the NAC deposition process.

Figure 5. Diffraction from NAC PC lattice; the large number of diffraction spots indicate that the film is in thin enough to be in the Raman-Nath regime; high quality of the sample is indicated by the lack of scattering between the diffraction maxima.

Furthermore, the numerous diffraction orders indicate that the phase grating formed by the structure is relatively thin. Note the absence of scattered light compared to the sample of Figure 2 indicating the high degree of filling obtained. Figure 6 is an optical micrograph of the same sample, where we can clearly see the conformally coated NAC as well as the spatial periodicity that produced the k-space pattern in Figure 5; only a few small defects are seen. For metal-doped NAC we expect that the skin depth of the fabricated films will generally be less than the physical film depth, thus the presence of the SU-8 template will have no effect optically;19 in certain cases it would be beneficial to leave the SU-8 template in place rather than to remove it by volatilization. While CVD affords a direct technique for depositing NAC onto photonic crystal templates that can be made by standard lithography, the technology would be somewhat limited if only NAC could be used. As discussed above, Intex has developed the ability to selectively dope NAC with various metals (such as molybdenum, tungsten, copper, etc.) such that both the optical and electrical properties of the material can be changed significantly. Several promising application for these materials are under consideration including infrared filters and emitters.

Figure 6. Triangular NAC PC lattice fabricated by CVD onto an SU-8 template; center-to-center separation is approximately 1.5 m.

4. Azobenzene copolymers as infiltrants for all-optical PC devicesWhile it has long been known that azobenzene’s exhibit optically induced changes in conformation that

can lead to both color change and refractive index change, recently new azobenzene copolymers have been developed with exceptional optically induced refractive index changes.20 Figure 7 illustrates the structure ofthese copolymers where a very high azobenzene concentration is achieved by combining monomers with two different azobenzene dyes. The index change in these materials is caused by a combination of molecular conformation change (cis-trans isomerism) and subsequent molecular alignment; the maximum index change is achieved with illumination between 450 and 500nm at cw intensities on the order of 1W/cm2. These materials can be readily processed from solution in common solvents like cyclohexanone and high quality, uniform films are readily obtained. More recently, we have developed a method for processing these materials from the melt as this is more appropriate for photonic crystal infiltration. For convenience, we have performed our initial infiltration experiments using two-dimensional silicon PC lattices, which have been fabricated at Georgia Tech by Ali Adibi’s

group. Note that silicon PC’s with photonic band gaps should generally maintain those band gaps when filled with azobenzene copolymer, since the index contrast is nearly 2.3.

Figure 7. Structure of azobenzene copolymer PCDY50 synthesized by T. Fukuda, National Institute of Advanced Industrial Science and Technology (AIST), Japan.

Optically induced birefringence experiments were performed to determine the size of the optically induced refractive index change. Thin copolymer films a few microns thick were deposited on glass by spin coating and visually inspected for uniformity and film quality. Birefringence was measured using the apparatus shown in Figure 8, where a 488nm argon ion laser is used as the pump and a 632.8nm HeNe as the probe. Figure 9 shows typical data, where the intensity vs. analyzer angle is plotted for different pump intensities. Through these and other measurements, the index change at 632.8nm was deduced to be approximately 0.2 for 1W/cm2 illumination at 488nm, a very large value compared to other

Figure 8. Setup for measuring optically induced birefringence in azobenzene copolymers.

O

O

O

NN

CN

n

OO

O

CH3

NN

NN

n

Ar+ Laser λ=488nm

LinearPolarizer

Compensator Azobenzene

Polarimeter

x

LinearPolarizer

HalfWave Plate

He-Ne Laserλ=633nm

3mm

Ar+ Laser λ=488nm

LinearPolarizer

Compensator Azobenzene

Polarimeter

xx

LinearPolarizer

HalfWave Plate

He-Ne Laserλ=633nm

3mm

Figure 9. Optically induced birefringence signal at 632.8nm as a function of the analyzer angle for two different pump intensities.

Both solution and melt processes were developed to enable filling of silicon PC samples provided by Prof. Ali Adibi from Georgia Tech. Figures 10(a) and 10(b) show the results of infiltration of azobenzene copolymers into silicon PCs using solution and melt processes, respectively. The solution deposited film is seen to have delaminated from the silicon substrate, a result of shrinkage that takes place during the drying process. However, the melt deposited film demonstrates 100% filling and excellent adhesion in a very small structure (170nm hole diameter). Ongoing research will address the fabrication of azobenzene/silicon PC devices that exploit that large refractive index change in the azobenzene copolymer.

Figure 10. Solution processed (a) and melt processed (b) infiltrated azobenzene copolymers.

Analyzer angle

Sig

nal

I = 1 W/cm2

I = 1.8 W/cm2

-40 -20 0 20 40 60 80 100 120 140

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

Analyzer angle

Sig

nal

I = 1 W/cm2

I = 1.8 W/cm2

-40 -20 0 20 40 60 80 100 120 140

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

(a) (b)

5. ConclusionAs photonic crystal technology begins to be considered for applications, there is a need to increase the

options that device engineers have for both materials and processes. We have investigated several different new organic infiltrants that are suitable for both two- and three-dimensional PCs, as well as processes ranging from conventional solution processing to chemical vapor deposition. It is clear that the development of materials and processes for photonic crystal devices is at an early stage, with numerous promising research paths ahead.

ACKNOWLEDGMENTS

We would like to thank the Office of Naval Research and Intex Corporation for their support. We would also like to acknowledge the assistance of Dr. T. Fukuda at NAIST for providing the azobenzene copolymers discussed in this paper. We further acknowledge the assistance of Prof. Ali Adibi and his research group at Georgia Tech, particularly with regard to supplying silicon PC samples.

REFERENCES

1. E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987).2. J. D. Joannopolous, R. D. Meade, and J. N. Winn, Photonic Crystals (Princeton Univ. Press, Princeton, NJ, 1995).3. M. Lončar, T. Doll, J. Vŭcković, and A. Scherer, J. Lightwave. Tech. 18, 1402 (2000).4. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, Science 284, 1819 (1999). 5. H. Miguez, C. Lopez, F. Meseguer, A. Blanco, L. Vazquez, R. Mayoral, M. Ocaña, V. Fornes, and A. Misfud, Appl. Phys. Lett. 71, 1148 (1997).6. C. Manolatou, S. G.. Johnson, S. Fan, P. R. Villeneuve, H. A. Haus, and J. D. Joannopolous, J. Lightwave Tech. 17, 1682 (1999). 7. M. Tokushima, H. Kosaka, A. Tomita, and H. Yamada, Appl. Phys. Lett. 76, 952 (2000).8. S. M. Kuebler, M. Rumi, T. Watanabe, K. Braun, B. Cumpston, A. A. Heikal, L. L. Erskine, S. Thayumanavan, S. Barlow, S. R. Marder, and J. W. Perry, J. Photopolymer Sci. Tech. 14, 657 (2001).9. S. Maruo, O. Nakamura, S. Kawata, Opt. Lett. 22, 132 (1997).10. S. W. Leonard, J. P. Mondia, H. M. van Driel, O. Toader, S. John, K. Busch, A. Birner, and U. Gösele, Phys. Rev. B. 61, R2389 (2000). 11. C. Lopez, Adv. Mat. 15, 1679 (2003). 12. N. Peyghambarian and R. A. Norwood, Optics and Photonics News, February 2005, p. 31. 13. H. Sumimura, T. Fukuda, J. Y. Kim, D. Barada, M. Itoh, and T. Yatagai, Jpn. J. Appl. Phys. 45, No. 1 (2006).14. B. Momeni and A. Adibi, Applied Physics B 77, 555 (2003).15. Brewer Science product information. 16 C. K. Ullal, M. Maldovan, E. L. Thomas, G. Chen, Y.-J. Han, and S. Yang, Appl. Phys. Lett. 84,5434 (2004).17. Intex Corporation product information. 18. M. U. Pralle, N. Moelders, M. P. McNeal, I. Puscasu, A. C. Greenwald, J. T. Daly, E. A. Johnson, T. George, D. S. Choi, I. El-Kady, and R. Biswas, Appl. Phys. Lett. 81, 4685 (2002). 19. R. Biswas, J. Ahn, T. Lee, J.-H. Lee, Y.-S. Kim, C.-H. Kim, W. Leung, C.-H. Oh, K. Constant, and K.-M. Ho, J. Opt. Soc. Am. B22, 2728 (2005).20. J. Y. Kim and T. Fukuda, Jap. J. Appl. Phys. 45, 456 (2006).