Waveguiding in photonic crystals - Semantic Scholar

VOL.2 NO.1 JANUARY 2008 www.nature.com/naturephotonics

Waveguiding in photonic crystals

SILICON PHOTONICS Signal regeneration SPECTROSCOPY Real-time Raman BIOLUMINESCENCE Firefly efficiency

nphoton-Cover-Jan-2008.indd 1

12/12/07 15:33:41

ARTICLES

Embedded cavities and waveguides in three-dimensional silicon photonic crystals STEPHANIE A. RINNE†, FLORENCIO GARCIÁ-SANTAMARIÁ† AND PAUL V. BRAUN* Department of Materials Science and Engineering, Beckman Institute, and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA † These authors contributed equally to this work. *e-mail: [email protected]

Published online: 16 December 2007; doi:10.1038/nphoton.2007.252

To fulfil the promise that complete-photonic-bandgap materials hold for optoelectronics applications, the incorporation of three-dimensionally engineered defects must be realized. Previous attempts to create and characterize such defects were limited because of fabrication challenges. Here we report the optical and structural characterization of complex submicrometre features of unprecedented quality within silicon inverse opals. High-resolution three-dimensional features are first formed within a silica colloidal crystal by means of two-photon polymerization, followed by a high-index replication step and removal of the opal template to yield embedded defects in three-dimensional silicon photonic crystals. We demonstrate the coupling of bandgap frequencies to resonant modes in planar optical cavities and the first waveguiding of near-infrared light around sharp bends in a complete-photonic-bandgap material. Photonic crystals (PhCs), or materials possessing a periodic modulation in their dielectric constant, have been proposed as media in which photons can be manipulated in a similar fashion to electrons in semiconductors. Specifically, PhCs can exhibit a complete photonic bandgap, or a range of frequencies over which light cannot propagate in any direction within the medium1–3. The unique optical properties of complete-photonic-bandgap (cPBG) materials provide the basis for numerous applications, such as low-threshold lasers3, low-loss waveguides4–6, on-chip optical circuitry7 and fibre optics8,9. For the majority of cPBG-based applications, functionality is provided through the precise, controlled incorporation of three-dimensionally engineered defects. Since the advent of PhCs, much work has been devoted to harness their ability to three-dimensionally manipulate photons10,11. Of paramount importance has been the need to develop a means to incorporate high-quality, pre-engineered defects within a cPBG material. Defects disrupt the PhC lattice periodicity, creating states for otherwise forbidden bandgap frequencies. The defect geometry, composition and position can be engineered to design PhCs with tailored functionalities. For example, embedded resonant cavities and waveguides can enable the creation of low-threshold lasers and optical circuits3,7. Although exquisitely controlled defects have been designed and fabricated in two-dimensional PhCs, the complete confinement of light can only be achieved by extending the PBG into the third dimension. However, the realization of three-dimensional (3D) systems containing defined defect structures has presented a difficult set of fabrication and materials challenges. A review of the recent efforts to introduce defects in 3D PhCs operating at optical frequencies is presented in ref. 12. Previous attempts to define defects in 3D PhCs have been limited in terms of the dimensionality and placement of the defects12. Also, defects have not been optically characterized in a

material that has a cPBG (ref. 13). In this article, we introduce defects of arbitrary shape and position within a cPBG material and optically characterize these features. Light is guided around sharp bends in a 3D PhC, demonstrating the first waveguiding of near-infrared (NIR) light within a cPBG material. This represents a major milestone in the use of 3D PhCs for cPBG applications10.

RESULTS Silicon inverse opals16,17 containing a variety of complex, highresolution, multidimensional embedded features, including waveguides and optical cavities, can be created from two-photon polymerization (TPP) in colloidal crystals14,15. Figure 1 presents vertical cross-sections of submicrometre-resolution air defects defined within silicon– air inverse opals exhibiting a variety of defect geometries, including multibend and vertical waveguides (Fig. 1a,b), Y-shaped splitters (Fig. 1c) and embedded planar cavities (Fig. 1d). The complexity and resolution of these features demonstrates the unprecedented potential and flexibility of this approach to add functionality to PhCs that can exhibit a cPBG in the NIR. The alignment of defects with respect to the PhC lattice is also a matter of paramount importance. This is made evident in twodimensional (2D) PhCs by the strong dependence of the quality factor of resonant cavities on small lattice variations18. The use of in situ fluorescence confocal microscopy during TPP affords good registration accuracy19. In Fig. 2, features written at the surface of a colloidal crystal were imaged using in situ fluorescence confocal microscopy (Fig. 2a,c) directly after TPP, and again using scanning electron microscopy (SEM) after silicon inversion (Fig. 2b,d–f ). Figure 2e,f best demonstrates the registration to the PhC lattice, the high edge resolution and that the silicon inversion produces a high-fidelity replication of the initial TPP features.

52

nature photonics | VOL 2 | JANUARY 2008 | www.nature.com/naturephotonics

© 2007 Nature Publishing Group

ARTICLES

Figure 1 Vertical cross-sections of air features embedded within silicon – air inverse opals. a – d, Cross-sections were exposed by means of FIB milling and imaged using SEM: complex embedded feature (a), straight vertical cylinder (b), vertical Y-splitter (c) and planar cavity (d). Scale bars: 3 m m. The sphere diameter is 725 nm.

FABRICATION

The realization of a 3D PhC that has both a cPBG and embedded pre-defined defect structures has posed a number of materials and fabrication challenges. Here, we present an overview of the fabrication procedure (Fig. 3) that was used (see Methods and Supplementary Information for a more detailed description). First, a modified vertical deposition technique20 using a temperature gradient17 was used to grow an artificial opal on a double-side-polished silicon substrate from pre-calcined21,22 silica spheres (725 nm or 925 nm in diameter). A thin conformal film of amorphous alumina was then grown around the spheres by means of atomic layer deposition23 (Fig. 3a), enhancing the mechanical stability of the sample and providing control over the degree of interpenetration between the spheres. The interpenetration is important, as it directly influences the size of the cPBG of the final structure24 and facilitates the etching of the silica colloids during the final inversion step (Fig. 3d). In the second major fabrication step, polymer features were embedded within the colloidal crystal by means of TPP (Fig. 3b)14,15, using a modulated beam rastering approach15. To facilitate in situ observation during the TPP process and the registration of the TPP features with the underlying PhC lattice, a fluorescent dye was added to the monomer solution9,25. Features with 500-nm linewidths and less than 100-nm edge resolutions were easily achieved with TPP. Under optimal alignment conditions, even narrower features (100 nm) have been reported26. Consequently, the creation of features smaller than the colloid-sphere diameter, a resolution sufficient for most features of interest, is straightforward. Next, the colloidal PhC containing embedded TPP features was infiltrated with a conformal amorphous silicon (a-Si) layer by means of chemical vapour deposition16 (CVD) (Fig. 3c) at 325 8C. Importantly, the embedded polymer is stable to the CVD conditions. In the final step, the a-Si overlayer was removed using reactive ion etching17,27 (RIE), exposing the top half of the alumina-coated silica colloids. The termination geometry obtained after RIE prevents the deleterious effects resulting from surface resonances, as reported in ref. 27. The alumina and silica were then chemically etched with an ethanolic solution of hydrofluoric

Figure 2 Micrographs of features defined in a PhC. a – d, Images of TPP features in an opal: confocal images taken near the surface of an opal (a,c); corresponding SEM images of the features in a silicon inverse opal (b,d). Scale bars in panels a – d, 5 mm. e,f, Higher magnification SEM micrographs of the features in d. Scale bars: 3 m m and 1 m m, for e and f respectively. Notice the high-fidelity replication, edge resolution and registration of the features with respect to the PhC lattice. The sphere diameter is 725 nm.

Figure 3 Overview of the fabrication scheme used to embed air features in a silicon – air inverse opal. a, Silica opal coated with a thin layer of alumina. b, Opal with embedded TPP features. c, Composite opal obtained by loading the pores with a-Si by means of CVD. d, The final silicon – air inverse opal structure that remains after removing the silicon overlayer, oxides and polymer. The dotted lines are a guide for the eye.

acid (Fig. 3d). The polymer features are transparent in the NIR, and thus were only removed for imaging purposes (Figs 1 and 2) by means of calcination at 500 8C in air or through the use of oxygen plasma etching28. COUPLING OF PHOTONS TO PLANAR CAVITIES

Designed defects are necessary to support optical modes that cannot exist in the surrounding cPBG material. Here we investigate the effect of an embedded planar cavity on the optical response of an opal-based structure before and after infilling with



53

1.0

1.0

0.8

0.8 Reflectance

Reflectance

ARTICLES

0.6 0.4

0.4 0.2

0

0

0.8

0.8 Transmittance

Transmittance

0.2

0.6

0.6 0.4 0.2

0.6 0.4 0.2

0 1.3

1.5 1.7 1.9 Wavelength (µm)

2.1

0 1.3 1.5 1.7 1.9 2.1 2.3 2.5 Wavelength (µm)

Figure 4 Optical spectra from a planar cavity embedded in a PhC. a,b, Reflection (top) and transmission (bottom) spectroscopy collected from the same PhC in regions with (red line) and without (blue line) a planar cavity after growing a thin alumina coating (13 nm thick) on the opal (a) and after filling 40% of the remaining pore volume with a-Si by means of CVD (b). The planar defect (dimensions 1 150 150 m m3) is embedded halfway through the thickness of the PhC. The sphere diameter is 725 nm.

a-Si (Fig. 4). Spectroscopy was carried out and compared between regions in the PhC with and without the planar cavity (dimensions 1 150 150 mm3), which was embedded halfway through the thickness of the PhC. Notches in the reflectance and transmittance of the first stop-band revealed that some frequencies coupled to optical modes supported by the embedded planar cavity29,30. Because the embedded cavities are written within an existing opal, which is subsequently inverted in a-Si, there is complete flexibility in the cavity shape, size and registration with respect to the PhC lattice, as well as the a-Si filling fraction. All these parameters can be independently varied, enabling future methodical studies to improve the bandwidth and transmittance of the defect mode. As the vertical cross-section in Fig. 1d shows, air planar cavities in silicon inverse opals can also be fabricated. TRANSMISSION OF LIGHT THROUGH WAVEGUIDES

For the realization of PhC-based optical circuitry, embedded waveguides that allow the propagation of cPBG frequencies will probably be necessary. Here, a silicon inverse opal was fabricated to contain straight (Fig. 5) and double-bend waveguides (Fig. 6) that extend through the PhC. Transmission of cPBG frequencies through the waveguides were observed (Fig. 5d, 6f ) using a narrow bandpass filter centred at 1.48 mm. In this fashion we demonstrated waveguiding of NIR light around sharp bends in a cPBG material (Fig. 6f ). When filters at frequencies outside the cPBG were used to collect transmission micrographs from the double-bend waveguides, they could not be differentiated from the background, supporting the existence of guided modes within the embedded features owing to the presence of a cPBG. There is no observable trend in the transmission output with waveguide geometry in Fig. 6f, probably because of slight truncation differences, which have been shown to greatly affect the coupling efficiency in PhC waveguides31,32. Likewise, in Fig. 5, where all the waveguides have similar dimensions, the transmission output varies significantly. In the future, theoretical guidance should enable the design of waveguides33 with improved coupling and throughput efficiencies. We hope this

Figure 5 Straight vertical waveguides. a,b, Cross-sections taken by means of in situ fluorescence confocal microscopy directly after TPP of 13 straight waveguides (each with a 5 5 mm2 square cross-section) extending through the thickness of a silica opal: horizontal cross-section (a), vertical cross-section (b). Scale bars: 50 m m and 25 m m for a and b, respectively. c,d, Corresponding SEM (c) and IR transmission (d) micrographs collected after inverting the sample in silicon and removing the colloidal template. Scale bars: 50 m m. The IR micrograph was collected with a bandpass filter centred at 1.48 m m. The sphere diameter is 925 nm.

initial demonstration of waveguiding in a cPBG material will encourage such efforts.

DISCUSSION In this article we have demonstrated a flexible route for the introduction of complex, multidimensional features within 3D PhCs, and have provided evidence of waveguiding of NIR light through such features. This represents the first use of a cPBG to manipulate NIR light within a 3D PhC. To further facilitate the creation of optically active devices, the techniques described in this paper could be used to incorporate optical emitters or nonlinear materials within embedded features. This fabrication route is also broadly compatible with PhCs formed by other means, including holographic25, conventional and two-photon lithographies34,35, extending the range of crystal structures in which complex defects can be formed. As we have demonstrated, it is now possible to fabricate and characterize complex features with unprecedented quality in cPBG materials. This represents a major step forward in adding functionality to PhCs (ref. 10), extending their viability for the 3D manipulation of photons in all-optical devices. It is our opinion that this work may lead to a paradigm shift in the PhC community, prompting theorists to design defects with sophisticated functionalities for 3D-photonic-bandgap materials.

METHODS The fabrication procedure used in this work involved the following steps: (1) artificial opal assembly; (2) first atomic layer deposition of alumina; (3) TPP; (4) second atomic layer deposition of alumina (optional); (5) CVD of a-Si; (6) reactive ion etching of top a-Si layer; (7a) wet etch of silica and alumina or

54



ARTICLES

Figure 6 Double-bend waveguides embedded in a 3D PBG material. a – c, Vertical fluorescence confocal cross-sections of waveguides (2 2 m m2 transverse cross-sections) in a silica opal. d, Horizontal confocal image taken near the surface of an opal with one straight and four double-bend waveguides with increasing lateral extents. e, SEM image of the top surface after inversion. f, Infrared micrograph showing light transmitted through waveguides using a bandpass filter centred at 1.48 m m, within the calculated 6% cPBG. Images d – f were collected from the same region, ensuring that the five bright spots in f correspond to the locations of the waveguide output. Four IR images were averaged to obtain f. Scale bars: 10 m m. The sphere diameter is 925 nm.

(7b) removal of polymer; (8) wet etch of silica and alumina (see Supplementary Information, Fig. S1)36. (See Supplementary Information for additional fabrication details and supporting data. Specifically, for details on microsphere calcination and self-assembly see Figs S2 and S3.) ATOMIC LAYER DEPOSITION OF ALUMINA

To obtain an interpenetrated array of spheres and enhance the mechanical stability of the colloidal crystal, a thin, conformal amorphous alumina (Al2O3) layer was grown by means of atomic layer deposition37 (ALD). (See Supplementary Information, Fig. S4, for an explanation of the impact of this layer on the cPBG.) A commercial ALD system (Savannah 100, Cambridge NanoTech) was used to controllably grow highly conformal, thin alumina films within artificial opals. Precursor chemicals with nitrogen as the carrier gas were injected at a flow rate of 20 s.c.c.m. in an alternating fashion through the sample chamber to form atomic monolayers. The opening times of the water and trimethylaluminium (C3H9Al) valves were set to 0.05 s and 0.10 s, respectively. The sample chamber was maintained at 200 8C and 2 torr. The process was automatically repeated for 80 – 130 cycles, depending on the requisite layer thickness. Accurate control over the degree of sphere interpenetration was confirmed by spectroscopy (see Supplementary Information, Fig. S5).

static CVD system was used with disilane (Si2H6, 98%, Gelest). For samples assembled from 725-nm-diameter spheres, two cycles were used (50 mbar, 15 h, 325 8C); opals made from 925-nm-diameter spheres required a third cycle (see Supplementary Information, Fig. S7). Structures were filled until the trigonal interstitial sites closed off. The ratio of the radius to the centre-to-centre separation of the silicon-defined air spheres was 0.408. ETCHING

Reactive ion etching was used to expose the silica colloids (see Supplementary Information, Fig. S8). A Uniaxis 790 series RIE was used with a power of 70 W, chamber pressure of 100 mtorr, and O2 and SF6 gas flow rates of 20 s.c.c.m. A typical etch lasted 1 min, and the d.c. voltage readout was 35– 40 V. Samples were masked by placing a 50-mm-thick Kapton film containing a 1 2 mm2 opening over the sample. This enabled the selective exposure of a small region during RIE, reducing the probability of sample lift-off during subsequent processing. The silica microspheres and ALD alumina were then removed by means of wet etching using freshly prepared, ethanolic hydrofluoric acid (5% in 50% ethanol, 45% water) for 40 min for thick samples. (See Supplementary Information, Fig. S9, for more information on oxide removal.)

TWO-PHOTON POLYMERIZATION

MICROSCOPY AND SPECTROSCOPY

The detailed experimental procedure for TPP has been described previously in the literature15. Here, a fluorescent dye was added to the monomer solution to enable in situ fluorescence confocal imaging19,25 (Fig. 2). When embedding the polymer features, care was taken to select regions with particularly low defect densities identified by means of in situ fluorescence confocal imaging. This was done to minimize unwanted coupling of light to intrinsic defects or cracks in the inverse opal (for example, see the small vertical crack at the bottom of Fig. 5c,d). Thermogravimetric analysis and spectroscopy revealed that this polymer is stable at 325 8C, the temperature used for silicon CVD (see Supplementary Information, Fig. S6). To obtain air defects, TPP features were removed using calcination, overnight, at 500 8C in air (before wet etch).

Electron micrographs were taken with an SEM (Hitachi S-4700) or dual-beam focused-ion-beam (FIB) microscope (FEI Strata DB-235). For FIB milling, the ‘si.mtr’ control file was used with a 5,000 – 7,000 pA ion aperture for roughing cross-sections, followed by a 100– 300 pA aperture for cleaning cross-sections. Milling times were 1 – 2 h per location. Spectroscopy38 was collected using a 4, 0.1 numerical aperture objective on an optical microscope (Hyperion 2000) coupled to a Fourier transform infrared spectrometer (Vertex 70) and outfitted with a spatial aperture to reduce the collection spot diameter to 75 – 150 mm. Reflection spectra were normalized to a protected silver mirror with 95% reflectivity over the wavelengths used (Melles Griot). Transmission spectra in the manuscript were normalized to air (in the Supplementary Information, spectra were normalized to transmission through the same silicon substrate from a region without a colloidal crystal). Infrared micrographs were collected with an InGaAs 320 256 pixel camera with three-stage cooling (XEVA-FPA320, XenICs). Bandpass filters (ThorLabs and Edmund Optics) had centre

CHEMICAL VAPOUR DEPOSITION OF SILICON

Silicon CVD allows the conformal growth of a high-quality, low-roughness silicon layer around the microspheres16 and TPP features within an opal. Here a nature photonics | VOL 2 | JANUARY 2008 | www.nature.com/naturephotonics


55

ARTICLES wavelengths of 1,300 nm, 1,350 nm, 1,400 nm, 1,480 nm and 1,550 nm, and full-width at half-maximums of 10.5 nm or 12 nm.

Received 5 August 2007; accepted 15 November 2007; published 16 December 2007. References 1. Bykov, V. P. Spontaneous emission from a medium with a band spectrum. Sov. J. Quant. Electron. 4, 861 –871 (1974). 2. John, S. Strong localization of photons in certain disordered dielectric superlattices. Phys. Rev. Lett. 58, 2486– 2489 (1987). 3. Yablonovitch, E. Inhibited spontaneous emission in solid-state physics and electronics. Phys. Rev. Lett. 58, 2059–2062 (1987). 4. Meade, R. D. et al. Novel applications of photonic band-gap materials — low-loss bends and high Q-cavities. J. Appl. Phys. 75, 4753– 4755 (1994). 5. Fan, S. H. et al. Guided and defect modes in periodic dielectric wave-guides. J. Opt. Soc. Am. B 12, 1267– 1272 (1995). 6. Mekis, A. et al. High transmission through sharp bends in photonic crystal waveguides. Phys. Rev. Lett. 77, 3787–3790 (1996). 7. Joannopoulos, J. D., Villeneuve, P. R. & Fan, S. Photonic crystals: Putting a new twist on light. Nature 386, 143– 149 (1997). 8. Fink, Y. et al. A dielectric omnidirectional reflector. Science 282, 1679–1682 (1998). 9. Russell, P. Photonic crystal fibers. Science 299, 358– 362 (2003). 10. Noda, S., Tomoda, K., Yamamoto, N. & Chutinan, A. Full three-dimensional photonic bandgap crystals at near-infrared wavelengths. Science 289, 604– 606 (2000). 11. Sozuer, H. S., Haus, J. W. & Inguva, R. Photonic bands—convergence problems with the plane-wave method. Phys. Rev. B 45, 13962–13972 (1992). 12. Braun, P. V., Rinne, S. A. & Garcıá-Santamarıá, F. Introducing defects in 3D photonic crystals: State of the art. Adv. Mater. 18, 2665– 2678 (2006). 13. Jun, Y. H., Leatherdale, C. A. & Norris, D. J. Tailoring air defects in self-assembled photonic bandgap crystals. Adv. Mater. 17, 1908– 1911 (2005). 14. Lee, W. M., Pruzinsky, S. A. & Braun, P. V. Multi-photon polymerization of waveguide structures within three-dimensional photonic crystals. Adv. Mater. 14, 271– 274 (2002). 15. Pruzinsky, S. A. & Braun, P. V. Fabrication and characterization of two-photon polymerized features in colloidal crystals. Adv. Funct. Mater. 15, 1995 –2004 (2005). 16. Blanco, A. et al. Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometres. Nature 405, 437– 440 (2000). 17. Vlasov, Y. A., Bo, X.-Z., Sturm, J. C. & Norris, D. J. On-chip natural assembly of silicon photonic bandgap crystals. Nature 414, 289– 293 (2001). 18. Song, B. S., Noda, S., Asano, T. & Akahane, Y. Ultra-high-Q photonic double-heterostructure nanocavity. Nature Mater. 4, 207 –210 (2005). 19. Nelson E. C. & Braun, P. V. Registration and optical properties of embedded two-photon polymerized features within self-organized photonic crystals. arXiv:0710.0851v1 ,http://arxiv.org/abs/0710.0851v1. (2007). 20. Jiang, P., Bertone, J. F., Hwang, K. S. & Colvin, V. L. Single-crystal colloidal multilayers of controlled thickness. Chem. Mater. 11, 2132– 2140 (1999). 21. Chabanov, A. A., Jun, Y. & Norris, D. J. Avoiding cracks in self-assembled photonic band-gap crystals. Appl. Phys. Lett. 84, 3573–3575 (2004). 22. Garcıá-Santamarıá, F. et al. Refractive index properties of calcined silica submicrometer spheres. Langmuir 18, 1942– 1944 (2002). 23. Groner, M. D., Elam, J. W., Fabreguette, F. H. & George, S. M. Electrical characterization of thin Al2O3 films grown by atomic layer deposition on silicon and various metal substrates. Thin Solid Films 413, 186 –197 (2002).

24. Busch, K. & John, S. Photonic band gap formation in certain self-organizing systems. Phys. Rev. E 58, 3896 –3908 (1998). 25. Scrimgeour, J. et al. Three-dimensional optical lithography for photonic microstructures. Adv. Mater. 18, 1557– 1560 (2006). 26. Takada, K., Sun, H. B. & Kawata, S. Improved spatial resolution and surface roughness in photopolymerization-based laser nanowriting. Appl. Phys. Lett. 86, 071122 (2005). 27. Garcıá-Santamarıá, F., Nelson E. C. & Braun, P. V. An optical surface resonance may render photonic crystals ineffective. Phys. Rev. B 76, 075132 (2007). 28. Garcıá-Santamarıá, F. et al. Nanorobotic manipulation of microspheres for on-chip diamond architectures. Adv. Mater. 14, 1144–1147 (2002). 29. Palacios-Lidoń, E., Galisteo-Lo´pez, J. F., Jua´rez, B. H. & Lo´pez, C. Engineered planar defects embedded in opals. Adv. Mater. 16, 341 –345 (2004). 30. Tetreault, N. et al. Dielectric planar defects in colloidal photonic crystal films. Adv. Mater. 16, 346 –349 (2004). 31. Kramper, P. et al. Highly directional emission from photonic crystal waveguides of subwavelength width. Phys. Rev. Lett. 92, 113901 (2004). 32. Frei, W. R., Tortorelli, D. A. & Johnson, H. T. Topology optimization of a photonic crystal waveguide termination to maximize directional emission. Appl. Phys. Lett. 86, 111114 (2005). 33. Lousse, V. & Fan, S. H. Waveguides in inverted opal photonic crystals. Opt. Express 14, 866 –878 (2006). 34. Cumpston, B. H. et al. Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication. Nature 398, 51–54 (1999). 35. Deubel, M. et al. Direct laser writing of three-dimensional photonic-crystal templates for telecommunications. Nature Mater. 3, 444 –447 (2004). 36. Pruzinsky, S. A. Two-Photon Polymerization of Defects in Photonic Crystals. Thesis, Univ. Illinois at Urbana-Champaign (2006). 37. Sechrist, Z. A. et al. Modification of opal photonic crystals using Al2O3 atomic layer deposition. Chem. Mat. 18, 3562–3570 (2006). 38. Vlasov, Y. A., Deutsch, M. & Norris, D. J. Single-domain spectroscopy of self-assembled photonic crystals. Appl. Phys. Lett. 76, 1627– 1629 (2000).

Acknowledgements This material was based on work supported by US Army Research Office grant DAAD19-03-1-0227, National Science Foundation grant DMR 00-71645 and US Department of Energy, Division of Materials Sciences grant DE-FG02-07ER46471, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign (UIUC). This work was carried out in part in the Beckman Institute Microscopy Suite, UIUC, and the Center for Microanalysis of Materials, UIUC, which is partially supported by the US Department of Energy under grants DE-FG02-07ER46453 and DE-FG02-07ER46471. We gratefully thank L.-S. Tan (US Air Force Research Laboratory) for providing the two-photon sensitive dye, and E.C. Nelson, A.D. Stewart and E. Zettergren of our laboratory for providing some of the colloids and colloidal crystals used in this work. Correspondence and requests for materials should be addressed to P.V.B. Supplementary Information accompanies this paper on www.nature.com/naturephotonics.

Author contributions S.A.R. carried out the TPP, FIB, SEM, confocal and IR microscopy. F.G.S. carried out the ALD, CVD and bandgap calculations. S.A.R. and F.G.S. both performed HF etching, spectroscopy, RIE, and grew colloidal crystals. All authors conceived and designed the project, participated in discussions about the research and wrote the manuscript. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

56

