Auxiliary Information
I. Experimental methods
Artificial opals were fabricated from 925nm silica microspheres using a vertical deposition technique
similar to that described elsewhere [1, 2]. The microspheres were fabricated using an emulsion method in
which silica nanoparticles are used as seeds for repeated re-growth cycles [3]. This method results in a
suspension of silica microspheres with a very low polydipersity (~1%). An 8nm thick layer of Al2O3 was
grown in the opals using atomic layer deposition (ALD) resulting in interpenetrated spheres. The opals
were subsequently filled with amorphous silicon using a low-temperature static CVD. The Si precursor gas
is disilane (Si2H6) and the pressure 400 mbar. The decomposition of disilane into Si is performed at 375˚C
for 15 hours.
Opals were optically characterized by FTIR spectroscopy using a Bruker Hyperion microscope coupled
into a Bruker Vertex 70 FTIR spectrometer. All measurements were carried out at normal incidence with a
custom made CaF2 (transparent in the middle infrared) objective with 2.4x magnification and 0.07
numerical aperture from circular regions with a diameter of 250μm. The opal surface geometry was
modified using a Uniaxis 790 Series Reactive Ion Etching system to remove surface silicon. The reactive
species were SF6/O2 at 20sccm, 100mTorr and 70W RF power for cycles of 30-90 seconds.
II. Computational methods
All finite-difference time-domain (FDTD) simulations were performed with the MIT Electromagnetic
Equation Propagation (MEEP) software [4]. The computation cell (Fig. A1) consists of three layers (A-B-C
in the fcc configuration) repeated in space to achieve the desired number of layers in multiples of three on a
Si substrate extended to the end of the computation cell. The silica sphere radius is 0.36 a, where a is the
fcc lattice parameter, to account for the interpenetration. The Si radius is 0.408 a corresponding to 86%
filling of the interstitial sites which is what we achieve through static CVD. Growth of Si on the substrate
from the CVD is also taken into account. All layer and sphere spacings are set to perfect FCC geometry.
The top layer of Si spheres has a larger radius to simulate the effect of a thick Si overlayer.
Figure A1 – Cross-section of the computation cell for FDTD simulations. The cell size in the zdimension (left to right) is 25 a. Silicon (n=3.45) is indicated by black, silica (n=1.45) by gray and air
(n=1.00) by white.
III. Effect of the overlayer thickness
The position of the resonances blue-shifts as the thickness of the silicon overlayer is reduced by means of
RIE. The experimental reflectance spectra obtained from a Si/SiO 2 monolayer is shown in Fig. A2(a-c).
Accordingly, simulations performed with FDTD show the same behavior when the RIE effect on the
overlayer was taken into account [Fig. A2(d-f)]. RIE was simulated by moving the Si overlayer in the zdirection to simulate anisotropic etching perpendicular to the substrate. The resolution was chosen to have a
minimum of 8 pixels per normalized optical wavelength of the highest frequency calculated in the highest
dielectric constant material [4].
Figure A2 – Comparison of reflectance spectra from a monolayer of Si/SiO2 spheres a) as grown and b)
after 1 and c) 2 cycles of RIE (30 seconds each). FDTD simulations are provided for varying silicon
overlayer thicknesses on a monolayer of close-packed spheres. Corresponding radii are d) 0.48 a e) 0.46
a and f) 0.43 a.
IV. Comparison of infilled vs. inverted opals
The optical response from composite (Si/SiO2) and inverted (Si/air) opals normal to the (111) planes (L
direction) is very similar. Figure A3(a and b) show the band structures for a composite and inverted opal,
respectively. The band structures have all of the same important features with the only differences being
wider stop bands due to greater index contrast and a blue shift of the band gaps due to a lower average
refractive index for the inverted case. The reflectance spectra calculated from FDTD simulations of the
composite and inverted [Fig. A3(c and d)] opals are virtually identical for the region containing the
resonance. Thus, there is no reason to perform our experiments on the inverted case since our results are
applicable to both.
Figure A3 – A comparison of Si/SiO2 composite opals and Si inverse opals. a) Photonic band
structure calculated with MIT Photonic Bands (MPB) for a Si/SiO2 composite structure. b) Photonic
band structure calculated with MPB for a Si inverse opal. c) FDTD reflectance spectra of a monolayer
of silica spheres with a 0.12 a thick Si overlayer. d) FDTD reflectance spectra of a monolayer of an
inverse silicon opal with 0.12 a thick Si overlayer.
V. Polarization effects.
FDTD simulations were carried out for two different polarizations. We found the effect of polarization
for the particular case of a monolayer of close-packed Si/SiO2 spheres is almost negligible [Fig. A4].
Figure A4 – Reflectance spectra for a monolayer of silica spheres with a 0.12 a thick Si overlayer for two
polarizations. The schemes on the right show the surface of the opal and the direction of the electric field
for (a) and (b).
References
[1] P. Jiang, et al., Chem. Mater. 11, 2132 (1999).
[2] Y. A. Vlasov, et al., Nature 414, 289 (2001).
[3] A. van Blaaderen, Private communication.
[4] FDTD simulations were performed using freely available software: A. Farjadpour, et al., Opt. Lett. 31,
2972 (2006).