Infiltration of planar 2D silicon photonic crystals with nanocomposites using a
simple melt processing technique is presented. The nanocomposites that were developed by
evenly dispersing functionalized TiO2 nanoparticles into a photoconducting polymer exhibit
high optical quality and tunable refractive index. The infiltrated photonic crystals show
tuning of the photonic band-gap that is controllable by the adjustment of the nanoparticle
loading level. These results may be useful in the development of tunable photonic devices,
hybrid light emitting diodes and photovoltaics.
We have investigated photorefractive (PR) properties of a polymer composite with low glass-transition temperature (Tg) in a symmetric reflection geometry. A diffraction efficiency of more than 30% is observed in 105μm thick devices. In low Tg photorefractive polymers, poling of the nonlinear optical chromophores at room temperature leads to birefringence in the material. The birefringence will alter the Bragg condition, as the propagation vectors for object and reference beams as well as the readout angle are influenced. We observed the Bragg-mismatch effect that caused a reduction in diffraction efficiency as the external field is increased. We have varied the angle of readout beam slightly at each bias field to get the highest efficiency.
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 effectively
removed 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 μm. 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.
We report the photorefractive properties of tetraphenyldiaminobiphenyl (TPD) based polymer composites
that have been developed for single pulse laser operation at 532 nm. With an optimized composite, we
demonstrate more than 50% diffraction efficiency using 4 mJ/cm2 single shot writing and 633 nm
continuous wave (cw) beam reading. The present devices showed a 300 μs fast response time. This
reveals the potential for these polymer devices in applications which require fast writing and erasure. Since
the writing pulse-width is in nanosecond time scale, the recording is totally insensitive to vibrations. These
devices can also be used as a stepping stone to realize all-color holography since they are sensitive to both
green (532nm) and red (633nm) wavelengths. The holograms can be written with either of these two
wavelengths and can be read by the same wavelength or the other wavelength with high diffraction
efficiency. This demonstrates that these devices have the advantage of performing two-color holography, a
step closer to a dynamic full-color holographic recording medium.
We describe the material characteristics and photorefractive properties of novel tetraphenyldiaminobiphenyl (TPD) based polymer composites that were developed for operation wavelengths up to 1 micron. With an optimized composite, we demonstrated more than 50% external diffraction efficiency coupled with a fast response time of about 35 ms at 980 nm. In addition to this high performing composite, we have developed a composite with high two beam coupling gain (300 cm-1). To accomplish these attractive photorefractive properties in the near-infrared, we explored the chemical flexibility of the guest-host approach. We employed a new dye with enhanced near-infrared absorption to extend the sensitivity into this long wavelength range. Styrene-based chromophores were utilized to enable high refractive index modulation. We explored ellipsometry as well as photo-conductivity measurements to optimize the composition of the composites. In addition to the composites that contain a single chromophore species, we also analyzed samples prepared with a mixture of chromophores. Our studies reveal the potential of this new polymer-composite family to extend the operation wavelength of the photorefractive materials to even longer wavelengths. Attractive photorefractive properties coupled with long wavelength sensitivity make these materials potential candidates for imaging and communication applications.