Photonic crystals (PC) can fundamentally alter the emission behavior of light sources by suitably modifying the
electromagnetic environment around them. Strong modulation of the photonic density of states especially by full three-dimensional
(3D) bandgap PCs, enables one to completely suppress emission in undesired wavelengths and directions
while enhancing desired emission. This property of 3DPC to control spontaneous emission, opens up new regimes of
light-matter interaction in particular, energy efficient and high brightness visible lighting. Therefore a 3DPC composed
entirely of gallinum nitride (GaN), a key material used in visible light emitting diodes can dramatically impact solid state
lighting. The following work demonstrates an all GaN logpile 3DPC with bandgap in the visible fabricated by a template
directed epitaxial growth.
We describe various three dimensional photonic crystals fabricated from two methods - step-and-repeat projection
lithography and multi level electron beam direct write - with bandgap in the optical frequency for potential sensor
application. The tungsten woodpile lattice fabricated with step-and-repeat photolithography exhibits a thermal emission
peak centered at ~ 2μm wavelength with less than 30% peak emission for wavelengths > 4 μm. The tungsten photonic
crystal has also been investigated for application as a damage sensor in structures under mechanical stress. Using a
multilevel electron beam direct write, we have fabricated prototype woodpile lattices of nano crystalline silicon,
amorphous silicon, gold and titanium oxide. The 4 layer silicon woodpile PC exhibits a stop band centered at 1.5 μm as
measured by micro reflectance and transmission spectroscopy. We have also introduced line and point defects in the 5th
layer of a 9 layer amorphous silicon lattice, in order to explore them as sensor structures. We also fabricated a 4 layer Au
woodpile lattice which shows broad high reflectivity at longer wavelengths with a sharp roll off near 1.5 μm.
Arrays of lead lanthanum zirconate titanate disks are fabricated on Pt electrodes by wet etching of thin films processed from sol-gel precursors, which may be applicable as defect cavities with electrically tunable resonance wavelength when embedded inside 2-D Si photonic crystals. Using e-beam lithography followed by sputtering and liftoff, Pt etch masks with diameters down to 1μm are deposited on pyrolyzed PLZT films. Wet etching using diluted hydrochloric acid generates discrete PLZT disks with integrated top and bottom Pt electrodes. The dimensions of the PLZT disks are determined by the diameter of the Pt etch mask, although undercutting becomes a significant issue as etch mask size decreased. The effects of varying PLZT pyrolysis temperature on the etching rate and film quality after sintering are examined. Dielectric testing of wet etched PLZT film after sintering showed that the devices have short circuited, suggesting that the deformation of the top Pt electrode over the undercut PLZT during sintering may significantly hinder the applicability of the current wet etching technique. Alternate methods for the patterning of PLZT for integration into photonic crystals are proposed.
Photonic crystal templating of optically active hydrogel sensors is a topic of growing interest in materials chemistry. When interactions between a mesostructured hydrogel and the analyte molecules cause a reversible dimensional change of the hydrogel, the corresponding change in optical diffraction can be detected either spectroscopically or visually. Using poly(styrene) photonic crystals as templates, we synthesized inverse opal hydrogels through
photopolymerization of 2-hydroxyethylmethacrylate and various functional monomers, and demonstrated the ability to sense pH and glucose at different ionic strengths and other experimental conditions. The diffraction of the pH sensitive hydrogel shifted from 544 nm to 850 nm when the pH was increased from 4 to 7, while the diffraction of the glucose sensitive hydrogel changed from 599 nm to 719 nm when the glucose concentration was raised from 0 mM to 100 mM. Diffraction response kinetics on the order of ~30 minutes were observed, which may be attributed to diffusion of analyte molecules through thin (12 - 24 μm) hydrogel samples. These mechanically robust inverse opal hydrogel sensors may form a starting point for chemical and biological sensing using diffractive three-dimensional mesostructures.