The development of porous nanostructured materials, such as polymer Bragg gratings, offer an attractive and unique platform for chemical and biological recognition elements. Much of the efforts in polymeric gratings have been focused on holographic polymer dispersed liquid crystal (H-PDLC) gratings with demonstrated applications in switching, lasing, and display devices. Here, we present the application of porous polymer photonic bandgap structures produced using a modified holographic method that includes a solvent as a phase separation fluid. The resulting gratings are simple to fabricate, stable, tunable, and highly versatile. Moreover, these acrylate porous polymer photonic bandgap structures were generated using a simple one-beam setup. In this paper, we describe the application of these nanoporous polymer gratings as a general template for biochemical recognition elements. As a prototype, we developed an oxygen (O₂) sensor by encapsulating the fluorophore (tris(4,7-diphenyl-1,10-phenathroline)ruthenium(II) within these nanostructured materials. Thus, the obtained O₂ sensors performed through the full-scale range (0%-100%) with a response time of less than 1 second. Most importantly, the use of the inherent property of these gratings to transmit or reflect a particular wavelength spectrum, based on the grating spacing, enables us to selectively enhance the detection efficiency for the wavelengths of interest.

Tunable photonic crystals offer an interesting possibility to adjust the photonic band gap (PBG) as per requirement. Various methods of achieving this have been tried that include polarization of liquid crystals, thermal effects and more. Electrochromic (EC) materials in which a reversible optical property change can be induced with the application of a small electric field offer a novel possibility to tune the PBG in a controlled and reversible way. The reversible chemical change and the ensuing change of optical constants in these periodically arranged EC materials make the PBG tunability possible. In a recent work we have demonstrated for the first time the PBG tunability of EC materials deposited in the form of periodic inverse opals. This earlier work was carried out with the well known Tungsten Trioxide (WO₃) EC thin films into which lithium intercalation was done by a dry lithiation method. In the present work we report on the fabrication of a simple tunable photonic crystal device based on electrochemical insertion/extraction of lithium based on WO₃ inverse opals.

The design and simulation of a novel resonant cavity optical modulator incorporating a hybrid silicon/electro-optic polymer slot waveguide structure is presented in this work. The device utilizes the electro-optic polymer in the cavity region to provide an active material for modulation and includes distributed Bragg reflectors in single mode silicon waveguide regions at each end of the cavity to create a narrow response peak at the resonant wavelength. Simulation results show that this electro-optic modulator design can simultaneously attain a large modulation depth, short device length and a low drive voltage, all of which are expected to be necessary for future high speed integrated optics devices. The high operating frequency and complex nature of the structure lead to a need for full 3D simulations in order to obtain accurate propagation characteristics, particularly concerning scattering losses. However, 3D simulations are very computationally expensive, especially during design optimization. Therefore, the periodicity of the device has been exploited to allow a cascade matrix approach to be employed to reduce the necessary computational resources required for accurate simulation of the propagation characteristics. The design and fabrication process have been chosen to allow for the majority of the fabrication to be completed before the electro-optic polymer is introduced into the process, which enables the use of well-established CMOS processing techniques, and should accelerate the transition to hybrid silicon/electro-optic polymer devices in future integrated optics applications.

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 5^th 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.

The filling is reported of the air holes of an InP-based two-dimensional photonic crystal with solid polymer and with liquid crystal 5CB. The polymer filling is obtained by thermal polymerization of an infiltrated liquid monomer, trimethylolpropane triacrylate. The filling procedure for both the monomer and liquid crystal relies on the capillary action of the liquid inside the ~ 200 nm diameter and < 2.5 μm deep air holes. The solid polymer infiltration result was directly inspected by cross-sectional scanning electron microscopy. It was observed that the holes are fully filled to the bottom. The photonic crystals were optically characterized by transmission measurements around the 1.5 μm wavelength band both before and after infiltration. The observed high-frequency band edge shifts are consistent with close to 100% filling, for both the polymer and the liquid crystal. No differences were observed for filling under vacuum or ambient, indicating that the air diffuses efficiently through the liquid infiltrates, in agreement with estimates based on the capillary pressure rise.

The fabrication of one dimensional photonic bandgap nanostructures is described and the optical properties of these structures are examined. Using a deposition technique known as a glancing angle deposition (GLAD), porous films with a predefined nanoscale geometry are created. Specifically, in the present work we consider GLAD fabricated thin films characterized by periodically varying refractive index in one-dimension. We introduce a variety of planar defect layers into the structures and investigate the resulting changes observed in the photonic bandgap of the system. Theoretical simulation of transmittance spectra of GLAD fabricated films is performed with the finite-difference time-domain (FDTD) method and the results are compared with experimental measurements. Modifications of the transmittance spectra are investigated by changing the geometry of the defect layer and varying the void region effective index. It is shown that the spectral width and location of states within the bandgap is controlled by the geometry of defect and film microstructure. Active tunability of the defect states is obtained by considering infilling of the void regions of the structure with nematic liquid crystals and then analyzing the optical spectrum for various orientations of the liquid crystal director axis.

Interference lithography of polymer dispersed liquid crystals allows rapid, facile fabrication of complex polymeric photonic structures that have an inherent electro-optic component for agile structures. The polymerization mechanism (step-growth or chain growth) strongly influences the morphology of the LC droplet and distribution within the polymer matrix. Using a multi-functional acrylate monomer that undergoes chain growth polymerization leads to asymmetrical LC droplets of random size and distribution, in contrast to the step-growth mechanism of thiol-ene formation where LC droplets form with a nearly uniform size distribution and spherical shape. Thiol-ene holographic polymer dispersed liquid crystals (H-PDLCs) diffraction structures have narrower bandwidth and less baseline scatter than the acrylatebased H-PDLCs. Furthermore, distributed feedback lasers constructed from thiolene-based H-PDLC lasers show marked improvement in the optical and electro-optical properties as evinced by the factor of two decrease in switching voltage and the reduction of lasing threshold from 0.17 mJ cm^-2 to 0.07 mJ cm^-2. These differences in optical and electro-optic properties directly correlate with the difference in microscale morphology of the H-PDLCs giving insight to the importance of microscale structure on macroscale phenomenon.

The optical properties of photonic bandgap (PBG) structures are highly sensitive to environmental variation. PBG structures thus are an attractive platform for biosensing applications. We experimentally demonstrate a label-free biosensor based on a two-dimensional (2-D) photonic crystal microcavity slab. The microcavity is fabricated on a silicon-on-insulator substrate and integrated with tapered ridge waveguides for light coupling. The Finite-Difference Time-Domain (FDTD) method is used to model the sensor. The resonance of the microcavity is designed to be around 1.58 μm. In order to capture the target biological materials, the internal surface of the photonic crystal is first functionalized. Binding of the targets is monitored by observing a red shift of the transmission resonance. The magnitude of the shift depends on the amount of material captured by the internal surface. Compared to 1-D PBG biosensors, 2-D devices require a smaller amount of target material and can accommodate larger targets. Experimental results are compared with the predictions obtained from the FDTD simulations.

Photonic crystals have many potential applications due to their unique abilities to control the propagation of electromagnetic waves. If their bandgap and dispersive properties are modulated by external means, more exciting applications emerge. In this work, we present novel applications and devices created by tuning the bandgap and dispersive properties of periodic photonic crystal structures. We present our designs in both high- and low-refractive index materials. Many tunability alternatives exist including thermal, optical, electrical, microfluidic, and liquid crystals-based. In this paper, we will utilize these methods to implement various photonic crystal-based devices and applications.

Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization ("pixellization"). We show that accuracy can be significantly improved by using a sub-pixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory, and compare it to other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.

Photonic crystal fiber (PCF) is considered as a special type of 2-dimensional photonic crystal structure. As the result of its large design flexibility, there are numerous different types of PCFs exhibiting vastly different optical properties. Among them, the small-core PCFs have extremely high effective nonlinearity, which makes them an ideal nonlinear medium for new frequency generation. However significant nonlinear process in fiber only occurs when the phase matching condition is satisfied. Therefore, the dispersion properties of PCFs are also critical. In this paperr, three different approaches are taken to extend the tunability of an Yb-fiber-based femtosecond source. All of the schemes are enabled by the unique dispersion and nonlinear properties of photonic-crystal-fibers (PCFs). In the first approach, by adopting the newly available PCF into an optical parametric oscillator (FOPO) and combining it with our newly-developed mode-locked Yb-fiber laser [1], we extend the tunability of this fiber-based system to over 200-nm around 1 μm [2]. The second scheme uses Raman soliton self-frequency shifting effect in PCFs. Femtosecond soliton pulses tunable from 1100 nm to 1300 nm are generated. In the third approach, by taking advantage of broadly variable phase matching point for the Cherenkov radiation in PCFs, broadly tunable femtosecond visible pulses from 450 nm to 630 nm are achieved [3].

There is a growing need for miniature low-cost chemical sensors for use in monitoring environmental conditions. Applications range from environmental pollution monitoring, industrial process control and homeland security threat detection to biomedical diagnostics. Integrated opto-chemical sensors can provide the required functionality by monitoring chemistry induced changes in the refractive, absorptive, or luminescent properties of materials. Mach-Zehnder (MZ) interferometers, using the phase induced from a chemically reactive film, have shown success for such applications but typically are limited to one chemical analysis per sensor. In this paper we present a MZ-like sensor using the dispersion properties of a photonic crystal lattice. Properly engineered dispersion guiding enables the creation of multiple parallel MZ-like sensors monitoring different chemical reactions in a device much smaller than a typical MZ sensor. The phase shift induced in one arm of the photonic crystal structure by the chemical reaction of a special film induces a change in the sensor output. The use of a dispersion guiding photonic crystal structure enables the use of lower refractive index materials because the creation of a bandgap is not necessary. This in turn increases coupling efficiency into the device. Other advantages of this type of structure include the ability to guide both TE and TM modes as well as reduced sensitivity to fabrication tolerances. Two-dimensional FDTD analysis is used to optimize and model the effectiveness of the structure.

This work investigates the current transport across two-dimensional PhCs dry etched into InP-based low-index-contrast vertical structures using Ar/Cl₂ chemically assisted ion beam etching. The electrical conduction through the PhC field is influenced by the surface potential at the hole sidewalls, which is modified by dry etching. The measured current-voltage (I-V) characteristics are linear before but show a current saturation at higher voltages. This behaviour is confirmed by simulations performed by ISE-TCAD software. We investigate the dependence of the conductance of the PhC area as a function of the geometry of the photonic crystal as well as the material parameters. By comparing the experimental and simulated conductance of the PhC, we deduce that the Fermi level is pinned at 0.1 eV below the conduction band edge. The method presented here can be used for evaluating etching processes and surface passivation methods. It is also applicable for other material systems and sheds new light on current driven PhC tuning.