Murtaza Askari

Compact wavelength demultiplexing using focusing negative index photonic crystal superprisms

Here, we demonstrate a compact photonic crystal wavelength demultiplexing device based on a diffraction compensation scheme with two orders of magnitude performance improvement over the conventional superprism structures reported to date. We show that the main problems of the conventional superprism-based wavelength demultiplexing devices can be overcome by combining the superprism effect with two other main properties of photonic crystals, i.e., negative diffraction and negative refraction. Here, a 4-channel optical demultiplexer with a channel spacing of 8 nm and cross-talk level of better than -6.5 dB is experimentally demonstrated using a 4500 microm(2) photonic crystal region.

Accurate post-fabrication trimming of ultra-compact resonators on silicon

We experimentally demonstrate an accurate post-fabrication trimming technique for the correction of the optical phase of silicon photonic devices using a single fabrication step. Using this technique, we reduce the random resonance wavelength variation of ultra-compact silicon resonators by a factor of 6 to below 50 pm.

Planar photonic crystals infiltrated with nanoparticle/polymer composites

Infiltration of planar two-dimensional silicon photonic crystals with nanocomposites using a simple yet effective melt processing technique is presented. The nanocomposites that were developed by evenly dispersing functionalized TiO2 nanoparticles into a photoconducting polymer were completely filled into photonic crystals with hole sizes ranging from 90to500nm. The infiltrated devices 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 crystal based devices and hybrid light emitting diodes and solor cells.

Strong angular dispersion using higher bands of planar silicon photonic crystals

We present experimental evidence for strong angular dispersion in a planar photonic crystal (PC) structure by properly engineering the modes in the second PC band. We show that by using the second photonic band of a square lattice PC, angular dispersion of 4 degrees /nm can be achieved. We also show that major challenges in designing practical PC devices using second band modes can be addressed by engineering the lattice and adding input/output buffer stages designed to eliminate unwanted effects.

Systematic Design of Wide-Bandwidth Photonic Crystal Waveguide Bends With High Transmission and Low Dispersion

We identify factors affecting transmission and dispersive properties of photonic crystal waveguide (PCW) bends, using 2-D simulations and present a method for systematic design of PCW bends to achieve high transmission and low dispersion over large bandwidths. The bends presented here have higher bandwidth and lower dispersion than bends already reported.

Efficient coupling of light into the planar photonic crystal waveguides in the slow group velocity regime

We present methods for systematic design of couplers for efficient coupling of light into the slow group velocity modes of photonic crystal waveguides (PCW).

Photonic crystal waveguide based sensors

Waveguide based sensors allow independent control over sensitivity and dynamic range, which is not possible in resonance based sensors. In this paper, we present a refractive index sensor based on using photonic crystal waveguides (PCWs) in an unbalanced Mach-Zehnder interferometer configuration. In this configuration the dynamic range of the sensor is determined by the path difference between the two arms and the sensitivity is controlled by the length of the PCW. We show that by using PCWs we can get a factor of 8 improvement in sensitivity over a ridge-waveguide based sensor. This enhanced sensitivity is achieved due to reduced group velocity in a PCW. By reducing the loss at low group velocities the sensitivity can be further improved.

An on-chip silicon grating spectrometer using a photonic crystal reflector

A compact and robust implementation of grating spectrometers on silicon is demonstrated. In the proposed device, two separate mirrors are used in a folded geometry for a smaller device footprint, and a photonic crystal-based reflector is used to improve the robustness against fabrication imperfections. Experimental results show a wavelength resolution of 2.5 nm around the center wavelength of 1560 nm in this device at a chip area of 800 µm × 480 µm. Device potentials for use as on-chip spectrometers are also investigated.

Absorbing boundary conditions for low group velocity electromagnetic waves in photonic crystals.

We present an efficient method for the absorption of slow group velocity electromagnetic waves in photonic crystal waveguides (PCWs). We show that adiabatically matching the low group velocity waves to high group velocity waves of the PCW and extending the PCW structure into the perfectly matched layer (PML) region results in a 15 dB reduction of spurious reflections from the PML. We also discuss the applicability of this method to structures other than PCWs.

High efficiency coupling of light from a ridge to a photonic crystal waveguide.

We present theoretical and experimental demonstration of two designs to achieve group velocity insensitive coupling of light from a ridge waveguide to a photonic crystal waveguide. We demonstrate an average improvement of 62% in coupling to low group velocity modes and an average coupling enhancement of 11.5% at large group velocities.