Simulations and Fabrication of Photonic Crystal Waveguides and a Computational Study of Two-Dimensional Photonic Crystal Cavity Resonators

Abstract

Photonics crystals are artificially made materials with a periodic dielectric constant where the size of the periodicity is comparable to the wavelength of light. These structures have a huge potential for on-chip applications because of their interesting dispersion characteristics and light confinement properties. This thesis is devoted to study and fabricate air-hole photonic crystal devices.

Photonic crystal waveguide slabs were fabricated by pattering a silicon-on-insulator (SOI) substrate using electron beam lithography (EBL) followed by an inductively coupled plasma reactive ion etch (ICP-RIE). Light were for the first time at NTNU successful coupled through a fabricated photonic crystal waveguide. The ICP-RIE recipe was optimized in order to reach the desired depth of the air holes, and an etch time of 45 seconds was proven to be sufficient.

Scanning electron microscopy (SEM) characterisation of the fabricated waveguides showed a high-quality structure with uniform air holes, minimal fabrication errors, and a high process reproducibility. Work remains on quantifying the degree of sidewall roughness and the measuring the angle of the vertically etched sidewalls, but the fabricated photonic crystals waveguides show great potential.

Optical transmission measurements of the fabricated photonic crystal waveguide using different parameters were performed using a butt coupling technique. Although the optical setup made it easy to couple light through the devices and to measure the transmission, the optical characterization process needs to be optimized in order to compare different fabricated photonic crystal devices.

Computer simulations of photonic crystal waveguides were performed using MIT Electromagnetic Equation Propagation (Meep) for finite-difference time-domain (FDTD) simulations in both two- and three-dimensions. The transmission properties were optimized for different photonic crystal lattice cut-positions and strip waveguide widths by using a triangular interface at the photonic crystal boundaries. Enlarging the width of the strip waveguide towards the photonic crystal boundary increased transmission for certain lattice terminations, while decreasing the strip waveguide width resulted in a higher transmission for large values of the strip waveguide width.

Numerical computations of two-dimensional photonic crystal cavity resonators were performed using FDTD simulations in Meep and by using the finite element method (FEM) in COMSOL. The FEM proved to be the most reliable and accurate tool when investigate simple photonics crystal resonator structures, but because FEM simulations are very time and memory consuming, FDTD simulations proved useful when investigation a large frequency area.