Mid-Infrared Applications of Quasicylindrical Waves at Dielectric Interfaces

Abstract

The work presented in this thesis focuses on the development of a fully dielectric subwavelength aperture groove structure optimised to collimate light in the mid-infrared by supporting quasicylindrical wave (QCW) propagation. The motivation behind implementing such a structure is to provide an alternative to metallic aperture-groove structures that support surface plasmon (SP) propagation to collimate light and change other beam characteristics such as polarisation. When integrating such a structure onto an active device, depositing metal is not desirable. An extra layer of insulation needs to be deposited to avoid a short circuit and in some cases an additional insulation channel is needed to prevent the device from a high malfunctioning rate. These added fabrication steps increase costs and complexity of manufacture. SPs also perform less optimally at longer wavelengths where losses are high. Increasing the power density output of light sources at these infrared wavelengths has numerous applications including gas-sensing, free space communications and remote sensing. Fully dielectric subwavelength aperture groove structures on mid-infrared band pass filters are designed, modelled and optimised using the finite difference time domain method (FDTD). By applying a resonant nanograting to the exit side of the filter the effect of quasicylindrical waves (QCWs) on the near and far fields are presented, showing a reduction in the full-width half-maximum (FWHM) of the output beam from 129.8° (without grooves) to 3.4°. The design was then patterned onto commercially available band pass filters using focused ion beam etching (FIB). Optical measurements are presented and analysis on the far field radiation patterned analysed. Experimental results were less conclusive with no collimation measured in the fully dielectric filter and collimation within experimental error found in a second filter