Mid-Infrared Resonant Cavity-Enhanced Photodetectors

Bainbridge, Andrew and Marshall, Andrew (2022) Mid-Infrared Resonant Cavity-Enhanced Photodetectors. PhD thesis, Lancaster University.

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Abstract

Resonant cavity-enhanced photodetectors can be utilised for spectroscopic sensing, due to their narrow, adjustable spectral responses. The narrow response can be designed to align with an absorption peak of a target molecule, whilst also not covering neighbouring absorption peaks. Therefore, these detectors can offer an intrinsically high spectral selectivity. This thesis reports on the design, fabrication and analysis of resonant cavity-enhanced photodetectors for sensing in the mid-infrared. The structures all use III-V semiconductor alloys and employ distributed Bragg reflectors as high-reflectivity, highly selective and highly tunable mirrors. Fabricated detectors demonstrate the expected narrow, enhanced responses with full-widths at half-maximum of < 30 nm and enhancement factors of > 10. The tunability of the spectral responses is also demonstrated through the fabrication of structures with the same layer materials, but differing resonance wavelengths. The dependence of the resonance wavelength on the layer thicknesses is utilised with a novel concept of wavelength chirped resonant cavity-enhanced photodetectors. In these structures, the layer thicknesses are graded across a single wafer to create an array of detectors with spectral responses that shift across the wafer. Fabricated arrays demonstrate the benefit of the spectral response variation for spectroscopic sensing. The hyperspectral measurements can determine significantly more detailed spectral information than is possible by a single resonant cavity-enhanced photodetector. This wavelength chirp concept opens up the possibility of a solid-state spectrometer based on resonant cavity-enhanced photodetectors.