Quasi-phase-matched materials for active sensing in the mid-infrared waveband

Abstract

This thesis aims to fabricate a new method of achieving nonlinear conversion via quasiphase-matching in GaAs structures. CASE sponsors DSTL sought a means of achieving broadband wavelength conversion across the infrared spectrum using a GaAs device that would be available in the UK without the export restrictions imposed by currently available technologies or the laborious processing steps presented in other methods. To meet the goal of QPM in GaAs, a new fabricated structure - ‘Zip-GaAs’ - was created via a dicing saw to create periodic trenches of required QPM periodicity. Dicing parameters were optimised for GaAs to produce surface roughnesses as low as 2.4 nm using a feed rate and spindle speed of 0.5 mm/s and 30 kRPM respectively. Controlling pitch and depth of cut, along with blade characteristics allowed successful mating of two combs of opposite crystal orientation to form the Zip-GaAs device. In order to address micron-scale air gaps between periodic structures, trials were performed with chalcogenide glasses, GaSb and processes involving no intermediate materials - these had limited success and thus a new development was made using Indium and Bismuth as deposited materials on GaAs substrates. The metals were diffused in a tube furnace to create a bond between GaAs plates. One trial used 112 nm of both metals, deposited sequentially on a GaAs wafer; two plates from this wafer were contacted and thermally diffused at a temperature of 964◦C for 1 hour forming a robust bond. This process resulted in transmission of the bonded stack of ≈ 2/3 the transmission of GaAs across the measured window of 1.6 - 18 µm. Some Zip-GaAs devices were fabricated by dicing at Brewster’s angle as an alternative to bonding. Structures with 50 domains showed transmission < 0.1% in GaAs and LiNbO3 and it is thought depth of cut and form of diced trenches are responsible. In this thesis, precision dicing is shown to create low surface roughness facets without the need for polishing in multiple optical materials. This, combined with Brewster’s angle machining, can lead to optical component manufacturing without the need for AR coating.

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ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

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