Glass-bonded quasi-phase-matched gallium arsenide crystals for nonlinear wavelength conversion into the mid-infrared

Abstract

Non-linear optical (NLO) devices for wavelength conversion of laser sources into the mid-infrared waveband (such as optical parametric oscillators) require the provision of non-linear materials. Quasi-phase matched (QPM) gallium arsenide crystals represent a promising alternative NLO material (high non-linear coefficient, low-optical loss) to conventional birefringent chalcopyrite crystals for use in the mid to far-infrared. To date, several approaches have been investigated to produce QPM GaAs crystals, including diffusion and fusion wafer bonding, orientation patterned growth and total internal reflection techniques. However, these require ultra-clean processing environments, relatively high bonding temperatures or are limited in crystal aperture. We present an approach to developing QPM GaAs crystals based on bonding using an index-matched chalcogenide glass. The glass-bonding (GBGaAs) technique forms low-loss bonds at moderate temperature and has several advantages over existing approaches. In particular, the technique is tolerant to GaAs wafer thickness variations and surface defects, and has the potential to produce large-aperture crystals. The glass-bonding process involves coating individual GaAs wafers with a thin-film of glass, deposited by RF sputtering, and then bonding assembled stacks of coated wafers in a vacuum oven under carefully controlled temperature and pressure conditions to form a single composite structure. To date, GBGaAs crystals consisting of up to 40 layers have been produced and optical losses per layer of less than 0.1% have been achieved. An outline of the production process for manufacturing GBGaAs crystals will be described together with details of optical assessment procedures. The impact of glass purity, sputtering conditions and pressing conditions on optical absorption levels will be reported. Techniques to minimise optical loss in fabricated crystals will be discussed.