Stimulated Brillouin Scattering on AlGaAs on Sapphire platform

On-chip stimulated Brillouin Scattering (SBS) has gained attention for its wide applications including narrow linewidth lasers, amplifiers and microwave filters [1]. The key to achieve on-chip SBS is the simultaneous confinement of the optical and acoustic waves. Searching for easily manufactured and mechanically robust semiconductor platforms that can confine both light and high-frequency sound remains a key challenge. AlGaAs on insulator (AlGaAsOI) is a promising nonlinear platform due to strong optical confinement, low propagation loss and high third-order nonlinear coefficient [2]. Recently, on-chip SBS has been observed in AlGaAsOI waveguides [3] , which can guide transverse acoustic modes. However, Brillouin gain using longitudinal acoustic modes can be much higher than using transverse modes. Unfortunately, the AlGaAsOI waveguide cannot support the longitudinal acoustic mode because of poor acoustic wave velocity contrast between core and cladding. In this work we propose and demonstrate feasibility of SBS on the AlGaAs on Sapphire (AlGaAsOS) platform [4] using longitudinal acoustic mode to achieve high Brillouin gain. To achieve high optical and acoustic confinement, the cladding should have a lower refractive index and support higher acoustic speed compared to the core waveguide. Al 2 O 3 with a refractive index around 1.7 and longitudinal acoustic speed of approx. 8800 m/s [5] is an excellent candidate of cladding, thus the AlGaAsOS waveguide can simultaneously confine optical and acoustic waves.