Stimulated Brillouin scattering in an on-chip microdisk resonator

Brillouin scattering is the scattering of light from sound waves in a medium. There are three waves involved in the process: two optical waves (pump and scattered light) and one acoustic wave, and the scattering process is restricted through the conservation of momentum and energy. Stimulated Brillouin Scattering (SBS) occurs when the interference between the pump and scattered light reinforces the acoustic wave by electrostriction or by optical absorption. SBS is considered to be the strongest material level optical nonlinearity with many applications including SBS lasers, microwave oscillators, optical phase conjugation and slow light. Nonlinear optical phenomena can be enhanced in resonant systems due to the significantly increased interaction times between photons. In this regard, the SBS nonlinearity has already been demonstrated in resonators such as microsphere and microcapillary. However all these previous demonstrations have been off-chip as it is extremely challenging to satisfy the phase- matching requirements in small resonators due to the fewer optical modes available. An on-chip SBS-based optomechanical microresonator might be interesting since it will be more stable against vibration, and it will also demonstrate the potential for integration with other on-chip optical devices and the generation of resonantly enhanced SBS slow light on a chip. In this thesis, we propose a released silicon nitride microdisk resonator coupled by means of waveguide and gratings to realize on-chip SBS. Our COMSOL simulations show that the phase matching condition is possible between the first- and second-order transverse optical modes in the wavelength range of interest (1.52 μm-1.57 μm) by adjusting the dimension of the microdisk. The fabrication work is carried out in the cleanroom in the Micro and Nanotechnology Laboratory at the University of Illinois. It is shown that the releasing of the microdisk with suitable undercut is the most challenging step due to the delicate suspended structure as well as the stress in the silicon nitride layer. The unreleased device is tested by a setup that is designed for grating coupler testing, with a key component that is a “V groove assembly”. It shows that the grating and the disk work properly in our system. The fabrication process needs to be further optimized to achieve high quality factor and low insertion loss