  Integrated silicon microwave photonics offers great potential in microwave
phase shifter elements, and promises compact and scalable multi-element chips
that are free from electromagnetic interference. Stimulated Brillouin
scattering, which was recently demonstrated in silicon, is a particularly
powerful approach to induce a phase shift due to its inherent flexibility,
offering an optically controllable and selective phase shift. However, to date,
only moderate amounts of Brillouin gain has been achieved and theoretically
this would restrict the phase shift to a few tens of degrees, significantly
less than the required 360 degrees. Here, we overcome this limitation with a
phase enhancement method using RF interference, showing a 360 degrees broadband
phase shifter based on Brillouin scattering in a suspended silicon waveguide.
We achieve a full 360 degrees phase-shift over a bandwidth of 15 GHz using a
phase enhancement factor of 25, thereby enabling practical broadband Brillouin
phase shifter for beam forming and other applications.
