Topological phononics at the nanoscale: All-optical excitation and read-out of helical vibrations on a chip

We present the design of and numerical simulations for an on-chip optomechanical device that supports helical vibrations and allows all-optical excitation and read-out thereof. The device is based on a so-called optomechanical crystal, i. e. a nanostructure that supports both mechanical and optical bulk band gaps. The optomechanical crystal of interest is a patterned silicon slab. The pattern of holes has been engineered to give rise to i) topologically distinct domains separated by a domain wall of chosen shape supporting a broadband mechanical helical edge state, ii) high quality factor optical cavities that are localized close to the domain wall and display a good coupling to the helical states. As usual in optomechanics, the optomechanical coupling can be boosted by driving the optical cavities using a laser. This allows the excitation and the high-precision read out of the helical vibrations propagating along the domain wall.