Position-squared coupling in a tunable photonic crystal optomechanical cavity

Abstract

We present the design, fabrication, and characterization of a planar silicon photonic crystal cavity in which large position-squared optomechanical coupling is realized. The device consists of a double-slotted photonic crystal structure in which motion of a central beam mode couples to two high-Q optical modes localized around each slot. Electrostatic tuning of the structure is used to controllably hybridize the optical modes into supermodes that couple in a quadratic fashion to the motion of the beam. From independent measurements of the anticrossing of the optical modes and of the dynamic optical spring effect, a position-squared vacuum coupling rate as large as g'/2π=245  Hz is inferred between the optical supermodes and the fundamental in-plane mechanical resonance of the structure at ω_m/2π=8.7  MHz, which in displacement units corresponds to a coupling coefficient of g'/2π=1  THz/nm 2. For larger supermode splittings, selective excitation of the individual optical supermodes is used to demonstrate optical trapping of the mechanical resonator with measured g'/2π=46  Hz.