Breaking time-reversal symmetry with acoustic pumping of nanophotonic circuits

Abstract: Achieving non-reciprocal light propagation via stimuli that break time-reversal symmetry, without magneto-optics, remains a major challenge for integrated nanophotonic devices. Recently, optomechanical microsystems in which light and vibrational modes are coupled through ponderomotive forces, have demonstrated strong non-reciprocal effects through a variety of techniques, but always using optical pumping. None of these approaches have demonstrated bandwidth exceeding that of the mechanical system, and all of them require optical power, which are both fundamental and practical issues. Here we resolve both of these challenges through breaking of time-reversal symmetry using an acoustic pump in an integrated nanophotonic circuit. GHz-bandwidth optomechanical non-reciprocity is demonstrated using the action of a 2-dimensional surface acoustic wave pump, that simultaneously provides non-zero overlap integral for light-sound interaction and also satisfies the necessary phase-matching. We use this technique to produce a simple frequency shifting isolator (i.e. a non-reciprocal modulator) by means of indirect interband scattering. We demonstrate mode conversion asymmetry up to 15 dB, efficiency as high as 17%, over bandwidth exceeding 1 GHz.