  The recent surge of interest in temporal modulation schemes to induce
magnet-free non-reciprocity has inspired several exciting opportunities for
photonic technology. Here, we investigate a scheme to realize free-space
isolators and highly non-reciprocal mirrors with weak modulation imparted by an
acoustic wave. Conventional optical mirrors are reciprocal: in a given plane of
incidence, reflection is independent of the sign of the angle of incidence,
which enables two people to simultaneously look at each other through their
reflection. In contrast, we propose a strategy to dramatically break this
symmetry by exploiting resonant interactions between a travelling acoustic wave
and highly resonant guided optical modes, inducing total reflection of an
optical beam at a given angle, and no reflection at the negative angle.
Different from conventional acousto-optic isolators, which are based on
non-resonant frequency conversion and filtering, our proposal operates at the
frequency of the optical signal by tailoring the resonant properties of the
structure as well as the acoustic wave frequency and intensity, enabling 50 dB
isolation with modest modulation requirements. Operation in reflection allows
for close-to-zero insertion loss, enabling disruptive opportunities in our
ability to control and manipulate photons.
