Time-reversal symmetry is a property shared by wave phenomena in linear stationary media. However, broken time-reversal symmetry is required for synthesizing nonreciprocal devices like optical isolators, circulators, and gyrators. Magnetic fields can enable such nonreciprocal behavior for electromagnetic waves, but this method does not conveniently translate to the chip-scale or to the acoustic domain, compelling us to search for nonmagnetic solutions. We have adopted a unique approach to address this challenge through the use of co-localized interacting modes of light and sound in resonator systems. The acousto-optical physics within these systems enable fundamental experiments having analogies to condensed matter phenomena, including phonon laser action, cooling, and electromagnetically induced transparency. This talk will describe our experimental efforts to exploit the momentum conservation rules intrinsic to light-sound interactions for producing strong nonreciprocal behavior. We have demonstrated that such systems can be used to produce complete optical isolation with low loss over a very compact footprint. Our results also reveal that chiral effects are pervasive throughout the phononic and photonic physical layers of these systems, for instance, showing that chirality can be dynamically imparted to phonon transport to suppress disorder-induced backscattering.
