High-Q optical and mechanical resonators have been utilized in ultra-high precision metrology, transducers, sensor applications, and even investigating quantum mechanics at mesoscales. However, these low-loss devices are often limited by sub-wavelength fluctuations within the host material, that may be frozen-in or even dynamically induced. Rayleigh scattering is observed in nearly all wave-guiding technologies today and can lead to both irreversible radiative losses as well as undesirable intermodal coupling. The mitigation of disorder-induced scattering is extremely challenging for micro and nanoscale devices, as surface roughness, which causes Rayleigh scattering, is unavoidable in microfabrication processes (Appl Phys Lett 85, 17, 2004; J Lightwave Technol 24, 12, 2006). Minimizing disorder-induced Rayleigh backscattering has thus been a significant challenge until now. It has been shown that backscattering from disorder can be suppressed by breaking time-reversal symmetry in magneto-optic (Sov Phys JETP, 59, 1, 1984; Phys Rev B, 37, 1988) and topological insulator materials (Phys Rev B, 38, 1988; Nature, 461, 7265, 2009). Yet, common monolithic dielectrics, which are basic building ingredients of high-Q resonators, possess neither of these properties.
Fortunately, we develop a novel technique to break time-reversal symmetry without magneto-optic in a high-Q optical cavity pumped by a single-frequency laser through parity-selective optomechanics. Such optomechanical interaction is achieved by Brillouin scattering, owing to the phase-matching condition. This method enables complete linear optical isolation without requiring magnetic fields. Instead, the isolation originates from a nonreciprocal induced transparency based on a coherent light-sound interaction, where light and sound are coupled bv a traveling-wave Brillouin scattering interaction. That breaks time-reversal symmetry within the waveguide-resonator system. Our result demonstrates that material agnostic and wavelength-agnostic optical isolation is far more accessible in chip-scale photonics than previously thought. However, isolators block backscattering from systems, but cannot prevent disorder-induced backscattering inherently.
In order to minimize disorder-induced backscattering, we experimentally demonstrate robust phonon transport in the presence of material disorder. This is achieved by explicitly inducing chirality through the parity-selective optomechanical coupling. We show that asymmetric optical pumping of a symmetric resonator enables a dramatic chiral cooling of clockwise and counterclockwise phonons, while simultaneously suppressing the hidden action of disorder. Surprisingly, this passive mechanism is also accompanied by a chiral reduction in heat load leading to optical cooling of the mechanics without added damping, an effect that has no optical analog. This technique can potentially improve upon the fundamental thermal limits of resonant mechanical sensors, which cannot be attained through sideband cooling.
This new mechanism can be also expanded to the optics domain, where Rayleigh scattering severely limits the performance of devices in the limit of microscale. We have demonstrated an optomechanical approach for dynamically suppressing Rayleigh light backscattering within optical resonators. Similar to the previous method, we achieve this by locally breaking time-reversal symmetry in a silica resonator through a Brillouin scattering interaction that is available in all materials. Near-complete suppression of Rayleigh backscattering is experimentally confirmed through three independent measurements -- the reduction of the back-reflections caused by scatterers, the elimination of a commonly seen normal-mode splitting effect, and by measurement of the reduction in intrinsic optical loss. More broadly, our results suggest that it is possible to dynamically suppress Rayleigh backscattering within any optical dielectric medium using time-reversal symmetry breaking, for achieving robust light propagation in spite of scatterers or defects. Our proposal is not limited by a specific form of time-reversal symmetry breaking through Brillouin scattering in optical cavities. It can be realized in linear waveguides under different time-reversal symmetry approaches such as acousto-optic, nonlinear-optics, and PT symmetry breaking technique
