State-of-the-art experimental implementations of these alternatives have succeeded in meeting various metrics of contrast, linearity, and bandwidth, but complete isolation with ultralow forward loss has remained elusive. In light of this challenge, several non-magnetic alternatives for breaking reciprocity 3 have been explored both theoretically 8– 13 and experimentally 14– 19. Unfortunately, this well-established technique 3 has proven challenging to implement in chip-scale photonics due to fabrication complexity, difficulty in locally confining magnetic fields, and significant material losses 4– 7. To date, the best method for achieving optical isolation with these characteristics has been through Faraday rotation via the magneto-optic response in gyrotropic materials 1, 2. In practice, isolators should also exhibit a broadband isolation response for robustness and usability across a wide range of applications. zero forward insertion loss) and zero transmission in the opposite direction – without any mode shifts, frequency shifts, or dependence on input signal power. Ideal optical isolators should exhibit complete linear isolation – where completeness implies perfect transmission one way (i.e. Our result demonstrates that material-agnostic and wavelength-agnostic optical isolation is far more accessible for chip-scale photonics than previously thought. The isolation originates from a nonreciprocal induced transparency based on a coherent light-sound interaction, with the coupling originating from the traveling-wave Brillouin scattering interaction, that breaks time-reversal symmetry within the waveguide-resonator system. Here we demonstrate that complete linear optical isolation can be obtained within any dielectric waveguide using only a whispering-gallery microresonator pumped by a single-frequency laser. However, none of these approaches have yet demonstrated linear optical isolation with ideal characteristics over a microscale footprint – simultaneously incorporating large contrast with ultralow forward loss – having fundamental compatibility with photonic integration in standard waveguide materials. The significant challenges that confront integration of magneto-optic nonreciprocal systems on chip have made imperative the exploration of magnet free alternatives. Low-loss optical isolators and circulators are critical nonreciprocal components for signal routing and protection, but their chip-scale integration is not yet practical using standard photonics foundry processes.
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