Electrically driven optical isolation through phonon-mediated photonic Autler–Townes splitting

Abstract

Optical isolators are indispensible components in nearly all photonic systems as they help ensure unidirectionality and provide crucial protection from undesirable reflections. While commercial isolators are exclusively built on magneto-optic (MO) principles they are not readily implemented within photonic integrated circuits due to the need for specialized materials. Importantly, the MO effect is generally weak, especially at shorter wavelengths. These challenges as a whole have motivated extensive research on non-MO alternatives. To date, however, no alternative technology has managed to simultaneously combine linearity (i.e. no frequency shift), linear response (i.e. input-output scaling), ultralow insertion loss, and large directional contrast on-chip. Here we demonstrate an optical isolator design that leverages the unbeatable transparency of a short, high quality dielectric waveguide, with the near-perfect attenuation from a critically-coupled absorber. Our design concept is implemented using a lithium niobate racetrack resonator in which phonon mediated Autler-Townes splitting (ATS) breaks the chiral symmetry of the resonant modes. We demonstrate on-chip optical isolators at wavelengths one octave apart near 1550 nm and 780 nm, fabricated from the same lithium niobate-on-insulator wafer. Linear optical isolation is demonstrated with simultaneously <1 dB insertion loss, >39 dB contrast, and bandwidth as wide as the optical mode that is used. Our results outperform the current best-in-class MO isolator on-chip on both insertion loss and isolator figures-of-merit, and demonstrate a lithographically defined wavelength adaptability that cannot yet be achieved with any MO isolator.