Nonlinear optical radiation of a lithium niobate microcavity

TL;DR

This paper demonstrates nonlinear optical radiation in lithium niobate microcavities, revealing enhanced free-space coupling and phase-matching-free frequency conversion, with applications in integrated gas sensing and optical interfaces.

Contribution

It uncovers the mechanism of nonlinear optical radiation in microcavities, enabling phase-matching-free frequency conversion and novel integrated sensing applications.

Findings

Enhanced nonlinear coupling between confined and free-space modes.

Observation of second-harmonic radiation indicating phase-matching-free conversion.

Development of an integrated Rb isotope sensor using this mechanism.

Abstract

The nonlinear optical radiation of an integrated lithium niobate microcavity is demonstrated, which has been neglected in previous studies of nonlinear photonic devices. We find that the nonlinear coupling between confined optical modes on the chip and continuum modes in free space can be greatly enhanced on the platform of integrated microcavity, with feasible relaxation of the phase-matching condition. With an infrared pump laser, we observe the vertical radiation of second-harmonic wave at the visible band, which indicates a robust phase-matching-free chip-to-free-space frequency converter and also unveils an extra energy dissipation channel for integrated devices. Such an unexpected coherent nonlinear interaction between the free-space beam and the confined mode is also validated by the different frequency generation. Furthermore, based on the phase-matching-free nature of the nonlinear radiation, we build an integrated atomic gas sensor to characterize Rb isotopes with a single telecom laser. The unveiled mechanism of nonlinear optical radiation is universal for all dielectric photonic integrated devices, and provides a simple and robust chip-to-free-space as well as visible-to-telecom interface.