Nonlinear Optical Radiation of a Lithium Niobate Microcavity

The nonlinear coupling between on-chip confined optical modes and free-space continuum modes is investigated. The radiation of second-harmonic wave due to the pump fields in integrated lithium niobate microcavities is experimentally observed. The mechanism of nonlinear optical radiation (NOR) is further verified by demonstrating the reversal process of NOR, i.e., the difference frequency generation between a free-space input and an on-chip pump field. The revealed mechanism of NOR is universal for all dielectric photonic integrated devices, which indicates unavoidable extra energy leakage channels of optical micro- and nanostructures, and also enables an alternative approach for spatial engineering of optical fields and light sources. For example, benefited from the phase-matching-free nature of NOR, an integrated atomic gas sensor for the absorption spectroscopy of Rb atoms is built.