  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.
