Ultrafast optical gating in a nonlinear lithium niobate microcavity.

Advances in optical simulation and computation have renewed interest in high-finesse optical cavities for enhancing light-matter interactions, engineering complex photonic band structures, and storing quantum information. However, the extended interaction times in these cavities dictate slow optical readout and limited control over system transients. Addressing this challenge, we demonstrate an ultrafast intracavity optical gating scheme in high-finesse, second-order nonlinear microcavities incorporating thin films of lithium niobate. A femtosecond optical gate pulse, tuned to the transparency region of the cavity's dielectric mirrors, instantaneously upconverts the intracavity field via sum-frequency generation. The upconverted pulse promptly exits the cavity, providing space- and time-resolved information on the intracavity field. We validate this approach by tracking the dynamics of multiple resonant modes excited in a plano-concave microcavity, comparing well with analytical models. In addition, we demonstrate that intracavity difference-frequency generation can efficiently instantiate the cavity fields. Fully compatible with low-temperature microcavity experiments, this gating scheme enables future real-time control of light-matter interactions or quantum-optical state manipulations.