Observation of a Triply Phase-Matched Five-Wave Acousto-optic Interaction

We observed in LiNb${\mathrm{O}}_3$ a collinear triply phase-matched interaction between two input infrared waves and two input ultrasonic shear waves that yielded an output visible wave near the optical second-harmonic frequency. The five-wave-interaction output was 35% of that from a related three-wave interaction under comparable conditions.