Continuous-wave optical modulation at the frequency of molecular motion

The novel scheme for lightwave modulation based on ultrafast modulation of polarizability induced by a coherently excited molecular motion has attracted attention as an alternative to a widely-used optical modulator based on an electro-optic effect to extend it to much higher frequency. A pulsed laser was first used to demonstrate the feasibility of this approach, and recently a cavity enhanced continuous-wave (cw) laser was successfully applied to the optical modulation based on the coherent molecular rotations. The efficiency of the sideband generation, however, was considerably low (~10^-6) for the cw laser modulation. In the report, the intensity of the cw-based sideband was dramatically improved, approaching 50% for an anti-Stokes sideband, by the fulfillment of a phase-matching condition of the sideband generation. A high-finesse optical cavity (HFC) consisting of a pair of broadband (750-930 nm) high reflective (R: ~99.98%) mirrors having a negative dispersion (~ -7 fs²) to enhance the intensity of a pump (ω₁: 847.9 nm) and a probe (ω_probe: 855.8 nm) beam was used in this study.