Fig. 1: Dual-comb optomechanical spectroscopy (DCOS).

a In DCOS, two combs excite the ro-vibrational transitions of gas molecules. The excited molecules release a portion of energy in the form of heat. The periodic heating, due to the beating between the paired dual-comb lines (within the absorption line profiles), yields ultrasonic waves which cause the displacement (dz_m) of a nanomechanical membrane placed in the middle of a high-finesse (finesse: \({{{{{\mathscr{F}}}}}}\)) optical cavity. Meanwhile, a probe field (wavelength:λ) is coupled into the cavity from the opposite direction. The displacement induces a phase change (\(d\varphi \propto \frac{{{{{{\mathscr{F}}}}}}}{\lambda }d{z}_{m}\))³⁶ of the probe field, which is magnified by the resonance between the field and the cavity and is detected interferometrically. b Illustration of the “light-sound-light” scheme. An ultrasound comb signal is produced by the dual-comb light via the photoacoustic effect and is then detected by the probe light through the optomechanical coupling.