High‐Sensitivity, High‐Resolution Stimulated Raman Photoacoustic Spectroscopy Enabled by Continuous‐Wave Intracavity Pump‐Stokes Beams

Raman spectroscopy is essential for chemical analysis by offering non‐invasive and selective detection of molecular vibrations. However, weak Raman signals from gas‐phase species pose significant challenges to detection sensitivity, even with high‐power laser sources. Here, cavity‐enhanced stimulated Raman photoacoustic spectroscopy is reported using continuous‐wave mW‐level near‐infrared lasers, achieving remarkable sensitivity and resolution. This approach involves coupling the pump and Stokes beams into a high‐finesse (>220 000) Fabry‐Perot cavity, enhancing the intracavity power of both lasers by four orders of magnitude. By modulating the light intensity of the pump laser, the stimulated Raman scattering‐induced acoustic waves are sensitively detected by a microphone. Additionally, a difference frequency scanning method is introduced that resolves narrow‐linewidth Raman transitions by tuning the length of the dual‐laser‐locked optical cavity. The effectiveness of the method is validated by measuring the Raman rotational transition S0(0) of hydrogen, achieving a minimum detection limit of 0.5 ppm with a 200‐s averaging time. This study establishes a new Raman spectroscopic platform for high‐sensitivity, high‐resolution molecular spectroscopy, with promising applications in chemical analysis and trace gas sensing.