Dual-comb spectroscopy (DCS) has revolutionized optical spectroscopy by providing broadband spectral measurements with unprecedented resolution and fast response. Photothermal spectroscopy (PTS) with a pump-probe configuration offers a highly sensitive gas sensing method, which is normally performed using a single-wavelength pump laser. The merging of PTS with DCS may enable a spectroscopic method by taking advantage of both technologies, which has never been studied yet. Here, we report dual-comb photothermal spectroscopy (DC-PTS) by passing dual combs and a probe laser through a gas-filled anti-resonant hollow-core fiber, where the generated multi-heterodyne modulation of the refractive index is sensitively detected by an in-line interferometer. As an example, we have measured photothermal spectra of acetylene over 1 THz, showing a good agreement with the spectral database. Our proposed DC-PTS provides opportunities for broadband gas sensing with super-fine resolution and high sensitivity, as well as with a small sample volume and compact configuration.

Photoacoustic dual-comb spectroscopy (DCS), converting spectral information in the optical frequency domain to the audio frequency domain via multi-heterodyne beating, enables background-free spectral measurements with high resolution and broad bandwidth. However, the detection sensitivity remains limited due to the low power of individual comb lines and the lack of broadband acoustic resonators. Here, we develop cavity-enhanced photoacoustic DCS, which overcomes these limitations by using a high-finesse optical cavity for the power amplification of dual-frequency combs and a broadband acoustic resonator with a flat-top frequency response. We demonstrate high-resolution spectroscopic measurements of trace amounts of C₂H₂, NH₃ and CO in the entire telecommunications C-band. The method shows a minimum detection limit of 0.6 ppb C₂H₂ at the measurement time of 100 s, corresponding to the noise equivalent absorption coefficient of 7 × 10⁻¹⁰cm⁻¹. The proposed cavity-enhanced photoacoustic DCS may open new avenues for ultrasensitive, high-resolution, and multi-species gas detection with widespread applications.

We demonstrate an ultrasensitive photoacoustic sensor using a low laser power (4 mW) and high-finesse ($>9000$) optical cavity. The Pound–Drever–Hall (PDH) method is adopted to lock the external cavity diode laser at 1531.58 nm to the Fabry–Pérot cavity. By placing a photoacoustic cell inside the 130-mm-long optical cavity, we obtain an enhancement of more than 630 times in laser power for acetylene (${\mathrm{C}}_2{\mathrm{H}}_2$) detection. The present photoacoustic spectroscopy (PAS) sensor achieves a normalized noise equivalent absorption coefficient of $1.1\times {10}^{-11}{\mathrm{cm}}^{-1}{\mathrm{WHz}}^{-1/2}$, which is unprecedented sensitivity among all the current PAS sensors. Our results demonstrate the feasibility of merging PAS with a high-finesse cavity using PDH locking for ultrasensitive trace gas detection.