Dual-comb optomechanical spectroscopy

Article https://doi.org/10.1038/s41467-023-40771-3 Dual-comb optomechanical spectroscopy Received: 8 March 2023 Accepted: 9 August 2023 1234567890():,; 1234567890():,; Check for updates Xinyi Ren1,8, Jin Pan1,8, Ming Yan 1,2,3 , Jiteng Sheng 1,4 , Cheng Yang1, Qiankun Zhang1, Hui Ma1, Zhaoyang Wen1, Kun Huang 1, Haibin Wu 1,4,5,6 Heping Zeng 1,2,3,7 & Optical cavities are essential for enhancing the sensitivity of molecular absorption spectroscopy, which finds widespread high-sensitivity gas sensing applications. However, the use of high-finesse cavities confines the wavelength range of operation and prevents broader applications. Here, we take a different approach to ultrasensitive molecular spectroscopy, namely dual-comb optomechanical spectroscopy (DCOS), by integrating the high-resolution multiplexing capabilities of dual-comb spectroscopy with cavity optomechanics through photoacoustic coupling. By exciting the molecules photoacoustically with dual-frequency combs and sensing the molecularvibration-induced ultrasound waves with a cavity-coupled mechanical resonator, we measure high-resolution broadband ( > 2 THz) overtone spectra for acetylene gas and obtain a normalized noise equivalent absorption coefficient of 1.71 × 10−11 cm−1·W·Hz−1/2 with 30 GHz simultaneous spectral bandwidth. Importantly, the optomechanical resonator allows broadband dual-comb excitation. Our approach not only enriches the practical applications of the emerging cavity optomechanics technology but also offers intriguing possibilities for multi-species trace gas detection. Highly-selective and ultrasensitive gas sensing, with widespread applications ranging from breath analysis1 to environmental monitoring2,3, constantly demands novel spectroscopic approaches. The emergence of optical combs, a coherent light source consisting of massive equidistant, ultra-sharp frequency lines, has enabled many revolutionary approaches to molecular spectroscopy4,5. Particularly, dual-comb spectroscopy (DCS), harnessing two combs of slightly different line spacings and a fast, single-pixel detector, enables multiplexed spectral acquisition without the use of moving parts6. As a result, it allows the simultaneous detection of non-neighboring molecular characteristic absorption lines with unprecedented spectral resolution, bandwidth, precision, and speed. Despite being challenging, integrating these features with ultrahigh sensitivity has proven to be essential for the tasks like selective multispecies detection3, reliable analysis of complex mixtures1, and real-time monitoring of trace gases2. Ultrasensitive DCS has been demonstrated using hollow-core fibers7, multi-pass cells8, and optical resonance cavities9–11. Among these demonstrations, cavity-enhanced schemes, with unmatched pathlength enhancement, yield the lowest detection limits — possibly down to the parts-per-trillion (ppt) level10. The resonance cavities, however, restrict the wavelength range of operation due to the technical difficulty of fabricating broadband high-reflection mirrors. Also, cavity-enhanced DCS needs extra efforts for comb-cavity coupling9 and intracavity dispersion control10,11. Meticulous electronic controls and cavity designs10,11 may alleviate these difficulties at the cost of increased system complexity and limited applicability. More generally, the above systems with long light-molecule-interaction lengths are bulky and may suffer from large sample volumes and low gas exchange rates, preventing their applications for real-time, in-situ gas monitoring. 1 State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China. 2Chongqing Key Laboratory of Precision Optics, Chongqing Institute of East China Normal University, Chongqing 401120, China. 3Chongqing Institute for Brain and Intelligence, Guangyang Bay Laboratory, Chongqing 400064, China. 4Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China. 5Shanghai Research Center for Quantum Sciences, Shanghai 201315, China. 6Shanghai Branch, Hefei National Laboratory, Shanghai 201315, China. 7Jinan Institute of Quantum Technology, e-mail: ; ; Jinan, Shandong 250101, China. 8These authors contributed equally: Xinyi Ren, Jin Pan. ; Nature Communications | (2023)14:5037 1 Article https://doi.org/10.1038/s41467-023-40771-3 Alternatively, one can achieve ultrasensitive gas sensing via enhanced photoacoustic spectroscopy (PAS), such as quartz-enhanced PAS (QEPAS)12 and cantilever-enhanced PAS (CEPAS)13,14. In contrast to optical detection, these techniques work at any molecular absorption wavelength and offer extreme sensitivity, without backgrounds, for a small gas volume. Their detection limits have reached the ppt-level or below with the normalized noise equivalent absorption (NNEA) coefficient down to 10−12 cm−1·W·Hz−1/2, but primarily due to the combination of optical resonance cavities14,15 and for one spectral element at a time, which has compromised the selectivity and reliability. Also, the narrow acoustic bandwidths of the cavity-enhanced photoacoustic (PA) systems (e.g. 1 Hz in ref. 15) limit their overall performances (such as acquisition speed and spectral width). Recently, comb-enabled multiplexed or broadband PAS16–20 and photothermal spectroscopy21 have been explored yet with sensitivities limited to sub-ppm (parts per million) levels or above, barely sufficient for trace gas detection. Hence, a novel strategy that improves the sensitivity within a wide acoustic bandwidth for real-time multiplexed PA sensing (that potentially works at any wavelength) is highly demanded. Recently, cavity optomechanical sensors have attracted a great deal of attention and have been recognized as a promising type of ultrasensitive sensors, benefiting from the significant sensitivity enhancement by both high-quality mechanical and optical resonators. Cavity optomechanical sensors have been widely employed in a variety of applications, from gravitational waves22 and dark matter detection23 to displacement24–26, acceleration27, mass28,29, and acoustic sensing30,31. Despite tremendous advancement, no experiments aimed at enhancing broadband molecular spectroscopy with cavity optomechanics have been reported. Here, we demonstrate ultrasensitive multiplexed spectroscopy by combing DCS and cavity optomechanics. Under the dual-comb excitation, a multiplexed molecular absorption spectrum is downconverted to a heterodyne ultrasonic signal and then transferred to the vibration of a mechanical resonator. The membrane-in-the-middle (MIM) cavity optomechanical system26 detects the vibration of a mechanical resonator in real time with a high displacement sensitivity. Such a dual-comb optomechanical spectroscopy (or DCOS) is promising for a wide scope of gas sensing applications due to its superior sensitivity and the advantageous spectral bandwidth, resolution, and acquisition speed. Results Basic principle Figure 1 illustrate (...truncated)