Optical frequency combs, consisting of equally spaced, phase-coherent teeth, enable precise measurements of optical frequencies and are essential light sources in optical metrology and spectroscopy. Dual-comb spectroscopy is a high-resolution, large-bandwidth spectroscopic technique that utilizes optical frequency combs. It records the interference pattern between two combs with slightly different repetition rates. Dual-comb spectroscopy has achieved remarkable results in infrared gas absorption spectroscopy.

Ultraviolet (UV) spectroscopy plays a pivotal role in studying electronic transitions in atoms and rotational transitions in molecules, providing unique insights into material structures. Its applications span diverse fields, including fundamental physics, atmospheric photochemistry, and astronomical observations. Thus, extending dual-comb spectroscopy to the UV range has been a long-standing aspiration in the academic community. However, due to the absence of directly emitting coherent UV light sources, nonlinear frequency upconversion becomes necessary for generating UV laser light. Unfortunately, shorter wavelengths necessitate more nonlinear conversion processes, leading to significant power losses, which significantly limits the practical application of UV dual-comb spectroscopy. This paper introduces a novel dual-comb spectroscopy method suitable for extremely low light levels. By employing photon counting technology, it achieves near-ultraviolet (NUV) dual-comb spectroscopy with both high resolution and quantum noise limit. Remarkably, its operating photon flux is over 106 times lower than that commonly used in dual-comb spectroscopy and other Fourier transform spectroscopy techniques based on frequency combs [1].

Figure 1. Schematic diagram of UV photon-counting dual-comb spectroscopy [1]

The research team demonstrates the potential of NUV dual-comb spectroscopy through nonlinear frequency conversion of near-infrared electro-optic frequency combs. Electro-optic systems exhibit low conversion efficiency in the UV range, making them ideal for testing the proposed photon-counting method. The frequency combs with slightly different repetition rates are generated by a continuous-wave laser modulated by an electro-optic modulator at a center frequency of 193 THz (1550 nm) with intensity and phase modulation. An acousto-optic modulator shifts the center frequency of one comb to measure the dual-comb spectrum without aliasing.

Figure 2. Experimental setup for NUV photon-counting dual-comb spectroscopy. (a) Generation of NUV comb light source, (b) Schematic diagram of NUV dual-comb spectroscopy [1] The generated near-infrared frequency combs undergo frequency doubling sequentially through PPLN and BIBO crystals, producing a pair of UV comb light sources with center wavelengths near 390 nm (as shown in Figure 3). One UV comb source, after passing through a heated cesium vapor cell, interferes with the other UV comb source with a slightly different repetition rate. The interference signal is then detected by a single-photon counter. The photon counts generated during this process are minimal, requiring over 20 comb repetition cycles for each count on average. By repeating the scanning and counting process 100,000 to 1 million times, the dual-comb interference signal becomes evident in the statistical histogram. As the accumulation time increases, the signal-to-noise ratio (SNR) also improves. Fourier transformation of the interference signal reveals a comb-like radiofrequency spectrum recording the absorption lines of cesium atoms.

Figure 3. Experimentally observed NUV line-resolvable photon-level dual-comb spectrum.

(a) NUV dual-comb spectrum, (b) Zoom-in view of three comb teeth, (c) Illustration of spectral agility [1] With an accumulation time of 100 s, a resolution of 500 MHz, and a comb power of 45 pW, the team measured the amplitude and phase spectra of the 6S1/2-8P1/2 and 6S1/2-8P3/2 transition lines of cesium atoms (133C). The average SNR of the spectral measurements reached 200, demonstrating the effectiveness and reliability of the proposed scheme. Additionally, the observed square-root dependence of the spectral SNR on the photon count rate confirms that the experiment achieves the quantum noise limit. Building upon this, they further measured the absolute optical frequencies of the transition lines with a relative uncertainty of 10-9, verifying the reliability of the experimental scheme and results. Vacuum and extreme UV frequency combs are generated solely as harmonics of near-infrared femtosecond mode-locked lasers, underscoring the importance of assessing their suitability for photon-counting dual-comb spectroscopy.

As depicted in Figure 4, researchers employed two erbium-doped mode-locked lasers with a repetition rate of 100 MHz and a center frequency of 192 THz. After frequency doubling through a 4 cm PPLN crystal, the generated visible light combs had a central wavelength of 384 THz and a frequency difference of -12.5 KHz. One of the visible light combs passed through a rubidium vapor cell, and both combs interfered at the beam splitter (BS), with one path directed to a photon counter and the other to a fast silicon photodiode for triggering photon counts. The visible dual-comb spectroscopy utilized a feedforward dual-comb technique to ensure the mutual coherence of the two combs.

Figure 4. Fourier transformation of the interference pattern from the visible dual-comb experimental setup reveals a transmission spectrum with resolved frequency comb lines (Figure 5). The spectrum spans 0.12 THz and contains 1200 frequency lines well above the noise level (Figure 5a). The average SNR of the measured Doppler-broadened 5S1/2–5P3/2 transition absorption baselines in 85Rb and 87Rb is 67, close to the calculated quantum noise limit of 69.

Figure 5. Photon-counting visible dual-comb spectrum.

(a) Dual-comb spectrum, (b) Zoom-in view of the Rb 5S1/2-5P3/2 transition spectrum, (c) Zoom-in view of four comb teeth In summary, this paper achieves high-resolution linear absorption NUV dual-comb spectroscopy at extremely low light levels through photon counting, attaining an SNR at the quantum noise limit. This represents a significant step toward precise spectroscopy in the extreme UV spectral region, potentially opening up new applications in precision spectroscopy, biomedical sensing, and environmental atmospheric detection.

Reference:

[1] Xu, B., Chen, Z., Hänsch, T.W.et al. Near-ultraviolet photon-counting dual-comb spectroscopy. Nature 627, 289–294 （2024）. https://doi.org/10.1038/s41586-024-07094-9