Vacuum Ultraviolet Dual-Comb Spectroscopy

TL;DR

This work extends dual-comb spectroscopy into the vacuum ultraviolet by implementing intracavity high-harmonic generation (iHHG) to produce the 5th and 7th harmonics, enabling absolute-frequency, Doppler-resolved absorption measurements at $\lambda=210$ nm and $\lambda=149$ nm for room-temperature acetylene and ammonia. Using two phase-locked Yb-fiber combs and enhancement cavities, the authors achieve broadband, high-resolution VUV spectra with well-calibrated absolute frequencies, validated against synchrotron cross-sections. A detailed noise analysis identifies detection noise and PMT feed-through as limiting factors and outlines concrete hardware improvements to improve signal-to-noise and averaging times, potentially scaling the figure of merit by nearly an order of magnitude. The demonstrated approach provides fast, high-resolution, absolute spectroscopy in the VUV, with implications for exoplanet photochemistry, plasma diagnostics, and fundamental tests of physics, and sets the stage for extending DCS further into the EUV.

Abstract

The optical frequency comb has made a significant impact in precision spectroscopy and on our ability to probe atomic, molecular and, recently, nuclear transitions to further our understanding of their fundamental properties and how their dynamics and complex interactions affect the observed world. To expand the energy scales and types of systems that can be studied, frequency comb sources from terahertz to vacuum ultraviolet frequencies and beyond have been pursued. Dual-comb spectroscopy, enabled by the development of these frequency comb sources, allows broadband absorption measurements of complicated spectra, exceeding the limitations of direct, single-comb spectroscopy. To date, however, the dual-comb approach has not been able to directly access many important transitions that lie at challenging vacuum ultraviolet wavelengths. Here, we demonstrate dual-comb spectroscopy in the vacuum ultraviolet utilizing intracavity high harmonic generation. This multi-harmonic source is used to measure molecular absorbance spectra at $λ=210$~nm and $λ=149$~nm from room-temperature samples of acetylene and ammonia, respectively. These measurements resolve the Doppler broadened structure of congested molecular spectra with absolute frequency accuracy. Noise contributions to the vacuum ultraviolet dual-comb spectroscopy measurements are characterized, guiding future efforts and technological development in this region.

Figures (6)

• Figure 1: Intracavity High Harmonic Generation and Dual-Comb Spectroscopy Beamline.a, Experimental schematic of intracavity high-harmonic generation showing each comb's out-coupled collinear harmonics aligned through a sample cell and into a nitrogen-purged chamber where they are overlapped on an ultraviolet beam splitter (BS) with piezo-controlled mirrors (PZT-M). Finally, the harmonics are separated with a CaF$_2$ prism, and the fundamental (H1) is measured on an infrared photodiode (IR PD), the 5$^\text{th}$ harmonic (H5) on an ultraviolet-enhanced avalanche photodiode (UV APD), and the 7$^\text{th}$ (H7) on a solar-blind photomultiplier tube (PMT). Vacuum chamber feed-throughs carry the high voltage (HV) bias to the PMT and output photocurrent from the PMT to an external trans-impedance amplifier (TIA). b, Low-resolution measurements of fundamental and harmonic spectra detected on various spectrometers.
• Figure 2: Dual-Comb Interferogram Detection and Averaginga-b, Detected 5$^{\text{th}}$ and 7$^{\text{th}}$ harmonic single-shot interferograms (gray) with centerbursts arbitrarily scaled to $\pm1$ unit are phase-corrected and averaged 10,000 times (a. blue, b. purple). c, For averaging time $\tau$, the noise floor of the absorbance spectra decreases with $\sqrt{\tau}$. Here, both 5$^{\text{th}}$ and 7$^{\text{th}}$ harmonic interferograms are processed to resolve $M=1000$ spectral elements across their respective optical bandwidths, giving $\Delta\nu_{res}=4$ and 7 GHz, respectively.
• Figure 3: Deep ultraviolet dual-comb spectroscopy of acetylene with the 5$^{\text{th}}$ harmonic at $\lambda=210$ nm. a, The dual-comb transmission spectrum processed with a resolution of 4 GHz and 10,000 averages (blue) and a smoothed pseudo-reference (black). b, The dual-comb absorbance spectrum (blue) is compared to the absorption cross-section of acetylene (black) measured at the SOLEIL synchrotron with a resolution of 15 GHz fleury2025high.
• Figure 4: Vacuum ultraviolet dual-comb spectroscopy of ammonia with the 7$^{\text{th}}$ harmonic at $\lambda=149$ nm. a, The dual-comb transmission spectrum processed with a resolution of 7 GHz and 15,000 averages (purple) and a smoothed pseudo-reference (black). b, The dual-comb absorbance spectrum (purple) is compared to the absorption cross-section of ammonia (black) measured at the SOLEIL synchrotron with a resolution of 7 GHz pratt2023high.
• Figure 5: Vacuum ultraviolet dual-comb spectroscopy noise analysis. For $M=1000$ frequency-resolved elements ($\Delta\nu_{res}=7$ GHz resolution across $\Delta\nu_{BW}=7$ THz optical bandwidth), $\tau=68$ s averaging time, and 0.6 $\mu$W power on detector per comb, our vacuum ultraviolet dual-comb measurement has an SNR near 20 (star). Projected SNR at other detected comb powers (red) is calculated from various noise source contributions (black).