Abstract
Photo-acoustic spectroscopy (PAS) is one of the most sensitive non-destructive analysis techniques for gases, fluids and solids. It can operate background-free at any wavelength and is applicable to microscopic and even non-transparent samples. Extension of PAS to broadband wavelength coverage is a powerful tool, though challenging to implement without sacrifice of wavelength resolution and acquisition speed. Here we show that dual-frequency comb spectroscopy (DCS) and its potential for unmatched precision, speed and wavelength coverage can be combined with the advantages of photo-acoustic detection. Acoustic wave interferograms are generated in the sample by dual-comb absorption and detected by a microphone. As an example, weak gas absorption features are precisely and rapidly sampled; long-term coherent averaging further increases the sensitivity. This novel approach of dual-frequency comb photo-acoustic spectroscopy (DCPAS) generates unprecedented opportunities for rapid and sensitive multi-species molecular analysis across all wavelengths of light.
Highlights
Germany. 3These authors contributed : Thibault Wildi, Thibault Voumard. ✉email: tobias.herr@cfel.de In PAS1–4, optical absorption of a modulated light source leads to periodic heating of a sample and the generation of an acoustic wave that can be detected by a microphone or an equivalent transducer (Fig. 1a)
We show that the resolution and speed limitations in broadband Photo-acoustic spectroscopy (PAS) can be overcome by combining the concept of dual-frequency comb spectroscopy (DCS)[20,21,22,23,24] with photoacoustic detection resulting in the new technique of dualfrequency comb photo-acoustic spectroscopy (DCPAS)
The superposition of all acoustic waves results in a series of interferograms, each with a duration of Δf Àre1p, that is detectable by a microphone or an equivalent transducer, provided all acoustic frequencies fn respect the bandwidth limitation of the transducer
Summary
As the detection relies on the acoustic waves (rather than a weak attenuation of an optical signal), photo-acoustic detection can be background-free, with high signal-to-noise ratio (SNR), and importantly, works at any wavelength of light. PAS is performed at one single probing laser wavelength This is not ideal for the study of multiple species or studies in the presence of uncontrolled background absorption. In order to achieve broadband wavelength coverage, photo-acoustic detection has been combined with Fourier-transform infrared spectrometers (FTIR-PAS)[15].
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