Abstract
We have developed a dispersive spectrometer by using a compact immersion grating for direct frequency comb spectroscopy in the long-wave infrared region of 8-10 μm for the first time. A frequency resolution of 460 MHz is achieved, which is the highest reported in this wavelength region with a dispersive spectrometer. We also demonstrate individual comb mode-resolved imaging by cavity filtering and apply this to obtain spectra of both simple and complex molecular spectra. These results indicate that the immersion grating spectrometer offers the next advancement for sensitive, high-resolution spectroscopy of transient and large/complex molecules when combined with cavity enhancement and cooling techniques.
Highlights
Mid-IR Direct Frequency Comb SpectroscopyDirect frequency comb spectroscopy (DFCS) is an attractive approach for molecular spectroscopy because of the afforded high sensitivity, broad spectral coverage, and rapid acquisition capabilities
As alternatives to a dispersive spectrometer, mid-IR DFCS has been demonstrated with dual-comb spectroscopy [5, 7,8,9,10] and Fourier transform spectroscopy [11,12,13]
We report in-depth characterization of this dispersive spectrometer, with focused discussions on diffraction efficiency, resolving power, noise performance, its use for high resolution molecular spectroscopy, and the potential for time-resolved spectroscopy
Summary
Direct frequency comb spectroscopy (DFCS) in the mid-infrared spectral region find important applications in both fundamental laboratory spectroscopy and remote sensing. The fast acquisition advantage allows for both real-time remote sensing [1] (seconds) and measurement of fast chemical kinetics [2,3,4,5] (microseconds) These combined advantages have been demonstrated in techniques of cavity-enhanced spectroscopy with a dispersive spectrometer [2,3,4, 6]. A comparably important consideration for these larger, complex molecules is that spectroscopic probing at longer infrared wavelengths alleviates spectral congestion due to IVR (intramolecular vibrational redistribution) processes, enabling quantum-state resolution [6, 20] These advantages motivate the recent construction of an 8-10 m mid-infrared frequency comb (an optical parametric oscillator (OPO) based on AgGaSe2) for DFCS [21]. The enabling technology for this spectrometer is the immersion grating, which has been fabricated for wavelengths > 5 m
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