Optical frequency metrology in the bending modes region

Abstract

Optical metrology and high-resolution spectroscopy, despite impressive progress across diverse regions of the electromagnetic spectrum from ultraviolet to terahertz frequencies, are still severely limited in the region of vibrational bending modes from 13 to 20 µm. This long-wavelength part of the mid-infrared range remains largely unexplored due to the lack of tunable single-mode lasers. Here, we demonstrate bending modes frequency metrology in this region by employing a continuous-wave nonlinear laser source with tunability from 12.1 to 14.8 µm, optical power up to 110 µW, MHz-level linewidth and comb calibration. We assess several CO2-based frequency benchmarks with uncertainties down to 30 kHz and we provide an extensive study of the v11 band of benzene, a significant testbed for the resolution of the spectrometer. These achievements pave the way for long-wavelength infrared metrology, rotationally-resolved studies and astronomic observations of large molecules such as aromatic hydrocarbons.

Bending modes metrology in the 12-15 µm region

“…Averaging over more than 20 acquisitions reduces the uncertainty to 30 kHz. The weighted deviation from the HITRAN line list is 0.34 MHz and fully consistent with the declared uncertainty [7]. An innovative spectrometer based on a comb-referred nonlinear laser source is demonstrated for high resolution molecular spectroscopy in the LWIR.…”

Absolute Frequency Referencing in the Long-Wave Infrared using a Quantum Cascade Laser Frequency Comb

“…The large free spectral range of 10 GHz of the QCL-OFC is practical for closed-loop combassisted spectroscopy over hundreds of MHz, as the problem of the heterodyne beat reaching near zero frequency and folding is avoided [21,34]. However, the available detectors can provide sufficient SNR for phase-locking only up to 1.5-GHz offsets, such that accessing the full optical spectrum requires changing the transition frequency of the locked laser.…”

“…Integrated photonics based on lithium niobate or silicon nitride promise small-footprint conversion with low pulse energy, but only to wavelengths up to 4.5 µm [8,14,[22][23][24]. In the long-wave infrared (8 µm to 12 µm) and beyond, several experiments successfully determined the absolute frequency of molecular transitions by performing nonlinear frequency conversion [25][26][27][28][29][30][31][32][33][34][35]. The obtained accuracies were typically in the 100-kHz range in the Doppler-limited regime and down to 22 Hz in the Doppler-free regime [10].…”

Comb-calibrated Spectroscopy in the 12–15 m Region

“…We also performed frequency metrology on the  band of CO2 in the 675-689 cm -1 region, which offers many isolated lines that can be used as frequency standards in the LWIR. Repeated measurements over the same lines allow to shrink the errors on the transition frequencies from 140 kHz (single measurement over 2 s with a signal to noise ratio of 300) to below 30 kHz [7]. An exemplary spectrum, on the R(14) line, is shown in Fig.…”