Optical frequency metrology in the bending modes region

M Lamperti,D Gatti,A Farooq,F Tamassia,P Laporta,M Marangoni,P Schunemann,R Gotti,E Cané,M K Shakfa

doi:10.1038/s42005-020-00441-y

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.

Highlights

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
An alternative approach is difference frequency generation (DFG) from cw Ti:sapphire[11,12] or diode[13] lasers, but this is accompanied by extremely low optical powers (10–100 nW range) that impair the acquisition of absorption spectra at a high signal-tonoise ratio (SNR)
The laser source is based on the DFG process between two MIR sources, namely a cw DFB-QCL and a CO2 laser, in an orientation-patterned gallium arsenide (OP-GaAs) crystal[24]

Summary

Introduction

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 longwavelength part of the mid-infrared range remains largely unexplored due to the lack of tunable single-mode lasers. We apply the spectrometer to the rich and congested absorption spectrum of the ν11 bending band of benzene, resolving 729 lines and retrieving the rotational parameters of the excited state with increased accuracy over the state-of-the-art These outcomes attest to the value of our approach for extending high-resolution spectroscopy and optical metrology to the yet unexplored longwavelength part of the mid-infrared spectrum

Methods

Results

Conclusion