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.

尽管在从紫外线到太赫兹频率的电磁频谱的各个区域都取得了令人瞩目的进步，但光学计量学和高分辨率光谱学仍然在13到20 µm的振动弯曲模式范围内受到严格限制。由于缺少可调谐的单模激光器，因此在中红外范围内的这一长波长部分仍未开发。在这里，我们通过使用可调谐范围为12.1至14.8 µm，光功率高达110 µW，MHz级线宽和梳状校准的连续波非线性激光源，演示了该区域的弯曲模式频率计量。我们评估了几个基于CO ₂的频率基准，不确定性低至30 kHz，并且我们对v _11进行了广泛的研究。苯谱带，这是光谱仪分辨率的重要测试平台。这些成就为长波红外计量学，旋转解析研究以及大分子（例如芳烃）的天文观测铺平了道路。