Abstract
We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.
Highlights
Nonlinear laser spectroscopy enables to overcome limitations in molecular spectroscopy regarding spectral resolution
We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration
We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line
Summary
Nonlinear laser spectroscopy enables to overcome limitations in molecular spectroscopy regarding spectral resolution. At terahertz (THz) frequencies, quantum-cascade lasers (QCLs) are suitable because they are powerful continuous-wave sources with a quantum-limited intrinsic linewidth of the order of 100 Hz [1]. Due to the much narrower absorption lines as compared to the mid-infrared range, challenges for Doppler-free spectroscopy are the required pump power in combination with a sufficiently small linewidth of the laser source and a sensitive detection method. We report on true Lamb dip spectroscopy in a collinear pump-probe configuration based on a free-running QCL at 3.3 THz, a free-space absorption cell, and a Ge:Ga photo-conductive detector This approach may be suitable for Lamb dip frequency stabilization of QCLs. Ip u m p /I0 m ax
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