Rapid passage signals from a vibrationally excited target molecule: a pump and probe experiment with continuous wave quantum cascade lasers

Abstract

Two 5 µm continuous wave quantum cascade lasers are used to perform a counterpropagating pump and probe experiment on a low pressure sample of nitric oxide. The strong pump field excites a fundamental rovibrational transition and the weaker probe field is tuned to the corresponding rotationally resolved hot band transition. When both light fields are in resonance, rapid passage is observed in the hot band absorption lineshape arising from a minimally damped and velocity-selected sample of molecules in the v=1 state. The measured rapid passage signals are well described by a two-level model based on the optical Bloch equations.

Polarization spectroscopy of a velocity-selected molecular sample

“…2(c) yields a decay period of ∼200 ns. This is a considerably longer period than we have previously reported (∼40 ns) under similar conditions using a 16 MHz bandwidth probe laser and is consistent with the reduced velocity distribution sampled by the narrower linewidth laser used here [11]. As expected from the optical Bloch equations, the absorption signal and dispersion signal are out of phase by π∕2.…”

Pump and probe spectroscopy with continuous wave quantum cascade lasers

“…Furthermore, as the probe chirp rate increases, τ decreases to a lower limit of ∼50 ns; this is the limit imposed by the 20 MHz bandwidth of the detector. The values of the decay constants reported here are significantly larger than those reported by Walker et al 12 (∼43 ns for pressures of ≤50 mTorr) as expected due to the smaller linewidth of the probe laser used in this study (800 kHz vs 16 MHz)-the current DFB laser probes a narrower range of velocities and thus the coherent transient signal dephases more slowly.…”

“…(1) was used, in which μ is half of the probe laser chirp rate and τ is a phenomenological decay constant. 12 The fit of Eq. (1) to the experimental data was made using a Nelder-Mead simplex algorithm to extract a value for τ , as well as the associated errors calculated from the square roots of the diagonals of the covariance matrix,…”

“…Pursuing these studies of coherent effects using dual laser pump and probe approaches, the population transfer induced by a cw QCL pump operating at 5 μm in a low pressure of NO was directly probed by another cw QCL in a counterpropagating arrangement. 12 In this case, the strong pump field excites a fundamental rovibrational transition and the weaker probe field is tuned to an appropriate rotationally resolved hot band transition. When both light fields are in resonance, RP is observed in the hot band absorption lineshape arising from a minimally damped and velocity-selected sample of molecules in the v = 1, J state.…”

“…As in Ref. 12, a velocity group is selected, and RP signals show the effect of improved lasing properties of the probe. The influence of gas pressure, laser power, and chirp rate on the RP signals is systematically studied.…”