Coherent transient spectroscopy with continuous wave quantum cascade lasers

Abstract

A high power continuous wave quantum cascade laser operating around 1900 cm(-1) has been used to conduct Lamb dip spectroscopy on a low pressure sample of NO. The widths of the Lamb dips indicate that the laser linewidth is 800 ± 60 kHz and the power sufficient to induce significant population transfer of up to 35%. While the Lamb dip signals are symmetric at low laser chirp rates, they become increasingly asymmetric as the chirp rate increases, further confirming the significant degree of population transfer. In addition rapid passage structure on the Lamb dip signal is observed after the weak probe beam is swept through the line center. This structure is sensitive to both the probe chirp rate and the underlying hyperfine structure of the rovibrational transition, and is accurately modeled using the optical Bloch equations.

Polarization spectroscopy of a velocity-selected molecular sample

“…The pure absorption signal, ΣI, in Fig. 1(a) demonstrates the velocity selection of the pump beam, having a width of 2 MHz, which is consistent with the previously measured linewidth of the laser [7]. The ΔI data are fitted using Eq.…”

“…In this case a commercial cw external cavity QCL (Daylight Solutions) covering the range 1776-1958 cm −1 , that emits up to 140 mW of optical power and exhibits a linewidth of ∼2 MHz [5,6]), is used as the pump field, and a distributed-feedback cw QCL (Maxion Technologies M575AH-NS) covering 1897-1904 cm −1 , with up to 250 mW of optical power and a linewidth of ∼800 kHz, [7,8], was the probe. The pump laser was circularly polarized using a λ∕4 waveplate (Alphalas) and its frequency tuned to a fixed frequency within the R14.5 1∕2 v 1 0 transition.…”

“…1(a) and 2(a)] is slightly asymmetric, and this is a sign of incipient RP effects, which are most noticeable in Figs. 2(c) and 2(f) [7].…”

Pump and probe spectroscopy with continuous wave quantum cascade lasers

“…8 Population transfer in a low pressure sample of nitric oxide (NO) has been shown to be chirp rate dependent with a cw QCL. 9 In that study 35% population transfer was achieved and RP structure observed within Lambdip spectra. Other coherent phenomena may also be observed with rapidly chirped cw QCLs and particularly striking is the work by Duxbury et al, 10,11 demonstrating the electric field induced Autler-Townes splitting in NO.…”

“…To increase the signal to noise ratio of the transient RP signals, an ensemble of traces was averaged over the Doppler absorption profile for each of the probed -doublets using a MATLAB program. 9 The frequency up and down chirp data were separated and the spectra realigned onto the most intense spectral feature to account for small frequency drifts in the probe laser. The averaging does not change either the overall shape of the signal or the decaying oscillations of the RP signals.…”