Sub-Doppler spectra of infrared hyperfine transitions of nitric oxide using a pulse modulated quantum cascade laser: Rapid passage, free induction decay, and the ac Stark effect

Abstract

Using a low power, rapid (nsec) pulse-modulated quantum cascade (QC) laser, collective coherent effects in the 5 μm spectrum of nitric oxide have been demonstrated by the observation of sub-Doppler hyperfine splitting and also Autler-Townes splitting of Doppler broadened lines. For nitrous oxide, experiments and model calculations have demonstrated that two main effects occur with pulse-modulated (chirped) quantum cascade lasers: free induction decay signals, and signals induced by rapid passage during the laser chirp. In the open shell molecule, NO, in which both Λ-doubling splitting and hyperfine structure occur, laser field-induced coupling between the hyperfine levels of the two Λ-doublet components can induce a large ac Stark effect. This may be observed as sub-Doppler structure, field-induced splittings, or Autler-Townes splitting of a Doppler broadened line. These represent an extension of the types of behaviour observed in the closed shell molecule nitrous oxide, using the same apparatus, when probed with an 8 μm QC laser.

The ac Stark effect in nitric oxide induced by rapidly swept continuous wave quantum cascade lasers

“…In the following paper 21 we demonstrate an alternative way of studying the interaction of light from a frequencychirped QC laser with NO by creating a fast scanning probe pulse that modulates the main QC laser output.…”

Pump and probe spectroscopy with continuous wave quantum cascade lasers

“…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. In this case the splita) E-mail: grant.ritchie@chem.ox.ac.uk ting, observed in a low pressure sample of NO over a pathlength of 100 m, was also found to be chirp rate dependent.…”

Coherent spectroscopy with fast frequency swept lasers

“…where the complex amplitude of the laser radiation is (8) and field E med (t, z) is related to coherent emission of the medium. Then, we have…”

“…A detailed comparison was not conducted in [4] because, on the one hand, the frequency sweeping rates of classical injection lasers were already at the limit, and, on the other hand, the speed of recording fast signals in the IR spectral range was also limited. With the advent of fast frequency swept quantum-cascade lasers and modern means of recording spectra, many researchers returned to investigation of these affects [6][7][8][9][10]. In general, the results obtained…”