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The Journal of Chemical Physics

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

GEOFFREY DUXBURY, James Floyd Kelly, Thomas A. Blake, Nigel Langford

Published May 7, 2012• 14 citations
Publisher

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.

DOI
10.1063/1.4710542

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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.
Free induction decay (FID), optical nutation, and rapid passage induced signals in nitrous oxide, under both optically thin and optically thick conditions, have been observed using a rapid current pulse modulation, or chirp, applied to the slow current ramp of a quantum cascade (QC) laser. The variation in optical depth was achieved by increasing the pressure of nitrous oxide in a long path length multipass absorption cell. This allows the variation of optical depth to be achieved over a range of low gas pressures. Since, even at the highest gas pressure used in the cell, the chirp rate of the QC laser is faster than the collisional reorientation time of the molecules, there is minimal collisional damping, allowing a large macroscopic polarization of the molecular dipoles to develop. This is referred to as rapid passage induced polarization. The resultant FID signals are enhanced due to the constructive interference between the field within the gas generated by the slow ramp of the laser (pump), and that of the fast chirp of the laser (probe) signal generated by pulse modulation of the continuously operating QC laser. The FID signals obtained at large optical depth have not been observed previously in the mid-infrared regions, and unusual oscillatory signals have been observed at the highest gas pressures used.
Rapid passage signals showing the effects of molecular alignment have been observed when low pressure samples of nitrous oxide are interrogated by radiation from a pulsed 7.84 µm quantum cascade laser. These effects occur when the sweep rate of the laser through a Doppler broadened absorption line is much faster than the collisional relaxation time, and when the power density of the linearly polarized laser radiation is sufficient to cause optical pumping. Using a laser pulse of duration 1.3 µs, the frequency sweeps approximately 90 GHz. The variation of the laser tuning rate during the laser pulse, from about100 MHz/ns at the beginning to about 20 MHz/ns at the end, allows the relationship between sweep rate and collisional damping to be investigated. It is shown, by comparing the experimental signals with those calculated by coupled Maxwell–Bloch equations, how the rapid passage effects in nitrous oxide are influenced by the number density, transition cross-section and reorientation lifetime.
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.
This thesis is concerned with coherent effects in high resolution mid-infrared gas phase spectroscopy using quantum cascade lasers (QCLs). An introductory chapter explains the importance of QCLs as radiation sources in the mid-infrared region of the spectrum and goes on to detail their development and structure. A discussion of coherent effects in spectroscopy follows, leading into the second chapter which discusses the theories relevant to the experimental sections of the thesis. In chapter 2 the theory underpinning direct and velocity selective, Doppler-free spectroscopy is discussed and a density matrix formalism is followed to derive the equations of motion that govern coherent excitation effects in two-level systems. In the final part of the chapter this treatment is extended to three-level systems. The equations derived in this chapter form the basis of quantitative interpretations of the phenomena observed in experimental data and presented in the remainder of the thesis. In chapter 3 the characterisation of a high power, narrow linewidth QCL is carried out. This laser is then used to perform direct and sub-Doppler resolution spectroscopy on NO, demonstrating non-linear absorption at high laser intensities and providing a measurement of the laser linewidth in the limit of slow frequency tuning. As the slow tuning rate increases, evidence of coherent transient effects is presented and density matrix theory used to model this behaviour. The data presented include the first observations of asymmetric Lamb dips and the onset of rapid passage oscillations from a Lamb dip. Pump-probe experiments on NO, utilising two cw QCLs are presented in chapter 4. The high level of velocity selection afforded by QCL excitation leads to coherent transient signals at far lower probe scan rates than previously reported. The effect of altering both the scan rate and the gas pressure and the importance of hyperfine structure are presented. A radio frequency noise source applied to one of the lasers is shown to broaden the laser linewidth, leading to rapid dephasing. A two-colour polarisation spectroscopy experiment is also presented which allows the measurement of both the absorption and the Doppler-free dispersion signals and the three-level density matrix formalism presented at the end of chapter 2 used to model the non-linear response of the system. The final chapter details the use of an acousto-optic modulator to create a pulse of mid-IR light using a cw QCL and the application of this to time resolved pump-probe spectroscopy. This capability suggests the prospect of achieving coherent population transfer by stimulated Raman adiabatic passage (STIRAP) using two such pulses. Simulations based on a simple three-level model and including Zeeman coherences are presented, which take the measured properties of the lasers used in this thesis as inputs to predict the potential population transfer achievable in NO as well as providing useful information about the angular momentum polarisation of the excited molecules. An experimental realisation of STIRAP would require the lasers to be stabilised, and so the final part of the chapter details experimental attempts to achieve stabilisation of an external cavity QCL, and suggests future avenues for improved implementation.
With a coupling laser locked to caesium 6S 1/2 F g =4–6P 3/2 F e =5 cycling transition and a co-propagating probe laser scanned across 6S 1/2 F g =4–6P 3/2 F e =3, 4 and 5 transitions, a novel scheme for sub-Doppler spectra in Doppler-broadened V-type three-level system is demonstrated by detecting the transmission of the coupling laser through a caesium vapour cell. The Autler–Townes doublet in the sub-Doppler spectra of the coupling laser is clearly observed. The effects of coupling laser intensity on the splitting and linewidth of the Autler–Townes doublet are experimentally investigated and the results agree well with theoretical predictions. Taking the multiple hyperfine levels of caesium atom into account, a brief analysis is presented.