Coherent transient spectroscopy with quantum cascade lasers
Keywords: QCL; Coherent; Spectroscopy

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.