Quantum cascade laser spectroscopy: non-linear optics and population transfer

This thesis is concerned with the non-linear effects observed in the absorption spectra obtained with swept continuous wave quantum cascade laser (cw-QCL) radiation. The slower chirp rate afforded by cw-QCLs, compared with pulsed systems, allows different aspects of the population transfer and polarisation induced in a molecular transition to be elucidated: namely, adiabatic rapid passage and coherent transient effects.
The thesis commences with a brief outline of the architecture of QCLs, and an introduction to the non-linear processes inherent in their utilisation. In the following chapter, an outline of the methods required to characterise and drive a QCL chip are described along with the theory and characterisation of a Herriott cell, the latter allowing optically thick but minimally damped samples to be investigated.
In chapter 3, a cw-QCL operating at &Tilde;10 μm is employed at chirp rates near to the adiabatic limit, in the range 0.1 – 1.1 MHz ns-1. A ro-vibrational overtone transition of OCS is studied at two path lengths within an astigmatic Herriott cell, 238 and 47 m. The population transfer and non-linear effects induced by the cw-QCL are described and modelled using the Maxwell-Bloch equations, which incorporate different experimental parameters. As such, the maximum population transfer found at a low pressure of 4 mTorr was determined to be 12 %. Additionally, the maximum amplitude of the coherent transient is forced to shift markedly to later times as the sample becomes increasingly optically thick, whilst remaining minimally damped.
Intrapulse spectroscopy with a cw-QCL operating at 4.6 μm is investigated in chapter 4. The laser’s response to short current pulses, in the range of 20 – 400 ns, applied in conjunction with a slow ramp, 2.75 kHz ns-1, is characterised. The initial chirp rate during the pulse is found to be &Tilde;6 MHz ns-1 by observation of the temporal width of transitions within the pulse. After the pulse a down chirp, at a rate of ~1 MHz ns-1, caused the laser frequency to relax back to the frequency position prior to the pulse. Subsequently, the effect of the pulse on a strongly absorbing N2O transition is monitored. In particular, the pulse position is altered with ii Quantum Cascade Laser Spectroscopy: Non-Linear Optics and Population Transfer respect to the transition line-centre and the coherent transient response of the molecular sample is probed. The free induction decay and RP effects noted are found to have a beat frequency which alters in line with the chirp rate of the laser, and the decay time of the transient signal was found to decrease as the range of velocity groups swept through is increased.
The non-linear response of optically thick NH3 is investigated in chapter 5 with a 30 mW laser operating at &Tilde;10 μm. The effect of changing the gas pressure, laser intensity, and chirp rate on optically thick transitions is investigated for this molecule, which has a markedly larger dipole moment than OCS, and a comparison between the three molecular systems studied in this thesis is presented. Due to the importance of velocity dephasing in this work, the linewidth of the QCL was measured by Lamb-dip spectroscopy and found to be &Tilde;3 MHz. Two noise sources are then employed to alter the linewidth of the QCL: a single frequency modulation and a random white noise source. The noise, applied through the bias-T of the laser, leads to a change in the linewidth and lineshape, and as such, user-selectable linewidths in the range 3 – 20 MHz can be created. Increasing the laser linewidth has been found to increase the saturation of the sample, and therefore leads to an increase in the population transfer, which is determined to be &Tilde;10% at a chirp rate of &Tilde;0.1 MHz ns-1.
The final chapter introduces the first use of a cw-QCL for population transfer within a molecular beam. The excited state is probed via resonantly enhanced multiphoton ionisation time of flight (REMPI-TOF) spectroscopy and a velocity selected population transfer of &Tilde;8 % is achieved. The effects of laser intensity, molecular beam carrier gas and laser linewidth on the population transfer are investigated. This thesis concludes with a discussion of some potential extensions to the work presented in this final chapter.

Advisor: Ritchie, G