Time-resolved spectroscopy of quantum cascade lasers

Abstract

The investigation of the physical properties of quantum cascade lasers can be achieved by time-resolved spectroscopy. This technique delivers precise information on amplitude and phase of the emitted light. These data give a more detailed view of the ultrafast physical processes like stimulated emission, absorption and intersubband transitions. Especially the possibility to directly measure the phase is very useful. It helps to describe the relation between stimulated emission and absorption. The benefit of this spectroscopy technique is the broad spectral range, a comparable low noise level, high sampling rates and also a high dynamic range. In particular this work also investigates the temperature dependence of the physical processes in quantum cascade lasers. The thermo-optic effect is a great issue in the acquisition of reliable data as we have to deal with phase distortion and change of the refractive index. This problem shows up at lasers operated in pulsed mode, which will therefor lead to a continuous cooling and heating of the laser medium altering the optical properties.<br />A method is presented which helps to overcome the negative influence of the temperature change during the measurement and provides more accurate phase information.