Towards multiple quantum well optically pumped far infrared lasers
Optically pumped lasers based on GaAs/AlGaAs multiple quantum well (MQW) structures are a potential coherent source in the far infrared (FIR) wavelengths. The FIR (~ 30 to 300 μm) is a region within the electromagnetic spectrum that has seen relatively little development. No practical solid-state lasers have been possible in the FIR, which is also referred to as the terahertz (THz) wave, until the very recent demonstration of THz quantum cascade lasers. Other existing FIR lasers are either bulky, expensive, or require magnetic fields for operation. Hence, progress in the FIR depends on the availability of compact, miniature, and inexpensive FIR sources. Optically pumped MQW lasers have advantages in simplicity of design, device processing, and lower free carrier loss, as compared to the electrically pumped lasers. Unlike traditional semiconductor lasers, the FIR MQW lasers involve only electron transitions between subbands of the conduction band. These subband energy levels in our stepped quantum well (QW) design can be tailored by selectively changing the thickness and alloy composition of the QW layers. However, electrons must first be confined to the subbands by doping of the MQW structure. Instead of placing the dopants at the middle of the barrier, the MQW is doped at the stepped well edge to minimise the parasitic distortion to the QW energy profile. FIR photoluminescence signal has been observed in a QW structure doped in the stepped well edge. Electron lifetime measurements were also conducted using a free electron laser (FELIX). Intersubband lifetimes of the order of few ps were obtained in the pump-probe experiment, which were longer than reported values for rectangular QWs. Moreover, a sub-ns slow decay due to multi-photon absorption was observed at high pump intensities. A rate equation model agreed well with the experimental result. A novel idea of stacking two MQW slabs face-to-face to improve the mode confinement of the laser system has been proposed. Possible errors in aligning the slabs were modelled, where the estimated increase in diffraction loss suggested that the stacking scheme is highly practical. Together with embedded heavily doped layers in the MQW structures, which utilise the surface plasmon guiding effects, a low modal loss and high mode confinement structure can be achieved. FIR optical signal systems were also studied, where detectors and optical materials required are different to those in the shorter wavelengths region. An optically pumped methanol gas laser was also built and tested for the work as an alternative FIR source.