Quantum Cascade Lasers (QCLs) are a new kind of semiconductor laser based on intersubband transitions within a quantum well (QW). Among the more attractive features of QCLs are their unipolar transport, high power emission, ultrafast modulation (>100GHz) and generation of wavelengths in the mid and far infrared, not ever obtained before. Considerable advances have been achieved in the development of the QCLs during the last few years, which have led to the appearance of the first commercial thermoelectrically cooled QCLs.
However, there are still many limitations of these lasers. A critical one is the unavailability of QCLs operating in continuous wave (CW) mode at room temperature (RT). Another is the absence of QCLs operating at short wavelengths, such as l=1.55mm, needed to develop ultrafast optical communications. Both of these drawbacks are due to insufficient electron confinement in the materials combinations presently used. Therefore, new systems based on wide band gap materials are required.
This program proposes to explore the use of wide bandgap II-VI materials for the development of QCLs. The large conduction band offsets and deep QWs offered by the II-VI semiconductors, such as ZnCdMgSe/ZnCdSe, provide the possibility of higher electron confinement (and consequently the opportunity to obtain QCLs operating at RT in CW mode) and of QCLs that operate at 1.55mm for a new generation of ultrafast optical communications lasers. An important issue in favor of using II-VI semiconductors for QCLs is the fact that these devices are unipolar and do not require p-type doping, which traditionally has been a problem in most of the II-VI systems. While GaN-based materials are an alternative, the difficulties associated with the growth of high quality multi-layered structures of the nitride materials make them less attractive than the near lattice-matched, well-developed II-VI material systems.
Several II-VI systems that are particularly promising for QCLs with low threshold current, operating at RT and some of them that should be able to generate emission at 1.55mm will be investigated. The QW properties, such as the energy levels available for intersubband transitions, will be investigated using modulation spectroscopy and other optical techniques. Such techniques have also been extensively utilized in the past by the PIs. Once a system has been identified to have potential for QCL applications, the QW structures will be optimized and incorporated in device structures to investigate the emission and lasing characteristics.