Towards room-temperature Terahertz Quantum Cascade Lasers : directions and design

Abstract

Terahertz Quantum Cascade Lasers (THz QCLs) are arguably the most promising technology today for the compact, efficient generation of THz radiation. Their main limitation is that they require cryogenic cooling, which dominates their ownership cost. Therefore, achieving room-temperature operation is essential for the widespread adoption of THz QCLs. This thesis analyzes the limitations of THz QCL maximum lasing temperature (Tmax) and proposes solutions. THz QCL Tmax is hypothesized to be limited by a fundamental trade-off between gain oscillator strength ful and upper-level lifetime [Tau]. This so-called "ful[Tau] tradeoff" is shown to explain the failure of designs which target [Tau] alone. A solution is proposed in the form of highly diagonal (low ful) active region design coupled with increased doping. Experimental results indicate the strategy to be promising, but heavily doped designs are shown to suffer band-bending effects which may deteriorate performance. In order to treat these band-bending effects, which are typically neglected in previous THz QCL designs, a fast transport simulation tool is developed. Scattering integrals are simplified using the assumption of thermalized sub bands. Results comparable to ensemble Monte Carlo are achieved at a fraction of the computational expense. Carrier leakages to continuum states are also investigated, although they are found to have little effect. Other work in this thesis includes the optimization of double-metal THz waveguides to enable Tmax ~ 200 K, a current world record. Furthermore, laser designs to investigate the leakages of carriers to high-energy subbands and continuum states were fabricated and tested; such parasitic leakages are suggested to be small. Finally, the design of gain media for applications is examined, notably the development of 4.7 THz gain media for OI line detection in astrophysics, and the development of broadband heterogeneous gain media for THz comb generation.