Quantum cascade lasers (QCLs) are unipolar semiconductor lasers, where the wavelength of emitted radiation is determined by the engineering of quantum states within the conduction band in coupled multiple-quantum-well heterostructures to have the desired energy separation. The recent development of terahertz QCLs has provided a new generation of solid-state sources for radiation in the terahertz frequency range. Terahertz QCLs have been demonstrated from 0.84 to 5.0 THz both in pulsed mode and continuous wave mode (CW mode).
A 2.7-THz QCL structure uses a metalmetal waveguide QCL with multiplequantum- well cascade medium to provide terahertz gain for subbands engineered to have the desired energy separation. The approach employs a resonant- phonon depopulation concept. The metal-metal (MM) waveguide fabrication is performed using Cu-Cu thermo- compression bonding to bond the GaAs/AlGaAs epitaxial layer to a GaAs receptor wafer. A laterally corrugated distributed feedback (DFB) grating is etched into a MM waveguide, as this is easily performed in a single photolithographic and etch step. Extended modeling is done for both the DFB cavity and the coupling with the waveguide via the integrated probe. The DFB structure QCL has an integrated waveguide probe suitable for mounting in a machined waveguide block. Following fabrication of the MM-waveguide, the wafer can be mounted top-down on a temporary support wafer, and the GaAs receptor substrate is thinned to a membrane with the assistance of an etch-stop layer.
Development of a demonstrator hornantenna coupled QCL at 2.7 THz with Gaussian output beam profile and high coupling efficiency capable of effectively pumping mixers at these frequencies is a major breakthrough in the spectroscopic studies for the Earth-observation and astrophysics community. The approach, which includes an integrated probe on the QCL device in a waveguide enclosure transitioning to a diagonal horn, may lead to compact, coherent, continuous- wave solid-state sources.
A phase-locked terahertz QCL source with high-quality beam profile and excellent output coupling efficiency operating at or above liquid nitrogen temperatures will be of great strategic importance for NASA’s astrophysics, Earth, and planetary mission capabilities. This will make these QCLs the local oscillator source of choice for the future NASA and European suborbital and orbital terahertz instruments for astrophysics missions such as the interferometric (ESPRIT) and other single- and multi-pixel heterodyne spectroscopic missions, as well as for Earth observing and planetary missions. A high-power QCL with good beam profile can also be used in biological and medical science instruments, security screening and illicit material detection, and nondestructive evaluation applications.