The frequency range 1-100 THz has long been devoid of compact semiconductor sources of coherent radiation, similar to diode lasers in near-infrared and visible. A breakthrough in this area occurred with the demonstration of a quantum cascade laser (QCL) in 1994. QCLs are unipolar devices based on intersubband transitions in a repeated stack of semiconductor superlattices. As a result, their emission frequency can be widely tailored within the same materials system. Currently, these devices can operate at room temperature in mid-infrared spectral range and at cryogenic temperatures in terahertz spectral range. There is a growing interest to utilize QCLs for a variety of applications, including chem/bio and environmental sensing, terahertz security screening, and spectroscopy.
I will give an introduction to the principles of QCLs, provide examples of QCL-based systems for chem/bio sensing, developed in our group, and then describe our progress towards developing a room-temperature terahertz QCL source. In particular, I will talk about our "traditional" THz QCLs, which currently operate at a record temperature of 178K, and a novel type of THz QCL source, operable at room temperature, based on intra-cavity terahertz difference-frequency generation in dual-wavelength mid-infrared QCLs engineered to possess giant second-order nonlinear susceptibility associated with intersubband transitions in the active region.