Superconducting waveguides for terahertz quantum cascade lasers

Quantum cascade lasers (QCLs) are the only compact sources of coherent terahertz (THz) radiation. This spectral region is very interesting for various applications like spectroscopy or imaging. Up to now THz QCLs are operating in the regime between 1 and 5 THz. One of the advantages of QCLs is that the wavelength of the device can be tailored by the design of the active region. In addition the waveguide determines the optical properties and the performance of the QCL. A double-metal (DM) waveguide, where the THz radiation is guided in between two metal layers, provides low losses and high confinement of the optical mode. Besides Fabry-Perot cavities also disk and photonic crystal resonators have been reported. The main reason for the waveguide losses is the absorption of the THz radiation by the metal layers.