Superconducting Microdisk Cavities for THz Quantum Cascade Lasers

We present superconducting waveguides for terahertz (THz) quantum cascade lasers (QCLs). Double-metal waveguides provide high confinement of the optical mode and low waveguide losses which are dominated by absorption of the radiation in the metal layers. Implementing novel waveguide materials like superconductors is one way to reduce these losses. In order to prove the compatibility with the THz QCL active region and waveguide we have replaced the commonly used gold or copper layers by superconducting niobium (Nb). We have simulated the temperature distribution inside the THz QCL in order to evaluate the operation conditions at which the critical temperature of the Nb layers is not exceeded. Experimental results of THz QCLs with Nb waveguides are presented which show lasing emission despite the fact that the energy of the THz radiation of the investigated active region f = 2.5 THz = 10.3 meV is higher than the superconducting energy gap of Nb 2Δ = 2.8 meV. Calculations show that improvements in terms of lower waveguide losses can be achieved using a superconductor with higher critical temperature and thus wider superconducting gap e.g., NbTiN or MgB₂.