Modes, emission beams and losses of THz heterostructure disk lasers: single-mode laser with vertical beam option

Properties of disk cavities for terahertz (THz) heterostructure lasers are discussed basing on observation of emission and beam from THz quantum cascade disk laser and analytical evaluation of modes, beams and losses of the laser disk cavities. The measured laser beam has beam peak at about 30° from vertical. To find condition for vertically emitted beam and for evaluation of the laser modes, emission beams and losses simple approach adopted in theory of slit (patch) antennas is used. With this approach results on observed laser beam are discussed and a possibility to create single-frequency disk laser with narrow vertical beam. This can be achieved if only single TEM1m cavity mode is exited. Several examples of such possibility for 100 µm wavelength with disk cavities from 18 to 200 µm in diameter and with cascade thickness 10 µm are presented. Depending on the diameter the cavity emission losses are higher than or of about that of THz quantum cascade laser waveguide losses or amplification. For wavelength longer than 100 µm and (or) with lower cascade thickness, the losses go down and cavity could be optimised for creation a single-mode powerful vertically emitted disk laser.