Controlling loss of waveguides for potential GaN terahertz quantum cascade lasers by tuning the plasma frequency of doped layers

We have analyzed the waveguide loss originating from various doping layers in double metal waveguides for potential GaN-based terahertz quantum cascade lasers (THz QCLs) by theoretical calculations. The optical field can be very well confined in the active QCL region. Average electron densities in the QCL active region and n+ contact layers should be controlled carefully. The loss in the low-frequency range (less than or similar to 3 THz) can be minimized by decreasing the electron density in the QCL layer. In the middle- and high-THz-frequency ranges (similar to 3 < f < 15 THz), the absorption by the heavily doped n+ contact layer dominates the waveguide loss. Consequently, the bulk plasma frequency, which is determined by electron density and shows a strong absorption peak, must be tuned to deviate from the target QCL frequency. The waveguide loss plus the cavity mirror loss can be controlled to be as low as similar to 24cm(-1). (C) 2018 The Japan Society of Applied Physics