Loss control between symmetric and anti-symmetric modes for single-lobed surface emitting of terahertz quantum...

Single-mode surface-emitting distributed feedback terahertz (THz) quantum cascade lasers (QCLs) operating around 2.76 THz are investigated in metal-metal waveguide. The design consists of one conventional long slit and two additional short slits aside in a single unit cell. Based on the symmetric modes, the two short slits provide radiating components in phase to form higher beam quality. Meanwhile the long slit provides the main feedback and lower extracted power with out-of-phased components, which contributes less to the beam. The surface-current analysis method is first introduced in QCL to analyze the losses between symmetric and anti-symmetric modes as the distance of the two short slits changes. The designed band-edge mode operates at a relative low loss, and a main single lobe is achieved with low side lobe level.