Distributed feedback ring resonators for vertically emitting terahertz quantum cascade lasers

In this paper, we have modeled and fabricated THz QCLs in a circular double-metal DFB resonator of much larger diameter, for which we compute the far-field shown in the inset. Devices are designed using a finite element solver. A second order grating with a double slit configuration was found to offer the best compromise between electrical and optical properties of the top electrode. Even without collection optics employed, the devices show peak power up to 10 mW, with a slope efficiency of ~25mW/A, demonstrating the good radiative efficiency of this resonator and directionality of the emitted power. We observe single-mode emission with a side-mode suppression of more than 20 dB. We believe the current work will further simplify the integration of THz QCLs into possible applications.