We describe the fabrication and operation of vertically emitting distributed feedback
quantum cascade ring lasers operating in the terahertz frequency range. A twofold increase
in radiation efficiency is observed as compared to Fabry–Pérot lasers. The emitters exhibit a
robust single-mode operation around 3.2 THz with a side mode suppression ratio higher
than 30 dB. Modal and threshold characteristics are investigated by performing finite
element simulations with results in good agreement with experiments. The ring-shaped …