Single-mode operation of terahertz quantum cascade lasers with distributed feedback resonators

Distributed feedback terahertz quantum-cascade lasers emitting at 4.34 and 4.43 THz are presented. Mode selection is based on a complex-coupling scheme implemented into the top-contact layer by a combination of wet chemical etching and ohmic-contact deposition. Single-mode emission stable at all injection currents and operating temperatures is shown, with a side-mode suppression ratio exceeding 20 dB. Peak output powers of up to 1.8 mW are obtained at low temperatures.