Terahertz master-oscillator power-amplifier quantum Cascade laser with controllable polarization

We report the realization of controllable linear-to-circular polarization states in single-mode terahertz master-oscillator power-amplifier quantum cascade lasers (THz-MOPA-QCLs). The MOPA device contains a first-order distributed feedback (DFB) laser as the master-oscillator, a preamplifier, and a 2D periodical antenna array as the power extractor. The polarization state is determined by the orientation and the phase relationship between the antennas. The antenna array is carefully designed to efficiently extract the THz radiation and not to induce field oscillation in the array or influence the mode oscillation in the DFB section. Each demonstrated device exhibits single-mode emission with a side mode suppression ratio of ∼26 dB and a single-lobed beam with a low divergence of ∼23°×30°. Realized in different devices, the degree of linear or circular polarization reaches as high as 97.5% or 99.3%. Both the operation frequency and the polarization state of the radiation are lithographically tunable.