Terahertz quantum cascade lasers with a record output power up to ∼0.23 W in continuous wave mode were obtained. We show that the optimal 2.9-mm-long device operating at 3.11 THz has a low threshold current density of 270 A/cm2 at ∼15 K. The maximum operating temperature arrived at ∼65 K in continuous wave mode and the internal quantum efficiencies decreased from 0.53 to 0.19 for the devices with different cavity lengths. By using one convex lens with the effective focal length of 13 mm, the beam profile was collimated to be a quasi Gaussian distribution.

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We demonstrate the highly efficient power extraction from terahertz quantum cascade laser (THz-QCL) via a grating coupler (GC). The GC, formed in the top metallization of a Fabry-Perot (FP) THz-QCL with a metal-metal (MM) waveguide, diffracts the THz wave into the free space when it oscillates between the two cleaved facets. The radiation loss can be tailored in a wide range by changing the structure of the GC. The asymmetric positioning of the GC enables near-unidirectional emission, the enlarged emission surface reduces the beam divergence, and the MM waveguide keeps the optical confinement factor near unity. Experimental results confirm that, when the operating frequency is near 2.7 THz, the laser with a GC shows superior performances than the conventional FP lasers in terms of output power and operation temperature.