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Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of$50-<\#comment/>170\mathrm{m}\mathrm{W}$are overlapped in the far field at frequencies ranging from$3.31-<\#comment/>3.54\mathrm{T}\mathrm{H}\mathrm{z}$. Collinear propagation with a maximum angular deviation of${0.3}^{\circ <\#comment/>}$is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as$25\mathrm{\%<\#comment/>}$. This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.

Plasmonic lasers suffer from low output power and divergent beams due to their subwavelength metallic cavities. We developed a phase-locking scheme for such lasers to significantly enhance their radiative efficiency and beam quality. An array of metallic microcavities is longitudinally coupled through traveling plasmon waves, which leads to radiation in a single spectral mode and a diffraction limited single-lobed beam in the surface normal direction. We implemented our scheme for terahertz plasmonic quantum-cascade lasers (QCLs) and measured peak output power in excess of$2\mathrm{W}$for a single-mode$3.3\mathrm{T}\mathrm{H}\mathrm{z}$QCL radiating in a narrow single-lobed beam, when operated at$58\mathrm{K}$in a compact Stirling cooler. We thereby demonstrated an order of magnitude increase in power and thirty-times higher average intensity for monolithic single-mode terahertz QCLs compared to prior work. The number of photons radiated from the cavity outnumber those absorbed within its claddings and semiconductor medium, which constitutes$><\#comment/>50\mathrm{\%<\#comment/>}$radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.