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The terahertz quantum-cascade (QC) VECSEL is a recently demonstrated approach to designing single-mode terahertz lasers based on the coupling of an amplifying reflect-array metasurface with an external optical cavity. The QC-VECSEL has demonstrated single-mode terahertz lasing with high output power and near-diffraction limited beam quality. The QC-VECSEL is also a natural candidate for demonstrating broadband, continuous, single-mode frequency tuning as the VECSEL’s lasing frequency is determined by the length of the its external cavity, which can be mechanically tuned. In this work, we use a piezoelectric translational stage to actively adjust the length of the QC-VECSEL’s external cavity and demonstrate >500 GHz of single-mode tuning around a center frequency of 3.5 THz (>20% fractional tuning). High-quality, circular beam patterns are observed with a divergence angle of ~15° throughout the tuning range, and tens of milliwatts of peak terahertz output power are observed. In order to maintain single mode behavior, the external cavity is made to be extremely short, increasing the spacing between the external cavity’s neighboring longitudinal resonances. Cavity lengths as short as 250 µm have been studied, but the free-spectral range of the external cavity could not be made larger than the gain bandwidth of the metasurface, providing testament to the bandwidth of both the metasurface and the QC-gain material.
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