Changing the length of the cavity is perhaps the simplest way to tune the wavelength of a laser, but is almost never used for continuous tuning over a large fractional range. This is because, to avoid multi-mode lasing and mode hopping, the cavity must be kept optically short to ensure a large free-spectral-range compared to the gain bandwidth of the amplifying material. The metasurface VECSEL architecture is shown to be an effective approach for widely tunable lasers based upon cavities that operate on low-order longitudinal modes. Since the gain resides in the amplifying reflectarray metasurface, and not a bulk medium, there is no gain/loss penalty to making the cavity length wavelength scale. Fractional tuning of a THz quantum-cascade laser up to 25% is observed in a multi-mode regime, and up to 19% in a single-mode regime with high quality beam pattern. We discuss the fundamental limits to broadband single-mode tuning using this approach.
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