Changing the length of a laser cavity is a simple technique for continuously tuning the wavelength of a laser but is rarely used for broad fractional tuning, with a notable exception of the vertical-cavity surface-emitting laser (VCSEL)^1,2. This is because, to avoid 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. Terahertz quantum-cascade lasers are ideal candidates for such a short cavity scheme as they demonstrate exceptional gain bandwidths (up to octave spanning)³ and can be integrated with broadband amplifying metasurfaces⁴. We present such a quantum-cascade metasurface-based vertical-external-cavity surface-emitting laser (VECSEL) that exhibits over 20% continuous fractional tuning of a single laser mode. Such tuning is possible because the metasurface has subwavelength thickness, which allows lasing on low-order Fabry–Pérot cavity modes. Good beam quality and high output power are simultaneously obtained.

改变激光腔的长度是一种用于连续调谐激光波长的简单技术，但很少用于宽分数调谐，除了垂直腔表面发射激光器（VCSEL）^1,2明显例外。这是因为，为了避免模式跳变，与放大材料的增益带宽相比，必须使腔在光学上保持较短，以确保较大的自由光谱范围。太赫兹量子级联激光器是此类短腔方案的理想选择，因为它们具有超凡的增益带宽（高达倍频程）^3，并且可以与宽带放大超颖表面集成^4。。我们提出了这样一种基于量子级联超表面的垂直外腔面发射激光器（VECSEL），该激光器显示出单个激光器模式的连续分数调谐超过20％。这样的调谐是可能的，因为超颖表面具有亚波长厚度，这允许在低阶Fabry-Pérot腔模上发射激光。同时获得了良好的光束质量和高输出功率。