Quantum cascade laser frequency comb at λ ≃ 5.3 µm via a double-plasmon waveguide architecture

We demonstrate what we believe to be a novel method for dispersion compensation in short-wavelength quantum cascade lasers (QCLs) for frequency comb operations. The addition of two highly doped InP plasmon layers below and above the active region, and the precise control of the distance between them and their dimensions, enables the engineering of waveguide structures with small group velocity dispersion around any central wavelength between 4 and 6 µm, while keeping the optical losses low. Based on this design, we show a highly-efficient frequency comb operation at 5.3 µm that exhibits up to 350 mW of output power, and operates in coherent comb mode up to 50°C. This result paves the way for robust fabrication of efficient, short-wavelength QCL frequency combs.