Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs.

We demonstrate dispersion compensation in mid-infrared quantum cascade laser frequency combs (FCs) emitting at 7.8 μm using the coupling of a dielectric waveguide to a plasmonic resonance in the top cladding layer of the latter. Devices with group velocity dispersion lower than 110  fs/mm were fabricated, and narrow beatnotes with FWHM linewidths below 1 kHz were measured on the entire operation range. At -20°C, the optical output power reaches 275 mW, and the optical spectrum spans 60  cm. The multi-heterodyne beating spectrum of two devices was measured and spans 46  cm, demonstrating the potential of dispersion-engineered waveguides for the fabrication of highly stable and reliable quantum cascade laser FCs with high output power across the mid-infrared.