Planarized tapered waveguides for high-temperature THz quantum cascade laser frequency combs

We present a novel planarized double metal waveguide platform for high-performance THz quantum cascade laser frequency combs. The waveguides are embedded in a low-loss polymer (BCB) and have an extended top metallization, resulting in lower waveguide losses and improved RF and thermal properties. A further performance improvement is observed for waveguides with a tapered profile due to nonlinearity-enhancing effects. These free-running devices produce strong single RF beatnotes up to almost -30 dBm at 90 K, nearly three orders of magnitude stronger than for ridge devices. The improved comb performance is maintained for higher operating temperatures, with a 200 GHz comb bandwidth and a single RF beatnote above -60 dBm at a heat sink temperature of 115 K.