Ultrabroadband air-dielectric double-chirped mirrors for laser frequency combs.

Dispersion engineering is critical for the creation of integrated broadband laser frequency combs. In the long wavelength infrared range (LWIR, 8-13 µm), frequency combs based on quantum cascade lasers are attractive since they are monolithic, fundamental oscillators with high power levels and efficiencies. One effective approach for expanding quantum cascade laser gain bandwidth is by stacking multiple gain media with different center lasing frequencies, as this leads to flatter broadband gain spectra. However, as the gain bandwidth is increased, dispersion becomes the main limiting factor for comb bandwidth. Therefore, achieving broadband combs requires schemes that can flexibly engineer the dispersion over broad bandwidths. Here, we demonstrate the ultimate nanophotonic dispersion compensation scheme: an air-dielectric slab double-chirped mirror, which we fully integrate with the quantum cascade laser gain section. This scheme relies on the highest possible index contrast and therefore provides the maximum correction per unit length over a very broad bandwidth. With this approach, we report the successful demonstration of a broadband room-temperature LWIR laser frequency comb on a gain medium that normally does not form combs without deliberate dispersion compensations. Our air-dielectric mirrors are versatile and can be extended to other integrated laser frequency combs in different material platforms and frequency bands.