Metrological-grade terahertz frequency combs

Metrological-grade frequency comb (FC) synthesizers are the ideal sources for quantum applications. At terahertz (THz) frequencies, electrically pumped quantum cascade lasers (QCLs) have shown quantum-limited frequency noise operation, phase/frequency absolute referencing and self-starting FC operation, albeit over a rather restricted dynamic range. Here, we conceive a technological approach to achieve FC operation over the entire available gain bandwidth and across almost the whole lasing regime at THz frequencies, by increasing the mirror losses of its Fabry-Perot cavity through coating the back facet with an epitaxially-grown multilayer graphene film.