Abstract
Optical frequency combs, consisting of well-controlled equidistant frequency lines, have been widely used in precision spectroscopy and metrology. Terahertz combs have been realized in quantum cascade lasers (QCLs) by employing either an active mode-locking or phase seeding technique, or a dispersion compensator mirror. However, it remains a challenge to achieve the passive comb formation in terahertz semiconductor lasers due to the insufficient nonlinearities of conventional saturable absorbers. Here, a passive terahertz frequency comb is demonstrated by coupling a multilayer graphene sample into a QCL compound cavity. The terahertz modes are self-stabilized with intermode beat note linewidths down to a record of 700 Hz and the comb operation of graphene-coupled QCLs is validated by on-chip dual-comb measurements. Furthermore, the optical pulse emitted from the graphene-coupled QCL is directly measured employing a terahertz pump-probe technique. The enhanced passive frequency comb operation is attributed to the saturable absorption behavior of the graphene-integrated saturable absorber mirror, as well as the dispersion compensation introduced by the graphene sample. The results provide a conceptually different graphene-based approach for passive comb formation in terahertz QCLs, opening up intriguing opportunities for fast and high-precision terahertz spectroscopy and nonlinear photonics.