Regular combs of modes in compact cavities based on slow-light dispersion engineering in active III-V planar photonic crystals

We experimentally validate the use of slow light dispersion engineering in active line-defect photonic crystal cavities to generate regular combs of modes around a wavelength of 1.55 µm. Our far-field and near-field optical spectroscopy measurements highlight that 11 frequency equidistant modes could be sustained over a bandwidth of 8.5 nm in a 45 µm-long cavity. The unprecedented use of slow light dispersion engineering with nearly constant group velocities between c/30 and c/40 in an active medium enables an 8-fold cavity length reduction compared to standard strip waveguides. We demonstrate that the equidistant comb of modes is related to the underlying dispersion-engineered slow-light guided mode, and we analyze its robustness against structural fluctuations due to fabrication. This paves the way towards the creation of compact and integrated mode-locked lasers.