Demonstration of optical frequency combs in photonic crystal cavities

We report on the design and fabrication of high-Q silicon photonic crystal cavities operating at telecom wavelength exhibiting quasi-equally spaced resonant modes and diffraction limited mode volumes. The cavities design is based on an a bichromatic geometry, resulting in a parabolic profile of the effective confinement potential in analogy with the quantum-mechanical harmonic oscillator. This naturally leads to a nearly equal spacing of the set of resonant modes and Gaussian envelope of the field profile. The fabricated devices exhibit Q factors exceeding one million on multiple modes, as experimentally determined by resonant scattering measurements, and comb-like spectral features. The extremely large field enhancement achievable with these devices, together with the peculiar spectral properties represent a promising platform for scalable integrated nonlinear optical applications based on optical frequency combs.