Femtosecond dissipative quadratic soliton mode-locking of cavity-enhanced second-harmonic generation

Abstract Nonlinear frequency conversion underpins numerous classical and quantum photonics applications but conventionally relies on synchronized femtosecond mode-locked lasers and dispersion-engineered enhancement cavities—an approach that imposes substantial system complexity. To address the challenges, here we report a fundamentally different paradigm: mode-locking of nonlinear frequency conversion enabled by the physics of dissipative quadratic soliton (DQS). We present the operating principle of femtosecond DQS mode-locking and experimentally validate it for the first time in a continuous-wave-pumped doubly resonant second-harmonic generator in free space, yielding bichromatic frequency combs spanning the visible and near-infrared. The observed DQSs exhibit 3 dB optical bandwidths and transform-limited pulse durations of 1.15 THz and 274 fs for the pump and 1.13 THz and 279 fs for the second harmonic. By harnessing phase-matched group-velocity-matched cascaded quadratic nonlinearities, we demonstrate an in-situ tunable effective Kerr nonlinearity that exceeds the intrinsic material response by over three orders of magnitude, enabling femtosecond DQS generation in both free-space and chip-scale cavities across normal and anomalous dispersion regimes. Our results establish a simple, flexible, and scalable approach to nonlinear frequency conversion without the need for synchronized femtosecond mode-locked lasers and expand the reach of soliton-based technologies across diverse cavity platforms and a wide range of challenging wavelengths that are otherwise inaccessible.