Dissipative quadratic soliton mode-locking of nonlinear frequency conversion

TL;DR

This paper introduces a novel dissipative quadratic soliton mode-locking technique in a CW-pumped second-harmonic generation cavity, enabling femtosecond pulse formation without dispersion engineering or complex synchronization.

Contribution

It demonstrates a new paradigm for nonlinear frequency conversion using engineered Kerr-like nonlinearity in a simple cavity setup, bypassing traditional complex systems.

Findings

Spontaneous formation of bichromatic femtosecond solitons spanning visible and near-infrared wavelengths

Achieved transform-limited pulses with durations around 274-279 fs

Engineered nonlinearity exceeds intrinsic Kerr response by over three orders of magnitude

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. Here, we report a fundamentally different paradigm: dissipative quadratic soliton (DQS) mode-locking in a continuous-wave (CW)-pumped, doubly resonant second-harmonic generation cavity. By leveraging a cascaded quadratic nonlinear process, we realize an effective Kerr nonlinearity (EKN) that exceeds the intrinsic material Kerr response by over three orders of magnitude and is tunable in both magnitude and sign via pump detuning. This engineered nonlinearity enables femtosecond DQS formation in a free-space lithium niobate cavity with normal dispersion, without dispersion engineering or synchronization electronics. Numerical simulations predict distinct dynamical regimes depending on phase detuning, and experiments confirm the spontaneous emergence of bichromatic femtosecond solitons spanning visible and near-infrared wavelengths. The observed DQSs exhibit spectral 3 dB 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. Our results establish a versatile platform for efficient and broadband nonlinear frequency conversion and frequency comb generation based on quadratic nonlinearities, with significant implications for scalable ultrafast and nonlinear photonics applications.