Ultrafast photonics underpins transformative technologies in remote sensing, precision metrology, biomedical imaging, ultrafast spectroscopy, and quantum information science, fundamentally reshaping how we observe and interact with the world. Extending these capabilities into the visible and ultraviolet has increasingly relied on nonlinear frequency conversion from mature wavelength bands such as telecommunications. However, this typically demands synchronously pumped, dispersion-engineered nonlinear cavities, substantially increasing system complexity and cost while often compromising efficiency—limitations that hinder scalability and real-world deployment.
In a recent paper published in eLight , a team led by Professor Shu-Wei Huang at the University of Colorado Boulder reports a fundamentally new approach to ultrafast nonlinear frequency conversion. By harnessing dissipative quadratic soliton (DQS) physics, the researchers generate bichromatic femtosecond pulse trains simultaneously at the fundamental and second-harmonic frequencies within a single quadratic nonlinear cavity driven by a continuous-wave laser. While DQSs have been extensively explored theoretically, this work constitutes the first experimental demonstration of femtosecond DQSs under continuous-wave pumping. The results mark a paradigm shift in femtosecond nonlinear frequency conversion, extending soliton-based technologies to diverse cavity platforms and previously inaccessible wavelengths, and opening new opportunities across a broad range of scientific and technological applications.
The researchers describe the novelty of their approach as follows:
“The core innovation lies in exploiting phase-matched, group-velocity-matched cascaded quadratic nonlinearities within a doubly resonant cavity, which enables precise control of the effective Kerr nonlinearity (EKN) via pump phase detuning and yields an in-situ tunable EKN more than three orders of magnitude stronger than the intrinsic material response. This represents a fundamental departure from prior DQS studies, where the EKN was assumed to arise from phase mismatch or group-velocity mismatch. As a result, our approach is markedly more efficient, enabling dissipative soliton formation in free-space cavities without relying on strongly confining platforms such as fibers or microresonators, while allowing scalability in both power and energy. Crucially, the in-situ EKN tunability enables compensation of both normal and anomalous dispersion, opening a broad design space for universal dissipative soliton formation across diverse platforms.”
“Our results demonstrate a simple, flexible, and scalable approach to nonlinear frequency conversion without the need for synchronized femtosecond mode-locked lasers,” they added. “Moreover, in-situ engineering of the effective Kerr nonlinearity introduces a powerful new degree of freedom in cavity nonlinear optics, with implications extending beyond frequency conversion and comb generation to applications such as quantum entanglement sources, photonic sensing, and programmable nonlinear photonic circuits for artificial intelligence.”
eLight
Femtosecond dissipative quadratic soliton mode-locking of cavity-enhance second harmonic generation