Widely Tunable Femtosecond Sources with Continuously Tailorable Bandwidth Enabled by Self-Phase Modulation

Generating ultrafast pulses with better spectrotemporal control is crucial for optimizing and characterizing nonlinear light-matter responses, yet it is limited by the gain bandwidth of laser media or the phase-matching geometry of nonlinear processes. This work proposes a simple approach to independently manage a femtosecond source's spectral location and bandwidth. Self-phase-modulation-enabled spectral broadening is first analyzed, which is potentially energy-scalable using hollow-core capillaries or multipass cells. It is demonstrated that the outmost lobes in the broadened spectrum show different dependencies on the initial pulse energy and duration. A simple yet effective toy model is introduced that successfully predicts broadband spectral tuning, and the impact of other nonlinear effects, dispersion, and input pulse asymmetry on the experimental scenario is also discussed. Thus a fiber-based versatile source is demonstrated, which is compressible down to its transform-limit duration, as short as 12.2 fs centered at 920 nm. In addition, bandwidth-dependent third-harmonic generation spectroscopy is performed from a dielectric metasurface with an optimized nonlinear response, and the dependency of laser bandwidth and pulse duration is investigated on the signal-to-background ratio of two-photon images. It is believed that this demonstration will advance the investigation of bandwidth-dependent nonlinear spectroscopy and microscopy.