Description
The demand for ultrafast, all-optical signal processing has pushed nonlinear photonics toward the extreme limits of light-matter interaction. While dynamically controlling macroscopic nonlinearities on the femtosecond timescale is highly desired for time-varying photonics, traditional integrated platforms are bottlenecked by perturbative physical mechanisms and the low optical damage thresholds of conventional nanoscale devices. Here, we report ultrafast spectral modulation of nonlinear optical signals in graphene. By utilizing a robust，polymer-free suspended-graphene platform that allows for both wide-range electrostatic gating and high optical damage thresholds, we observe dramatic frequency shifts (up to 8 THz) in third-harmonic generation (THG) and sum-frequency generation (SFG) driven by pump-induced nonequilibrium carrier dynamics. The magnitude and even the direction of this spectral shift can be reversibly controlled by the Fermi level and excitation conditions. A unified quasi-equilibrium theoretical framework based on hot-carrier dynamics quantitatively reproduces the measured spectral evolution, elucidating the critical interplay between carrier heating and the Fermi level. These findings establish a universal mechanism for carrier-mediated spectral control, providing a practical route toward high-speed, gate-tunable nonlinear photonic architectures.
