Femtosecond low-threshold all-optical switching enabled by giant broadband optical nonlinearity from heteroatom doping

Ultrafast all-optical switching (AOS) is pivotal for advancing integrated photonic devices, from high-speed photonic information processing to next generation all-optical computing and communication networks. However, conventional nonlinear materials suffer from sluggish response time, high power threshold, weak and narrow-bandwidth optical nonlinearities, critically limiting their viability. Here, we report a heteroatom engineering strategy to overcome these limitations by designing zero-dimensional nitrogen-doped carbon quantum dots (N-CQDs) with nonlinear optical performance far exceeding the state-of-the-art. Leveraging spatial self-phase modulation (SSPM) and ultrafast pump-probe technique, we first demonstrate an all-in-one AOS platform, where femtosecond laser pulses serve dual roles as control and signal beams. The AOS simultaneously realizes ultrafast response time (520 fs), ultralow threshold energy (2.2 Wcm-2), and giant nonlinear refraction indexes (10-5 cm2/W) in the wide spectral range (400-1064 nm), yielding performance surpassing state-of-the-art nonlinear carbon materials (i.e. carbon nanotube) by orders of magnitude. Spectroscopic and bandgap analyses attribute these exotic performances to enhanced n-pi interaction enabled by nitrogen doping, which amplifies nonlinear polarization dynamics. Crucially, ultrafast fluorescence spectroscopy reveals a large two-photon absorption cross-section of the N-CQDs, challenging the conventional cognition that broadband SSPM necessitates single-photon excitation. This discovery unveils a multi-channel AOS rooted in synergistic single-photon and two-photon processes.. This work demonstrates a new paradigm for achieving ultrafast, broadband, and energy-efficient AOS by heteroatom doping engineering.