Self-localized ultrafast pencil beam for volumetric multiphoton imaging

The formation of organized optical states in multidimensional systems is crucial for understanding light-matter interaction and advancing light-shaping technologies. Here, we report the observation of a self-localized, ultrafast pencil beam near the critical power in a standard multimode fiber (MMF) and demonstrate its application in volumetric multiphoton imaging. We show that self-focusing in step-index MMFs, traditionally considered detrimental, can facilitate the formation of a nonlinear spatiotemporal localized state with a sidelobe-suppressed Bessel-like beam profile, exhibiting markedly improved stability and noise characteristics. By simply launching an overfilled on-axis Gaussian beam into a standard MMF, a high-quality ultrafast pencil beam can be generated through a self-localized process and readily integrated into an existing multiphoton point-scanning microscope. Moreover, in contrast to existing focus extension methods limited to low NA ($\le$0.7), this approach readily produces pencil beams compatible with high-NA (1.05) multiphoton imaging systems, exhibiting significantly reduced sidelobes and enhanced aberration robustness. Finally, we apply this self-localized pencil beam to two-photon imaging of intact mouse enteric nervous systems, benchmarking with diffraction-limited Gaussian beams and outperforming conventional Bessel beams. Our findings provide new insights into the nonlinear dynamics of multidimensional optical systems and offer a promising approach for generating high-quality ultrafast pencil beams for various beam-shaping applications, including microscopy.