Nonlinear flying focus pulses for ultrafast 3D nonlinear microscopy

Ultrafast 3D nonlinear multiphoton imaging holds great promise for the visualization of complex biological structures. However, its practical implementation remains constrained, primarily due to the limited longitudinal scanning speed achievable with tightly focused laser pulses within the sample. In this work, we propose a 3D nonlinear imaging concept that harnesses a spatiotemporal Kerr self-focusing process for depth-resolved imaging without mechanical scanning. We exploit Townes solitons to generate ultrafast nonlinear flying focus pulses that are well-suited for nonlinear fluorescence imaging. A key feature of this approach is the power-dependent longitudinal displacement of the self-focusing point within the sample. This displacement, combined with the power-limiting effects from conical-wave emissions at multiple wavelengths, enables precise control and calibration of the beam’s focal position in three dimensions. Consequently, by implementing a temporal pulse encoding, either by splitting and delaying an initial pump pulse or by introducing an inter-pulse amplitude modulation, we can induce multiple self-focusing events at different time bins. This allows us to probe multiple axial planes in a single laser shot. As a result, a full 3D image can be reconstructed from a single 2D transverse scan, significantly accelerating imaging speed. Additionally, the stable nonlinear propagation of these filaments leads to collinear supercontinuum generation in the form of self-guided light beams. Overall, our method presents a pathway for ultrafast, high-resolution 3D nonlinear imaging.