Nonlinear flying focus pulses: Townes soliton for ultrafast 3D nonlinear microscopy

We experimentally demonstrate that Townes solitons generated in a Nd:YAG crystal enable the formation of ultrafast nonlinear flying focus pulses that can be exploited for nonlinear fluorescence imaging. The power dependent longitudinal shift of the self-focusing point along the crystal, accompanied by a power-limiting effect caused by conical-wave emission at different wavelengths, permits a precise spatial calibration of the beam profile. Consequently, by splitting and delaying an initial pump pulse or, equivalently, by introducing an inter-pulse amplitude modulation to implement a temporal pulse encoding, one can generate different self-focusing points at different time bins for the same pump pulse. This permits to analyze a biological sample at different depths in a single laser shot. Such a method avoids repeating multiple transverse scans while imaging at different depths. The nonlinear propagation of such stable filaments also leads to collinear supercontinuum generation in the form of self-guided polychromatic filaments. Our method paves the way for ultrafast 3D nonlinear multiphoton imaging.