Light-field amplitude control for multi-photon fluorescence imaging

Laser-induced fluorescence has been a staple technique for in-situ probing of radicals and atomic species for decades. The multi-photon equivalent is prevalent in a large variety of applications, where such excitation schemes enable the study of otherwise elusive species and systems. However, these variants inherently suffer from low signal intensities, impeding the broader use of multi-photon imaging. While many novel ultrafast approaches have been utilizing the temporal characteristics through pulse shaping to optimize experiments, we present Light-field Amplitude Control - a strategy that shapes the spatial distribution of the light-field energy through constructive interference. This local field enhancement yields a non-linear amplification of the signal response that amplifies the fluorescence signal generation. In addition, the constructive interference used in this work allows for multi-order Lock-in analysis, a strategy that yields a further signal-to-noise amplification. The combination of the two approaches opens up for two dimensional two-photon laser-induced fluorescence wide-field imaging of atomic distributions with excellent signal to noise ratios. We believe this new methodology for excitation, detection, and analysis of multi-photon processes will enable the imaging of transient and stochastic processes within ultra-fast sciences and applications.