Complementing two-photon fluorescence detection with backscatter detection to decipher multiparticle dynamics inside a nonlinear laser trap

Using wide-field and point detection modalities, we show how optical trapping dynamics under femtosecond pulsed excitation can be explored by complementing detection of two-photon fluorescence with backscatter. Radial trajectories of trapped particles are mapped from correlated/anti-correlated fluctuations in backscatter pattern whereas temporal evolution of two-photon fluorescence is used to mark the onset of trapping involving multiple particles. Simultaneous confocal detection of backscatter and two-photon fluorescence estimates axial trap stiffness, delineating short-time trapping dynamics. When a second particle is being trapped an oscillatory signal is observed which is due to interference of backscatter amplitudes, revealing inter-particle interactions within the trap. These findings are crucial steps forward to achieve controlled manipulation by harnessing optical nonlinearity under femtosecond pulsed excitation.

Automated summary

By combining wide-field and point detection modalities, this study explores the dynamics of optical trapping under femtosecond pulsed excitation. The radial trajectories of trapped particles are mapped from backscatter fluctuations, while the temporal evolution of two-photon fluorescence marks the onset of trapping involving multiple particles. The simultaneous confocal detection of backscatter and two-photon fluorescence estimates axial trap stiffness, revealing short-time trapping dynamics and inter-particle interactions within the trap. These findings are crucial for achieving controlled manipulation using optical nonlinearity under femtosecond pulsed excitation.