Characterizing colloidal clusters with femtosecond optical tweezers

We present a method to characterize the cluster formed within an optical trap using two-photon fluorescence (TPF) signals from trapped polystyrene beads. We employed the power spectrum method to analyze the TPF signal based on the Brownian motion of a spherical particle trapped under a harmonic potential. Based on our work, we can assess the homogeneity of clusters by examining the corner frequencies of single- and double-particle traps. The presented method can be readily extended and used to characterize clusters formed by nanometer-sized particles of unknown size.