Differential optical trapping of nanoparticles with a single femtosecond laser beam

Abstract

Unlike conventional continuous-wave (CW) lasers, we theoretically demonstrate that a single femtosecond pulsed laser beam with a Gaussian intensity profile can simultaneously trap and distinguish nanoparticles—all possessing a refractive index higher than that of the surrounding medium—based on their differing nonlinear optical properties. Our model reveals the formation of three discrete trapping sites: one at the focal center and two symmetrically positioned off-center, enabling simultaneous multi-site trapping using a single, tightly focused Gaussian beam. We refer to this phenomenon as “differential trapping”. This differential trapping is governed by key system parameters such as laser power, particle number density, pulse repetition rate, pulse width, numerical aperture of the objective, and beam polarization. Notably, the polarization direction influences the spatial alignment of the trapped nanoparticles. This mechanism offers promising potential for non-contact, non-invasive micromanipulation and selective sorting of nanoparticles based solely on their intrinsic optical nonlinearities.