Photophoretic forces: a new paradigm in optical trapping of absorbing particles in air

Abstract

Photophoretic forces [1] have opened a new vista in the optical trapping of absorbing mesoscopic particles in air, as these forces, having a thermal origin, are almost four orders of magnitude higher than optical radiation pressure or dipole forces, when acting on particles of the same size. Thus, these forces can easily balance inertial effects due to gravity, and also lead to spontaneous motion of the particles. However, extensive work on using such forces optimally has commenced only recently, and there is fair amount of work in the use of such forces to trap [2], controllably manipulate [3], or even rotate[4] particles in air using rather simple experimental configurations and without the use of tight focusing objective lenses typically warranted in optical gradient force trapping. However, there is still limited understanding of the nature of these forces, since there does not exist literature that quantifies the effects of these forces and observe their manifestations in the Brownian motion of trapped particles in comparison to the extensively studied problem of trapping using optical gradient forces. For this, we measure first the Brownian motion of a trapped cluster of absorbing particles and determine the determine the power spectral density, mean squared displacement, and normalized position and velocity autocorrelation functions in order to characterize the photophoretic body force in a quantitative fashion for the first time [5]. We also measure the power spectral density of the Brownian motion, from which we are able to measure the mass of the particle cluster to reasonable accuracy. Moreover, the large magnitude of these forces should also enable very simple means of trapping and manipulation, in order to motivate large scale applications of these forces. We thus develop a very simple optical fiber based setup where use a single fiber to generate Gaussian and Hermite-Gaussian (HG) beam modes, and use these to trap and manipulate single particles. The HG modes are seen to be more efficacious than the fundamental Gaussian mode in trapping and manipulation.