Brownian motion of a Kerr nanobead in a single beam optical trap

Abstract

We investigate Brownian motion of a Kerr nanobead in a single beam optical trap. Kerr nanobeads refer to small beads whose refractive index is governed by n₂ = n₂⁽⁰⁾ + n₂⁽¹⁾E*E, where n₂⁽⁰⁾ is the linear component of the refractive index, n₂⁽¹⁾ is the nonlinear component, and E*E is the intensity of linearly-polarized laser beam. Thermal fluctuation of the surrounding medium (water in this case) of the optical trap is apparent when the nanobead radius is appreciably small in the nanometric range. Theoretical model of three dimensional motion of Kerr nanobead under water appropriates the use of Langevin equation to account for the Brownian force and radiation force on the Kerr nanobead. Diffusion of 30 nm-sized dielectric bead requires 3.1 k_bT (T = 300 K) of total energy to confine a Kerr nanobead within 0.4 μm from the central optical trap based from the probability density calculations.