Stability and instability for low refractive-index-contrast particle trapping in a dual-beam optical trap.

Alison Huff,Linda S Hirst,Jay E Sharping,Charles N Melton

doi:10.1364/boe.6.003812

Abstract

A dual-beam optical trap is used to trap and manipulate dielectric particles. When the refractive index of these particles is comparable to that of the surrounding medium, equilibrium trapping locations within the system shift from stable to unstable depending on fiber separation and particle size. This is due to to the relationship between gradient and scattering forces. We experimentally and computationally study the transitions between stable and unstable trapping of poly(methyl methacrylate) beads for a range of parameters relevant to experimental setups involving giant unilamellar vesicles. We present stability maps for various fiber separations and particle sizes, and find that careful attention to particle size and configuration is necessary to obtain reproducible quantitative results for soft matter stretching experiments.

Highlights

We present experimental data along with simulation results illustrating that the landscape for giant unilamellar vesicles (GUVs) trapping will shift between stable and unstable behavior for relevant refractive indices, fiber separations and particle sizes
By understanding trapping characteristics at low refractive index contrasts, we can increase the effectiveness of our GUV trapping apparatus and more accurately quantify the forces present within the trap
Our simulations and experimental data show a shift between stable and unstable trapping behavior for some fiber separations and particle sizes which is due to the interplay between net gradient and net scattering forces in the axial direction

Summary

Ray r Refracted

A light beam interacts with a particle made from an optically denser material than its environment. In the case where the refractive index ratio between the particle and medium is relatively large (nparticle/nmedium > 1.01) and the particle size satisfies the criteria for a Mie scattering treatment, 2πR/λ 1 [12], the particle will trap halfway between the two fibers in the dual beam optical trap In this case, the scattering forces are dominant along the beam axis. We have observed that this trap exhibits unusual trapping behaviors when the refractive indices between the dielectric particle and the surrounding liquid are nearly equal (nparticle/nmedium < 1.01) This occurs because Fs decreases more rapidly than Fg as a function of the refractive index contrast and eventually Fg becomes dominant in the axial direction as well. By understanding trapping characteristics at low refractive index contrasts, we can increase the effectiveness of our GUV trapping apparatus and more accurately quantify the forces present within the trap

Epoxy Trapping Space

Findings

Conclusions