Characterising Conical Refraction Optical Tweezers

TL;DR

This paper investigates the trapping characteristics of conical refraction optical tweezers, analyzing trap stiffness and particle behavior in different regions created by conical refraction through biaxial crystals.

Contribution

It provides a detailed quantification of trap stiffness and particle dynamics in conical refraction optical tweezers, highlighting differences between Raman spots and Lloyd/Poggendorff rings.

Findings

Lower Raman spot acts as a gradient force trap.

Upper Raman spot causes optical levitation due to radiation pressure.

Lloyd/Poggendorff rings offer rotational control with lower trap stiffness.

Abstract

Conical refraction occurs when a beam of light travels through an appropriately cut biaxial crystal. By focussing the conically refracted beam through a high numerical aperture microscope objective, conical refraction optical tweezers can be created, allowing for particle manipulation in both Raman spots and in the Lloyd/Poggendorff rings. We present a thorough quantification of the trapping properties of such a beam, focussing on the trap stiffness and how this varies with trap power and trapped particle location. We show that the lower Raman spot can be thought of as a single-beam optical gradient force trap, while radiation pressure dominates in the upper Raman spot, leading to optical levitation rather than trapping. Particles in the Lloyd/Poggendorff rings experience a lower trap stiffness than particles in the lower Raman spot but benefit from rotational control.