Enhancing trap stiffness and Raman scattering in optical traps

Abstract

Optical tweezers have found applications in basic physics, medicine, biology, and nanotechnology. For a small particle in a Gaussian laser beam, the trapping force arises as a competition between gradient forces pulling the particle to the beam focus and scattering forces pushing the particle along the optical axis. Balancing these competing forces means that the optimal behaviour on a trapped particle occurs when the radius of the particle is approximately the beam waist of the trapping light.

In this work, I explore the application of a short wavelength laser into an optical trap to reduce the beam waist and obtain a substantial improvement in the stiffness per unit of power for subwavelength silica particles. I then harness the high optical scattering cross section of the particles at short wavelengths to realise a Raman optical tweezers setup for improved micro-plastic detection.

For very small particles, one cannot continue to move to shorter wavelength lasers. To generate sufficient trapping forces, one needs to overcome the diffraction limit and confine light even more tightly. Here, my collaborators and I exploit a Fano resonance effect in an all-dielectric quadrupole nanostructure to realize tight light confinement, enabling a high effective trap stiffness of 1.19fN/nm for a 100nm diameter polystyrene nanoparticle with 4mW/μm2 illumination. We furthermore show the capability of our nanostructure to trap two particles at different locations within the array.

Date of Award	2 Dec 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Kishan Dholakia (Supervisor) & Graham Bruce (Supervisor)