Gain-assisted optical tweezing of plasmonic and large refractive index microspheres

TL;DR

This paper theoretically demonstrates that optical gain in microspheres enhances optical tweezing capabilities, enabling trapping and manipulation of large refractive index and plasmonic particles that are otherwise difficult to trap.

Contribution

It introduces a gain-assisted optical tweezing method using Mie-Debye theory, allowing trapping and control of particles with high refractive index or plasmonic properties.

Findings

Gain enhances trapping forces for plasmonic and high-index particles.

Adjusting gain via pump power shifts the equilibrium position.

Gain-functionalized particles can be trapped where passive particles cannot.

Abstract

We have theoretically investigated optical tweezing of gain-functionalized microspheres using a highly focused single beam in the nonparaxial regime. We employ the Mie-Debye theory of optical tweezers to calculate the optical force acting on homogeneous and core-shell Mie microspheres with gain. We demonstrate that the optical gain plays a crucial role in optical manipulation, especially to optimize the restoring force and thus allowing for trapping of large refractive index and plasmonic particles. Indeed we demonstrate that one can trap such particles, which is usually not possible in the case of passive media, by functionalizing them with an optical gain material. We show that by varying the value of the gain, which can be realized by changing the pump power, one can not only achieve trapping but also manipulate the equilibrium position of the tweezed particle. Altogether our findings open new venues for gain-assisted optomechanics, where gain functionalized systems could facilitate optical trapping and manipulation of plasmonic nanoparticles in particular, with potential applications in self-assembling of nanoparticle suspensions and on a chip.