Direct observation of the effects of spin dependent momentum of light in optical tweezers

Debapriya Pal,Subhasish Dutta Gupta,Ayan Banerjee,Nirmalya Ghosh

doi:10.1063/5.0015991

Abstract

We demonstrate that tight focusing of a circularly polarized Gaussian beam in optical tweezers leads to spin-momentum locking—with the transverse momentum density (Poynting vector) being helicity-dependent, while the transverse spin angular momentum density becomes independent of helicity. We further use a stratified medium in the path of the trapping beam in our optical tweezers setup to enhance the magnitude of the transverse momentum and the electric field intensity in the radial direction with respect to the beam axis and cause them to significantly overlap. This overlap allows us to experimentally observe the circular motion of a birefringent particle, trapped off-axis, in response to an input circularly polarized fundamental Gaussian beam carrying no intrinsic orbital angular momentum (OAM). The circular motion is dependent on the helicity of the input beam so that we can identify it as the signature of the elusive Belinfante spin in propagating light beams obtained in our optical tweezers configuration. Our results can be extended to beams carrying intrinsic OAM leading to simple routes for achieving complex manipulation of micro-machines or other mesoscopic matter using optical tweezers.

Highlights

Light carries both orbital and spin angular momentum
Po is responsible for generating orbital angular momentum (OAM) l (l = r × Po, where r is the distance from the beam axis) that is directly manifested in experiments by the rotation of mesoscopic particles about the beam axis in optical tweezers
When we use a coverslip with refractive index (RI) 1.814 for the sample chamber of the optical tweezers, we observe in our simulations that the transverse extent of both the TM and field intensity increases with the axial distance from the beam focus

Summary

INTRODUCTION

Light carries both orbital and spin angular momentum. The Poynting vector—considered to be the vector representative of the flow of energy—has contributions from both the canonical and spin part of the momentum. It has recently been observed that a longitudinal component of the field—phase-shifted with respect to the transverse component— plays a major role in the appearance of spin (polarization) dependent transverse momentum and spin (polarization) independent transverse spin angular momentum (TSAM).2,5–9 This particular feature is well known as spin momentum locking in condensed matter physics in the context of topological insulators, where special states exist at the outermost surface of the insulator, which falls within the bulk energy gap and permits surface metallic conduction. We proceed to verify the simulation results experimentally by observing the spin-dependent rotational motion of a birefringent particle around the beam axis for input circularly polarized light propagating through a stratified medium and carrying no intrinsic orbital angular momentum. The circular motion is dependent on the helicity of the input beam so that we can identify it to be the signature of the elusive Belinfante spin, the manifestations of which—to the best of our knowledge—has not been previously demonstrated for the propagating light

THEORY

NUMERICAL SIMULATIONS

EXPERIMENT

CONCLUSIONS