Transverse optical torque from the magnetic spin angular momentum.

Area of Science:

• Optics
• Nanophotonics
• Optical Mechanics

Background:

• Optical forces and torques are crucial for manipulating micro/nanoparticles.
• The interplay between electric and magnetic responses dictates particle behavior under light.
• General understanding suggests electric response dominates for dielectric particles, especially in the Rayleigh limit.

Purpose of the Study:

• To investigate the origin of transverse optical torque on spherical particles.
• To derive an analytical expression for transverse optical torque applicable to particles of arbitrary size.
• To challenge conventional understanding of optical forces in dielectric particles.

Main Methods:

• Full-wave electromagnetic simulations.
• Derivation of an analytical expression for transverse optical torque.
• Analysis of optical torque contributions from different physical mechanisms.

Main Results:

• A transverse optical torque is demonstrated on isotropic spherical particles, perpendicular to wave propagation.
• The transverse optical torque is exclusively attributed to the magnetic component of spin angular momentum.
• For non-magnetic dielectric particles, even in the sub-wavelength regime, the magnetic response is the dominant source of transverse optical torque.

Conclusions:

• The magnetic response plays a critical, often underestimated, role in optical torques, even for non-magnetic dielectric particles.
• This finding necessitates a re-evaluation of light-matter interactions in optical manipulation, particularly concerning spin angular momentum transfer.
• The derived analytical expression provides a versatile tool for understanding transverse optical torques across various particle sizes.