Crossover from positive to negative optical torque in mesoscale optical matter

Area of Science:

• Optics and Photonics
• Nanotechnology
• Materials Science

Background:

• Circularly polarized light transfers spin angular momentum to micro- and nanostructures, typically inducing positive optical torque.
• The handedness of the optical torque usually matches the spin angular momentum of the photons.

Purpose of the Study:

• To investigate the possibility of achieving negative optical torque in mesoscopic arrays of metal nanoparticles.
• To explore the influence of particle arrangement and interactions on optical torque.
• To demonstrate the reconfigurability of optical torque in engineered nanoparticle systems.

Main Methods:

• Assembling metal nanoparticles (NPs) into mesoscopic arrays within circularly polarized optical traps.
• Utilizing electrodynamics simulations to clarify the underlying physical mechanisms.
• Analyzing the crossover from positive to negative optical torque based on array parameters.

Main Results:

• Demonstrated that negative optical torque can be achieved in mesoscopic optical matter arrays of metal nanoparticles.
• Observed a crossover from positive to negative optical torque influenced by the number, separation, and configuration of nanoparticles.
• Identified many-body interactions as the key factor responsible for the observed torque reversal, as confirmed by simulations.

Conclusions:

• Both positive and negative optical torque can be readily controlled in optical matter arrays.
• The reconfigurable nature of these arrays enables the creation of programmable materials.
• Potential applications include optomechanics, microrheology, and biological studies.