Anomalously enhanced transverse optical torque on a dipolar plasmonic nanoparticle in two-wave interference.

Area of Science:

• Nanophotonics and Plasmonics
• Optical Forces and Torques

Background:

• Optical torque (OT) is crucial for manipulating microparticles using light.
• Plasmonic nanoparticles offer unique light-matter interactions for optical manipulation.
• Understanding factors influencing optical torque is key for advanced applications.

Purpose of the Study:

• To investigate the anomalous enhancement of transverse optical torque on dipolar plasmonic nanoparticles.
• To explore the role of core-shell structures and multipole expansions in optical torque.
• To identify potential applications in optically driven microparticle rotation.

Main Methods:

• Theoretical analysis based on multipole expansion theory.
• Modeling of transverse optical torque on spherical nanoparticles in dual plane waves.
• Comparison between homogeneous gold (Au) and gold-silver (Au-Ag) core-shell nanoparticles.

Main Results:

• Anomalous enhancement of transverse optical torque was observed for dipolar plasmonic nanoparticles.
• Au-Ag core-shell nanoparticles with ultra-thin shells showed over two orders of magnitude greater OT than homogeneous Au nanoparticles.
• The enhanced torque is dominated by electric quadrupole excitation, not just dipole approximation.

Conclusions:

• The study reveals a significant enhancement mechanism for transverse optical torque in core-shell plasmonic nanoparticles.
• Electric quadrupole interactions play a dominant role in the enhanced optical torque, necessitating advanced theoretical approaches.
• Findings provide a deeper physical understanding of optical torque and suggest applications in optically driven rotation of plasmonic microparticles.