Topologically protected optical pulling force on synthetic particles through photonic nanojet

Area of Science:

• Optics and Photonics
• Soft Matter Physics
• Nanotechnology

Background:

• Dielectric microspheres generate photonic nanojets (PNJs) that mediate forces, enabling them to move towards near-infrared lasers.
• Janus particles, with asymmetric compositions, were previously thought incapable of forming stable PNJs due to opaque layers.
• Optical torques can arise in Janus particles due to their inhomogeneous structure, even under non-resonant, linearly polarized light.

Purpose of the Study:

• To investigate the formation of topologically protected photonic nanojets (PNJs) by synthetic Janus particles.
• To observe and analyze the backaction optical forces exerted on these Janus particles.
• To explore the potential applications of PNJ-mediated forces for particle manipulation.

Main Methods:

• Fabrication of synthetic Janus particles with specific material compositions.
• Illumination of Janus particles with counter-propagating laser beams.
• Optical microscopy and force measurements to analyze PNJ formation and particle movement.
• Investigation of power-dependent optical forces and hysteresis effects.

Main Results:

• Demonstrated the generation of topologically protected PNJs from synthetic Janus particles.
• Observed that counter-propagating beams can form PNJs on opposite sides, attracting the Janus particle towards each source.
• Identified hysteresis in the backaction force with respect to laser power, similar to dielectric counterparts.
• Noted variations in backaction force magnitude during power increase and decrease cycles.

Conclusions:

• Synthetic Janus particles can generate stable, topologically protected PNJs.
• PNJ-mediated forces can be utilized for directed particle manipulation, including pulling irregular particles.
• The observed hysteresis and power-dependent forces offer avenues for advanced particle sorting and classification applications.