Enhanced optical trapping by concentrated local fields in metallic semi-continuous films

Area of Science:

• Plasmonics
• Nanotechnology
• Optical physics

Background:

• On-chip optical trapping requires substrates with strong near-field forces and thermal stability.
• Metallic nanostructures are crucial for plasmonic optical trapping applications.

Purpose of the Study:

• To engineer metallic semi-continuous films (SCFs) as a transformative platform for high-efficiency optical manipulation.
• To investigate the relationship between film morphology and optical trapping performance.

Main Methods:

• Controlled sputter deposition to create gold films in nanoparticle (NP), semi-continuous film (SCF), and continuous film (CF) morphologies.
• Optical trapping experiments using 500-nm polystyrene spheres.
• Electromagnetic simulations and experimental trajectory analysis.

Main Results:

• SCFs achieved a peak trapping stiffness of 0.0955 ± 8.0 × 10^-4 pN/µm, a 16.9× and 6.2× enhancement over NP and CF substrates.
• Performance is attributed to sub-12-nm nanogaps in SCFs concentrating electromagnetic fields via coupled gap-plasmon modes.
• SCFs dissipate localized heating, mitigating thermal instability observed in CFs.

Conclusions:

• SCFs offer a superior platform for optical trapping by balancing field enhancement and thermal dissipation.
• The morphology-driven design principle enables scalable lab-on-chip optical manipulation systems.
• This work overcomes the trade-off between field enhancement and thermal stability in plasmonic optical trapping.