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

On-chip optical trapping necessitates substrates that generate strong near-field forces while mitigating thermal instability. Here, metallic semi-continuous films are established as a transformative platform for high-efficiency optical manipulation. Through controlled sputter deposition, we engineer gold films across three morphological regimes: discontinuous nanoparticles (NPs), semi-continuous films (SCFs), and continuous films (CFs). Optical trapping experiments with 500-nm polystyrene spheres reveal that SCF substrates achieve a peak stiffness of 0.0955 ± 8.0 × 10^{- 4} pN/µm, representing 16.9× and 6.2× enhancements over NP and CF substrates, respectively. The performance arises from sub-12-nm nanogaps within percolated gold networks, which concentrates electromagnetic fields via coupled gap-plasmon modes, intensifying optical gradient forces. Concurrently, SCFs' interconnected pathways dissipate localized heating that destabilizes trapping in CFs. Electromagnetic simulations and experimental trajectory analysis confirm that SCFs balance field enhancement and thermal effect, overcoming the persistent trade-off in plasmonic optical trapping. This work provides a morphology-driven design principle for scalable lab-on-chip optical manipulation systems.