Comparison of optical trapping between gold and silver nanostructures
Date
2024
Authors
Haddad, Layla
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Abstract
This thesis explores the optical trapping capabilities of silver (Ag) and gold (Au) nanostructures. Double nanohole (DNH) apertures with a gap size of ≈ 32 nm were fabricated in each nanostructure to trap 20 nm polystyrene (PS) nanospheres. The surfaces of each metal were either modified using an aqueous solution of 5 mM monodisperse thiol polyethylene glycol (mPEG) (the “experiment” group), or immersed in deionized (DI) water only (the “control” group). A solution containing the PS nanospheres was then transferred and confined to a chamber spacer attached to the modified surface (for the “experiment” group) or the unmodified surface (for the “control” group). To evaluate the trapping performance of each group, the trapped data was acquired as a function of time. The power spectrum density (PSD) of the trapping signal was analyzed using the Brownian motion theory of trapped beads, and the data was fitted with a Lorentzian function to extract the corner frequency and time constant. The trap stiffness (force constant) of the optical trap was also determined.
The results indicated that increasing the trapping power led to higher trap stiffness for silver DNHs compared to gold in the DI water environment, suggesting a stronger force holding the nanoparticles in the trap. The surrounding medium significantly impacted the trapping performance, with the mPEG-thiol monolayer reducing the trap stiffness for both silver and gold DNHs. Additionally, a linear dependence of trap stiffness and time constant on laser power was observed, confirming that increasing power enhances trapping strength and reduces the time for the nanoparticle to fall into the trap. This research provides nuanced insights into the comparative effectiveness of silver and gold nanostructures in optical trapping and demonstrates the influence of the surrounding medium on trapping performance. These results provide pathways for future advancements in aperture-based trapping.