This work shows elliptical nanoholes on an extended gold film are a simple yet effective achiral platform for chiroptical analysis based on linearly polarized light. A well-designed elliptical nanohole can simultaneously generate chiral near-field for chiroptical analysis and act as a nano-optical trap to capture dielectric and plasmonic nanospheres. The figure shows two distinct optical potentials (Uxy) landscapes of the same elliptical hole upon illumination with different linear polarizations. The in-plane optical force (Fx and Fy) exerted on a polystyrene nanoparticle can trap or repel the particle according to its chirality (κ). This makes the platform powerful for enantioselective optical trapping. Using this achiral platform with linearly polarized illumination, false chiroptical signals due to nanostructures can be eliminated. Moreover, the compatibility of the platform with typical optical microscopes is greatly improved because the problems due to the distortion of circularly polarized light by the optics of a microscope are avoided. The platform is ideal for sensitive chiroptical analysis in combination with nanoparticles-based solid-state extraction and pre-concentration, which further enhances chemical selectivity and sensitivity.

Funded by: 
Deutsche Forschungsgemeinschaft,
Ministry of Science and Technology of Taiwan 

In the image above:
Plasmonic optical trap by elliptical nanohole. (a) Schematic of the elliptical nanohole on the gold film. (b, c) Trapping potential contour maps of the in-plane optical force (𝐹𝑥 and 𝐹𝑦) exerted on a chiral PS nanosphere (diameter = 20 nm). The chirality parameter κ is ±1. The red and blue traces on the x-z and y-z projection planes are the cross-sectional potential cut by y = 0 and x = 0 planes. The black dashed lines on the x-y projection planes mark the boundaries of the elliptical nanohole. The calculations were made for an injected power in the aperture of 1 W.