Figure 1. Experimental setup and optical image of doughnut-like microstructures consisting of selectively assembled Ag nanoparticles.

a, Aqueous solution of Ag nanoparticles (NPs) of various shapes and sizes, including Ag nanorods (NRs), are dropped between a substrate (glass slide) and a cover slip. A red diode laser (wavelength = 660 nm, power = 12.0 mW) or a Nd³⁺:YVO₄ laser (wavelength = 1064 nm, power = 23.8 mW) is used as the light source for optical tweezing and is focused on the substrate surface for the fixation of Ag NRs. An oil immersion lens is used as the objective lens for laser focusing. b, Extinction spectrum of the suspension of Ag NPs. c, Dark-field optical image (scattering image) of ring-like arrangement of Ag NPs produced under the irradiation of radially polarized Nd³⁺:YVO₄ laser with diameter ~2 μm.