Optical trapping of gold microparticles using linearly polarized, centrally obstructed Gaussian beams

Metallic microparticles larger than the illumination wavelength are commonly considered poor optical trapping candidates due to their high extinction coefficient. This paper presents a numerical and experimental study on the three-dimensional (3D) trapping of gold microparticles using a centrally obstructed Gaussian beam based on the T-matrix method. The range of particle size for stable optical trapping is determined. For the trapping numerical aperture of 1.32 and illumination wavelength of 1.064 µm, numerical analysis proves that 3D trapping of gold microparticles with a radius bigger than 1.0 µm can be readily achieved. By imprinting a digital lens to the spatial light modulator, we slightly defocus the centrally obstructed Gaussian beam to shift the trapping location to the focal plane for clear observation. Experimental results demonstrate stable trapping of gold microparticles with a radius greater than 1.4 µm at high-power illumination, agreeing well with the theoretical predictions. The presented work should be of interest to the community applying metallic microparticles to relevant research.