Subwavelength photonic nanojet processing of thin metal films using optical tweezers

Abstract

We demonstrate subwavelength optical trap-assisted nanojet patterning of gold/palladium thin films, achieving feature sizes as small as 240 nm, which correspond to 0.45λ, and thus are narrower than the diffraction limit. Photonic nanojets are generated at the back surface of 2-μm silica spheres, optically trapped in water by a 1064-nm CW laser. For nanojet formation, the trapped microspheres are irradiated with a 532-nm ns laser. During this process, the formation of large cavitation bubbles can lead to explosive boiling of water, accompanied by the generation of a shock front. This effect destabilizes the trapped microspheres and disrupts the patterning process. To address this challenge, we simulate the water boiling dynamics and identify laser processing parameters that mitigate boiling effects. We employ Mie theory and Finite Element Modeling to numerically solve electromagnetic wave scattering on the microspheres and determine the optimum sphere-substrate distance and laser beam characteristics for nanojet processing. Photonic nanojet technology is particularly attractive for enabling laser processing of surfaces at spatial resolutions below the diffraction limit. Contactless, subwavelength processing of metallic surfaces with high positioning precision offers promising applications in miniaturized electronic circuits and plasmonic metamaterials.