Continuously adjustable hollow beam for ultrafast laser fabrication of size-controllable nanoparticles.

The focused vortex beam generates a hollow beam, which has been widely used for size-controlled nanoparticle formation on various materials. However, the size variation of the vortex beam is limited by the integral order of the 2π phase wrap, while the waste is caused by the large side lobe around the center. In this study, we propose a method for hollow beam generation by splitting a femtosecond laser and imparting opposite phases to the outer annular region and the central Gaussian region. After focusing, these two regions overlap at the focal spot, resulting in a hollow beam due to phase cancellation. By modulating the relative dimensions of these two regions, the hollow center can be continuously varied. When such a hollow beam is used for surface processing, the thermal capillary effect facilitates the convergence of the molten material toward the center, ultimately leading to the formation of nanoparticles. This ability to control size allows precise control of nanoparticle size with a diameter range from 140 nm to 940 nm. This method holds great promise for guiding research into nanoparticle properties that are influenced by size effects.