Laser Micro Machining Using an Oblique Photonic Nanojet With Focused Laser Beam Irradiation

Three-dimensional microstructures in the sub-micrometer scale exhibit unique properties. A flexible machining method to fabricate such structures is desired. Photonic nanojet (PNJ) is high intensity laser beam with sub-micrometer scale beam diameter and micrometer scale depth of focus. PNJs have a longer depth of focus than tightly focused laser beams with a high numerical aperture. In this study, we investigate the angular control of PNJs by controlling the propagation direction of incident light in order to realize flexible laser micro machining using PNJs. By controlling the position of the microsphere in the focused laser beam with a large defocus, the propagation direction of the laser beam incident on the microsphere is changed, and the angle of the PNJ can be controlled. Laser machining experiments on a silicon substrate showed that the PNJ angle can be controlled by incident laser angle. Furthermore, sub-micrometer scale laser machining was achieved even when using an oblique PNJ. The simulation results and experimental results are in good agreement. In conclusion, the angle control of the photonic nanojet can be applied to flexible multi-axis laser micro machining.