Spatial-Control of Laser-Written in-Chip Si Structures with Bessel Beams

Gaussian beam profile is the most widely used beam type in laser-material processing. However, these beams are practically limited when they are focused, due to their diffraction-limited focal depth. Consequently, in order to fabricate elongated, high-aspect-ratio structures with 3D laser-writing of transparent materials, it is generally required to scan the Gaussian beams multiple times. In contrast, Bessel beams, due to their "non-diffracting" nature, can produce sub-micron core diameters and can have extended focal depths with a single scan [1, 2]. For instance, Bessel beams have successfully been used in bulk processing of glasses to realize high-aspect-ratio structures [2]. Although, analogous laser modifications have also been created in silicon, their feature sizes remains at 1μm scale [3]. Here, using spatial light modulator (SLM)-generated Bessel beams, we demonstrated subsurface ("in-chip") structures with adjustable structure diameters and elongations. We also observed submicron modifications buried inside Si with an aspect ratio > 100. Potentially, such in-chip modifications can provide novel phase control for in-chip holograms operating in the near-IR part of the spectrum [3].