Spatially controlled nano-structuring of silicon with femtosecond vortex pulses

Abstract

engineering material properties is key for development of smart materials and next generation nanodevices. this requires nanoscale spatial precision and control to fabricate structures/defects. Lithographic techniques are widely used for nanostructuring in which a geometric pattern on a mask is transferred to a resist by photons or charged particles and subsequently engraved on the substrate. However, direct mask-less fabrication has only been possible with electron and ion beams. that is because light has an inherent disadvantage; the diffraction limit makes it difficult to interact with matter on dimensions smaller than the wavelength of light. Here we demonstrate spatially controlled formation of nanocones on a silicon surface with a positional precision of 50 nm using femtosecond laser ablation comprising a superposition of optical vector vortex and Gaussian beams. Such control and precision opens new opportunities for nano-printing of materials using techniques such as laserinduced forward transfer and in general broadens the scope of laser processing of materials. The fundamental limit to spatial resolution of any optical system is governed by diffraction and is approximately half the wavelength of light 1. Diffraction also dictates how tightly a laser beam can be focused, which in turn determines the feature size one can achieve in laser ablation of materials. Therefore, shorter wavelengths (ultraviolet) are often used in combination with lithographic techniques to produce sub-wavelength features as small as 50 nm 2. Driven primarily by the semiconductor industry, research efforts are ongoing to use coherent and non-coherent extreme ultraviolet light to produce features smaller than 10 nm to meet the ever increasing demand for miniaturization 3,4. Concurrently, alternate methods are also being explored to overcome the diffraction limit of light that does not involve the use of a photomask. These fall into two categories-near field and far field approaches. Nanofabrication using near field approach exploits local field enhancement around a nanoparticle to confine light to sub-wavelength dimensions and thereby induce local deformations (melting or ablation) of the substrate. Large scale periodic array of nanoholes were fabricated by laser irradiation of a monolayer of microspheres 5-a multistep process with no direct control on the position of nanostructures, analogous to lithographic techniques 6. Alternately, controlled fabrication of individual nanostructures can be achieved using scanning probe microscope either directly 7 or by irradiating the tip with light 8. Direct laser processing of materials is a far field approach that exploits the nonlinear nature of the light-matter interaction and localized energy deposition. Using ultrashort laser pulses, three-dimensional (3D) control was achieved in transparent materials 9,10 and sub-wavelength structures were created with enhanced spatial precision in a cold ablation process due to negligible lateral heat transport to the surrounding material. Exploiting n...

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