Spatially controlled nano-structuring of silicon with femtosecond vortex pulses.

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 laser-induced forward transfer and in general broadens the scope of laser processing of materials.

Mapping complex polarization states of light on a solid.

Polarization states of light, represented by different points on a Poincaré sphere, can be readily analyzed for a Gaussian beam by a combination of wave plates and polarizers. However, this method cannot be extended to higher-order Poincaré spheres and complex polarization patterns produced by coherent superpositions of vector vortex (VV) beams. We demonstrate the visualization of complex polarization patterns by imprinting them onto a solid surface in the form of periodic nano-gratings oriented parallel to the local structure of the electric field of light. We design unconventional surface structures by controlling the superposition of VV beams. Our method is of potential interest to the production of sub-wavelength nano-structures.