We fabricated conical nanostructures on silicon with a tip dimension of ∼ 70 nm using a single twisted femtosecond light pulse carrying orbital angular momentum (ℓ=±1). The height of the nano-cone, encircled by a smooth rim, increased from ∼ 350 nm to ∼ 1 μm with the pulse energy and number of pulses, whereas the apex angle remained constant. The nano-cone height was independent of the helicity of the twisted light; however, it is reduced for linear polarization compared to circular at higher pulse energies. Fluid dynamics simulations show nano-cones formation when compressive forces arising from the radial inward motion of the molten material push it perpendicular to the surface and undergo re-solidification. Simultaneously, the radial outward motion of the molten material re-solidifies after reaching the cold boundary to form a rim. Overlapping of two irradiated spots conforms to the fluid dynamics model.

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...

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.

Storage capacity of a conventional optical disc can be enhanced significantly by recording data within the three-dimensional volume. However, spherical aberrations and light-scattering limit the number of layers that can be efficiently recorded and retrieved. In this paper, we show that by optimizing the multi-layer data recording and reading parameters, the number of fabricated layers could be tripled to 60 in commercial grade poly-methyl-methacrylate polycarbonate and reduce the read laser power by a factor of 7 by the 20th layer. Influence of spherical aberration on write laser spot size was studied by using vector diffraction theory and compared with experimental values obtained by monitoring the threshold energy of the write laser with depth. Scattering losses of the read laser were studied by simulating the propagation of light through multi-layered micro-structured material. The main advantage of our technique is its practical implementation, as it refrains from the use of index-matching liquids and specialized optics during the read–write process.