Polarization sensitive anisotropic structuring of silicon by ultrashort light pulses

Imprinting of anisotropic structures on the silicon surface by double pulse femtosecond laser irradiation is demonstrated. The origin of the polarization-induced anisotropy is explained in terms of interaction of linearly polarized second pulse with the wavelength-sized symmetric crater-shaped structure generated by the linearly polarized first pulse. A wavefront sensor is fabricated by imprinting an array of micro-craters. Polarization controlled anisotropy of the structures can be also explored for data storage applications

Femtosecond laser nanostructuring for high-topological charge vortex tweezers with continuously tunable orbital angular momentum

It is well known that the light carries linear and angular momentum that can be transferred to the irradiated objects. Angular momentum of the beam is comprised of spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is associated to the beam's polarization and is always intrinsic. OAM comes from the azimuthal phase variations of the beam and can be both extrinsic and intrinsic. The beam with helical phase phi = l.Phi, where phi is phase, Phi is polar angle and l is positive or negative integer number, possesses well-defined OAM with l.h [1]. Such beams are often referred to as optical vortices and are exploited in optical tweezer experiments enabling the rotation of trapped particles. Changing the wavefront's helicity, also the geometry of the beam is changed. The higher is |l|, the larger is the diameter of the beam. In order to change the total angular momentum of the beam, either the shape of the beam or the photon density has to be changed. As a result, the experiments which require fixed beam size and intensity are limited to fixed OAM. Recently, we implemented optical tweezers with tunable angular momentum, there OAM could be changed from -1 to 1 by controlling ellipticity of the incident laser beam. Here we extend this technology and demonstrate the generation of optical vortices of high topological charge up to 100 (Fig. 1(a)-(i)) using femtosecond laser written polarization converters (the S-waveplate) [2]

Engineering anisotropy in glass with ultrafast laser assisted nanostructuring

Recent applications of femtosecond laser assisted self-assembled nanostructures will be overviewed. Specifically, polarization sensitive optical elements and 5-dimensional optical data storage with practically unlimited life-time will be demonstrated and discussed

Optical vortex production mediated by azimuthal index of radial polarization

[EN]Special light beams are becoming more and more interesting due to their applications in particle manipulation, micromachining, telecommunications or light matter-interaction. Both spin and orbital angular momenta of light are exploited often in combination with spatially varying linear polarization profiles (e.g. radial or azimuthal distributions). In this work we study the interaction between those polarization profiles and the spin-orbit angular momenta, finding the relation involved in the mode coupling. We find that this manipulation can be used for in-line production of collinear optical vortices with different topological charges, which can be filtered or combined with controlled linear polarization. The results are valid for continuous wave and ultrashort pulses, as well as for collimated and focused beams. We theoretically demonstrate the proposal, which is further confirmed with numerical simulations and experimental measurements with ultrashort laser pulses.This work was partially funded by Junta de Castilla y León (SA287P18) and FEDER Funds; Spanish Ministerio de Economía y Competitividad (MINECO) (FIS2017-87970-R, EQC2018-004117-P); European Research Council (ENIGMA)

Femtosecond laser printed microoptics in hydrogenated amorphous silicon

Conventional optics (e.g. lenses or mirrors) manipulates the phase via optical path difference by controlling thickness or refractive index of material. Recently, a promising type of optics emerged which exploits geometric phase shift, when a lightwave is transformed by parameter other than optical path difference, e.g. polarization. Here, wavefront is modified by introducing spatially varying anisotropy and is a result of Panchatraman-Berry phase [1]. Theoretically any phase pattern can be achieved solely by means of geometric phase with efficiencies reaching 100% [2]. This allows continuous optical phase shifts and without phase resets, in stark contrast to conventional elements, wherein phase profiles are encoded as discrete optical path variations in refractive index or thickness, limiting performance. The geometric phase optics is a promising alternative for controlling and manipulating light, but it stumbles on the lack of adequate fabrication technology

Engineered optical materials by ultrafast laser nanostructuring

This thesis is focused on ultrafast laser induced modification in optical materials including transparent dielectrics, high-index semiconductors, and glass-metal nanocomposites. Under certain conditions, ultrafast laser direct writing through a nonlinear light-matter interaction enables a high-precision nanostructuring. This type of the modification exhibits form birefringence and/or dichroism enabling the fabrication of polarization sensitive optical elements. The main activities involved in the research are the optimization of light-mater interaction processes, engineering the optical properties of materials, design and fabrication of optical elements, and implementation of engineered optics into the multidisciplinary fields. The pioneering steps were taken towards a practical exploitation of femtosecond laser imprinted space-variant optical elements and development of a novel scheme for optical trapping. As a result, a set of novel optical components with high efficiency, high phase density and low losses were successfully developed and demonstrated, including optical dichroic elements, polarization gratings, arrays of polarization micro-lenses and micro converters, and computer generated Fourier holograms. A novel type of optical tweezers with tunable orbital angular momentum was also designed and developed, which has attracted attention from the beam-shaping and optical micro-manipulation communities. The record high topological charge torque with high-precision control of trapped micron-size objects was achieved. Practical laser imprinted optical elements in materials other than fused silica were demonstrated. A number of optical elements were realized in amorphous silicon thin-films. It was demonstrated that the laser-induced periodic thin-film structures exhibit giant birefringence and was implemented in space-variant polarization and phase manipulations. Surface texturing with 30 nm resolution was demonstrated by potassium hydroxide wet etching and ultrafast laser nanostructuring of silica, leading to the fabrication of dichroic glass-metal patterns. Other laser material processing approaches such as single and double pulse irradiation of crystalline silicon, or irradiation of optical materials with tightly focused cylindrical vector beams were implemented. The femtosecond laser shaping of silver nanoparticles embedded in soda-lime glass was studied. The developed approach can be employed to control the anisotropy of the glass-metal nanocomposites

High-performance geometric phase elements in silica glass

High-precision three-dimensional ultrafast laser direct nanostructuring of silica glass resulting in multi-layered space-variant dielectric metasurfaces embedded in volume is demonstrated. Continuous phase profiles of nearly any optical component are achieved solely by the means of geometric phase. Complex designs of half-wave retarders with 90% transmission at 532 nm and >95% transmission at >1 µm, including polarization gratings with efficiency nearing 90% and computer generated holograms with phase gradient of ~0.8pi rad/µm, were fabricated. Vortex half-wave retarder generating single beam optical vortex with tunable orbital angular momentum of up to ±100ℏ is shown. High damage threshold of silica elements enables simultaneous optical manipulation of large number of micro-objects using high-power laser beams. Thus, the continuous control of torque without altering the intensity distribution was implemented in optical trapping demonstration with a total of 5 W average power, which is otherwise impossible with alternate beam shaping devices. In principle, the direct-write technique can be extended to any transparent material that supports laser assisted nanostructuring, and can be effectively exploited for the integration of printed optics into multi-functional optoelectronic systems

Unambiguous evidence of two plasmon decay during ultrafast laser writing in glass

The interaction of femtosecond laser pulses with transparent media has been a focus of research due to its unique properties. It has been established that above a certain threshold, self-assembled nanogratings in silica glass can be induced [1]. Although the mechanism that triggers the nanostructure is still unclear, a theory has been introduced involving the mechanism of nanogratings formation based on two plasmon parametric decay [2]. A signature of two-plasmon decay is the generation of the 3/2 harmonic. Previously, in transparent media, only a weak 3ω/2 emission was observed at high energy thresholds [3]. Thus it remains unclear if two plasmon decay can be associated with self-assembled nanogratings formation. Here we present a thorough survey of 2nd, 3rd and 3/2 harmonic generations in fused silica for varying laser fluences within multiple regimes of optical laser writing and self-assembled nanostructures. We demonstrate that 3ω/2 can be observed at the energies close to the threshold of permanent material modification