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In this work we present a novel method for depolarization compensation based on spatially variable wave plate. Thermally induced depolarization losses were reduced from 14.3% to 1.3% and bifocusing eliminated in a double-pass Yb:YAG amplifier.

Beams exhibiting long focal lines and small focal spot sizes are desired in a variety of applications and are called optical needles, with Bessel beams being a common example. Conical prisms are regularly used to generate Bessel beams, however, this method is usually plagued by an appearance of on-axis oscillations. In this work, we consider an optical element based on the space-domain Pancharatnam-Berry phase (PBP) to generate a high-power optical needle with a smooth and constant on-axis intensity profile. The phase in PBP elements is not introduced through optical path differences but results from the geometric phase that accompanies space-variant polarization manipulation. Our implementation is based on a type 2 modification of bulk transparent glass material, resulting in the formation of nanogratings with slow axes aligned perpendicular to the grating corrugation. We investigate both numerically and experimentally the stability of an optical needle generation under imperfect conditions. Influences of misalignments in the optical schema are investigated numerically and experimentally.

Here, we present thorough research into high-order vector Bessel-Gauss beams from a theoretical perspective, with experimental realization and application to transparent-material laser micromachining. Efficient, stable, high-quality, and power-vector Bessel-Gauss beam generation using two geometric-phase-based optical elements, a high-order $S$-wave plate and an axicon in combination with simple optical elements, such as lenses, wave plates, and polarizers, is demonstrated. Ultrashort pulse-vector Bessel-Gauss beams are used to induce modifications in the glass, creating three-dimensional tubular structures of type I and type II modification in D263T glass. Furthermore, samples containing tubular modifications are tested for etch selectivity in the femtosecond-laser-induced chemical-etching method with KOH solution by etching throughout channels.

Bessel beams of the zeroth order are notorious for characteristic of having diffraction-less focal zone. Due to this property they became desirable for a number of laser micromachining applications, such as Bragg grating inscription, microchannel forming, photopolymerization and more [1] . By means of spatial light modulators or diffractive optical elements Bessel like beam variations with shaped transverse intensity distribution can be obtained which also proved beneficial in glass cleaving, for example [2] . Higher order Bessel beams also known as Bessel vortex beams also have interesting properties like carrying orbital angular momentum [3] . A combination of Bessel vortices of different topologies allows to form high width/depth ratio beams with complex transverse intensity distribution [4] , [5] . Such beams can be realized with geometrical phase elements which are manufactured by inscribing birefringent nanograting inside the bulk of fused silica [6] . By controlling retardance and the local angle of nanograting one can manufacture numerous beam shaping elements.

During the last decade the zeroth order Bessel-Gauss laser beam has found many uses in the transparent material processing. The high aspect ratio channels can be created that slice through various thin transparent materials and increase the efficiency of cutting. However, the generation of high-quality Bessel-Gauss beam remains a challenge due to imperfection of glass axicon manufacturing, i.e. rounded tip, not smooth surface etc. These imperfections generate intensity modulation along propagation axis or even modify transversal central core intensity distribution, that results in worsening of micro-machining quality. The diffractive optical element (DOE) is a great alternative that do not suffer from previously mentioned problems. In this study we show the possibility of generating high quality Bessel-type beams with geometric phase optical elements (GPOEs) (manufactured by Workshop of Photonics). These elements act as precise flat DOEs that have very high diffraction efficiency (>90%), high optical damage threshold and can be freely customized for specific needs. Therefore, with the use of high-power laser they can be applied to process transparent materials. In this work, controllable phase shifts are implemented in axicon phase masks to create unique and fanciful Bessel-type beams as well as asymmetric core beams for thin glass modification/cutting application. Using numerical simulations and experimental data we compare performance of GPOEs and demonstrate thin glass processing using powerful laser with reshaped intensity distribution by GPOE.

Pancharatnam-Berry phase enables various flat special optical elements such as top-hat converters. We present a study on engineering efficient vectorial top-hat converters inscribed in the glass by high-power femtosecond laser pulses. We start with a phase-encoded top-hat converter and demonstrate how its efficiency can be further increased by adjustment of phase masks and various parameters. We use self-organized nanogratings inscribed by a femtosecond laser for the creation of the converter (produced by 'Workshop of Photonics'). Experimental verification of the concept is also presented.

We put forward a co-axial pump (optical)-probe (x-rays) experimental concept and show performance of the optical component. A Bessel beam generator with a central 100 µm diameter hole (on the optical axis) was fabricated using femtosecond (fs) laser structuring inside a silica plate. This flat-axicon optical element produces a needle-like axial intensity distribution which can be used for the optical pump pulse. The fs-x-ray free electron laser (X-FEL) beam of sub-1 µm diameter can be introduced through the central hole along the optical axis onto a target as a probe. Different realisations of optical pump are discussed. Such optical elements facilitate alignment of ultra-short fs-pulses in space and time and can be used in light–matter interaction experiments at extreme energy densities on the surface and in the volume of targets. Full advantage of ultra-short 10 fs-X-FEL probe pulses with fs-pump (optical) opens an unexplored temporal dimension of phase transitions and the fastest laser-induced rates of material heating and quenching. A wider field of applications of fs-laser-enabled structuring of materials and design of specific optical elements for astrophotonics is presented.

Nondiffracting beams are known for their long line of focus, which has various applications in laser materials processing. Zeroth order Bessel beam is usually generated with an axicon and has a distinct circular spatial spectra. The nature of higher order Bessel beams, elliptical and parabolic nondiffracting beams is also conical and their spatial spectra have their own azimuthal modulation. We study numerically and verify experimentally generation of vortical Bessel beams, their superpositions along with elliptical and parabolical beams using an axicon. Laser induced modifications in glasses for various durations and beam powers using generated pulsed beams are analyzed.

Beam profile engineering, where a desired optical intensity distribution is generated by phase shifting and/or amplitude changing elements, is a promising approach in various laser-related applications. For example, vector geometrical phase elements enable various flat special optical elements such as top-hat converters. We present a study on engineering efficient top-hat converters inscribed in the glass by femtosecond laser pulses. We start with an amplitude encoded top hat converter and demonstrate how its efficiency can be further increased by introduction of phase masks and by the polarization of the incident beam. Experimental verification of the concept is also presented.

Zeroth order Bessel beams are widely used in laser micromachining of transparent materials. The small diameter of central core and elongated focus enables to generate high aspect ratio voids. The simplest way to generate this beam is to induce a conical shape phase with an axicon. However, the quality of the axicon tip is very crucial to generate smooth Bessel beams since it is known that a blunt axicon tip induces large intensity modulation in propagation direction. Alternative Bessel beam generation method is to use a Diffractive Optical Elements (DOEs) that do not suffer from previously mentioned problem. In this work we demonstrate generation of a zeroth order Bessel beam with Geometric Phase Optical Elements (GPOEs) (manufactured by Workshop of Photonics) acting as a diffractive beam shaping element. Having absolute control of induced beam phase, we have modified mask phase so that half of it had additional phase shift or spatial transposition resulting in creation of fanciful induced beam phase patterns. With the use of laser beam propagation numerical modeling we show that these new phase masks can create various beam transverse intensity patterns such as asymmetrical central core, generation of multiple peaks or even large rings that are highly demanded for various laser micromachining applications. We have chosen couple of most perspective beam shapes and manufactured GPOEs to generate them. The experimentally generated beams were compared to numerical simulations. As the GPOEs are able to work with high power pulses we have also investigated induced transparent material modifications.

We put forward a co-axial pump(optical)-probe(X-rays) experimental concept and show performance of the optical component. A Bessel beam generator with a central 100 micrometers-diameter hole (on the optical axis) was fabricated using femtosecond (fs) laser structuring inside a silica plate. This flat-axicon optical element produces a needle-like axial intensity distribution which can be used for the optical pump pulse. The fs-X-ray free electron laser (X-FEL) beam of sub-1 micrometer diameter can be introduced through the central hole along the optical axis onto a target as a probe. Different realisations of optical pump are discussed. Such optical elements facilitate alignment of ultra-short fs-pulses in space and time and can be used in light-matter interaction experiments at extreme energy densities on the surface and in the volume of targets. Full advantage of ultra-short 10 fs X-FEL probe pulses with fs-pump(optical) opens an unexplored temporal dimension of phase transitions and the fastest laser-induced rates of material heating and quenching. A wider field of applications of fs-laser-enabled structuring of materials and design of specific optical elements for astrophotonics is presented.

Non-diffracting beams have contributed greatly to recent developments in high power applications such as laser micromachining. Well known for their elongated focal region (also known as a focal line) are zeroth-order Bessel and Bessel vortical beams which allow achieving high aspect ratio modifications inside transparent materials. A superposition of several optical Bessel vortices of different topological charges allows generating high aspect ratio beams with complexly engineered transverse intensity patterns. Non-diffracting Bessel vortices commonly are created using a conical prism — an axicon. Therefore, they can be referred to as axicon beams. In this work, we have studied the formation of high power and energy axicon vortical beams both numerically and experimentally. We have inscribed sub-wavelength birefringent structures inside fused silica glass to create geometrical phase elements for the azimuthal modulation of the spatial spectra of a zeroth-order axicon (Bessel) beam which allows creating a high power superposition of several Bessel vortices. To prove the concept, we apply azimuthally modulated axicon vortical beams to create modifications inside fused silica glass samples.

Invariant Bessel–Gauss beams have attracted great interest for transparent material microprocessing applications. The small central peak and elongated focal region of the zeroth-order Bessel–Gauss beam provide advantages in laser micromachining. However, many transparent material processing applications need asymmetric beams to induce directional cracking. Therefore, in this work, we investigate analytically, numerically, and experimentally a beam that was generated by spatially displaced axicon halves, which induces noncylindrical symmetry in the beam phase profile. Analytical solutions were found and verified experimentally. High power and quality beams were generated by writing a needed phase alteration of the beam with a geometric phase element. These beams were able to demonstrate applicability to micromachine processing of transparent material.

A subpicosecond laser system featuring a fiber chirped pulse amplification-based seed laser and a double-pass end-pumped Yb:YAG crystal power amplifier was investigated. The key novelty of the system was the application of depolarization compensation using a specially designed spatially variable wave plate. To the best of our knowledge, this method was applied for the first time. The presented laser system produced pulses of 441 fs duration, 116 µJ pulse energy at 116 W average power with a beam quality of M 2 ∼ 2.1 , featured optical-to-optical efficiency of 32% at room temperature ( T = 20 ∘ C ), and had residual depolarization level of 2.7%.

Nondiffracting Mathieu beams are one of the candidates for laser microprocessing of transparent materials due to generation of high aspect ratio nanochannels [1], and ability to control micro crack formation [2]. Thus, they can enable an efficient cutting of transparent materials, because the direction, in which the crack propagates is controlled by the asymmetry of the beam profile [2]. Vector Mathieu beams are a more exotic than scalar version of the nondiffracting beams and their vector nature may bring additional benefits for micro-manufacturing of materials. Flexible generation of vector Mathieu beams can be achieved using a vectorial diffractive optical element, where the so-called geometrical phase is controlled by changing the orientation of nanograting inscribed in the bulk of the glass by a linearly polarized femtosecond laser pulse [3].

Geometrical phase (also known as Pancharatnam-Berry phase [1,2].) elements and their applications are booming nowadays: from devices like high NA metalenses to special optics like s-waveplates [3], from wavelengths in the ultraviolet to the THz diapason. The reason behind such flexibility is due to variety of different production approaches — lithography based, sculptured coatings etc. and due to the varying orientation and individual properties of sub-elements of the GPE. Such elements can act as both scalar and vectorial diffractive optical elements (or as shapers of the spatial spectra of the input beam), but usually their vectorial properties are largely ignored due to the complexity of mastering phases and amplitudes of both transverse components.

Bessel beams are perfect candidates for laser microprocessing of transparent materials due to generation of high aspect ratio micro voids [1], and ability to control micro crack formation enables to efficiently cut them [2, 3]. The crack propagation direction is controlled by inducing asymmetry to beam profile [3], and allows ultra-fast glass cutting over non-straight lines. The laser-matter interaction of various nondiffracting beams is heavily investigated and their utilisation for laser microprocessing is soon to be expected. Vector beams that are more exotic than scalar counterparts and can have additional beneficial properties for micro-manufacturing of materials. For instance, vector Bessel beam has a doughnut shaped intensity profile with complex spatial distribution of polarisation and can be de-composed to two transverse polarisation intensity components [4].

Ultrafast relaxation processes in diamond-like carbon (DLC) thin films with embedded Cu nanoparticles (DLC:Cu nanocomposites) were investigated by means of transient absorption spectroscopy focusing on localized surface plasmon resonance (LSPR) of photoexcited Cu nanoparticles. Absorption spectra of the composite films correspond to the sum of absorption spectra of DLC matrix and Cu nanoparticles; however, Cu nanoparticles strongly dominate in the transient differential absorption. Excitations of DLC matrix and of Cu nanoparticles relax independently revealing no strong interaction. High sensitivity measurements enabled to obtain the hot electron relaxation dynamics in Cu nanoparticles in the low excitation intensity conditions. The relaxation time was found to be independent of the excitation intensity up to tens of microjoule per square centimeter per pulse and to increase at higher intensities. The relaxation time obtained at low excitation intensity was also found to increase by about 30 % in the samples with high Cu concentration, where larger nanoparticles were formed.

In this work, we present efficient generation of a high-quality vector Bessel beam using an S-wave plate (radial/azimuth polarization converter) together with an ordinary glass axicon. We examine laser-induced modifications in glass with different pulse durations. We achieve material cracking and observe dominant crack propagation directions caused by the generated beam’s intensity asymmetry. By translating the beam, we demonstrate potential application of vector Bessel beams and their transverse polarization components for microprocessing of transparent materials using ultra-short pulses.

Engineering of sophisticated synthetic 3D scaffolds that allow controlling behavior and location of the cells requires advanced micro/nano fabrication techniques. Ultrafast laser micro-machining employing a 1030 nm wavelength Yb:KGW femtosecond laser and a micro-fabrication workstation for micro-machining of commercially available 12.7 and 25.4 µm thickness polyimide (PI) film was applied. Mechanical properties of the fabricated scaffolds, i.e., arrays of differently spaced holes, were examined via custom-built uniaxial micro-tensile testing and finite element method simulations. We demonstrate that experimental micro-tensile testing results could be numerically simulated and explained by 2-material model, assuming that 2-6 µm width rings around the holes possessed up to 5 times higher Young's modulus and yield stress compared with the rest of the laser intacted PI film areas of “dog-bone” shaped specimens. That was attributed to material modification around the micro-machined holes in the vicinity of the position of the focused laser beam track during trepanning drilling. We demonstrate that virgin PI films provide a suitable environment for the mobility, proliferation, and intercellular communication of human bone marrow mesenchymal stem cells and discuss how cell behavior varies on the micro-machined PI films with holes of different diameters (3.1, 8.4, and 16.7 µm) and hole spacing (30, 35, 40, and 45 µm). We conclude that the holes of 3.1 µm diameter were sufficient for metabolic and genetic communication through membranous tunneling tubes between cells residing on the opposite sides of PI film but prevented the trans-migration of cells through the holes. This article is protected by copyright. All rights reserved.

Manipulation of liquids at the lowest levels of volume and dimension is at the forefront of materials science, chemistry and medicine, offering important time and resource saving applications. However, manipulation by mixing is troublesome at the microliter and lower scales. One approach to overcome this problem is to use passive mixers, which exploit structural obstacles within microfluidic channels or the geometry of channels themselves to enforce and enhance fluid mixing. Some applications require the manipulation and mixing of aggressive substances, which makes conventional microfluidic materials, along with their fabrication methods, inappropriate. In this work, implementation of an optimized full scale three port microfluidic mixer is presented in a slide of a material that is very hard to process but possesses extreme chemical and physical resistance—alumina. The viability of the selected femtosecond laser fabrication method as an alternative to conventional lithography methods, which are unable to process this material, is demonstrated. For the validation and optimization of the microfluidic mixer, a finite element method (FEM) based numerical modeling of the influence of the mixer geometry on its mixing performance is completed. Experimental investigation of the laminar flow geometry demonstrated very good agreement with the numerical simulation results. Such a laser ablation microfabricated passive mixer structure is intended for use in a capillary force assisted nanoparticle assembly setup (CAPA).

The Bessel-Gauss beams being distinguished by an elongated focal zone have attracted great interest and potential in various laser micro-machining applications. The ability to create high width/depth ratio modifications inside transparent materials has opened new perspectives, ranging form micro-channel fabrication [1] to efficient cutting of various glasses [2] . The mostly used optical element to produce Bessel beam is an axicon. However, the shape of the axicon must be almost ideal, because any irregularities of its surface and especially a rounded tip greatly reduces the quality of the generated beam by inducing unwanted axial intensity modulation [3] . Hence, alternatives have emerged to substitute glass axicons to increase the overall quality of the beam or create more complex patterns, these are diffractive optical elements, sub-wavelength metasurfaces or liquid crystals (spatial light modulators) [4] , [5] to name a few. Ability to choose the Bessel beam generation method allows one to have a high level of freedom in application of them in various specific manufacturing processes. However, the price or low optical damage threshold of the beam shaping elements may prevent from wide applicability. Alternative beam shaping method can be utilised - the so-called geometric phase optical elements (GPOEs). These elements are based on the anisotropy of nanogratings inscribed by laser inside the fused silica [6] . The control of orientation and retardance of nanogratings enables to create various phase retarders that can work as custom flat beam shaping optical elements.