We demonstrate a novel approach to femtosecond microfabrication of transparent dielectrics, which employs nondiffracting Bessel beams instead of the conventionally used Gaussian beams. The main advantage of Bessel beams is the possibility of recording linear photomodified tracks, extending along the lines of nondiffractive beam propagation without sample translation, as would be required for Gaussian beams. Recording of patterns with an aspect ratio of up to 102–103 in vitreous silica using amplified femtosecond Ti:saphire laser pulses is demonstrated.

Application of Bessel Beams for Microfabrication of Dielectrics by Femtosecond Laser : Instrumentation, Measurement, and Fabrication Technology

Diffraction is a phenomenon related to the wave nature of light and arises when a propagating wave comes across an obstacle. Consequently, the wave can be transformed in amplitude or phase and diffraction occurs. Those parts of the wavefront avoiding an obstacle form a diffraction pattern after interfering with each other. In this review paper, we have discussed the topic of non-diffractive beams, explicitly Bessel beams. Such beams provide some resistance to diffraction and hence are hypothetically a phenomenal alternate to Gaussian beams in several circumstances. Several outstanding applications are coined to Bessel beams and have been employed in commercial applications. We have discussed several hot applications based on these magnificent beams such as optical trapping, material processing, free-space long-distance self-healing beams, optical coherence tomography, superresolution, sharp focusing, polarization transformation, increased depth of focus, birefringence detection based on astigmatic transformed BB and encryption in optical communication. According to our knowledge, each topic presented in this review is justifiably explained.

Bessel Beam: Significance and Applications-A Progressive Review.

Molecular hydrogen is expected to display metallic properties under high pressures, but so far experiments performed at low temperatures (100 K) have showed that hydrogen remains insulating up to 300 GPa. A transformation of normal molecular hydrogen to a conductive and metallic state at room temperature is now observed above 220 GPa.

Conductive dense hydrogen

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.

High-Order Vector Bessel-Gauss Beams for Laser Micromachining of Transparent Materials

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.

Void and micro-crack generation in transparent materials with high-energy first-order vector Bessel beam

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.

All femtosecond optical pump and x-ray probe: holey-axicon for free electron lasers

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.

Laser induced modifications in transparent materials using azimuthally modulated axicon beams

Long focal lines with transverse spot sizes as small as a few wavelengths are called optical needles. A zeroth order Bessel beam being a good example is widely used in such applications as laser micromachining. In practice Bessel beam generated with an axicon has a peak in axial intensity distribution and is not only due to aberrations caused by planar dielectric material interface. Here, we investigate optical needles with controlled axial intensity distribution via intensity modulation of the incoming beam. We have chosen to generate constant axial intensity Bessel beam and propose spatial transmission mask to do so. Experimental verification is presented using diffractive optics elements based on Pancharatnam-Berry phase. We demonstrate a flattening of the axial intensity profile of the Bessel beam without the alteration to the optical needle diameter.

Justas Baltrukonis

Papers

High-Order Vector Bessel-Gauss Beams for Laser Micromachining of Transparent Materials

Void and micro-crack generation in transparent materials with high-energy first-order vector Bessel beam

All femtosecond optical pump and x-ray probe: holey-axicon for free electron lasers

Laser induced modifications in transparent materials using azimuthally modulated axicon beams

Formation of optical needles by Pancharatnam-Berry phase element for laser-induced modifications in transparent materials