Femtosecond laser pulses have proven to provide valuable insight into the dynamics of microscopic systems by using pump–probe techniques. Applied to atomic clusters even a single pulse of varying pulse duration can reveal how and when energy from the laser pulse is transferred effectively to the cluster. We review the main experimental observables for energy transfer to a cluster and the major theoretical approaches which have been devised. Most importantly, we compare the cluster response to standard 780 nm light pulses with the response to 100 nm pulses, already obtained at a VUV free electron laser (FEL) source, and with 3 nm light which will be available from x-ray FEL sources.

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Mechanisms of cluster ionization in strong laser pulses

Plasma wakefield accelerators produce gradients that are orders of magnitude larger than in conventional particle accelerator, but beams tend to be disrupted by transverse forces. Here the authors create an extended hollow plasma channel, which accelerates positrons without generating transverse forces.

https://hal-polytechnique.archives-ouvertes.fr/hal-01337399/document

Demonstration of a positron beam-driven hollow channel plasma wakefield accelerator.

An analytical solution for the scattering of an acoustic Bessel beam of any order by a deformable sphere centered on the beam is used to calculate the acoustic radiation force acting along the wave propagation axis. Situations are noted where, even in the absence of absorption, the radiation force of a high-order Bessel beam acting on the sphere is opposite to the direction of beam propagation. In the present research, the cases of a fluid and solid elastic spheres are considered with particular emphasis on how the mechanical properties and resonances of spheres as well as the beam parameters affect the negative radiation force. Conditions for the negative attracting force on hexane (fluid), aluminum and gold spheres are established, which help in designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.

https://iopscience.iop.org/article/10.1088/1751-8113/42/24/245202/pdf

Negative axial radiation force on a fluid and elastic spheres illuminated by a high-order Bessel beam of progressive waves

Three-dimensional arrays of optical bottle beams

Ultrafast laser materials processing has undergone an important change with the development of non-diffracting beams. These beams enable overcoming many of the difficulties usually encountered with standard Gaussian-beam focusing in materials. We review the techniques of non-diffracting and accelerating beam shaping that generates lines, tubes or curved segments of focused light on distances that exceed the Gaussian Rayleigh range by several orders of magnitude. We review the benefits and applications of nondiffracting beams for laser micro- and nano-processing in the general context of materials processing with ultrashort pulses in the filamentation regime. We highlight applications on ultra-high aspect ratio nano-drilling and direct laser processing along curves.

https://hal.archives-ouvertes.fr/hal-02392845/document

[INVITED] Ultrafast laser micro- and nano-processing with nondiffracting and curved beams: Invited paper for the section : Hot topics in Ultrafast Lasers

A high power, hollow Bessel beam (J5) is generated using an axicon and a phase plate in combination. The optical breakdown of a gas target and generation of a tubular plasma fiber with such a beam is realized. Hydrodynamic simulations of the hollow beam-plasma breakdown and heating are in reasonable agreement with interferometric measurements of the plasma time evolution.

Tubular plasma generation with a high-power hollow bessel beam

An investigation was made of the possibility of conversion of high-power laser beams into hollow Bessel beams with the first maximum in the radial field distribution in the form of a ring and a slight variation of the field in the direction of propagation. An analysis was made of the influence of real conditions during formation of such beams on their parameters. The limits were found on the divergence of the intial laser radiation and on the characteristics of one of the conversion components (phase screw). These limits had to be kept for the formation of the field distribution in the form of a hollow tube. Experiments were carried out with the aid of a beam former consisting of an axicon and a phase screw, and also with the aid of kinoform elements equivalent to a combination of an axicon and a phase screw. The results confirmed the validity of the theoretical analysis. Hollow beams were formed with radial distributions described by the Bessel functions of the first, second, and fifth orders. The beam length was 20 cm and the diameters of the principal maxima were 20-60 μm.

https://iopscience.iop.org/article/10.1070/QE1996v026n02ABEH000607/pdf

Formation of high-power hollow Bessel light beams

A model of the appearance of periodic structures in the plasma channel of an extended laser spark is considered. It is shown that sideways delivery of the heating radiation by a Bessel beam to the breakdown region and expansion of the heated plasma result in inhomogeneous refraction and, consequently, give rise to structures with a longitudinal period ~λ/γ2 (λ is the radiation wavelength and γ is the angle of inclination of the beam to the axis). This model is used to account for the results of experiments demonstrating that such structures are not observed in the case of a low-density gas (at pressures of 0.05 bar or less) and short (~0.1 ns) heating pulses.

Generation of periodic inhomogeneities in optical breakdown by a Bessel laser beam

The plasma formed by the breakdown of gases (argon was used in the experiments, whereas argon and helium were considered in the calculations) at atmospheric pressure in light beams was studied experimentally and modelled numerically. The field amplitude in these beams was virtually constant in the direction of propagation and described by a high-order Bessel function in the radial direction. The initial stage in the evolution of a spark, when the tubular configuration of the heating-radiation caustic exerts the greatest influence on the characteristics of the plasma channel, was investigated.

http://iopscience.iop.org/article/10.1070/QE1999v029n03ABEH001458/pdf

The formation of a plasma channel in the optical breakdown of a gas by tubular Bessel beams

The shadow technique was used to reveal a change in the refractive index of water during propagation of a powerful Bessel light beam (λ = 1.06 μm). The investigation was carried out on a beam 12 cm long with the diameter of the caustic 10 μm. The heating radiation power was 2.5–250 MW. The axial region near the acoustic, 1 cm in diameter, was investigated. The region was probed parallel to the heating beam axis. When the heating radiation power exceeded 40 MW, the shadow photographs showed characteristic inhomogeneities of dimensions which increased with the power. These inhomogeneities were a system of fringes and they were attributed to diffraction at the boundary between illuminated and unilluminated regions of water. The refractive index step at this boundary was estimated to be −3×10-6 when the electric field of the heating radiation was 105 V cm-1.

L Ya Margolin

Papers

Tubular plasma generation with a high-power hollow bessel beam

Formation of high-power hollow Bessel light beams

Generation of periodic inhomogeneities in optical breakdown by a Bessel laser beam

The formation of a plasma channel in the optical breakdown of a gas by tubular Bessel beams

Nonlinear propagation of a Bessel light beam of near-breakdown intensity in water