A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the K shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the L shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.

http://absimage.aps.org/image/DPP09/MWS_DPP09-2009-000843.pdf

Injection and Trapping of Tunnel Ionized Electrons into Laser Produced Wakes

To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA), it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. One of the purposes of this paper is to evaluate the stability properties of intense laser pulses propagating extended distances (many tens of Rayleigh ranges) in plasma channels. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects such as group velocity dispersion, as well as wakefield and relativistic nonlinearities. It is shown that in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. This cancellation can result in pulse propagation over extended distances, limited only by dispersion. Since relativistic focusing is not effective for short pulses, the plasma channel provides the guiding necessary for long distance propagation. Long pulses (greater than several plasma wavelengths), on the other hand, experience substantial modification due to Raman and modulation instabilities. For both short and long pulses the seed for instability growth is inherently determined by the pulse shape and not by background noise. These results would indicate that the self-modulated LWFA is not the optimal configuration for achieving high energies. The standard LWFA, although having smaller accelerating fields, can provide acceleration for longer distances. It is shown that by increasing the plasma density as a function of distance, the phase velocity of the accelerating field behind the laser pulse can be made equal to the speed of light. Thus electron dephasing in the accelerating wakefield can be avoided and energy gain increased by spatially tapering the plasma channel. Depending on the tapering gradient, this luminous wakefield phase velocity is obtained several plasma wavelengths behind the laser pulse. Simulations of laser pulses propagating in a tapered plasma channel are presented. Experimental techniques for generating a tapered density in a capillary discharge are described and an example of a GeV channel guided standard LWFA is presented.

Wakefield Generation and GeV Acceleration in Tapered Plasma Channels

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.

Ultrafast lasers have revolutionized material processing, opening a wealth of new applications in many areas of science. A recent technology that allows the cleaving of transparent materials via non-ablative processes is based on focusing and translating a high-intensity laser beam within a material to induce a well-defined internal stress plane. This then enables material separation without debris generation. Here, we use a non-diffracting beam engineered to have a transverse elliptical spatial profile to generate high-aspect-ratio elliptical channels in glass of a dimension 350  nm×710  nm and subsequent cleaved surface uniformity at the sub-micron level.

Single-shot ultrafast laser processing of high-aspect-ratio nanochannels using elliptical Bessel beams.

The extended focal depth of Bessel beams is a very attracting property for glass cutting applications. However, Bessel beam generation with a non-ideal conical lens induces beam pattern distortions. We present our novel results on bulk modifications of soda-lime glass using a non-ideal axicon-generated Bessel beam. Modelling of the Bessel beam pattern and experimental measurements indicated ellipticity of the central core diameter. That resulted in the formation of cracks in a transverse direction inside the bulk of glass. Furthermore, we demonstrate the possibility to control the transverse crack propagation direction, which is crucial in the case of glass cutting applications.

Non-ideal axicon-generated Bessel beam application for intra-volume glass modification

It is known that Bessel beam generation with a non-ideal axicon induces beam pattern distortions. In this paper, we introduce a simple method for non-ideal axicon-generated Bessel beam reconstruction by tilting the axicon perpendicular to its optical axis. We found an optimum axicon tilt angle where beam distortions can be compensated by inducing additional astigmatic aberrations. At optimal tilt angle, the central spot symmetry and focal depth was increased. By this method we could control crack formation symmetry in the bulk of glass, which is essential for many transparent material processing applications.

Aberration-controlled Bessel beam processing of glass.

We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a specially designed asymmetrically shocked gas jet. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. Particle in cell simulations confirm the role of the shock and the density transition in the electron injection mechanism. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

https://link.aps.org/pdf/10.1103/PhysRevAccelBeams.23.093401

Demonstration of stable long-term operation of a kilohertz laser-plasma accelerator

In this paper the possibility to optimize the glass dicing process by controlling the axicon-generated Bessel beam ellipticity is presented. Single-shot intra-volume modifications in soda-lime glass followed by dicing experiments of 1 mm-thick samples are performed. The Bessel beam ellipticity is essential for glass dicing process. Such beam generates intra-volume modifications with transverse crack propagation in dominant direction. Orientation of these modifications parallel to the dicing direction gives significant advantages in terms of processing speed, glass breaking force and cutting quality.

Glass dicing with elliptical Bessel beam

Laser-based fabrication can be an alternative technology to mechanical grinding and polishing processes. However, the performance of these elements in real applications still needs to be validated. In this paper, we demonstrate that the subtractive fabrication technology is able to produce high-quality axicons from fused silica, which can be efficiently used for glass processing. We comprehensively investigate axicons, fabricated by ultrashort pulsed laser ablation with subsequent CO2 laser polishing, and compare their performance with commercially available axicons. We show that laser-fabricated axicons are comparable in quality with a precision commercial axicon. Furthermore, we demonstrate the intra-volume glass modification and dicing, utilising mJ-level laser pulses. We show that the tilting operation of the laser-fabricated axicons results in the formation of directional transverse cracks, which significantly enhance the 1 mm-thick glass dicing process.

Juozas Dudutis

Papers

Non-ideal axicon-generated Bessel beam application for intra-volume glass modification

Aberration-controlled Bessel beam processing of glass.

Demonstration of stable long-term operation of a kilohertz laser-plasma accelerator

Glass dicing with elliptical Bessel beam

Laser-fabricated axicons challenging the conventional optics in glass processing applications.