2ND-HARMONIC LIGHT GENERATION IN THE INTERACTION OF LASER-BEAMS WITH UNDERCRITICAL PLASMAS

The process of second harmonic generation (SHG) in undercritical plasmas is studied. It is shown that filamentation and self-focusing of the laser beam in the plasma can break the plasma density symmetry and lead to SHG by free electrons. In turn, second harmonic emission may be used to investigate the plasma parameters and to diagnose the process of laser beam filamentation itself.

Interaction of intense laser pulses with preformed density channels

The interaction of a high-intensity laser pulse with a plasma density channel preformed in a gas jet target has been studied. At neutral densities below 3.0 X 10(19) cm(-3) a strong interaction between the pulse and the channel walls was observed, there was clear evidence of pulse confinement, and the laser irradiance was significantly increased compared to an interaction with neutral gas. At higher gas densities, however, the radial uniformity and length of the channel were both found to be adversely affected by refractive defocusing of the prepulse used to generate the channel.

Dynamics of intense laser propagation in underdense plasma: Polarization dependence

We present a comprehensive numerical study of the dynamics of an intense laser pulse as it propagates through an underdense plasma in two and three dimensions. By varying the background plasma density and the polarization of the laser beam, significant differences are found in terms of energy transport and dissipation, in agreement with recently reported experimental results. Below the threshold for relativistic self-focusing, the plasma and laser dynamics are observed to be substantially insensitive to the initial laser polarization, since laser transport is dominated by ponderomotive effects. Above this threshold, relativistic effects become important, and laser energy is dissipated either by plasma heating (p-polarization) or by trapping of electromagnetic energy into plasma cavities (s-polarization) or by a combination of both (circular polarization). Besides the fundamental interest of this study, the results presented are relevant to applications such as plasma-based accelerators, x-ray lasers, and fast-ignition inertial confinement fusion. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737151]

Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet

Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein's seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency >10 4 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure. 

Laser-driven ion acceleration: State of the art and emerging mechanisms

Ion acceleration driven by high intensity laser pulses is attracting an impressive and steadily increasing research effort. Experiments over the past 10-15 years have demonstrated, over a wide range of laser and target parameters, the generation of multi-MeV proton and ion beams with unique properties, which have stimulated interest in a number of innovative applications. While most of this work has been based on sheath acceleration processes, where space-charge fields are established by relativistic electrons at surfaces of the irradiated target, a number of novel mechanisms has been the focus of recent theoretical and experimental activities. This paper will provide a brief review of the state of the art in the field of laser-driven ion acceleration, with particular attention to recent developments.

Fast-electron refluxing effects on anisotropic hard-x-ray emission from intense laser-plasma interactions

Fast-electron generation and dynamics, including electron refluxing, is at the core of understanding high-intensity laser-plasma interactions. This field is itself of strong relevance to fast ignition fusion and the development of new short-pulse, intense, x-ray, gamma-ray, and particle sources. In this paper, we describe experiments that explicitly link fast-electron refluxing and anisotropy in hard-x-ray emission. We find the anisotropy in x-ray emission to be strongly correlated to the suppression of refluxing. In contrast to some previous work, the peak of emission is directly along the rear normal to the target rather than along either the incident laser direction or the specular reflection direction.

High-order harmonic generation from controlled plasma mirrors

Ultrashort, high contrast laser pulses when focused to high intensity and reflected from a steep solid density 'plasma mirror (PM)' contain coherent XUV radiation in the form of high-order harmonics. The emission can either be due to the relativistically driven oscillating PM (ROM) [1] or due to Coherent wake emission (CWE) [2]. Selective control over the mechanisms and the characteristics of these harmonics and understanding the physics is crucial for the development of intense attosecond light sources. © 2013 IEEE.

Vacuum birefringence in strong inhomogeneous electromagnetic fields

Birefringence is one of the fascinating properties of the vacuum of quantum electrodynamics (QED) in strong electromagnetic fields. The scattering of linearly polarized incident probe photons into a perpendicularly polarized mode provides a distinct signature of the optical activity of the quantum vacuum and thus offers an excellent opportunity for a precision test of nonlinear QED. Precision tests require accurate predictions and thus a theoretical framework that is capable of taking the detailed experimental geometry into account. We derive analytical solutions for vacuum birefringence which include the spatio-temporal field structure of a strong optical pump laser field and an x-ray probe. We show that the angular distribution of the scattered photons depends strongly on the interaction geometry and find that scattering of the perpendicularly polarized scattered photons out of the cone of the incident probe x-ray beam is the key to making the phenomenon experimentally accessible with the current generation of FEL/high-field laser facilities.

Modifications of Surface Properties of Beta Ti by Laser Surface Treatment

β -type Ti-alloy is a promising biomedical implant material as it has a low Young’s modulus but is also known to have inferior surface hardness. Various surface treatments can be applied to enhance the surface hardness. Physical vapour deposition (PVD) and chemical vapour deposition (CVD) are two examples of this but these techniques have limitations such as poor interfacial adhesion and high distortion. Laser surface treatment is a relatively new surface modification method to enhance the surface hardness but its application is still not accepted by the industry. The major problem of this process involves surface melting which results in higher surface roughness after the laser surface treatment. This paper will report the results achieved by a 100 W CW fiber laser for laser surface treatment without the surface being melted. Laser processing parameters were carefully selected so that the surface could be treated without surface melting and thus the surface finish of the component could be maintained. The surface and microstructural characteristics of the treated samples were examined using X-ray diffractometry (XRD), optical microscopy (OM), 3-D surface profile & contact angle measurements and nano-indentation test.

Modifications of Surface Properties of Beta Ti by Laser Gas Diffusion Nitriding

Beta-type Ti-alloy is a promising biomedical implant material as it has a low Young’s modulus and is also known to have inferior surface hardness. Various surface treatments can be applied to enhance the surface hardness. Physical vapor deposition and chemical vapor deposition are two examples of this but these techniques have limitations such as poor interfacial adhesion and high distortion. Laser surface treatment is a relatively new surface modification method to enhance the surface hardness but its application is still not accepted by the industry. The major problem of this process involves surface melting which results in higher surface roughness after the laser surface treatment. This paper will report the results achieved by a 100 W continuous wave (CW) fiber laser for laser surface treatment without the surface being melted. Laser processing parameters were carefully selected so that the surface could be treated without surface melting and thus the surface finish of the component could be maintained. The surface and microstructural characteristics of the treated samples were examined using x-ray diffractometry, optical microscopy, three-dimensional surface profile and contact angle measurements, and nanoindentation test.