Multifocal Electroretinography as a Method of Functional Assessment of Retinal Laser Injury in Experimental Studies

Purpose: to investigate local functional changes in the rabbit retina by multifocal electroretinography under pulsed laser radiation.Materials and methods. Transpupillary applications by single laser pulses (Nd:YAG laser, 532 nm, 50 ms) with the diameter of the laser beam spot on the retina surface of 132 µm (15 eyes) and 200 µm (15 eyes) were performed on 30 eyes of 15 rabbits. In each eye were applied 6 applications of different laser pulse power (15, 30, 50, 100, 150 and 200 mW). The diameter of the injury zone was assessed ophthalmoscopically and histologically. Multifocal electroretinography was performed before and 30 minutes after exposure using a module for multifocal electroretinography Neuro-ERG (Neurosoft, Russia), topographically comparing a pattern of 61 hexagons with an ophthalmoscopic fundus picture. The amplitude (µV) of the P1 peak and the implicit times (ms) of the P1 and N1 peaks were estimated in the first-order response in the hexagons corresponding to the laser damage zones.Results. When using a laser spot on the retina of 132 microns and 200 microns, the threshold level of laser radiation power for the development of significant local changes in the bioelectric activity of the retina was 50 and 30 mW, respectively (p < 0.05). The minimal diameter of the laser damage at which significant functional changes are recorded by multifocal electroretinography with a pattern stimulator consisting of 61 hexagons was 127.2 ± 6.4 µm (spot 132 µm, energy 50 mW), while a significant relationship was found between changes in the peak amplitude P1 and ophthalmoscopic and histological dimensions of the damage zone (r = 0.73 and r = 0.71, respectively, p < 0.01).Conclusion. The use of multifocal electroretinography can be used to quantify functional changes in local laser damage to the retina in experimental studies on rabbits.

The Influence of Laser Alloying of Ti13Nb13Zr on Surface Topography and Properties

The laser alloying is a continually developing surface treatment because of its significant and specific structuration of a surface. In particular, it is applied for Ti alloys, being now the most essential biomaterials’ group for load-bearing implants. The present research was performed on the Ti13Nb13Zr alloy subject to laser modification in order to determine the treatment effects on surface topography and its some mechanical properties like nanohardness, Young’s modulus, roughness. A pulse laser Nd:YAG was applied at three different laser pulse regimes: either 700 W, 1000 W or 1000 W treatment followed by 700 W modification at a pulse duration of 1 ms. The surface topography and morphology were examined using light microscopy and scanning electron microscopy with spectroscope of X-ray energy dispersion. The mechanical properties were determined by nanoindentation tests and surface roughness with a use of profilograph. The wettability was tested with a goniometer. The obtained results demonstrate complex behavior of the material surface: decrease in penetration distance and increase in hardness after first laser treatment, maintenance of this trend when machining using a higher laser pulse power, followed by an increase in penetration and decrease in hardness after additional laser treatment at lower power input, due to which a surface with fewer defects is obtained. The change in Young’s modulus follows the change in other mechanical properties, but not a change in roughness. Therefore, the observed hardening with the increase of the laser pulse power and then a small softening with the use of additional treatment with lower power can be attributed to some processes of remelting, diffusion and crystallization, sensitive to the previous surface state and heat energy flux. Despite that, the laser treatment always caused a significant hardening of the surface layer.

Electron energy relaxation under terahertz excitation in (Cd1− x Zn x )3As2 Dirac semimetals

We demonstrate that measurements of the photo-electromagnetic effect using terahertz laser radiation provide an argument for the existence of highly conductive surface electron states with a spin texture in Dirac semimetals (Cd1− x Zn x )3As2. We performed a study on a range of (Cd1− x Zn x )3As2 mixed crystals undergoing a transition from the Dirac semimetal phase with an inverse electron energy spectrum to trivial a semiconductor with a direct spectrum in the crystal bulk by varying the composition x. We show that for the Dirac semimetal phase, the photo-electromagnetic effect amplitude is defined by the number of incident radiation quanta, whereas for the trivial semiconductor phase, it depends on the laser pulse power, irrespective of wavelength. We assume that such behavior is attributed to a strong damping of the interelectron interaction in the Dirac semimetal phase compared to the trivial semiconductor, which may be due to the formation of surface electron states with a spin texture in Dirac semimetals.

Physics of short pulse laser plasma interaction by multi-dimensional particle-in-cell simulations

Interaction of relativistically strong laser pulses with under- and overdense plasmas is studied by 3D particle-in-cell simulations. We show that electrons in the underdense plasmas can be accelerated not only by the plasma wake field, but also by direct laser push in self-generated magnetic and electrostatic fields. These two mechanisms of acceleration manifest themselves in the electron energy spectra as two effective “temperatures.” We show that the fast electrons transport a significant part of the laser pulse power through the overdense plasma in the form of magnetized jets. We also find high collective stopping because of an anomalous resistivity of the plasma.

Relativistic and Nonlinear Radiation Interaction Between Laser Beams and Plasmas

Starting from a combination of Maxwell's laws for the electromagnetic field and the conservation. equations for a fully ionized plasma, the appropriate equations describing electrodynamic laser propagation and plasma dynamic particle motion are developed and solved. Calculations for multiply ionized transient conditions are carried out to yield electric field amplitudes, radial electron number density distributions and the progress of formation of a self-focused beam filament as a function of the target plasma density distribution and the laser pulse power-time history, among other parameters. Separate solutions emphasizing field-induced plasma motion on the one hand and significant beam contraction on the other are illustrated.