We report here evidence of phase-matched optical wave mixing in the extreme ultraviolet (XUV) region. This process has been studied with a collinear two-colour high-order harmonic generation scheme. An 800 nm, 30 fs driving field is used to produce a small bandwidth comb of odd harmonic orders (wavelength around 30 nm) in a long cell filled with argon gas. Mixing frequencies in this spectral range are produced by applying a second weak control-field of 1,400 nm, 40 fs. Low order (third- and fifth-order) nonlinear optical wave mixing is observed to be a phase-matched process. The dependence of the intensity of the harmonic orders and the mixing frequencies on different control-field intensities, gas pressure, and interaction length is analysed to verify the phase matching process.
We study the use of a second driving beam to enhance the phase matching and also to create wave mixing and parametric amplification in extreme ultraviolet region. New methods for studying coherent processes in atoms and molecules and for imaging with high spatial resolution have been proposed and developed
Dynamics of a laser-induced optical breakdown in the bulk of fused silica initiated by a sub-nanosecond laser pulse of an energy fluence as high as 8.7 kJ/cm2 was investigated by using femtosecond time-resolved shadowgraphy. Plasma ignition, growth of the damaged region and accompanying hydrodynamic motion were recorded from the moment directly before the arrival of the driving laser pulse, in the time steps adapted to the rate of the occurring processes. The growth rate of the plasma channel, curvature radii and velocities of the wave fronts were extracted from the shadowgrams. It was found that the plasma channel develops with a supersonic velocity and the first observed shock front tends to transform itself from the initial bowl-like shape to the final spherical one characterising an acoustic wave. Appearance of multiple fronts accompanying the main shock front was registered and used in more detailed analysis of the optical breakdown dynamics in the transparent dielectrics.
We present characterization of structural modification triggered by tightly focused single pulses of a nanosecond laser inside single-crystal sapphire. Structural changes induced in the shock compressed region were investigated using high resolution transmission electron microscopy (HRTEM). Analysis of the zone around cavity in the bulk of sapphire reveals loss of crystalline order and formation of a mixture of amorphous/poly-crystalline structure. The properties of the laser-affected solid and possible routes of material transformation to the final state long after the pulse end is discussed. The results suggest that transformations to amorphous/poly-crystalline state occur as a result of sufficient heating of the shell region. This creates a localized molten zone which solidifies so rapidly that crystallization is by-passed.
Amount of energy deposited by a single, tightly focused nanosecond laser pulse in different transparent materials was measured vs. either the incident intensity or energy fluence. An integrating sphere enabled quantification of the energy scattered during the long-lived breakdown process. It was shown that absorptance dependence on the photon flux can be analytically described by the sigmoidal Hill function. We suggest using this analytical description to quantify empirical laser-induced breakdown threshold (LIBT). The structured changes in the breakdown area were analysed by the means of different kinds of microscopy, especially by the high-resolution-transmission- electron-microscopy (HRTEM). The analysis revealed nanocrystallisation in the densified material surrounding the void.
A single pulse of a nanosecond laser was tightly focused in the bulk of transparent materials (soda lime glass,
borosilicate glass, fused silica , sapphire and Gorilla Glass) to a beam spot diameter of ~ 2.1μm. A value of
the total energy absorbed in the materials was measured with corrections for the transmitted, scattered and
reflected components of the incident energy. It was found that 3-11% of the incident radiation was scattered
but the total absorption still achieved a very high level of up to 88%. Absorptance dependence on the incident
fluence was reasonably approximated by the sigmoidal Hill function. Here we suggest using this analytical
description to identify empirical intrinsic laser-induced breakdown threshold (LIBT). Optical damage threshold
(ODT) was identified by optical inspection. The results for some materials suggest significantly lower breakdown
threshold than that reported earlier for more loosely focused beams. A study of the damage area morphology
with a scanning electron microscope (SEM) and a high resolution transmission microscope (HRTEM) revealed
existence of the shock waves-affected area with a localized nano-crystallization. Spectroscopic study of the light
emission accompanying breakdown showed typical quasi-continuum emission with temperature as high as 8917K