Atom-wall interactions play an unexpectedly important role in the atomic spectroscopy. J.L. Cojan was the first who observed and then interpreted the effects of the atom-wall interactions on the reflection spectra in the vicinity of the atomic spectral line. His observation was made on the mercury vapors of such a low concentration that the Doppler width was much larger than the homogeneous width of the atomic transition. Surprisingly, the width of the spectral line he observed in reflection was much smaller than the Doppler width. He pointed out that the atoms those leave the window posses a transient rather than the stationary polarization. This is the reason why their contribution to the reflected field differs from what was expected. M. Ducloy employed the tiny distortions of these narrow resonances in reflection spectra to measure for the first time the van der Waals constants in the excited atomic states. In our work we considered reflection from a narrow slice of atomic vapors and found a manifold of spectral line shapes depending on the width of the vapor slice that have nothing in common with the Fabri-Perot resonances. It was not until the invention of an Extremely Thin Cell (ETC) by D. Sarkisyan that the observation of these effects becomes possible in the optical domain. In the subsequent years ETC proved to be a very powerful tool of modern spectroscopy.
Damage of a metal spherical nanoparticle by femtosecond laser pulses is analyzed by splitting the overall process into two steps. The fast step includes electron photoemission from a nanoparticle. It takes place during direct action of a laser pulse and its rate is evaluated as a function of laser and particle parameters by two approaches. Obtained results suggest the formation of significant positive charge of the nanoparticles due to the photoemission. The next step includes ion emission that removes the excessive positive charge and modifies particle structure. It is delayed with respect to the photo-emission and is analyzed by a simple analytical model and modified molecular dynamics. Obtained energy distribution suggests generation of fast ions capable of penetrating into surrounding material and generating defects next to the nanoparticle. The modeling is extended to the case of a nanoparticle on a solid surface to understand the basic mechanism of surface laser damage initiated by nano-contamination. Simulations predict embedding the emitted ions into substrate within a spot with size significantly exceeding the original particle size. We discuss the relation of those effects to the problem of bulk and surface laser-induced damage of optical materials by single and multiple ultrashort laser pulses.
Granular films of alkali and coinage metals are the most popular objects for exploring plasmonic effects. They are easy to obtain via physical vapor deposition and to study via optical means. In this contribution we show several ways not only to record but also to modify the granular metal films using thermal and nonthermal optical effects.
The objective of this combined experimental and theoretical research is to study the dynamics and mechanisms of nanoparticle
interaction with ultrashort laser pulses and related modifications of substrate surface. For the experimental effort,
metal (gold), dielectric (SiO2) and dielectric with metal coating (about 30 nm thick) spherical nanoparticles deposited on
glass substrate are utilized. Size of the particles varies from 20 to 200 nm. Density of the particles varies from low (mean
inter-particle distance 100 nm) to high (mean inter-particle distance less than 1 nm). The nanoparticle assemblies and the
corresponding empty substrate surfaces are irradiated with single 130-fs laser pulses at wavelength 775 nm and different
levels of laser fluence. Large diameter of laser spot (0.5-2 mm) provides gradient variations of laser intensity over the
spot and allows observing different laser-nanoparticle interactions. The interactions vary from total removal of the nanoparticles
in the center of laser spot to gentle modification of their size and shape and totally non-destructive interaction.
The removed particles frequently form specific sub-micrometer-size pits on the substrate surface at their locations. The
experimental effort is supported by simulations of the nanoparticle interactions with high-intensity ultrashort laser pulse.
The simulation employs specific modification of the molecular dynamics approach applied to model the processes of
non-thermal particle ablation following laser-induced electron emission. This technique delivers various characteristics
of the ablation plume from a single nanoparticle including energy and speed distribution of emitted ions, variations of
particle size and overall dynamics of its ablation. The considered geometry includes single isolated particle as well a
single particle on a flat substrate that corresponds to the experimental conditions. The simulations confirm existence of
the different regimes of laser-nanoparticle interactions depending on laser intensity and wavelength. In particular,
implantation of ions departing from the nanoparticles towards the substrate is predicted.
We present the results of analytical and numerical study of spatial distribution of the fluctuating electric field near the
surface of a model ionic crystal as a function of temperature. The potential variance is calculated and it is shown that the
fluctuating field power density decays outside of the crystal as h-3, where h is the distance from the crystal suface. When
h > 6a (a is the lattice period), the fluctuating field exceeds the static field, which decays exponentially. At these
distances, the chatacteristic strength of the fluctuating field lies in the range of 106V/cm. The fluctuating potential
variance squared grows linearly at high temperatures and does not vanish at 0 K. The effect of the fluctating electric field
on formation of nanostructures on the crystal surface is considered.
We report the results of theoretical study of damage, induced by Coulomb forces, in (a) solid nanoparticles, and (b) the
surface of solid dielectric, ionized by ultrashort laser pulses (USLP). The basic assumption of proposed model is that the
damage occurs due to the laser-induced disturbance of charge equilibrium in solid with the further electron emission
from irradiated area. When electrons outflow from crystal, the non-compensated positive charge creates a strong
electrostatic field, causing the movement of the charged sites and micro- and/or macro- destruction of the condensed
matter.
The shape of the luminescence spectrum of Cs atoms flying near a sapphire surface is explored experimentally and described phenomenologically for the detuning range of 5 to 50 cm-1 on both sides of D2 line. The numerical fitting of one theoretical parameter and a small variation of another parameter ensure a satisfactory agreement with the experimental spectrum. The experimentally determined abnormally high intensity of the antistatic wing is explained by the effect of phase relaxation of the atomic transition, caused by fluctuations of the electric field of oscillating ions of the sapphire crystal lattice.
The results of both analytical and numerical study of the optical damage of a solid nano-size particle, partly ionized by
an ultra-short laser pulse (USLP) are presented. The comparison of the results that has been obtained analytically and
numerically shows that the proposed method allows to describe the main features of nano-particle damage induced by
Coulomb forces, that arise in solid due to the charge equilibrium distortion under USLP action.
The kinetics of energy spectra of the spreading ions has been analyzed taking into account the Coulomb repulsive forces
and the retarding processes that restrict ion motion inside the particle.
The results of theoretical investigation of processes resulting from disturbance of charge equilibrium in a solid nanoparticle,
induced by fast ionization under high-intensity ultra-short laser pulse action are presented. Analytical and
numerical methods were used to study time evolution of space and energy distributions of moving ions during their
spreading away from the particle for various sizes and shapes of the particle as well as various degrees of ionization of
its material. The final aim of the presented study is to analyze the influence of the disturbance of charge equilibrium
induced by high-intensity ultra-short laser pulses in a solid on parameters of laser-induced damage of the material, and
the regularities of the atoms and ions motion of the media in damage area under the action of local laser-induced
Coulomb potential.
The thermo-induced processes leading to the abrupt growth of the light transparency in the high-scattering media due to its homogenization under the intense laser beam action has been studied and discussed. It has been studied experimentally the light beam penetration into the solid dispersed stearin during its softening and melting under the laser-light action. The description and explanation of the experimental results has been done in the framework of the model based on the assumption, that the light scattering is defined by some phenomenological parameter namely the “order parameter” which is linked with the grain boundaries concentration in the non-homogeneous medium.
Alkali adsorption on the surface of the monocrystalline sapphire was studied by means of laser stimulated desorption. The photo-desorption yield was found to be proportional to the laser fluency up to the threshold of the thermo-desorption process. The feature of the experimental apparatus was the possibility to control the coverage by means of the surface temperature provided the adsorbate was in dynamic equilibrium with a vapor of constant pressure. It was found that alkali atoms are adsorbed mostly on the centers with the surface concentration of about 1013 cm-2. Although the temperature dependence of surface coverage is consistent with the conventional Langmuir model for adsorption on the isolated centers, the estimated adsorption energy of about 1 eV is too large to account for the quantum yield of photo-desorption process which is as high as 5 multiplied by 10-5. To resolve this contradiction we assumed that adsorption centers make up chains along the steps on the surface. Lateral interaction between neighboring adatoms were accounted for by means of methods developed for Ising model. Theoretical results are in agreement with the experiment.
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