S. G. Przhibel'skii

Granular metal films on the surfaces of transparent dielectric materials studied and modified via optical means

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.

Destruction of the nano-size solid particles under femtosecond laser pulse action

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.

Dynamics of laser-induced damage of spherical nanoparticles by high-intensity ultrashort laser pulses

Abstract. 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.

Shaping of surface nanostructures via non-thermal light-induced processes

Non-thermal light-induced surface processes hold out hope for the development of new approaches to surface nanostructuring. Although they do not seem to be as flexible and universal as photolithography a number of niche application are waiting for a specific cheap and easy process. The light induced atomic desorption is a reliable tool to control the surface number density of the adsorbed atoms in the course of the physical vapor deposition process. The strong enough illumination diminishes the number density of the adsorbed atoms below the threshold value needed for the beginning of the nucleation process. Hence, the deposition pattern reproduces the distribution of the illumination intensity over the surface. In some cases plasmonic nanostructures are obtained via self organization of metal deposits on dielectric substrates. Their performance is severely hindered by the broad size and shape distributions. The non-thermal light-induced detachment of surface atoms from the metal nanoparticles may be used to tune their size and shape. In particular, we find that a narrow dip in the shape distribution of the metal nanoparticles may be obtained by the laser treatment at the specific wavelengths.

Metal nanosphere at an interface: revival of degeneracy of a dipole plasmon

Metal nanoparticles supporting surface plasmon modes are used in many areas of science and technology. Often it is important to know the exact location of the metal nanoparticle relative to a larger dielectric object. In this paper, we demonstrate that this goal may be achieved by monitoring the localized surface plasma resonance splitting in the course of the nanoparticle movement. In particular, we simulate splitting of the plasma resonance localized in a metal nanosphere while it approaches and penetrates the interface of two dielectric media. Numerical simulations show that splitting goes through two maxima at the beginning and at the end of the penetration process while the plasmon modes become exactly degenerate at some distance near the midpoint of the nanosphere trajectory. These results may be used to real time monitoring of the exact position of the nanoparticles while they approach and penetrate different targets. Applications in the drug delivery, photodynamic therapy and other biomedicine branches are envisioned.

Photo-ionization and modification of nanoparticles on transparent substrates by ultrashort laser pulses

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.

Effect of illumination on the electron transport in the metal island film

In the present work the mechanisms of the optical radiation action on the Na metal island films, representing ensemble of nanoparticles deposited on a dielectric substrate and interacting with each other due to the electron transport between the islands, were investigated. The major effect of optical radiation action on the film conductivity was found to be caused by photons with the energies above the threshold of the photoeffect in Na. The appreciable action of illumination with the wavelengths greater than the photoelectric threshold was also detected and interpreted.

Ultra-narrow spectroscopic cells in atomic spectroscopy: reflection, transmission, fluorescence, and nonadiabatic transitions at the walls

Ultra-narrow cells with the thicknesses in the range from several wavelengths to the small fractions of the wavelength brought a number of new opportunities for atomic spectroscopy. Depending on the cell thickness, spectral lines recorded in ultra-narrow cells are either Doppler-free or Doppler-broadened. With careful selection of the cell thickness hyperfine structure may be easily resolved without resorting on the multibeam nonlinear optical techniques. Moreover, frequent collisions with the walls leads to the important modifications of velocity selective optical pumping resonances. Finally, ultra-narrow cells provide with the unique opportunity to study collisions of the excited atoms with the solid surfaces. In this contribution several examples of the use of the ultra-narrow spectroscopic cells filled with the alkali atomic vapour is presented. First, we discuss general aspects of the transient polarisation that defines all peculiarities of an ultra-narrow cell as a spectroscopic tool. Second, we demonstrate the resolution of the magnetic sublevels in the transition from Zeeman to Paschen-Back regime in the Cs hyperfine structure. Third, new aspects of velocity selective optical pumping resonances in reflection and transmission of resonant radiation by the 6 wavelengths thick cell filled with Cs are discussed. Forth, the experimental evidences of the nonadiabatic transitions between excited states of Rb atoms in the course of collisions with the sapphire surface are presented.

Atom-wall interactions and their role in the spectroscopy of spatially constrained atomic vapors

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.

Spectroscopy of the atom-wall interactions in a nanocell

The optical spectroscopy of rubidium atoms confined to a nanocell was employed to study the quenching and energy transfer processes in the course of atom-wall collisions. The distance between the sapphire windows of the cell varies between 150 and 500 nm forming a vapour wedge. At moderate pressures, the frequency of the atom-wall collisions greatly exceeds the frequency of atom-atom collisions. Hence, the line shapes and intensities recorded in a nanocell provide valuable information about interactions of excited atoms with the surface of the window material.