T. A. Vartanyan

Optical properties of nanostructured gold-silver films formed by deposition of small colloid drops

The formation of gold-silver cluster structures on surfaces of glasses has been investigated. The properties of these structures are analyzed in dependence on the weight ratio of metal nanoparticles and the particle concentration in the colloid. It is shown that the optical properties of these structures may change significantly with a change in the morphological properties and chemical composition of the structures.

Fine structures and switching of electrical conductivity in labyrinth silver films on sapphire

Changes in electrical resistance of silver films were measured in the range from 1013 to 103 Ω during thermal deposition on sapphire in a high vacuum, after the deposition over time, and under an applied voltage. The dependences of the electrical resistance of the films on their thickness and deposition rate were determined. It was established that, with an increase in the film thickness from 2 to 10 nm during the deposition at rates of 0.6 and 0.1 Å/s, the resistance decreases by 7.5 and 4 orders of magnitude, respectively. The measured dependences of the resistance on the deposition time were found to be close to exponential. The room-temperature resistance of 10-nm-thick films deposited at different rates changed spontaneously by 3–4 orders of magnitude in different ways: the resistance of the slowly deposited films spontaneously increased, whereas in the rapidly deposited films, it spontaneously decreased. After fine annealing, the steady-state resistance changed also differently: it increased by 2 orders of magnitude in the former case and by 9 orders of magnitude in the latter case. Under voltages above 5 V, the resistance of the rapidly deposited films abruptly decreased from ∼1012 to ∼106 Ω, and these films became ohmic. After fine annealing, they became again high-ohmic. Under voltages above 5 V, the high-ohmic films thus obtained became again low-ohmic. This cycle of electrical conductivity switching was reproduced many times. The observed phenomena were explained in the framework of the hypothesis of the formation of fine metastable structures in channels of labyrinth films, namely, protrusions and bridges that bring together the boundaries of islands and connect them into conducting clusters, respectively.

Mutual modification of silver-nanoparticle plasmon resonances and the absorptive properties of polymethine-dye molecular layers on a sapphire surface

Extinction spectra of a hybrid material comprised of molecular layers of polymethine dyes and granulated silver films preliminarily deposited onto sapphire substrates are studied experimentally. Specific features of the dye layer composition and broadening of plasmon absorption bands are discussed. In hybrid samples, both an increase and a decrease in the silver-nanoparticle plasmon resonance frequency upon spreading of dyes have been observed, as well as a considerable increase in the dye molecule absorption. Possible mechanisms of the phenomena are discussed.

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.

Evolution of the optical properties and morphology of thin metal films during growth and annealing

The formation kinetics of an island silver film on a dielectric substrate and changes in the film morphology during heating have been investigated using optical methods. The results obtained are interpreted within a model relating the film optical properties with the shape and size of the nanoparticles forming this film. The proposed interpretation is confirmed by electron microscopy data.

Correlation between the morphological characteristics and extinction spectra of sodium nanoparticles and modification of nanoparticles through photoatomic emission

The morphological characteristics of ensembles of metal nanoparticles on transparent dielectric substrates have been studied by the methods of hole-burning and linear absorption spectroscopy. The size and shape of particles were changed under the exposure to the optical radiation that induced photodetachment of atoms and their photostimulated diffusion over the surface of nanoparticles. The correlation between holes burned in different absorption bands is explained by the morphological features of the island film. The relaxation times of individual modes of collective plasma oscillations and the shape parameters of particles modeled by three-axial ellipsoids have been determined.

Determination of the structure of hyperfine sublevels of Rb in strong magnetic fields by means of the coherent population trapping technique

The splitting of hyperfine sublevels of the 85Rb atom in strong magnetic fields has been studied by means of the coherent population trapping technique. Narrow resonances with a high signal-to-noise ratio have been detected in a 30-μm-thick spectroscopic cell. The magnetic field in the direction transverse to the laser beams has been created by permanent magnets and has reached 1600 G. Owing to the exclusive narrowness of the cell, the field in it is almost uniform. The break of the coupling between the electronic and nuclear moments, as well as the transition to the Paschen-Back regime in magnetic fields above 600 G, has been observed. The derivatives of the frequency shifts of the observed resonances and their asymptotic values in strong magnetic fields have been determined in terms of the magnetic field strength. The experimental results have been interpreted within a theoretical model based on the known constants of the hyperfine structure of the Rb atom.

Fine-structure mixing of excited states of cesium atoms upon an impact on the surface of a sapphire single crystal

The spectrum of resonantly excited luminescence of Cs atoms in a microcell with sapphire windows has been studied. Spectral wings of atomic luminescence near the windows have been revealed near (±50 cm−1) the D2 line of the excited 6S1/2–6P3/2 transition. At the same time, the radiation of the D1 line has been found. The features of the spectrum of the far wings, namely, the exponential shape and the relatively small difference between the static and the antistatic wings, as well as the radiation of the D1 line, have been explained by the mixing of fine-structure states due to interactions of Cs atoms with the sapphire windows. From the relative intensity of the lines, it has been determined that the probability of luminescence quenching at the window surfaces differs from unity by no less than 1/700.

The influence of the reduced dimension on the dephasing time of surface plasmon excitation in gold nanoparticles

In this contribution, we present measurements of the ultrafast dephasing time T2 of surface plasmon polariton excitation in gold nanoparticles by means of persistent spectral hole burning. T2 is an essential parameter that does not only reflect the role of different dephasing and deexcitation mechanisms but also allows one to determine the field enhancement factor that is of great importance for many applications of nanoparticles. In our experiments gold nanoparticles were first fabricated in ultrahigh vacuum on sapphire substrates by deposition of atoms, followed by diffusion and nucleation, i.e. Volmer-Weber growth. Subsequently, systematic measurements of T2 in the size range between r = 7 nm and 14 nm were carried out. The most essential among the numerous results is the observation of the influence of the reduced dimension on the dephasing time. While T2 = 14 fs has been measured for r = 12 nm which is, within the error bars, consistent with the damping contained in the bulk dielectric function, the value of T2 shrinks to, for example, T2 = 11 fs for r = 7 nm. This reduction of T2 can be attributed to surface scattering of the electrons. Further experiments are in progress to confirm the predicted 1/r law for the variation of T2.

Comparative investigation of the effect of heat and optical radiation on the structure of island metal films by optical fluctuation microscopy

Variations in the structural parameters of island sodium films on the quartz surface, caused by heating or exposure to optical radiation, are investigated by the optical method proposed earlier. A correct scheme of processing the measured data is developed. It is shown that to determine the structure parameters, the measurements of both the mean values and fluctuations of transmission and reflection coefficients of the film and of their correlator are required. It is also demonstrated that under a number of assumptions, it is possible to determine the character of the process of evaporation of islands and to reveal the evolution of the distribution function by using the data of measurements. It was found experimentally that variations in the structural parameters of a film during its evaporation caused by heating of the substrate or irradiation by light can noticeably differ. This fact is explained by a difference in the mechanisms of action of heating and irradiation on an island.