Fedor A. Pudonin

Imaging of surface plasmons with a near-field microscope

Imaging of surface plasmon polaritons (SPPs) with a photon scanning tunneling microscope (PSTM) combined with shear force feedback is experimentally investigated. A contrast correction factor, that accounts for spatial frequency filtration performed by an uncoated fiber tip of the PSTM, is introduced and evaluated from the measurements of a standing wave interference pattern formed by two counter- propagating evanescent waves, that are generated by total internal reflection of light beams inside a glass prism. Influence of propagating field components stemming from inelastic SPP scattering on the resultant intensity distribution and, consequently, on the near-field optical images obtained with the PSTM is discussed. Optical images taken at different tip-surface distances are used to evaluate the contribution from propagating field components in near-field optical images taken with shear force feedback. The approach developed is applied to experimental studies of elastic scattering of SPPs excited at the wavelength of 633 nm along smooth and rough surfaces of gold and silver films.

Quantum size effects in the optical conductivity of ultrathin metal films

Abstract Optical and electrical properties of ultrathin (0.15–7 nm) metal films (Nb, Cu, Mo, W, Ni and Ti) deposited by radio-frequency sputtering have been studied. Infrared (IR) reflectivity, transmittance as well as direct current (DC) conductivity measurements have been used to characterise these films. The periodic thickness dependence of the optical and DC conductivity was found. The periods of oscillations for different metals are of the order of the electron Fermi wavelength. Thus it was proposed that these oscillations are due to quantum size effects.

Thickness dependence of optical second-harmonic generation from ultrathin niobium films

Abstract The optical second-harmonic generation from ultrathin niobium films embedded in a dielectric is measured as a function of the film thickness (in the range 6–42 A) for the p ω -to- p 2 ω and s ω -to- p 2 ω polarization configurations. Using a pump wavelength of 532 nm, it is found that the p ω -to- p 2 ω second-harmonic intensity exhibits a pronounced maximum for a film thickness of ∼ 15 A. The s ω -to- p 2 ω second-harmonic signal exhibits a step-like increase at the same thickness. By modeling the metal film as a symmetric quantum well, a microscopic local-field calculation of the second-harmonic generation is established, and numerical results for the thickness and angular dependencies of the second-harmonic energy reflection coefficient are presented. Using simply infinite-barrier wave functions together with a self-field approximation the theoretical calculation qualitatively accounts for the observed thickness dependence of the second-harmonic generation.

Out-of-Plane and In-Plane Magnetization Behavior of Dipolar Interacting FeNi Nanoislands around the Percolation Threshold

Magnetic properties of inhomogeneous nanoisland FeNi films were studied by SQUID magnetometry. The FeNi films with nominal thickness ranging from 0.6 to 2.0 nm were deposited by rf sputtering on Sitall glass substrates and covered by a protecting Al2O3 layer on the top. The SQUID data indicate pronounced irreversibility behavior for the out-of-plane temperature-dependent magnetization response measured at H≃100 Oe using zero-field cooling ZFC and field-cooled warming FCW after the applied dc magnetizing field Hm≃2 T for the FeNi samples with nominal thickness 1.1 nm ≲d≲1.8 nm, below the percolation threshold. The positive difference between the FCW and ZFC data identifies two irreversibility temperature scales, TB≈50 K and T⁎≈200 K, which can be associated with the superparamagnetic and superferromagnetic behavior in inhomogeneous nanoisland FeNi films, respectively. However, above the film percolation threshold, we observed a crossover from the out-of-plane to in-plane magnetization orientation. Here, the in-plane FCW-ZFC difference implies negative remanent magnetization response in the temperature range TB≲T≲T⁎. The observed magnetization properties can be associated with the presence of the superferromagnetic phase in self-assembled clusters of quasi-2D metallic magnetic FeNi nanoislands.

Interplay of electron correlations and localization in disordered β-tantalum films: Evidence from dc transport and spectroscopic ellipsometry study

We report the dc transport (5 K ≲ T ≲ 380 K) and spectroscopic ellipsometry (0.8 eV ≤ hν ≤ 8.5 eV, T ≃ 300 K) study of β-Ta films prepared by rf sputtering deposition as a function of their thickness in the range 2.5 nm ≲ d ≲ 200 nm. The dc transport of the β-Ta films with a thickness d ≳ 25 nm is characterized by negative temperature coefficient of resistivity (TCR) caused by localization effects peculiar of highly disordered metals. Their dielectric function spectra display non-metallic-like behavior due to the presence of the pronounced band at 2 eV. We found that with increasing TCR absolute value, specifying elevated degree disorder, the optical spectral weight (SW) of free charge carriers decreases. The associated SW is recovered in the range of Mott-Hubbard transitions, indicating the mechanism of localization enhancement by electronic correlations in disordered metals.

Spectral filtration of XUV radiation with the help of thin film filters in the TEREK-C solar telescope and RES-C spectroheliograph

The characteristics of different types of thin film XUV filters for the TEREK-C solar telescope and the REC-C spectroheliograph developed in the framework of the CORONAS-I program are presented. We describe the design and properties of Al-Formvar composite filters on rigid mesh supports with a spacing of 0.7 mm and 2 mm, which have been mounted in front of the XUV spectral channels of both instruments. It was proposed to use thin films of SiC as bandpass filters with high transparency in the 13 - 30 nm spectral region and effective cutoff blocking of strong solar Lα (121.7 nm) line radiation. The Al and SiC thin films on fine porous polymer membranes with through pores were placed in close proximity of the entrance plane of the image detectors, designed as a combination of open type microchannel plate and CCD-matrix. The problem of elimination of the influence of diffraction effects on X-ray images in the telescope focal plane is considered. All filters have sustained without damage the launching overloads (the launch took place on March 12, 1994) and successfully worked in the TEREK-C and the RES-C instruments.

SiC: filter for extreme ultraviolet

It is proposed to use thin films of silicon carbide as Extreme Ultraviolet bandpass filters transparent within 135-304 A band and with excellent cutoff blocking of the strong L(alpha ) 1216 A line radiation. Mesh or particle track porous membrane supporting 200-800 A thickness SiC filters have been made by RF sputtering techniques. We describe the design and performance of these filters. Such type SiC filter was used in front of the microchannel plate detector of the TEREK X-Ray Telescope mounted on the Solar Observatory CORONAS-I which was successfully launched on March 2, 1994.

Linear optical properties and second-harmonic generation from ultrathin niobium films: A search for quantization effects

We present the experimental investigations of the optical properties of ultrathin niobium films embedded in a dielectric that form quantum well structures. The linear optical properties of the Al/sub 2/O/sub 3/-Nb-Al/sub 2/O/sub 3/ structures in the near infrared and visible spectral ranges exhibit periodic dependencies on the Nb layer thickness that can be explained by the influeuce of quantum size effects on the electron scattering rate. The absorption bands that can be ascribed to the intersubband transitions have not been found in the linear optical spectra. The investigations of the optical second-harmonic generation from the Nb films show that the thickness dependencies of the second-harmonic intensity reveal resonant behavior that can be accounted for by the intersubband transitions in the niobium quantum wells. The microscopic local-field calculations qualitatively explain the thickness dependence of the SHG observed in the experiment. >

Infrared reflectivity and dielectric permeability of ultra-thin Cu and Al films

In this report we present an experimental investigation of the reflectivity (R) and the dielectric permeability (e) for Cu and Al ultra-thin films ranging in thickness from a few monolayers to 12 nm at infrared and visible wavelengths. The metal films were prepared by RF-sputtering on SiO2 (glass) and Si substrates. IR reflectivity was measured at 9.2 μm, while e was measured with the help of laser ellipsometer at a wavelength of 632.8 nm. Two types of oscillations on R(d) and e(d) were discovered for two thickness regions determined by the critical thickness value d*. Oscillations at d d* (thickness between 6–12 nm) we discover a new type of strong oscillation of R(d) and e(d) with an oscillating period of ∼0.2 nm. For thickness larger than 12 nm all the oscillations tend to disappear and R and e behave almost as their volume values. A possible explanation for the appearance of these two kinds of oscillations is based on the introduction of the critical film thickness d*.

Optical second-harmonic generation from ultrathin niobium films

Experimental and theoretical investigations of the optical second-harmonic generation (SHG) from ultrathin niobium films embedded in a dielectric are presented. The dependence of the intensity of second-harmonic emission on the film thickness (in the range from 6 to 42 angstroms) and the angular dependencies of SHG are investigated for different polarization configurations. The thickness dependencies of the second-harmonic intensity reveal resonant behavior: the p(omega )-to-p2(omega ) second-harmonic intensity exhibits a pronounced maximum for a film thickness of approximately 15 angstroms while the s(omega )-to-p2(omega ) second-harmonic signal exhibits a step-like increase at the same thickness. By modeling the metal film as a symmetric quantum well, a microscopic local-field calculation of the second-harmonic generation is performed, and numerical results for the thickness and angular dependencies of the second-harmonic energy reflection coefficient are presented. Using simply infinite-barrier wave functions together with a self-field approximation the theoretical calculations qualitatively describe the observed thickness dependence of the second-harmonic generation which can be accounted for by the intersubband transitions in the quantum wells formed by the niobium films.