Near-field scanning optical microscopy studies of thin film surfaces and interfaces

In this article, the results of the modeling of topography related artifacts appearing in near-field scanning optical microscopy measurements are presented. The results obtained for near-field scanning optical microscope operation in reflection mode with off-axis far field detector position are compared with experimental results. It is shown that the chosen numerical method - Finite Difference in Time Domain method (FDTD) - can be used for efficient modeling of main topography related artifact. It is also seen that the far field detector position can have large influence on the resulting reflection mode optical images.     

Temperature dependent photoluminescence down to 4.2 K in EuTFC

The temperature dependence of photoluminescence in Europium tris[3-(trifluoro-methylhydroxymethylene)-(+)-camphorate] (EuTFC) embedded in polymer films has been examined from 40 K down to 4.2 K with the goal of preparing sensor films for low-temperature thermal imaging. The behavior of EuTFC showed significant difference when based on polystyrene compared to poly(n-alkyl methacrylate)s. In poly(n-alkyl methacrylate)s prepared by standard methods for imaging applications, the photoluminescence is fully saturated below 30 K, whereas in polystyrene films there is a strong temperature dependence even down to 4.2 K. By optimizing the preparation procedure for films made of poly(butyl methacrylate) (PBMA) and poly(methyl methacrylate), also these polymers became very sensitive down to liquid helium temperature. The maximum temperature sensitivity of EuTFC in PBMA is found to be 1.0%/K at 4.2 K. The problem of delamination and cracking of the polymer film at cryogenic temperature is also avoided by the special preparation method.     

Electron-vibrational interaction in 4f-5d optical transitions in Ba, Ca, Sr thiogallates doped with Eu ions

This study is devoted to the problem of the electron-vibrational interaction in 4f-5d optical transitions. We analyze the room temperature experimental data on the vibronically assisted broad bands arising from the 4f-5d transitions in Ba, Ca, and Sr thiogallates doped with Eu²⁺ ions. An approximate simple expression is given for the shape function of the broad vibronic bands with allowance for the terms corresponding to the emission (absorption) from both ground and first excited vibrational levels. We estimate the vibronic coupling parameters and the Stokes shifts for these systems. The theoretical results are in a good agreement with the experimental data.     

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al³⁺ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn_i or V_OZn_i) to valence band and/or deep acceptor states (VB, V_Zn or O_i or O_Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al³⁺ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.     

A new blue-emitting phosphor, SrZnO2:Pb, synthesized by the adipic acid templated sol-gel route

A new blue-emitting phosphor, Sr_1−xPb_xZnO₂, was prepared by a novel adipic acid templated sol-gel route. Photoluminescence and crystalline properties were investigated as functions of calcination temperatures and the Pb²⁺ doping levels. It was found that under UV excitation with a wavelength of 283 or 317 nm, the phosphors gave emission from 374 to 615 nm with a peak centered at 451 nm. This broad-band was composed of UV and the visible range was attributed to an impurity-trapped exciton-type emission. The maximum emission intensity of the Sr_1−xPb_xZnO₂ phosphors occurred at a Pb concentration of x=0.01. The decay time was observed to be ∼33 ms for the compound doped with 1 mol% Pb prepared at 1000 °C. Diffuse reflectance spectra revealed the characteristic absorption peaks and the bandgap energy of SrZnO₂ was found to be 3.4 eV. SEM analysis indicated that phosphor particles have an irregularly rounded morphology and the average particle size was found to be approximately 1 μm.     

Nonresonant two-photon generation of excitonic matter in a CuCl pellet

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.     

First-principles study on electronic structures of BaWO4 crystals containing F-type color centers

The electronic structures of BaWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded that F and F⁺ color centers have donor energy level in the forbidden band. The optical transition energies are 2.449 and 3.101 eV, which correspond to the 507 and 400 nm absorption bands, respectively. It is predicted that 400-550 nm absorption bands originate from the F and F⁺ color centers in BaWO₄ crystals.     

Surface state density distribution at vacuum-annealed InP(1 0 0) surface as derived from the rigorous analysis of photoluminescence efficiency

The surface state density distribution N_SS(E) and surface Fermi level E_Fs position on a sequentially ultra-high vacuum-annealed n-InP(1 0 0) sample are investigated using rigorous computer analysis of dependences of the room temperature band-edge photoluminescence efficiency (Y_PL) on the photon flux density (Φ). We have found that the minimum density of a U-shaped N_SS(E) distribution as well as the donor-like surface state density are reduced by one order of magnitude after the annealing at 250 and 300 °C. This can be assigned to the decrease in the disorder in the unintentionally formed InP native oxides. On the other hand, we demonstrate that the annealing simultaneously generates discrete surface states probably due to missing group V element (P) in the interface region which may account for electrical interface instabilities observed in InP-based devices. The results are discussed quantitatively and compared to other reports.     

Cs^＋-K^＋ Exchange in Z-Cut KTiOPO4 Crystal Covered with Chromium Masking Layer

Cs^＋-K^＋ ion exchanges are performed on z-cut KTiOPO4 crystals with chromium coating covered. The temperature of ion exchange is 430℃, and the time range from 15min to 30min. The dark mode spectra of the samples are measured by the prism coupling method. The channel structures on the samples are observed by a microscope and the near field pattern of the channel waveguides are measured by the end-fire coupling method. The refractive index of the samples increases and the increments at surface are modulated due to the existence of Cr film. In the region covered by Cr film, the refractive index of the samples at the surface increases dramatically in a shallow layer. The results of energy dispersive x-ray spectra indicate that in the region covered with Cr film, Cr ions participate in the ion exchange process, and enhance the refractive index. The results may provide a possibility that achieves index enhancement and Cr doping synchronically.     

Off-diagonal magnetoimpedance in stress-annealed amorphous ribbons

The influence of creep-induced magnetic anisotropy on the off-diagonal magnetoimpedance in amorphous Co₆₇Fe₄Cr₇Si₈B₁₄ ribbon is investigated. Hard-ribbon-axis anisotropy is produced by continuous stress annealing. On applying DC bias current, the off-diagonal impedance becomes an antisymmetric function of applied field with a quasi-linear part around the zero field. Theoretical explanation of the phenomenon is based on classical electrodynamics. It is shown that the maximum on the frequency dependence of off-diagonal impedance, observed around 1 MHz, results from competition between the electromagnetic induction and the skin effect. The quasi-linear characteristic can be utilized in low-cost high-sensitive magnetic field sensors.