Diffractive structure of semitransparent hologram overlays applied to prevent document forgery was analysed by a combination of microscopic and spectroscopic methods. It was shown that the diffractive structure is fully covered by a polymer. The shortest period was possible to detect by confocal optical microscope. The properties of diffracted light were analysed by spectrogoniometer; good correlation with the period of the structure was obtained. Therefore spectrogoniometric measurements may be applied to detect the period of diffractive structure inside semitransparent hologram overlays.
COBISS.SI-ID: 4626458
Picosecond (40 ps) pulsed Nd:YAG laser irradiation of a WTi thin film on silicon with a wavelength of 532 nm and a fluence 2.1 J/cm2 was performed in air. This led to significant changes of the chemical composition and morphology on the surface of the WTi thin film. The results show an increase in surface roughness, due to formation of conical structures, about 50 nm wide in the base, and a very thin oxide layer composed of WO3 and TiO2, with a dominant TiO2 phase at the top, within the depth of about 20 nm. The thickness of the oxide layer was dependent on the number of laser pulses. The samples were analyzed by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy.
COBISS.SI-ID: 23406375
The results on surface modification of bimetal (WTi), multilayer (Ni/Ti) and hard (TiAlN/TiN) coatings with different composition, structure and morphology by exposure with short and ultra-short laser pulses are described. The pulsed laser systems were used: picosecond Nd:YAG laser and femtosecond Ti:Sapphire laser. Various analytical techniques were used for characterization of the sample before and after laser irradiation. The laser-induced modifications showed dependence on laser pulse duration, pulse count and laser wavelength. Modification is associated with significant change of the absorption coefficient for the laser radiation and thus influences the temperature field in the material and level of properties modification. The main part of the absorbed laser energy was rapidly transformed into heat, producing intensive modifications of composition and morphology on the target surface. The following morphological changes were observed: (i) ablation/exfoliation of the materials, (ii) appearance of hydrodynamic features such as resolidified material, and (iii) formation of nano-sized structures in a mosaic, cone and grain patterns. Laser treatment of samples in air is accompanied with formation of oxide layer on the surface. At lower laser fluences formation of laser induced parallel/ripple surface structures was evident, with a periodicity which depends on the wavelength and polarization of the used radiation.
COBISS.SI-ID: 25629223
Goniospectrophotometric space curves were obtained by summation of spatially under-sampled bidirectional reflectance distribution function over all directions and repeating this for all wavelengths in the visible spectral region. This gives a 3D goniospectrophotometric curve called an xDNA graph. Systematic analysis applying 19 measurement geometries confirms existence of characteristic shapes of the graph for all optically similar samples. This enables distinguishing between differently rough samples, interference effect on various transparent layers and selective spectral absorption of light in differently thick pigmented coatings. Therefore the considered goniospectrophotometric space curves could serve as an appearance fingerprint of such samples.