Marek Škereň

Effect of Actual Indenter Shape on the Results of Spherical Nanoindentation

Actual shape of the diamond spherical indenter of nominal radius 20 μm was investigated in this study. 3D reconstruction was performed by atomic force microscope and by the method of stereopair using SEM images of the tip taken under several different angles. The results were compared with the shape obtained indirectly by the calibration performed on specimens with known Young’s modulus. It was found that lower effective values of tip radius for the small penetration depths are caused by the irregular geometry of contact between indenter and specimen surface. With increasing penetration depth the radius increased to the theoretical values and it decreased again for high penetration depths. The stress-strain curves were determined using corrected effective indenter radius.

Numerical analysis of color holograms based on surface-plasmons

High selectivity of surface-plasmon polaritons on wavelength can be used for reconstruction of color holograms illuminated with white light. In such a case, a combination of plasmonic filter and holographic microstructure is used. Compared to other techniques for creating holograms for white light reconstruction, such as rainbow holograms, the plasmonic approach offers full angle color reconstruction. The plasmonic structure consists of a holographic grating recorded in a photo-resist material and a thin metallic layer on top of the grating. The parameters of both components are tuned to transmit only a selected spectral range when illuminated with white light. For RGB reconstruction three independent structures can be combined on a single substrate. An excitation of plasmonic effects usually requires a relatively complicated setup containing prisms for in-coupling of optical waves. In this paper, a modification of the plasmonic hologram is proposed, which consists of two gratings recorded on opposite surfaces of a substrate. The first grating serves as an in-coupling element which also partially tunes the spectral transmittance of the structure. The second grating is metallized and out-couples the energy stored in the surface plasmon to a transmitted wave. However, there is still a problem of diffraction efficiency of such transmission holographic structures. In order to optimize the transmittance and spectral selectivity, numerical simulations of light propagation through the plasmonic structure have been performed using MEEP software. The dependencies of transmitted light on various parameters of the diffraction gratings and metallic layer are discussed in detail.

Photopolymer recording material with AgBr nanoparticles for optical holography

A new self-developing recording material with silver bromide nanoparticles and photopolymerization system has been prepared and tested as a recording medium for optical holography. Through the method of the real-time measurement of a diffraction grating growth, we have shown that an efficient volume phase grating is formed during a holographic exposure. The refractive index modulation of the formed grating is several times higher than in the case of standard acrylamide-based photopolymers with a similar composition. The material response on different exposure parameters has been measured and obtained results have been discussed. The transfer of nanoparticles within the recording layer, which is the main cause of the grating growth, has been experimentally verified through the direct observation of the grating micro-structure with the scanning electron microscope.

Preparation of nanostructured ultrathin silver layer

Silver is widely used for a fabrication of plasmonic devices due to its unique optical constants. Nanostructured Ag layer can exhibit strong localized surface plasmon resonance, which mainly affects its optical behavior in visible and near infrared spectra. The nanostructure of the Ag layer is mainly influenced during the initial stage of the silver nucleation. Therefore we focused our attention on the study of this stage of the silver growth. The nanostructured ultra-thin silver layers were prepared by means of the magnetron sputtering. The nucleation mode and the resulting nanostructure was controlled by the deposition conditions. The initial stage of the nucleation and the layer growth was studied by means of an optical monitoring, which is based on a principle of spectrophotometric measurement of sample reflectivity. The measured data were fitted to a model of layered structure. The non-continual (Volmer-Weber) mode of the layer nucleation was clearly distinguished in the monitored data. Thus we were able to estimate the point of the non-continual layer coalescence as well as the subsequent evolution of the surface roughness. The prepared nanostructured Ag layers were analyzed by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Optical properties were studied by spectroscopic ellipsometry and spectrophotometry.

Optical security elements based on waveguide effects

Optical document security represents an important field of application of analogue and synthetic diffractive structures. Most of the security elements are based on visual effects formed by diffraction on a structure with the details in the order of hundreds of nanometers. However, to improve the anti-counterfeiting properties of these structures, various types of hidden features are included within the area of the security elements. They are not visible under normal lighting but it is possible to easily reveal the hidden information under specially-defined geometry and/or type of illumination. In this paper, theory and application of a novel type of hidden diffractive security element are presented. It combines standard visual properties of synthetic holograms with waveguide effects. The hidden information is recorded using a special grating, which is not visible under normal observation geometry. The encoded image can be reconstructed only when the proper guided mode appears in a substrate. During the reconstruction, light is coupled into a waveguide (holographic foil) using a grating coupler and after traveling through the substrate in a chosen direction it is selectively out-coupled within the areas containing the hidden information. Several elements with different properties have been designed, fabricated and compared with theory. Principles of diffraction and waveguide effects, realization technology and properties of the realized test samples are presented. The advantage of the combination of diffractive and waveguide effects is that the resulting hidden effect is sophisticated but easily readable with no additional tools.

New recording material with AgBr nanoparticles for optical holography

A new recording material with silver bromide nanoparticles and photopolymerization system has been prepared and tested as a recording medium for optical holography. Through the method of the real-time measurement of a diffraction grating growth, we have shown that an efficient volume phase grating is formed during a holographic exposure. The material response on different exposure parameters has been measured and obtained results have been discussed. The transfer of nanoparticles within the recording layer, which is the main cause of the grating growth, has been experimentally verified through the direct observation of the grating micro-structure with the scanning electron microscope.

Advanced optical document security elements

ABSTRACT Synthetic diffractive structures represent an important tool in the optical document security. Their macroscopic visual behavior is based on properties of a very fine micro-structure which cannot be copied using common copying techniques. The visual effects can be easily observed by a common observer without any special inspection tools. However, when a high level of security is needed, additional features are often included based on an optical encryption of information. In this paper, a novel encryption technique is presented, which is based on utilizing the plastic holographic foil as a waveguide and special diffractive structures as coupling elements. When an in-coupling area is illuminated with a defined light beam, the light is coupled into the waveguide and travels to an out-coupling part. The encrypted information is encoded either in the shape of the out-coupling area or it can be formed from an out-coupling hologram in free space above the element. Both laser and normal white light sources can be used for reading the information. The coupling areas can be mixed with diffractive micro-structures forming visual effects and can be invisible during a normal observation of the hologram. The couplers can be realized using the technology fully compatible with the standard process for mastering and replication of the security elements. Several extensions of the described idea of waveguide cryptograms are also included. Finally, a set of real samples of the security elements is presented, which were realized using an advanced matrix laser lithography technique.

Design, realization, and applications of diffractive structures for laser beam manipulation

This paper deals with the design, fabrication, and applications of the synthetic diffractive elements. Selected design algorithms such as the Iterative Fourier Transform Algorithm and others have been researched and improved to give better results for particular applications. Interesting fabrication technologies such as the matrix laser lithography are also presented. Finally, several applications are described that have been solved at the Department of Physical Electronics of the Faculty of Nuclear Sciences and Physical Engineering.

Optical Manipulation with Particles Using the Holographic Optical Tweezers on Special Microfluidic Substrates

Optical manipulation is presented with various micro-particles on special relief substrates working as the microfluidic devices. The substrates are prepared using the laser lithography. The manipulation is realized using the holographic optical tweezers.