Filip Mika

Three-Dimensional Optical Trapping of a Plasmonic Nanoparticle using Low Numerical Aperture Optical Tweezers

It was previously believed that larger metal nanoparticles behave as tiny mirrors that are pushed by the light beam radiative force along the direction of beam propagation, without a chance to be confined. However, several groups have recently reported successful optical trapping of gold and silver particles as large as 250 nm. We offer a possible explanation based on the fact that metal nanoparticles naturally occur in various non-spherical shapes and their optical properties differ significantly due to changes in localized plasmon excitation. We demonstrate experimentally and support theoretically three-dimensional confinement of large gold nanoparticles in an optical trap based on very low numerical aperture optics. We showed theoretically that the unique properties of gold nanoprisms allow an increase of trapping force by an order of magnitude at certain aspect ratios. These results pave the way to spatial manipulation of plasmonic nanoparticles using an optical fibre, with interesting applications in biology and medicine.

Photonic Crystal Structure and Coloration of Wing Scales of Butterflies Exhibiting Selective Wavelength Iridescence

The coloration of butterflies that exhibit human visible iridescence from violet to green has been elucidated. Highly tilted multilayers of cuticle on the ridges, which were found in the scales of male S. charonda and E. mulciber butterflies, produce a limited-view, selective wavelength iridescence (ultraviolet (UV)~green) as a result of multiple interference between the cuticle-air layers. The iridescence from C. ataxus originates from multilayers in the groove plates between the ridges and ribs. The interference takes place between the top and bottom surfaces of each layer and incoherently between different layers. Consequently, the male with the layers that are ~270 nm thick reflects light of UV~560 nm (green) and the female with the layers that are ~191 nm thick reflects light of UV~400 nm (violet). T. aeacus does not produce the iridescent sheen which T. magellanus does. No iridescent sheen is ascribed to microrib layers, which are perpendicular to the scale plane, so that they cannot reflect any backscattering. The structures of these butterflies would provide us helpful hints to manipulate light in photoelectric devices, such as blue or UV LEDs.

Very low energy scanning electron microscopy in nanotechnology

The group of low energy electron microscopy at ISI AS CR in Brno has developed a methodology for very low energy scanning electron microscopy at high image resolution by means of an immersion electrostatic lens (the cathode lens) inserted between the illumination column of a conventional scanning electron microscope and the sample. In this way the microscope resolution can be preserved down to a landing energy of the electrons one or even fractions of an electronvolt. In the range of less than several tens of electronvolts the image signal generation processes include contrast mechanisms not met at higher energies, which respond to important features of the 3D inner potential of the target and visualise its local crystallinity as well as the electronic structure. The electron wavelength comparable with interatomic distances allows observation of various wave–optical phenomena in imaging. In addition, the cathode lens assembly secures acquisition of electrons backscattered from the sample at large angles with respect to the surface normal, which are abandoned in standard microscopes although they provide enhanced crystallinity information and surface sensitivity even at medium electron energies. The imaging method is described and illustrated with selected application examples.

Imaging with STEM Detector, Experiments vs. Simulation

Although there are a great many Scanning Electron Microscopes (SEMs) and Scanning Transmission Electron Microscopes (STEMs) currently in existence, obtaining quantitative information from the electron signal is proving difficult to establish [1]. In order to create good quantitative procedures, a comparison between experimental and theoretical signals from well defined samples needs to be carried out. Since the theoretical understanding of electron transport at low energies remains relatively poor, studies which make use of the higher energy scattered primary electrons would be a good place to start such a comparison. Hence, one could compare the signal obtained from a backscattered electron detector in an SEM with what one might expect from a Monte Carlo (MC) simulation. Another approach would be to consider the transmitted electrons (TEs) through thin films of elemental materials in a STEM. It is the latter approach that we will concentrate upon in this report. This is because calibrating the STEM detector using the primary beam is possible using the transmitted beam when no sample is present. However this approach is not possible for backscattered measurements. The aim of this work is to find out the angular and energy distribution of the TEs that are very important for the understanding of image formation in STEM.

The effect of amorphous and crystal sodium warfarin and its content uniformity on bioequivalence of tablets

&NA; Warfarin is intensively discussed in terms of generic substitution due to particular cases of bleeding, which are attributable to fluctuations in API content or the substitution of crystalline (WSC) for amorphous (WSA) warfarin. The aim of this study was to assess to what extent the in vitro release was affected by the form of API depending on the composition and technology. Bioequivalent tablets containing 5 mg of WSA or WSC prepared by wet granulation or direct compression were used. Furthermore, tablets of the same composition with WSC or WSA prepared by direct compression were evaluated. Raman spectroscopy was used to confirm the presence of WSA or WSC. The dissolution was more influenced by the technology than by the form of API but even tablets with dissimilar profiles were bioequivalent. This is probably due to the precipitation of WSA and WSC in the stomach on a poorly soluble acidic form, which subsequently dissolves in the neutral environment of the small intestine. Recrystallization was demonstrated in the in vitro assay at a pH of 1.2 and 4.5 using Raman spectroscopy and X‐ray diffraction. In summary, the content uniformity appears to be the main factor affecting the safety of the treatment.

Factor analysis in optimization of formulation of high content uniformity tablets containing low dose active substance

&NA; Warfarin is intensively discussed drug with narrow therapeutic range. There have been cases of bleeding attributed to varying content or altered quality of the active substance. Factor analysis is useful for finding suitable technological parameters leading to high content uniformity of tablets containing low amount of active substance. The composition of tabletting blend and technological procedure were set with respect to factor analysis of previously published results. The correctness of set parameters was checked by manufacturing and evaluation of tablets containing 1–10 mg of warfarin sodium. The robustness of suggested technology was checked by using “worst case scenario” and statistical evaluation of European Pharmacopoeia (EP) content uniformity limits with respect to Bergum division and process capability index (Cpk). To evaluate the quality of active substance and tablets, dissolution method was developed (water; EP apparatus II; 25 rpm), allowing for statistical comparison of dissolution profiles. Obtained results prove the suitability of factor analysis to optimize the composition with respect to batches manufactured previously and thus the use of metaanalysis under industrial conditions is feasible. Graphical abstract Figure. No caption available.

Noise of low-energy electron beam

Noise diagnostics were performed on a tungsten hair-pin cathode operating in a vacuum (3 · 10-3 Pa). The cathode operates in thermal emission mode at several temperatures based on the level of heating. Several acceleration voltages have been used starting at 5 kV, 10 kV and 20 kV up to 30 kV. There were two modes - with and without electron beam scanning. Both setups were compared from the point of view of the noise spectral density value. Shot noise is about one order below the current noise spectral density of the electron beam. Therefore, in this case the shot noise spectral density can be neglected. Noise spectral density is 1/f type at the frequency band 0.1 to 10 Hz for cathode with thin film of tungsten oxides. For accelerating voltage from 5 to 20 kV noise in the frequency band from 2 to 20 Hz can be described by the g-r noise with noise spectral density proportional to square of a beam current. At accelerating voltage 30 kV a new g-r noise component appears which is related to electric field distribution among anode; condensers, stigmators and objective.

SEM characterization of carbon nanotubes based active layers of chemical sensors

Characterization method for active layers for microelectrodes of electrochemical and gas sensors using scanning electron microscopy (SEM) is presented. In order to achieve maximum sensitivity and selectivity of the sensor, it is necessary to obtain precise knowledge of electrodes surface. Characterization was made using FEI Magellan SEM equipped with TLD secondary electron (SE) and CBS back-scattered electron (BSE) detectors, Energy-dispersive X-ray spectroscopy (EDS) and beam deceleration system. SEM at lower energies offers several advantages, among them the most important are: increase of material contrast, high ratio SE, BSE signal and noise, smaller interaction volume and elimination of charging effects.

Strategies for Collection of Secondary Electrons in the SEM

The standard way of secondary electron (SE) detection in the scanning electron microscope (SEM) is to use the Everhart-Thornley (ET) detector. Only weak electrostatic field attracts low energy SEs. Let us call this system the standard detector. Although the ET detector has been around for more than fifty years, it remains the most frequently used type of detector in SEMs. Modern SEMs have improved their image resolution by so called immersion systems, allowing a strong magnetic field of the objective lens to penetrate into the specimen region. In that case, two ET detectors are usually used: one is located above the objective lens, and the other below it (upper and lower detector). The resulting contrast of the SE images depends on SE energy and on the angular sensitivity of detectors, which is a result of specific distributions of electrostatic and magnetic fields in the specimen region.