Jaromír Pištora

Anisotropy of quadratic magneto-optic effects in reflection

Quadratic or second-order magneto-optic effects in reflection significantly effect in-plane magnetization measurements. While the magneto-optic effects linear in magnetization are independent of orientation of cubic crystal axes, the amplitude and sign of the quadratic effects change significantly under crystal rotation. Theoretical formulas for the magneto-optic effects have been derived using a permittivity tensor including terms quadratic in magnetization. A method for separation of the diagonal and off-diagonal quadratic magneto-optic tensor components (G11−G12) and 2G44 is proposed. The theory was completed by an experimental observation of the quadratic effect anisotropy in an epitaxial Fe layer prepared on a MgO substrate. The influence of the magnetization components on the magneto-optic vector magnetometry is discussed for an interface, a single layer, and exchange coupled bilayer system for a general magnetization direction including the quadratic magneto-optic effect anisotropy.

Transport and optical properties of the gapless Heusler compound PtYSb

This work presents a systematic study on the optical and transport properties of the Heusler compound PtYSb. The optical properties were investigated in a wide spectral range from 10 meV to 6.5 eV and compared to ab-initio calculations. For photon energies below 2.5 eV, the optical absorption increases linearly with photon energy. This is related with the conical shape of the electronic structure in the vicinity of the Fermi energy. The optical spectra reveal a maximum band gap of about 60 meV. Furthermore, the temperature dependence of thermal conductivity, electrical resistivity, Seebeck coefficient and Hall mobility were investigated. PtYSb exhibits very good thermoelectric properties with a high figure of merit ZT of 0.2 and a Hall mobility μh of 300 cm2/Vs at 350 K, which is the highest value obtained for Heusler compounds up to now. The carrier concentration ranges from 5 × 1018 at low temperature to 1019 cm−3 at 400 K.

Physical properties of Al doped Ba hexagonal ferrite thin films

We developed the thin film microwave magnetic material, M-type barium hexagonal ferrite (BaM) doped with Al, for signal processing devices operating above 40 GHz with little to no applied magnetic field. Al was chosen as the dopant material because it significantly increases the already strong anisotropy field of BaM. A series of thin film BaAlxFe12-xO19 samples, x ranging from 0 to 2 in 0.25 steps, were deposited on Pt templates using a metal-organic decomposition growth technique. The resulting films are polycrystalline and highly textured, with the hexagonal c-axis directed out of plane. These films are also self-biasing; easy axis hysteresis loops have a high squareness ratio, s, in the 0.83-0.92 range. As expected, the anisotropy field increases with x, ranging from 1.34 to 2.19 × 106 A/m (16.9-27.5 kOe) for x = 0-2, while the saturation magnetization Ms decreases with x, ranging from 0.334 to 0.175 × 106 A/m (4πMs = 4.2-2.2 kG) for x = 0-2. These values were measured at room temperature, but the tem...

Coupled mode enhanced giant magnetoplasmonics transverse Kerr effect

We show that the enhancement of the transverse magneto-optical Kerr effect of a smooth magnetic dielectric film covered by a noble metal grating, is strongly dependent on the precise geometry of this grating. Up till now this magnetoplasmonic enhancement was solely attributed to a nonreciprocal shift of the dispersion of the surface plasmon polariton resonances at the interface with the magnetized substrate. It is demonstrated that by hybridization of surface and cavity resonances in this 1D plasmonic grating, the transverse Kerr effect can be further enhanced, extinguished or even switched in sign and that without inverting or modifying the films magnetization. This strong geometrical dispersion and the accompanying anomalous sign change of the magneto-plasmonic effects in such systems has never been considered before, and might find interesting applications in sensing and nanophotonics.

Convergence properties of critical dimension measurements by spectroscopic ellipsometry on gratings made of various materials

Spectroscopic ellipsometry (SE) in the visible/near-UV spectral range is applied to monitor optical critical dimensions of quartz, Si, and Ta gratings, namely, the depth, linewidth, and period. To analyze the SE measurements, the rigorous coupled-wave theory is applied, whose implementation is described in detail, referred to as the Airy-like internal reflection series with the Fourier factorization rules taken into account. It is demonstrated that the Airy-like series implementation of the coupled-wave theory with the factorization rules provides fast convergence of both the simulated SE parameters and the extracted dimensions. The convergence properties are analyzed with respect to the maximum Fourier harmonics retained inside the periodic media and also with respect to the fineness of slicing imperfect Ta wires with paraboloidally curved edges.

Null ellipsometer with phase modulation

A new null ellipsometer is described that uses photoelastic modulator (PEM). The phase modulation adds a good signal-to-noise ratio, high sensitivity, and linearity near null positions to the traditional high-precision nulling system. The ellipsometric angles Delta and psi are obtained by azimuth measurement of the analyzer and the polarizer-PEM system, for which the first and second harmonics of modulator frequency cross the zeros. We show that the null system is insensitive to ellipsometer misadjustment and component imperfections and modulator calibration is not needed. In addition, a fast ellipsometer mode for fine changes measurement of ellipsometric angles is proposed.

Imperfectly geometric shapes of nanograting structures as solar absorbers with superior performance for solar cells

The expectation of perfectly geometric shapes of subwavelength grating (SWG) structures such as smoothness of sidewalls and sharp corners and nonexistence of grating defects is not realistic due to micro/nanofabrication processes. This work numerically investigates optical properties of an optimal solar absorber comprising a single-layered silicon (Si) SWG deposited on a finite Si substrate, with a careful consideration given to effects of various types of its imperfect geometry. The absorptance spectra of the solar absorber with different geometric shapes, namely, the grating with attached nanometer-sized features at the top and bottom of sidewalls and periodic defects within four and ten grating periods are investigated comprehensively. It is found that the grating with attached features at the bottom absorbs more energy than both the one at the top and the perfect grating. In addition, it is shown that the grating with defects in each fourth period exhibits the highest average absorptance (91%) compared with that of the grating having defects in each tenth period (89%), the grating with attached features (89%), and the perfect one (86%). Moreover, the results indicate that the absorptance spectrum of the imperfect structures is insensitive to angles of incidence. Furthermore, the absorptance enhancement is clearly demonstrated by computing magnetic field, energy density, and Poynting vector distributions. The results presented in this study prove that imperfect geometries of the nanograting structure display a higher absorptance than the perfect one, and provide such a practical guideline for nanofabrication capabilities necessary to be considered by structure designers.

Investigation of optical absorptance of one-dimensionally periodic silicon gratings as solar absorbers for solar cells

A rigorous design using periodic silicon (Si) gratings as absorbers for solar cells in visible and near-infrared regions is numerically presented. The structure consists of a subwavelength Si grating layer on top of an Si substrate. Ranges of grating dimensions are preliminary considered satisfying simple and feasible fabrication techniques with an aspect ratio defined as the ratio of the grating thickness (d) and the grating lamella width (w), with 0 < d/w < 1.0. The subwavelength grating structure (SGS) is assumed to comprise different lamella widths and slits within each period in order to finely tune the grating profile such that the absorptance is significantly enhanced in the whole wavelength region. The results showed that the compound SGS yields an average absorptance of 0.92 which is 1.5 larger than that of the Si plain and conventional grating structures. It is shown that the absorptance spectrum of the proposed SGS is insensitive to the angle of incidence of the incoming light. The absorptance enhancement is also investigated by computing magnetic field, energy density, and Poynting vector distributions. The results presented in this study show that the proposed method based on nanofabrication techniques provides a simple and promising solution to design solar energy absorbers or other energy harvesting devices.

Vectorial magnetometry using magnetooptic Kerr effect including first- and second-order contributions for thin ferromagnetic films

A new combination of different vectorial magnetometry techniques using magnetooptic Kerr effect is described. The processing of the experimental data contains the separation of linear and quadratic parts of the magnetization curves and determination of all three components of the magnetization vector in units of Kerr rotation without any normalization to the saturation values. The experimental procedure includes measurements with parallel and perpendicular polarized incident light and an external magnetic field parallel and perpendicular to the plane of incidence of light. The determination of the complex Kerr amplitude and the theoretic description of the data processing in assumption of small angles of incidence and also for larger angles of incidence using adequate scaling to the mean saturation value validate this vectorial magnetometry method. In the case of an absent out-of-plane component of the magnetization vector, the complete reversal process can easily be reconstructed and interpreted by monodomain states and domain splitting. The measurement procedure and the processing of the data are demonstrated for an ultra-thin epitaxial Fe film on MgO(0 0 1).

Specular spectroscopic ellipsometry for the critical dimension monitoring of gratings fabricated on a thick transparent plate

Specular-mode spectroscopic ellipsometry is applied to analyze the optical response of gratings fabricated on a thick transparent plate substrate. The principles of the optical response of the gratings are described by employing incoherent contributions due to backreflections in the finite transparent substrate medium. A special function identifies a “diminution effect” caused by deflecting the secondary contributions from the primary beam axis. Two different methods are used to measure the ellipsometric response, a liquid solution method with the backreflections eliminated and a method including the incoherent backreflections. The grating parameters deduced by fitting from the measurement using the first method are applied to simulate the ellipsometric response using the second method. The spectral dependencies yielded by both methods are compared with remarkable agreement between the simulations and the measurements, which suggests the high usability of the backreflection method in the metrological char...