Transparent Regions Research Articles

Color Micro‐ and Nanopatterning with Counter‐Propagating Reaction–Diffusion Fronts

Thin films of ionically doped gelatin have been color-patterned with submicrometer precision using the wet-stamping technique. Inorganic salts are delivered onto the gelatin surface from an agarose stamp, and diffuse into the gelatine layer, producting deeply colored precipitates. Reaction fronts originating from different features of the stamp cease within < 1 μm of each other, leaving sharp, transparent regions in between.

Synthesis and non‐linear optical properties of new ionic species: tolan and diphenylbutadiyne with trimethylammonio and dimethylamino groups

AbstractAs new ionic organic species for second‐order non‐linear optical (NLO) materials, 4‐{[4‐(dimethylamino)phenyl]ethynyl}phenyltrimethylammonium iodide (1a), 4‐{[4‐(dimethylamino)phenyl]butadiynyl}phenyltrimethylammonium iodide (2a) and their derivatives with 4‐substituted benzensulfonate instead of iodide were successfully synthesized. The transparent wavelength region of 1a is wider than that of a typical NLO molecule, p‐nitroaniline (pNA), whereas 2a and pNA have similar transparent regions. The experimental and calculated first hyperpolarizabilities (β) of 1a and 2a were found to be about 7 and 11 times, respectively, larger than those of pNA. Even when the volume factor was taken into account, the cationic part of 1a was found to be a better second‐order NLO material than pNA derivatives. Four second‐order NLO‐active crystals were found out of 12 salts synthesized. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Interference Effects in Photoacoustic and Reflectance Spectroscopies on TiO2/Si Structures and TiO2 Band Gap

Experimental results of photoacoustic (PAS) and reflectance (RS) spectroscopies of titanium dioxide thin films (TiO2), deposited on Si substrates, are compared in a wide optical range including transparent and absorbent regions of TiO2. Due to the fact that the light modulation frequency f used in the photoacoustic experiments was so low that the thermal diffusion length of the TiO2 (mu = 100 microm) is always larger than the thickness of the studied films, the PAS turns out to be complementary to RS over the entire range. The presence of multiple reflection interference effects makes difficult a direct evaluation of the TiO2 band gap from the PAS signal. However, by employing k(lambda) values, obtained from transmission experiments on equivalent TiO2 films deposited on transparent fused quartz substrates, the PAS spectra for the films deposited on silicon are reconstructed by using those theoretical models that consider multiple reflections. The reasonable agreement of the simulated and experimental PAS spectra allows one to obtain reliable Eg values for the TiO2 films deposited on opaque silicon substrates.

The structure of low-frequency standing Alfvén waves in the box model of the magnetosphere with magnetic field shear

The paper is concerned with the influence of magnetic field shear on the structure of Alfvén waves standing along field lines in the one-dimensionally inhomogeneous box model of the magnetosphere, enclosed between two parallel, infinitely conducting planes (ionospheres). We consider the transverse small-scale Alfvén waves whose azimuthal component of the wave vector $k_y$ satisfies the condition $k_y l\,{\gg}\,1$, where $l$ is the distance between the ionospheres. For this model, the Alfvén resonance condition has been established. It is shown that resonance can also occur at a constant Alfvén velocity if the field-line inclination to the ionosphere is changed. On resonant magnetic shells there occurs a singularity of the wave field of the same kind as in the absence of shear. Moreover, there are found many resemblances between Alfvén-wave behavior in our one-dimensionally inhomogeneous model and in two-dimensional inhomogeneous models with plasma and magnetic field parallel inhomogeneity taken into account. Thus, the presence of shear leads to a difference of the frequencies of poloidal and toroidal oscillations of field lines, and to the dependence of the wave's frequency on the transversal components of wave vector. Then, in the sheared magnetic field with highly conductive boundaries the source excites multiple standing Alfvén harmonics at different locations. In general, the localization regions of different longitudinal harmonics overlap. However, in the small but finite shear limit, a total wave field represents a set of mutually isolated transparent regions corresponding to different harmonic numbers. In each of these regions the waves are found to be travelling across the magnetic shells, and the transparent region is limited in the coordinate $x$ by two turning points, at one of which the mode is poloidally polarized, and the other point it is toroidally polarized (it is at this latter point where Alfvén resonance occurs). Furthermore, the phase velocity of the wave is directed toward the poloidal point, and the group velocity is directed at the toroidal point.

Determination of the dielectric functions of MBE‐grown Zn1−xMgxTe II‐VI semiconductor alloys

AbstractWe have explored the dielectric functions of ternary Zn1−xMgxTe thin films using a variable angle spectroscopic ellipsometer for samples between x = 0 and x = 0.52. We obtained values for the complex dielectric function for Zn1−xMgxTe in both the transparent and absorption regions by incorporating a threelayer model to simulate the experimental data. To this end, we also used the previously published relations of the dispersion of the indices of refraction (in the transparent region) of Zn1−xMgxTe measured using a combination of prism coupler and reflectivity. We have fitted the second derivatives of both the real and the imaginary parts of the dielectric function to obtain the critical point parameters corresponding to the higher order electronic transitions in the lattice. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Absorption and reflection infrared spectra of MgO and other diatomic compounds

Oxide and sulphide minerals are expected to occur in diverse astronomical environments. However, optical constants for such minerals are either lacking or poorly characterized. Minimizing errors in laboratory data, while extrapolating over wide frequency ranges, is the focus of this report. We present reflection and absorption spectra of single-crystal MgO from about ∼100 to 18 000 cm −1 (∼100 to 0.5 µm), and derive emissivity, dielectric and optical functions (n and k) using classical dispersion analysis and supplementary data to ensure that the reflectivity values are correct at the low- and high-frequency limits. Absorbance spectra of thin films of oxides (MgO, CaO, FeO and ZnO) and sulphides (MgS, CaS and FeS) are in good agreement with available reflectivity measurements, and provide information on the various effects of chemical composition, structure and optical depth. The greatest mismatch occurs for MgO, connected with this compound having the broadest peak in reflectance. The ferrous compounds (FeO and FeS) have relatively weak infrared features and may be difficult to detect in astronomical environments. Previous optical data based on transmission spectra of dispersions have underestimated the strength of the main infrared features because this approach includes spectral artefacts that arise from the presence of opaque particulates, or from non-uniform optical depth. We show that areal coverage, not grain size, is the key factor in altering absorption spectra from the intrinsic values, and discuss how to account for ‘light leakage’ in interpreting astronomical data. Previous reflectivity data on polycrystals differ from intrinsic values because of the presence of additional, internal reflections, creating errors in the derived optical functions. We use classical dispersion analysis and supplemental data from optical microscopy to provide correct n- and k-values for FeO from the far-infrared to the visible, which can then be used in radiative transfer models. Thin-film absorption data are also affected by internal reflections in the transparent regions: we show how to recognize these features and how to obtain the absorption coefficient, n, and k from thin-film infrared data on CaO, CaS and MgS using the damped harmonic oscillator model.

Spatial phenomenology requires potential illumination

Collapsing three-dimensional space into two violates Lehar's “volumetric mapping” constraint and can cause the visual system to construct illusory transparent regions to replace voxels that would have contained illumination. This may underlie why color constancy is worse in two dimensions, and argues for Lehar to revise his phenomenal spatial model by putting “potential illumination” in empty space.

Poisson image editing

Using generic interpolation machinery based on solving Poisson equations, a variety of novel tools are introduced for seamless editing of image regions. The first set of tools permits the seamless importation of both opaque and transparent source image regions into a destination region. The second set is based on similar mathematical ideas and allows the user to modify the appearance of the image seamlessly, within a selected region. These changes can be arranged to affect the texture, the illumination, and the color of objects lying in the region, or to make tileable a rectangular selection.

Birefringence in two-dimensional bulk photonic crystals applied to the construction of quarter waveplates

We have experimentally measured the birefringence in bulk two-dimensional hexagonal photonic crystals in transparent spectral regions above and below the fundamental band gap. Data is presented for structures with different numbers of layers and two different air-filling fractions. We have used these data to design a photonic crystal quarter waveplate and provide independent experimental demonstrations of its operation.

A study of the optical transfer function for annular binary filters

Summary The annular binary pupil filters consist of two transparent annuli obstructed by an annular mask. The optical transfer functions for them are analytically derived under paraxial approximation, and three- and two- as well as one-dimensional numerical plots are presented. It is shown that by changing the parameters of transparent regions it is possible to improve the axial resolution whilst the transverse resolution remains unchanged, compared with an annular lens. Results indicate, as is well known, that this kind of filters can be applied to three-dimensional confocal microscopy.

Theory of azimuthally small‐scale Alfvén waves in an axisymmetric magnetosphere with small but finite plasma pressure

An analytical and numerical study is made of the influence of plasma finite pressure on the structure of Alfvén waves with large numbers of the azimuthal wave number, m ≫ 1, as well as of the conditions under which the waves can be poloidally polarized. The study is based on using the axisymmetric model of the magnetosphere assuming the background plasma inhomogeneity along field lines and across magnetic shells, the curvature of field lines, and an equilibrium electric field. The poloidality condition of the mode can imply that in the region where many azimuthal wavelengths fit in the region where the propagation of the wave is possible (the transparent region). The broader is the transparent region, the less stringent conditions are imposed on azimuthal wave numbers of the poloidally polarized Alfvén wave. In this paper it is shown that with nonzero plasma pressure, the transparent region is much broader when compared with cold plasma case. For instance, if finite pressure for the second standing harmonic along a field line of the wave (N = 2) is accounted for, the width of the transparent regions several thousand kilometers, and poloidal Alfvén waves can exist when m ≫ 10. This is consistent with experimental data on radially polarized Pc4 pulsations in the magnetosphere which have m from 50 to 150, whereas in the case of zero pressure the transparent region does not exceed a few tens of kilometers; therefore, in β = 0 case, poloidally polarized waves must have too large values of m in excess of 1000, which is not observed. A further important result of this study implies that near the ring current maximum there must exist a cavity, and high‐m oscillations enclosed within this cavity must be standing oscillations across magnetic shells.

Dynamics of wave-pulse penetration into an evanescent region.

The initial problem of plasma wave dynamics in the presence of a sharp density jump that divides the space into transparent and opaque regions is studied. A wave packet is assumed to be initially localized in the transparent region. The transient process of field penetration beyond the density barrier during the wave packet reflection from the density jump is investigated. Signal velocity beyond the barrier is defined as the speed at which some small, but finite, value of the field amplitude appears in the evanescent region. This velocity, which is proved to be always less than the speed of light, is determined analytically for the case of quasi-Gaussian wave packet. Further insight into the field dynamics in opaque region is gained by considering a steplike initial wave packet.

Noble gas systematics of the Réunion mantle plume source and the origin of primordial noble gases in Earth’s mantle

New noble gas data of ultramafic xenoliths from Réunion Island, Indian Ocean, further constrain the characteristics of primordial and radiogenic noble gases in Earth’s mantle plume reservoirs. The mantle source excess of nucleogenic 21Ne is significantly higher than for the Hawaiian and Icelandic plume reservoirs, similar to excess of radiogenic 4He. 40Ar/ 36Ar of the Réunion mantle source can be constrained to range between 8000 and 12 000, significant 129Xe and fission Xe excess are present. Regarding the relative contribution of primordial and radiogenic rare gas nuclides, the Réunion mantle source is intermediate between Loihi- and MORB-type reservoirs. This confirms the compositional diversity of plume sources recognized in other radioisotope systematics. Another major result of this study is the identification of the same basic primordial component previously found for the Hawaiian and Icelandic mantle plumes and the MORB reservoir. It is a hybrid of solar-type He and Ne, and ‘atmosphere-like’ or ‘planetary’ Ar, Kr, Xe (Science 288 (2000) 1036). 20Ne/ 22Ne ratios extend to maximum values close to 12.5 (Ne-B), which is the typical signature of solar neon implanted as solar corpuscular radiation. This suggests that Earth’s solar-type noble gas inventory was acquired by small (less than km-sized) precursor planetesimals that were irradiated by an active early sun in the accretion disk after nebular gas dissipation, or, alternatively, that planetesimals incorporated constituents irradiated in transparent regions of the solar nebula. Previously, such an early irradiation scenario was suggested for carbonaceous chondrites which follow common volatile depletion trends in the sequence CI–CM–CV–Earth. In turn, CV chondrites closely match Earth’s mantle composition in 20Ne/ 22Ne, 36Ar/ 22Ne and 36Ar/ 38Ar. This indicates that mantle Ar could well be a planetary component inherited from precursor planetesimals. However, a corresponding conclusion for mantle Kr and Xe is less convincing yet, but this may be just due to the lack of appropriate ‘meteoritic’ building blocks matching terrestrial composition. Alternatively, heavy noble gases in Earth’s mantle could be due to admixing of severely fractionated air, but this effect must have affected all mantle sources to a very similar extent, e.g. by global subduction before the last homogenization of the mantle reservoirs.

Ellipsometric Determination of Anisotropic Optical Constants in Electroluminescent Conjugated Polymers

Structural information about the degree of chain alignment in thin polymer films can be obtained by ellipsometry. A combination of reflection and transmission ellipsometry has been employed to determine the ordinary and extraordinary optical constants in conjugated polymer films in both the absorbing and transparent regions. Optical constants obtained by this technique will allow accurate modeling of the optical structure of polymer light-emitting diodes (LEDs) and photodiodes.

Transparent and quasi-transparent regional solutions to minority-carrier transport in arbitrarily doped semiconductors

The exact analytical regional solution to minority-carrier transport is derived in arbitrarily doped transparent semiconductor regions. By using this solution, new regional quasi-transparent solutions for emitter light-generated current density are derived in both the Cuevas and Balbuena approach (Cuevas and Balbuena, IEEE Trans. Electron Devices, vol. 36, pp. 553-560, 1989) and the Hamel approach (Hamel, IEEE Trans. Electron Devices, vol. 46, pp. 104-109, 1996) . Either of the new third-order quasi-transparent expressions is shown to be more accurate than both the local second-order quasi-transparent expression of Cuevas and Balbuena and the third-order regional expression of Bisschop et al (IEEE Trans. Electron Devices, vol. 37, pp. 358-364, 1990). In particular, while the new expression derived according to Hamel is more accurate at passivated surfaces, the new expression derived according to Cuevas and Balbuena is always more accurate, except for the case of a negligible surface recombination, where it is as accurate as the third-order regional expression of Bisschop et al.

Experimental and theoretical studies of absorption in microwindows of the ν4-band of methane and methane–hydrogen

The absorption coefficients of room temperature CH 4 and CH 4–H 2 mixtures were measured in the 1300– 1360 cm −1 region of the ν 4 band at about 295 K as a function of pressure. These results were analyzed at frequencies in the most transparent regions of various microwindows in order to study the far wings of self- and H 2-broadened lines in the R-branch of the ν 4 band. The experimental results were compared with synthetic spectra computed from the addition of Lorentzian line contributions. From such comparisons it appears that the high-frequency wings of the lines of the Q-branch decay more rapidly than do Lorentz wings. At higher wave numbers, the measured absorption tends to be equal and then less than the computed Lorentz absorption, although these deviations may not be significant in view of the uncertainties of the measurements and line parameters used in the computed spectra. Nonetheless, the computed absorption when including line coupling is in better overall agreement with the experiment and confirms the role of the Q-branch in producing the sub-Lorentzian absorption noted above.

Structure of magnetosonic eigenoscillations of an axisymmetric magnetosphere

The spatial structure of fast magnetosonic eigenoscillations of an axisymmetric magnetosphere is investigated theoretically. It is shown that transparent regions of the eigenmodes where the bulk of the fast magnetosonic oscillation energy is concentrated lie near the equatorial plane. The transparent regions have a narrow channel from the magnetosphere to the solar wind near the equatorial plane. This enables the magnetosonic waves to penetrate from the solar wind deep into the magnetosphere, as well as some of the energy of intramagnetospheric magnetosonic oscillations to escape to the solar wind. Some of the eigenmodes have an isolated portion of the transparent region above the plasmapause that forms a well‐known magnetospheric cavity for fast magnetosonic waves.

Dynamics of a microcapillary discharge plasma using a soft x-ray laser backlighter

We have used the new technique of soft x-ray laser shadowgraphy in combination with traditional plasma emission spectroscopy and theoretical modeling to study the dynamics of a plasma column created by a discharge through a 380 &mgr;m diameter evacuated microcapillary. The transient microcapillary plasma was imaged with high-spatial and temporal resolution using a tabletop discharge pumped 46.9-nm laser backlighter. Model computations show that the sharp boundary observed between the absorbent and transparent regions of the shadowgrams is defined by the spatial distribution of weakly ionized ions that are strongly photoionized by the probe laser. The plasma was observed to rapidly evolve from an initially nonuniform distribution into a column with good azimuthal symmetry and minimum density on axis [computed electron density on axis n(e)=(1-3)x10(19) cm(-3)]. This concave electron density profile constitutes a plasma waveguide for laser radiation. Heated solely by Joule dissipation from relatively small excitation currents (1.5 kA), this dense plasma reaches substantial electron temperatures of T(e)=15-20 eV as a result of the absence of significant hydrodynamic losses and reduced radiation losses caused by large spectral line opacities. The results illustrate the potential of tabletop soft x-ray lasers as a new plasma diagnostic tool.

Fast Volume Rendering and Data Classification Using Multiresolution in Min‐Max Octrees

Large‐sized volume datasets have recently become commonplace and users are now demanding that volume‐rendering techniques to visualise such data provide acceptable results on relatively modest computing platforms. The widespread use of the Internet for the transmission and/or rendering of volume data is also exerting increasing demands on software providers. Multiresolution can address these issues in an elegant way. One of the fastest volume‐rendering alrogithms is that proposed by Lacroute &amp; Levoy 1 , which is based on shear‐warp factorisation and min‐max octrees (MMOs). Unfortunately, since an MMO captures only a single resolution of a volume dataset, this method is unsuitable for rendering datasets in a multiresolution form. This paper adapts the above algorithm to multiresolution volume rendering to enable near‐real‐time interaction to take place on a standard PC. It also permits the user to modify classification functions and/or resolution during rendering with no significant loss of rendering speed. A newly‐developed data structure based on the MMO is employed, the multiresolution min‐max octree, M 3 O, which captures the spatial coherence for datasets at all resolutions. Speed is enhanced by the use of multiresolution opacity transfer functions for rapidly determining and discarding transparent dataset regions. Some experimental results on sample volume datasets are presented.

Preparation of damage-free glass TEM specimens

A jet polishing technique to chemically thin glass specimens for transmission electron microscopy (TEM) after a preliminary mechanical dimpling step has been developed. Slightly modified commercial equipment is used with automatic optical termination of the polishing process to produce foils exhibiting large, high quality, electron transparent regions.