Optical implementation of a wavelet transform by the use of dynamic holographic recording in a photorefractive material

An optical system that employs holographic recording in a photorefractive material is proposed and experimentally demonstrated for the implementation of a wavelet transform of two-dimensional mages. A scaling operation, to derive the family of wavelet filters from a mother wavelet filter, is performed by the use of an optical feedback loop. The selection of a desired wavelet filter from the family and the correlation for a wavelet transformation are made by the use of a holographic recording in a photorefractive material. The principle of operation of the system relies on the frequency detuning introduced inside the loop and the subsequent variation in the holographic grating diffraction. Experimental results on wavelet-filter selection and wavelet transformation are presented. This nonlinear optical wavelet-transform system is advantageous for pattern recognition applications.     

Dispersion implementation in optical filter design by the Fourier transform method using correction factors

The Fourier transform method to design graded-index optical filters, that relates the desired reflection spectrum and the index profile through the use of a Q function, has two important drawbacks: (1) It relies on approximate Q functions, and (2) it does not account for the dispersion of the index of refraction. The former is usually addressed by an iterative correction process. We propose to address the latter by scaling the wavelength in the Fourier transform by the optical thickness of the filter and to multiply the Q function by a wavelength-dependent correction factor. We demonstrate the high effectiveness of this approach by the performance of optical filters designed with such correction factors using the optical properties of SiO2/TiO2 mixtures.     

Noise filtering in the interference pattern by dynamic holographic recording in photorefractive Bi(12)TiO(20) crystals

The possibilities of stabilization of the interference pattern by filtration of a random-phase noise caused by vibrations, turbulence, and other local changes in the wave front in interferometric measurements are investigated. Dynamic holographic recording in photorefractive Bi(12)TiO(20) crystals is used. The parameters of the holographic recording are presented for determination of the dynamic range for filtering. Noise filtering takes place in real time and contributes to the enhancement of the contrast and the signal-to-noise ratio of the interference pattern. This results in a considerable increase in the sensitivity and the accuracy of the interferometric measurements.     

Long-term reading experiment on a photorefractive holographic memory with the hologram sustainment technique by optical feedback

We demonstrate a long-term continuous readout of a two-dimensional image in a photorefractive holographic memory with a BaTiO₃ crystal. A considerable extension of reading time is achieved by use of a hologram sustainment technique with an optical feedback circuit. Hologram rewritings by the simultaneous illumination of the reading beam and the feedback beam, which is incident on a crystal from the opposite direction to the reading beam, contribute to all-optical hologram sustainment without any fixing technique. In this paper, the effectiveness of the hologram sustainment technique is explained by the analysis of the temporal property of the amplitude of the index grating in a crystal. By calculating the temporal properties of the reconstructed beam intensity, we reveal the feedback rate and the coupling strength for high output efficiency. We perform an experiment on two-dimensional image reading and writing with a 45° cut BaTiO₃ crystal and show that a recorded image can be reconstructed over 20?min without critical image degradation whereas a reconstructed image fades away within 20?s in the conventional readout technique.     

Continuous-Wave and Passively Mode-Locked Operation of a Cunyite (Cr(4+):Ca(2)GeO(4)) Laser

Continuous-wave and mode-locked Cr(4+):Ca(2)GeO(4) lasers that use a fiber laser pump source were demonstrated. The continuous-wave Cr(4+):Ca(2)GeO(4) laser yielded a maximum output power of 415 mW at 1420 nm and a tuning range of 1335-1492 nm. With a saturable-absorber mirror, 60-ps pulses and 110-mW maximum output power were generated from a passively mode-locked Cr(4+):Ca(2)GeO(4) laser.     

Ambipolar field effect in the ternary topological insulator (Bi(x)Sb(1-x))2Te3 by composition tuning

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi(x)Sb(1-x))2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.     

In situ detection of F2 laser-induced oxidation on hydrogen-terminated Si(111) and Si(110) surfaces by Fourier transform infrared transmission spectroscopy

Although oxidation of silicon has been intensively investigated experimentally and theoretically, the composition and structure of the interface region between the bulk oxide and silicon are not very well known. In this work, we report on the vacuum UV laser-induced oxidation of well-defined atomically flat Si(111)-(1×1):H and Si(110)-(1×1):H surfaces, prepared by wet-chemical processing, using an F₂ laser (157 nm). The silicon samples were irradiated under ultra high vacuum conditions in water and oxygen atmospheres. The photo-induced partial oxidation of the first monolayer, controlled by the oxidant pressure, was monitored by in situ Fourier transform infrared transmission spectroscopy with different photon detectors. The chemical reactions occurring at the surface were monitored by the shift and decreasing strength of the stretching vibration of Si---H surface bonds. The IR absorption spectra recorded during surface processing yield detailed information on the rearrangement of chemical bonds with a sensitivity reaching monolayer resolution.     

Doping effect on the optical, electro-optic, and photoconductive properties of Bi12MO20 (M = Ge, Si, Ti)

The doping effect on the optical, electro-optic, and photoconductive properties of Czochralski-grown Bi₁₂MO₂₀ (M = Ge, Si, Ti) sillenite-structure single crystals was studied. The dopants were introduced into the growth charge in the form of oxides. Bi₁₂TiO₂₀ crystals were doped with V, Zn, Cu, P, and Nb; and Bi₁₂SiO₂₀ crystals were doped with Cd and Mo. The results indicate that the doping level has a significant effect on the photoconductivity of the Bi₁₂TiO₂₀〈Zn〉, Bi₁₂TiO₂₀〈Cu〉, Bi₁₂SiO₂₀〈Cd〉, and Bi₁₂SiO₂₀〈Bi₂₄CdMoO₄₀〉 crystals, which may exceed that of undoped crystals at low doping levels and may be substantially lower than it, down to zero, at increased dopant concentrations. Niobium doping of bismuth titanate has no effect on its photosensitivity and electro-optic properties. Phosphorus and vanadium enhance the photosensitivity of bismuth titanate over the entire composition range studied but have little effect on its electro-optic coefficient r ₄₁. A slight increase in r ₄₁ was only observed at high vanadium concentrations. The axial impurity distribution in the crystals is shown to be nonuniform, which reflects in their photoresponse: the photoconductivity of the copper-doped crystals near the seed end is 3 times that near the tail end.     

Effect of fluorination on preparation of Bi(Pb)SCCO 2 2 2 3 by the citrate precursor process

Citrate precursor technology was used to prepare fluorine substituted Bi(Pb)SCCO 2 2 2 3 superconducting phase. Samples with the nominal composition of up to three F atoms per formula were synthesized. A number of experimental methods have been used to characterize the samples and to trace the phase transformations during the preparation process: DTA/TGA, XRD, EPMA (WDS and EDS), atomic absorption, potentiometry with fluoride selective electrode, resistance and inductive measurements. Fluorine was shown to enhance considerably the formation of the 2 2 2 3 phase. Thermodynamic calculations of P-T-X equilibrium in the Bi-Pb-Sr-Ca-Cu-O-F-C-H-N system were made in a wide temperature range to determine the composition of the vapours coexisting with the solid phases at different stages of the preparation process.A part of the results was presented at the M²S-HTSC IV Conference in Grenoble, France, 1994.     

Structure and properties of thermoelectric materials based on Bi2(SeTe)3 and (BiSb)2Te3 solid solutions prepared by equal-channel angular pressing

Using X-ray diffraction and scanning electron microscopy, we have studied general aspects of defect structure formation in thermoelectric materials in different stages of plastic flow in the equal-channel angular pressing process with three channels. The results demonstrate that materials prepared using this deformation configuration have a fine-grained, homogeneous microstructure with a favorable texture, such that the cleavage planes of the grains are oriented along the extrusion axis. Studies of the structure and properties of the thermoelectric materials allowed us to optimize the equal-channel angular pressing temperature, which should be below the recrystallization onset temperature.     

Fabrication and optical properties of Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) thin films on Si substrates using the PLD method.

Epitaxial 0.67Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3)-0.33PbTiO(3) (PMN-PT) thin films with electro-optic effects were fabricated on (PMN-PT) thin films with electro-optic effects were fabricated on (La0(0.5)Sr0(0.5))CoO(3) (LSCO)/CeO(2)/YSZ-buffered Si(001) substrates using double-pulse excitation pulsed laser deposition (PLD) method with a mask placed between the target and the substrate. Epitaxial growth of PMN-PT thin films was undertaken using the two-step growth method of PMN-PT film. The PMN-PT seed layer was deposited at 500 degrees C on the LSCO/CeO(2)/YSZ/Si, which temperature was the same as that used for LSCO deposition. The PMN-PT thin films were deposited on the PMN-PT seed layer at 600 degrees C, which enables growth of high-crystallinity PMN-PT films with smooth surfaces. We obtained optimum fabrication conditions of PMNPT film with micrometer-order thickness. Resultant films showed high crystallinity with full width at half maximum (FWHM) = 0.73 deg and 1.6 mum thickness. Electro-optic properties and the refractive index value were measured at 633 nm wavelength using the prism coupling method. The obtained refractive index was 2.59. The electro-optic coefficients r(13) and r(33) were determined by applying the electrical field between a semitransparent, thin top electrode of Pt and a bottom LSCO electrode. The electro-optic coefficient was r(13) = 17 pm/V at transverse electric field (TE) mode and r(33) = 55 pm/V at transverse magnetic field (TM) mode.     

A realistic topological p-n junction at the Bi2Se3 (0001) surface based on planar twin boundary defects

We propose a realistic topological p-n junction (TPNJ) by matching two Bi2Se3 (0001) slabs with opposite arrangements of planar twin boundary defects.The atomistic modeling of such a device leads to dislocation defects in the hexagonal lattice in several quintuple layers.Nevertheless,total energy calculations reveal that the interface relaxes,yielding a smooth geometrical transition that preserves the nearest-neighbors fcc-type geometry throughout these defect layers.The electronic,magnetic,and transport properties of the junction have then been calculated at the ab initio level under open boundary conditions,i.e.,employing a thin-film geometry that is infinite along the electron transport direction.Indeed,a p-n junction is obtained with a built-in potential as large as 350 meV.The calculations further reveal the spin texture across the interface with unprecedented detail.As the main result,we obtain non-negligible transmission probabilities around the Γ point,which involve an electron spin-flip process while crossing the interface;     

化学共沉淀法制备BaFe_(12-2x)CO_xTi_xO_(19)系列磁性材料的研究

本文阐述了利于批量生产的、用化学共沉淀法制备 BaFe_(12-2x)Co_xTi_xO_(19)微粒磁粉的工艺过程,并进行了 X 射线衍射、电镜分析及一些磁参数的测最,获得了比饱和磁矩σ_(?)、矫顽力 H_c、单畴临界尺寸 R_c、超顺磁性临界尺寸 R_T 随配比成分 x 及烧结温度 T 变化的关系曲线。分析结果表明,Co~(2+)-Ti~(4+)离子从优占据磁铅石结构中的12K 位,而且具有一定的温度依赖性,共沉淀粉料在750℃开始发生由非晶态向结晶态的转变,未发现中间相 BaFe_2O_4;在850℃左右可以获得满意的粒子形貌及分布。     

Effect of silver on the formation of Bi2Sr2Ca2Cu3O10 (Bi-2 2 2 3) superconductors by sol–gel process

Bi-2 2 2 3 superconductors with silver additions are synthesized by sol–gel process. The effect of silver on the formation of Bi-2 2 2 3 phase is studied. The addition of silver brings down the reaction temperature as well as annealing time for the formation of Bi-2 2 2 3 phase. The presence of silver affects the Tc of the Bi-2 2 1 2 phase. The heat treatment of silver added Bi-2 2 2 3 samples above 1113 K show poor superconducting properties due to partial melting of Bi-2 2 2 3 phase. © 1998 Chapman & Hall     

Experimental study on a low-temperature SCR catalyst based on MnO(x)/TiO(2) prepared by sol-gel method

A catalyst based on MnO(x)/TiO(2) was prepared by sol-gel method for low-temperature selective catalytic reduction (SCR) of NO with NH(3). Focusing on the effects of the operating parameters, the SCR reaction was investigated at temperatures from 353 to 523K under steady and transient states. Under the optimal conditions, the efficiency of NO removal could exceed 90% at temperature of 423K. Furthermore, within the range investigated, the reaction order of NO, NH(3), O(2) was determined to be 1, 0, and 0.5, respectively. Apparent activation energy was also calculated to be 38kJ/mol, lower than that for most of the catalysts reported by previous investigations.     

A real-time, high-resolution optical fibre dosemeter based on optically stimulated luminescence (OSL) of KBr:Eu, for potential use during the radiotherapy of cancer

A real-time optically stimulated luminescence (OSL) dosimetry system for potential in vivo use during radiotherapy treatments is proposed. Single-crystal europium-doped KBr samples were grown in a Bridgman furnace, and characterised using optical absorption techniques. An algorithm for the processing of the OSL signal was defined for use in real-time measurements, and its performance was studied on data obtained with a home-built reader, using optical-fibre-coupled dosemeters. OSL dose-response, fading properties and temperature dependence of the signal were investigated in correlation with the concentration of Eu(2+) dopant in the sample.     

Structural,chemical, morphological and optical properties of thin films based on potassium bis(2-methyllactato) borate hemihydrate doped by iodine

Journal of Materials Science: Materials in Electronics - Complex composite films of potassium bis(2-methyllactato) borate hemihydrate (KMB) doped with iodine (I2) were dip-synthesized on glass...     

Fabrication of N-level binary optical elements by use of m mask patterns with N in the range of 2(m-1) + 1 </= N </= 2(m)

Diffraction characteristics of N-level (5 相似文献   

Dual surface plasmon resonances in Zn nanoparticles in SiO(2): an experimental study based on optical absorption and thermal stability

Metallic zinc nanoparticles (NPs) of 5-15?nm in diameter, formed in silica glass (SiO(2)) by Zn ion implantation of 60?keV, showed a strong ultraviolet absorption peak at around 4.8?eV, which has been assigned as the surface plasmon resonance (SPR) of Zn NPs, and another small peak at 1.2?eV, which has never been reported before. To identify the origin of the 1.2?eV peak, the correlations of thermal stability between the two peaks and Zn NPs were evaluated under annealing both in a vacuum (pure thermal stability) and in oxygen gas (thermal oxidation stability). The well-correlated stability between the 1.2?eV peak, the 4.8?eV peak and Zn NPs indicates that the 1.2?eV peak is not ascribed to radiation-induced defects but to the Zn NPs. The 1.2?eV peak can be ascribed to an SPR of Zn NPs in SiO(2), because the peak satisfies the criterion of the SPR of metallic NPs. Since the 4.8?eV peak is also expected to satisfy the criterion, Zn NPs in SiO(2) have two SPRs at 1.2 and 4.8?eV.