ESR- und ENDOR-Spektren von F-Zentren in RbCl und RbBr

ESR- and ENDOR-spectra of F-centers in RbCl and RbBr have been measured, giving the hyperfine constants for 5 resp. 6 shells of neighbouring ions. The resolved structure with a characteristic distance of 36 G in the ESR-spectrum of RbCl is due to the interaction with the nearest Rb⁸⁵ nuclei. All results fit well with the properties of F-centers in other alkali halides.     

Microstructural evolution of ZnS during sintering monitored by optical and positron annihilation techniques

Positron lifetime and optical absorption techniques were employed to track the microstructural evolution of polycrystalline ZnS grown by Chemical Vapor Deposition (CVD). As grown material and material treated with Hot Isostatic Pressure (HIP) was sintered at temperatures ranging from 400 to 1000°C for 2–18 h. A 290 ps defect lifetime could be resolved in all samples, while an additional longer lifetime (=430 ps) was found only in samples annealed at low temperatures. This component gradually disappeared during annealing at 800°C. Associated with the disappearance of the long-lived component, the apparent bulk lifetime of the material changed from 235 to 215 ps. A 215±2 ps bulk parameter was also found for HIP-treated material annealed at temperatures greater than 400°C and hence is taken to represent the delocalized state of the positrons in ZnS. Optical absorption measurements showed that annealing at 800°C also caused the absorption profiles of the CVD and HIP samples to converge. The rate of the bulk lifetime transition correlates with the absorption changes. The observed sharpening of the absorption profile is attributed to a decrease in scattering from grain boundaries and voids, and a decrease in absorption from point defects. The 430 ps lifetime is believed to be due to trapping at voids and grain boundaries, while the 290 ps lifetime likely is due to a monovacancy stabilized as a small complex.Paper presented at the 132nd WE-Heraeus-Seminr on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Emission and decay time studies on Pb2+ centers in KBr,RbBr, and RbCl

The emission spectra and the luminescence decay times of KBr, RbBr, and RbCl crystals doped with Pb²⁺ and excited in the A-absorption band have been studied in the temperature range 5–300 K. The emission-lineshape spectra have been analysed in terms of skew-Gaussian bands. New bands have been observed in RbCl and RbBr at very low temperatures. While the luminescence decay of KBr:Pb²⁺ and RbBr:Pb²⁺ show only a single component with a decay time τ ～ 20 ns, RbCl:Pb²⁺ shows a short and a long component. The reason for the missing long component in KBr:Pb²⁺ and RbBr:Pb²⁺ is tentatively attributed to an anomaly in the structure of the adiabatic potential energy surface (APES) of the excited states.     

Strain-optical coefficients and deformation potential constants of the valence band at K = 0 for RbCl,RbBr and RbI

Using static and ultrasonic methods, absolute values of the strain-optical coefficients, p₁₁, p₁₂ and p₄₄ are determined for RbCl, RbBr and RbI in the wavelength region 240–410 nm. Following the band-structure approach, deformation potential constants of the valence band at K = 0 are evaluated for RbBr and RbI.     

Role of stochastic heating in wakefield acceleration monitored by optical injection

It is shown that stochastic heating can play an important role in Laser Wake Field Acceleration. When considering low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wake field. The influence of stochastic acceleration on the trapping of electrons is compared to the one of cold injection by considering several polarizations of the colliding pulses. For some value of the plasma density and pulse duration, a transition from an injection due to stochastic acceleration to a cold injection dominated regime – regarding the trapped charge – has been observed from PIC code simulations. When the plasma density exceeds some value, stochastic heating becomes important and is necessary in some circumstances to get electrons trapped into the wakefield.     

Europium doped strontium borate glasses and their optical properties

Optical absorption and luminescence spectra of europium doped strontium borate glasses prepared in different conditions are studied. It is found that the percentage of Eu³⁺ ions varies from 100 to 30% being controlled by the conditions of preparation. The mechanism, favoring reduction of europium to Eu²⁺ state in polycrystalline strontium tetraborate, is much weaker in glasses of the same composition. In samples containing mixed valence europium at densities of 8×10²⁰ cm⁻³, the efficient transfer of optical excitation from Eu³⁺ to Eu²⁺, suppressing the Eu³⁺ luminescence, has been found. The most reliable way of monitoring the percentage of europium ions in different valences for strontium borate glasses is the measuring of absorption at f-f transition ⁷F₀→⁵D₂ of Eu³⁺.     

Self-assembled nanostructures of redox-functionalized terpyridines monitored by optical second-harmonic generation

Adsorption and self-organization of ferrocenyl-functionalized terpyridines on gold substrates were investigated in situ by optical second-harmonic generation (SHG). Film growth of these organometallic compounds could be detected in the submonolayer regime. The SHG data have been compared to the results of ellipsometric measurements and show that monolayer films are formed. Film growth is followed by self-assembly of ordered redox-active ferrocenyl nanostructures. PACS 81.07.Nb; 81.16.Dn; 42.65.Ky; 68.43.Mn; 81.70.Fy     

Calcium precipitation in NaCl monitored by the Eu2+ -fluorescence

Taking into account that the optical spectroscopy of the Eu²⁺ ion is quite sensitive to the crystalline environment in which this ion is located, in the present investigation the fluorescence of a small concentration (8 ppmm) of divalent europium incorporated into a NaCl crystal which was also doped with ≈4500 ppmm of Ca²⁺ has been systematically investigated as a function of different thermal treatments in order to study the calcium-precipitation processes in the host NaCl. The data presented in this paper strongly suggest that the annealing of quenched samples at 200°C produces the incorporation of Eu²⁺ into the stable dihalide phase CaCl₂, as well as into the metastable precipitated CaCl₂-like plate zones parallel to the {111} planes of the NaCl matrix. This fact is associated with the increase in intensity of two overlapping emission bands peaking at 430 and 432 nm. On the other hand, the aging of quenched samples at room temperature produces the growth of three emission bands peaking at 400, 414, and 447 nm. The former two emission bands have been associated with Eu²⁺ embedded into calcium precipitates, the structures of these precipitates being different from that of CaCl₂, while the band at 447 nm has been ascribed to europium ions incorporated into the metastable precipitated CaCl₂-like plate zones parallel to the {310} planes of the NaCl matrix. Some of the characteristics of the different calcium second phase precipitates have been obtained by measuring the crystal field splitting (10 Dq) of the 4f⁶5d configuration of the divalent europium ions when they were located inside them. Values for the 10 Dq splitting were determined from the excitation spectrum of each of the emission bands associated with the different calcium-precipitated phases.     

Thermoluminescence in single crystals of RbBr:OH− and RbBr:Ca2+

Optical absorption, thermoluminescence glow and emission spectra of RbBr:Ca²⁺ and RbBr:OH⁻ have been studied and analysed. It is observed that both Ca²⁺ and OH⁻ ions enhance theF-centre concentration.F _Z1 band in RbBr:Ca²⁺ appears at 1.55 eV. TL glow peak corresponding toF _Z1 centre on analysis gives a trap depth of 0.84 eV. OH⁻ ions in the crystal seem to act as TL ‘killers’. Spectral distribution of emission under the glow peaks shows five bands around 1.5, 1.8, 2.1, 2.5 and 2.9 eV. Probable models of TL mechanism are suggested to explain the observed TL emission bands.     

Displacement measurements in structural elements by optical techniques

Speckle metrology and holographic interferometry (HI) have been used in several civil engineering applications. We present the results obtained by applying speckle photography (SP) to the study of two quadratic shearwalls with different boundary conditions, and the potential of the technique in the study of this kind of structures is described. The analysis of Young's fringes obtained with this technique at certain points on each shearwall provides the whole field of displacement measurements. HI has been used to measure the three components of absolute displacement, verifying that the bulging phenomenon does not affect the in-plane components when the applied load remains on the same plane as the shearwall. A qualitative analysis is carried out following an electronic speckle pattern interferometry (ESPI) technique. The results obtained by optical techniques are compared to the numerical results obtained by the finite element method (FEM), finding good correlation between them in all the cases.     

Laser tattoo removal as an ablation process monitored by acoustical and optical methods

Monodisperse hollow silica nanospheres have been prepared by using a polystyrene nanosphere template-assistant approach and their potential as antireflection (AR) coatings for window applications has been discussed. The as-prepared hollow silica nanospheres have a typical inner diameter of 200 nm and a shell thickness of 15–20 nm. The AR effect over the ultraviolet-visible-near infrared spectral region has been observed for the hollow silica nanospheres, with a minimized reflection of about 5.2 % at 500 nm, compared to 8.5 % of a plain float glass substrate. By modifying the structural features of the hollow silica nanospheres, their AR properties can be further enhanced.     

对差分光学吸收光谱法的监测数据进行实时预测研究

差分光学吸收光谱法（DOAS）已经成为测量大气中微量气体浓度广泛应用的方法.通过对大气差分吸收光谱的分析可以得到它们的浓度.但在恶劣气候条件下，DOAS系统不能获得连续的实时监测数据，因此不能满足我国环境监测条例对环境监测子站的要求.文中提出了一种利用逐步回归分析的方法，对DOAS系统的监测数据进行实时预测.通过大量的对比实验表明，利用该方法得到的预测结果与实际测量结果一致，目前该技术已经被应用于DOAS监测仪器中. 关键词： 差分光学吸收光谱法（DOAS） 逐步回归分析 预测     

Oligonucleotide hybridization observed by surface plasmon optical techniques

Surface plasmon spectroscopy and microscopy are employed to study the hybridization reaction of an oligonucleotide ten-mer from solution to its complement immobilized to a solid support. It is shown that a dilute target monolayer is necessary to allow for an effective dimerization process. Parallel multi-site detection of the DNA binding is demonstrated to be possible with surface plasmon microscopy without the need for any extra labeling of the reaction partners. The simultaneous quantitative evaluation of the degree of dimerization to many different target areas, each as small as a few μm² and each functionalized by different oligonucleotide sequences, can be accomplished by two-dimensional image analysis and should be relevant for DNA sequencing protocols.     

Structural and optical characterization of CdS nanoparticles prepared by chemical precipitation method

In the present study we have synthesized CdS semiconducting quantum dots by the chemical precipitation method using Thioglycerol as the capping agent. X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) are employed to characterize the size, morphology and crystalline structure of the as-prepared material. The synthesized QPs have a mixture of cubical and hexagonal crystal symmetry with 12 nm average diameter. Ultraviolet-visible (UV-vis) absorption spectroscopy is used to calculate the band gap of the material and blue shift in absorption edge. Confinement of the optical phonon modes in the QPs is studied by Raman spectroscopy, while FTIR for identification of chemical bonds in the nanomaterial. Multiple cadmium and sulphur defects were observed by employing the photoluminescence (PL) method.     

Understanding of optical readout accuracy with micromechanical sensor cantilever monitored by surface plasmon resonance

This paper reports a concept of micromechanical sensing of environmental condition using the surface plasmon resonance phenomenon. We calculate the resolution in the cantilever bending monitoring using the transfer matrix numerical method. We show that the cantilever deflection can be monitored with a resolution in the nanometer range. The SPs resonance behavior of the multilayer stack in the case of gold cantilever is discussed. We believe that this concept permits a low cost and ease of fabrication for a large bi-dimensional array of sensors with an enhanced signal-to-noise ratio.     

Optical properties of arachidate monolayers by integrated optical techniques

The refractive indices of thin dielectric films on the surface of an optical waveguide were determined from the mode patterns, i.e., effective indices in the guide as a function of selected visible laser wavelengths. These more accurate results are compared with previous measurements and illustrate the advantages of this method.     

Measurement of ultrashort optical pulses by two-photon photoconductivity techniques

Two-photon conductivity in GaAs and CdS_0·5Se_0·5 were investigated with modelocked Nd:glass laser pulse excitation, with a view to determine their suitability for picosecond pulsewidth measurement. The contrast ratio using GaAs as twophoton conductor was 1.8 and it increased to 2.4 when CdS_0·5Se_0·5 was used. The improved contrast ratio in the case of CdS_0·5Se_0·5 was partly due to the improved resolution of the crystal (2 ps) and partly due to the extended square-law region.     

Resolution enhancement techniques for optical lithography and optical imaging theory

Production of a fine pattern is necessary to get a high integration degree of integrated circuits. The conventional methods which utilize high numerical aperture and short wavelength exposure are limited by designing and manufacturing of a practical lens and make the focus depth narrow. Resolution enhancement techniques (RETs) have, therefore, been required and proposed. This paper introduces a phase-shifting mask, a typical RET, points out the problems and inconsistencies of conventional optical imaging theory and explains the image formation concept of expansion of plane waves. Essentially using this concept, an attempt is also made to describe some other typical RETs with potential.     

Mixing in a model gas turbine combustor studied by panoramic optical techniques

Using planar optical methods based on laser-induced fluorescence and particle image velocimetry instantaneous velocity fields and passive tracer concentration are measured simultaneously in a model of GT-combustor at realistic flow rates. Spatial distributions of velocity pulsations and passive tracer concentration pulsations are measured at air flow rate about 0.4 kg/s. Correlations of velocity and concentration pulsations are measured. The most intense turbulent mass flux in the region of swirling flow mixing layer was observed. The contribution of advective and turbulent components in the transfer of a passive tracer in the axial direction was estimated.     

Acoustically driven charge separation in semiconductor heterostructures sensed by optical spectroscopy techniques

We demonstrate a method of using a two-layer sandwich structure, which includes a LiNbO₃ plate and a semiconductor heterostructure to create an inhomogeneous stress and piezoelectric harmonic potential in the semiconductor. Both the GaAs/AlGaAs quantum well (QW) structures and SiGe/Si heterostructures are attempted, working with and without using a piezoelectric field in the semiconductor layer. The standing-wave fields generated in the semiconductor and the electron and hole distributions driven by the piezoelectric field are computed by finite element method (FEM) techniques. It is experimentally shown that, in a GaAs/Al_xGa_1-x As asymmetric double quantum well structure, the resonance enhancement of the narrower QW photoluminescence band is observed, which may be explained by the resonant charge transfer between the wider and narrower QWs. It is also shown that the piezoelectric fields quench the pure LO-phonon lines in the Raman spectra, whereas the coupled LO-phonon-plasmon mode strengthens. Experimental results indicate that the charge separation occurs in the plane of the QWs due to the piezoelectric fields. The recombination of carriers in the SiGe/Si heterostructures can be effectively enhanced by the presence of ultrasonic stress, displaying features consistent with varying electrical activity at dislocations.