Effect of heterophase inclusions on the optical losses in high-purity IR optical materials

Using a tungstate-tellurite glass and high-purity zinc sulfide—promising IR optical materials—as test systems, we have developed an approach for evaluating the limits of the sensitivity of optical losses to the presence of heterophase impurity inclusions and second-phase inclusions differing in chemical nature and size. We have calculated the volume fraction of disperse phases and concentration of inclusions which ensure optical losses in these materials below a predetermined level.     

Diffusion and electrical conductivity studies in some heterophase glasses containing bismuth

Self-diffusion coefficients of sodium in the temperature range 300 to 375° C have been measured for glasses in the system Na₂O-B₂O₃-Bi₂O₃-SiO₂, by using a radioactive tracer technique. All these glasses have a two-phase structure. The d.c. resistivities of these glasses were also measured over the temperature range 30 to 375° C. The correlation factors,f, of all the samples have been calculated by using the diffusion coefficients and d.c. resistivity values for temperatures above 300° C. The anomalously largef values in the case of Bi₂O₃-containing glasses are thought to be due to a distribution of sodium ions in the dispersed as well as the continuous phases in these systems.     

Design and fabrication of omnidirectional reflectors in the visible range

An omnidirectional reflector operating in the visible range with the centre wavelength around 532?nm is designed and fabricated. By the calculations of electrical field intensity distributions of the incident light along the propagation direction in the photonic crystal (PC) structure, a minimum period number for producing one-dimensional PCs with an omnidirectional photonic band gap (PBG) is determined. A 12-layer-pair stack of TiO₂/SiO₂ was grown by the electron-gun evaporation method. Transmission spectra measured with transverse-magnetic- and transverse-electric-polarized incident lights at various angles show that an omnidirectional PBG of 24?nm width is obtained, which is in agreement with theoretically simulated results. This kind of reflector may find applications in engineering measurements, laser beam security, optical filters, biosensors and so on.     

Characterization of ZBGAN glasses

The physical and electrical properties of a ZrF₄-BaF₂-GdF₃-AIF₃-NaF (ZBGAN) glass have been studied. The addition of NaF increases the glass-forming ability of the base composition glass yet does not degrade the optical characteristics. The viscosity decreased with increasing NaF, suggesting that the increased glass-forming ability was due to the confusion principle. The electrical conductivity decreased with increasing NaF for low temperatures but increased at higher temperatures. The activation energy showed an increase with increasing NaF. The data suggest that the conductivity is not due to the total fluorine ion concentration.     

Nature of heterophase inclusions in high-purity optical fiber materials as studied with 3D laser ultramicroscopy

3D laser ultramicroscopy (3D LUM) is intended specially for determining the concentration and size distribution of submicron inclusions in the bulk samples of high-purity materials for visible and IR fiber optics. In this work the 3D LUM technique is shown to be able to identify the nature of individual inclusions detected. The measurement of the light scattered by an inclusion at a varied probe beam wavelength and polarization and at a varied scattered light collection angle makes it possible to determine the inclusion refractive index. The 3D LUM possibilities are illustrated by the example of studying the inclusion nature in the As₂S₃ glass samples prepared by the direct synthesis from elements in a quartz container at elevated temperatures.     

Radiation characteristics of a dense cesium plasma in the visible range

Calculations are made of the radiation spectrum from a cesium plasma column. It is shown that this plasma may serve as a light source with a high proportion of visible radiation and an almost continuous spectrum. The visible spectrum is formed mainly by bright recombination 6P and 5D continua whose thresholds are shifted in the long-wavelength direction. Pis’ma Zh. Tekh. Fiz. 23, 40–45 (December 26, 1997)     

Negative refraction in the visible range in water-infiltrated opal photonic crystals

A negative refraction effect has been found in opal photonic crystals in the visible range. We have calculated the dispersion branches of a photonic crystal and determined the position of its photonic band gap. The frequency range has been identified where the refractive index of the opal is negative. An experimental arrangement is proposed for focusing a light beam by passing it through a plane-parallel opal photonic crystal and experimental evidence is presented in favor of negative refraction in the visible range.     

Characterization of fluorine-doped silica glasses

Fluorine-doped silica glasses containing up to 2 wt% were prepared by the vapour-phaseaxial-deposition (VAD) sintering process. The characteristics of these glasses were investigated by Raman spectroscopy, vacuum-ultraviolet-ultraviolet (v.u.v-u.v.) spectroscopy, and viscosity measurements. From the Raman spectroscopic investigation, it has been shown that the Si-F bond structure in the fibres is the same as that in bulk glass and is not affected by codoped additives such as B₂O₃. From the u.v.-v.u.v, spectroscopic investigation, it has been shown that the absorption band at 7.6 eV in high-purity silica glass is removed with the addition of fluorine. This addition produces a silica glass with most excellent transparency in u.v. and v.u.v. regions. Also it was observed that the optimum addition is around 1 wt%. This fact proves that fluorine incorporation in glass prevents the generation of defects related to optical loss in fibres. From the viscosity study, it was found that the viscosity decreases with an increase of the fluorine content in glass and the activation energy also decreases with increasing fluorine content.     

Superconducting single-photon detector for the visible and infrared spectral range

Abstract

We present results on dark count rates and spectral sensitivities of superconducting single-photon detectors in the visible and near-infrared spectral range. The active detector element is a nanometre-sized (a few nanometres thick and less than 100nm wide) meander line carrying a supercurrent. The superconducting materials are NbN and Nb, respectively. The NbN detector exhibited a flat spectral sensitivity up to about 2.4μm. Fluctuations of the superconducting order parameter are considered as a major source of dark count events. A simple model and its limitations to explain the observed dark count rates is discussed.     

Intercomparison of reflectances observed by GOME and SCIAMACHY in the visible wavelength range

We compare the Earth reflectances of the spectrometers Global Ozone Monitoring Experiment (GOME) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) over their overlapping wavelength range (240-800 nm). The goal is to investigate the quality of the radiometric calibration of SCIAMACHY using calibrated GOME data as a reference. However, severe degradation of the GOME instrument in the UV since 2001 prevents it from being a reliable reference below 500 nm. Above 500 nm, GOME is reliable and we find substantial disagreement between GOME and SCIAMACHY, of the order of 15%-20%, which we can attribute completely to the current calibration problems of SCIAMACHY. These numbers are supported by a previous study in which SCIAMACHY was compared with the imager Medium Resolution Imaging Spectrometer (MERIS) onboard the Envisat satellite.     

Point defects as the centers of titanium dioxide sensitization in the visible spectral range

The formation of surface point defects in the initial stage of TiO₂ reduction (x ≤ 10¹² cm⁻²) has been studied by mass spectrometry and ultraviolet photoelectron spectroscopy (UPS). Heating to 720 K or UV illumination in ultrahigh vacuum creates surface color centers in TiO₂ with an energy spectrum extending from the Fermi level to the valence band top. The continuous photoelectron spectrum exhibits a peak at 2.7 eV, which varies in a manner correlated with the behavior of the optical absorption bands at 2.55 and 2.81 eV assigned to oxygen vacancies in the TiO₂ crystal structure. The interaction of the surface point defects with molecular oxygen has been studied and a special form of the photoadsorbed oxygen (with E _des = 1.37 eV) is found. It is shown that the surface color centers may serve as centers of TiO₂ sensitization in the visible spectral range.     

Characterization of a visible spectrum division-of-focal-plane polarimeter

The development of high resolution division-of-focal-plane polarimeters in the visible spectrum allows real-time capture of two chief properties of interest, the degree of linear polarization and the angle of polarization. The accuracy of these two parameters can be influenced by a number of factors in the imaged scene, from the incident intensity and wavelength to the lens used for image capture. The alignment, transmission, and contrast ratios of the pixel matched filters also impact the measured parameters. A system of measurements is presented here that shows how these factors can determine the quality of a division-of-focal-plane polarimeter.     

Method and apparatus for determination of the total directional emissivity of opaque materials in the temperature range 300 to 600 K

A novel device for measurements of the normal and directional total emissivity of solid surfaces in the range 300–600 K is described. The measurements were performed using the radiation comparison method. A novel black-body radiation source is used as a standard. The detection system consists of a pyroelectric head, a preamplifier, a chopper, and a matching lock-in amplifier with analog recorder ourput. The device is simple and well suited for routine measurements. Comparison of our results with those of previous investigations shows a good agreement.     

Multiwavelength sun-photometers for accurate measurements of atmospheric extinction in the visible and near-IR spectral range

A new multiwavelength sun-photometer has been developed to obtain accurate measurements of the atmospheric attenuation of direct solar radiation at seven window wavelengths in the visible and near-infrared spectral range. The instrument's optical features and performance are described. The photometer was calibrated by following a careful procedure based on the Langley method. An additional eighth bandpass filter was used to obtain measurements of the precipitable water. Five examples of this instrument have been constructed and calibrated for a network of mountain stations in Southern Italy to monitor Sahara dust transport episodes.     

Frequency transmission from the infrared to the visible spectrum range when using nonlinear crystals

Translated from Izmeritel'naya Tekhnika, No. 5, pp. 16–17, May, 1989.     

Palladium-based plasmonic perfect absorber in the visible wavelength range and its application to hydrogen sensing

We report on the experimental realization of a palladium-based plasmonic perfect absorber at visible wavelengths and its application to hydrogen sensing. Our design exhibits a reflectance <0.5% and zero transmittance at 650 nm and the operation wavelength of the absorber can be tuned by varying its structural parameters. Exposure to hydrogen gas causes a rapid and reversible increase in reflectance on a time scale of seconds. This pronounced response introduces a novel optical hydrogen detection scheme with very high values of the relative intensity response.     

可见光区一维光子晶体纳米膜偏振带通滤波器的设计

应用一维时域有限差分方法研究各种条件下一维二元光子晶体的偏振带通滤波特性,具体数值分析了掺杂层位置、厚度、电磁参数、入射角度四种因素对偏振滤波特性的影响.数值结果表明传统意义上的光学多层膜是一维二元光子晶体在光学厚度满足四分之一波长时的特例;可见光区的偏振滤波器的窄带滤波特性与掺杂层的位置有关,掺杂层在整个膜中间位置时偏振分离效果好,掺杂层的厚度与周期层厚度相差越大则分离效果越好,两组元折射率相差越大越易形成禁带,入射角越大禁带越窄,偏振的分离度越好.特别是P偏振局域模更多;在线度参数相同的情况下介质电磁参数对禁带有较大影响,禁带只有在两组元折射率相差越大才能形成,介质损耗同样是不可忽略的因素;光源的入射角对禁带有重大影响.本文的研究对光子器件的设计有一定的指导作用.     

Peculiarities of the magnetization reversal in heterophase nanocomposite permanent magnets

We have studied the process of magnetization reversal in a thin-film Fe/Sm₂Co₇ exchange coupled bilayer structure under the action of an in-plane external field. An analysis of the local magnetization changes, as measured using the magnetooptical indicator film technique, showed that the magnetization reversal proceeds by inhomogeneous rotation of the magnetic moments in Fe and SmCo layers, both in plane and in the perpendicular direction. It is established that, because of the exchange interaction between layers, the magnetization reversal along the easy axis in the entire structure is determined primarily by the formation of exchange-induced spin helices and domain walls in the magnetically soft layer, whereas the magnetization reversal at an angle of α with respect to the easy axis plays a significant role in the magnetically hard layer and becomes dominating for α=90°.