Short-wavelength Band Research Articles

Short wavelength band edge thermometry during molecular beam epitaxial growth of GaN on SiC substrates and detected adatom self-heating effects

Band edge thermometry was used to measure the temperature of 4H SiC substrates, band gap of 3.25 eV, during heat-up followed by molecular beam epitaxial growth of GaN structures. The surface of the rotating wafer was illuminated with a white light source which provided sufficient scattered signal for SiC substrate temperature measurements of single side polished substrates and unmetallized double side polished substrates. Substrate temperatures above 400 °C could be measured with good signal-to-noise ratio. At 750 °C the precision was ±1 °C despite the small temperature dependence of the SiC band gap. A complete spectral interference oscillation was observed for a layer thickness of 800 Å. The scattered intensity was found to be dependent on whether gallium- or nitrogen-rich growth conditions were used. During conventional gallium-rich GaN growth on unmetallized, double sided polished substrates, the thermometer detected an increase in wafer temperature due to the presence of surface gallium. A similar adatom self-heating effect was observed with indium.

Solution-Processable Crystalline Platinum-Acetylide Oligomers with Broadband Absorption for Photovoltaic Cells

A series of solution-processable and crystalline platinum−acetylide oligomers containing a thienyl−benzothiadiazole−thienyl core and oligothiophene alkynyl ligands are synthesized and characterized. X-ray crystallography analysis indicates a two-dimensional arrangement of oligomers through CH−π interactions in single crystals. These oligomers show two intense and broad absorption bands in the visible spectral region, with the short-wavelength absorption band being strongly dependent on the oligothiophene length. In neat films, all the oligomers form large crystalline domains of several hundred nanometers in size upon thermal treatment and exhibit space-charge limited current (SCLC) mobilities on the order of 10−5−10−4 cm2 V−1 s−1. The photovoltaic properties of these oligomers were evaluated by fabricating bulk heterojunction devices with fullerene derivatives (PC61BM and PC71BM) and some of these devices showed high-power conversion efficiencies (PCEs) of up to 3% and a peak external quantum efficiency (EQE) to 50% under AM 1.5 simulated solar illumination. The present work suggests that well-defined platinum oligomers with desirable light-absorbing and self-assembly properties have potential for solution-processed organic photovoltaics.

UV-light induced luminescence processes in Al 2O 3 bulk and nanosize powders

UV-light induced photoluminescence (PL) and thermoluminescence (TL) have been studied in the alumina (Al 2O 3) bulk and nanosize powders. It has been found that luminescence properties of the studied alumina powders are determined mainly by presence of the uncontrolled impurities of titanium, iron and chromium. Despite the general similarity of PL characterized by two main emission band around 400 and 750 nm a number of differences has been observed in relative yield and position of the emission bands as well as in the excitation spectrum of the short wavelength band. Essential differences have been observed also in the intensity of the TL signal, TL curves and TL emission spectra revealed by bulk and nanosize alumina powders. Peculiar properties of nanopowder, such as presence of the 395 nm band excited at 210 and 250 nm and ascribed to intrinsic surface-perturbed defect F s or F s + , as well as very low intensity of the TL signal due to surface quenching, may be considered as manifestation of the surface effects, characteristic to nanostructures.

Boradipyrromethenecyanines derived from conformationally restricted nuclei

Abstract A series of meso -polymethine-substituted 4,4-difluoro-4-bora-3a,4a-diaza- s -indacenes were synthesized based on the reaction of meso -methyl borondipyrromethene with a number of hemicyanine derivatives. The dyes obtained exhibited a weak short-wavelength absorption, a strong long-wavelength absorption and weak fluorescence. Upon protonation, the long-wavelength band disappeared while the intensity of the short-wavelength band increased markedly. The properties of the dyes were closely related to those merocyanine dyes rather than borondipyrromethenes.

Pyrometric Method of Temperature Measurement with Compensation for Solar Radiation

Pyrometric Method of Temperature Measurement with Compensation for Solar RadiationOutdoor remote temperature measurements in the infrared range can be very inaccurate because of the influence of solar radiation reflected from a measured object. In case of strong directional reflection towards a measuring device, the error rate can easily reach hundreds per cent as the reflected signal adds to the thermal emission of an object. As a result, the measured temperature is much higher than the real one. Error rate depends mainly on the emissivity of an object and intensity of solar radiation. The position of the measuring device with reference to an object and the Sun is also important. The method of compensation of such undesirable influence of solar radiation will be presented. It is based on simultaneous measurements in two different spectral bands, shor-twavelength and long-wavelength ones. The temperature of an object is derived from long-wavelength data only, whereas the short-wavelength band, the corrective one, is used to estimate the solar radiation level. Both bands were selected to achieve proportional changes of the output signal due to solar radiation. Knowing the relation between emissivity and solar radiation levels in both spectral bands, it is possible to reduce the measurement error several times.

WIDE-BAND HYBRID AMPLIFIER OPERATING IN S-BAND REGION

Wide-band hybrid amplifier is theoretically proposed using a series configuration of Thulium-doped fiber amplifier (TDFA) and fiber Raman amplifier (FRA), which using the similar type of pump laser. The operating wavelength of this amplifier covers the bandwidth of entire short wavelength band (S-band) region by combining the gain spectrum of TDFA and FRA. The theoretical gain varies from 20 to 24dB within a wavelength region from 1460 to 1525nm and which is in a good agreement with the experimental result. The development of reliable high-power diode lasers in the 1420nm wavelength range will make this type of wide-band hybrid amplifier an interesting candidate for S-band optical telecommunication systems.

Enhancement of fluorescence of porous silicon upon saturation by liquid crystal

The fluorescence of samples of porous silicon of various morphologies that are filled with a liquid crystal (LC), n-pentyl-n′-cyanobiphenyl (5CB), is studied. The fluorescence spectra of the sample, along with the long-wavelength band of porous silicon with a maximum in the range 627–667 nm, exhibit a short-wavelength band of 5CB with a maximum in the range 385–410 nm. The radiative relaxation times of porous silicon and 5CB lie in the micro- and nanosecond ranges, respectively. It is found that the filling of pores with 5CB enhances the fluorescence of porous silicon by two to three times. This enhancement is caused by non-radiative energy transfer from 5CB to the porous matrix as a result of efficient interactions between LC molecules and pore walls. Using IR spectroscopy, it is shown that the formation of hydrogen bonds between cyano groups of 5CB molecules and silanol groups of pore surface is the predominant type of these interactions. A transfer mechanism is suggested according to which excited associates of 5CB molecules transfer their energy via surface channels to excitons of porous silicon, enhancing its fluorescence.

EXPLORATIONS BEYOND THE SNOW LINE:SPITZER/IRS SPECTRA OF DEBRIS DISKS AROUND SOLAR-TYPE STARS

We have observed 152 nearby solar-type stars with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. Including stars that met our criteria but were observed in other surveys, we get an overall success rate for finding excesses in the long wavelength IRS band (30-34 micron) of 11.8% +/- 2.4%. The success rate for excesses in the short wavelength band (8.5-12 micron) is ~1% including sources from other surveys. For stars with no excess at 8.5-12 microns, the IRS data set 3 sigma limits of around 1,000 times the level of zodiacal emission present in our solar system, while at 30-34 microns set limits of around 100 times the level of our solar system. Two stars (HD 40136 and HD 10647) show weak evidence for spectral features; the excess emission in the other systems is featureless. If the emitting material consists of large (10 micron) grains as implied by the lack of spectral features, we find that these grains are typically located at or beyond the snow line, ~1-35 AU from the host stars, with an average distance of 14 +/- 6 AU; however smaller grains could be located at significantly greater distances from the host stars. These distances correspond to dust temperatures in the range ~50-450 K. Several of the disks are well modeled by a single dust temperature, possibly indicative of a ring-like structure. However, a single dust temperature does not match the data for other disks in the sample, implying a distribution of temperatures within these disks. For most stars with excesses, we detect an excess at both IRS and MIPS wavelengths. Only three stars in this sample show a MIPS 70 micron excess with no IRS excess, implying that very cold dust is rare around solar-type stars.

Calibration and Performance of the AKARI Far-Infrared Surveyor (FIS) — Slow-Scan Observation Mode for Point-Sources

Abstract We present the characterization and calibration of the Slow-Scan observation mode of the Far-Infrared Surveyor (FIS) onboard the AKARI satellite. The FIS, one of the two focal-plane instruments on AKARI, has four photometric bands between 50–180$\mu$m with two types of Ge:Ga array detectors. In addition to the All-Sky Survey, FIS has also taken detailed far-infrared images of selected targets by using the Slow-Scan mode. The sensitivity of the Slow-Scan mode is one to two orders of magnitude better than that of the All-Sky Survey, because the exposure time on a targeted source is much longer. The point spread functions (PSFs) were obtained by observing several bright point-like objects, such as asteroids, stars, and galaxies. The derived full widths at the half maximum (FWHMs) are $\sim$30$^{\prime\prime}$ for the two shorter wavelength bands and $\sim$40$^{\prime\prime}$ for the two longer wavelength bands, being consistent with those expected by optical simulation, although a certain amount of excess is seen in the tails of the PSFs. A flux calibration was performed by observations of well-established photometric calibration standards (asteroids and stars) over a wide range of fluxes. After establishing the method of aperture photometry, the photometric accuracy for point-sources is better than $\pm$15% in all of the bands, expect for the longest wavelength.

Hole-Assisted Fibers With $\lambda _{0}$ Around 1000 nm and Holey Fibers With $\lambda _{0}$ Around 500 nm

Microstructured optical fibers (MOFs) with air holes within the structure, such as hole-assisted fibers (HAFs) and holey fibers (HFs), having unique dispersion properties in short-wavelength bands, have captured considerable attention recently thanks to their wide range of practical applications. In particular, supercontinuum (SC) source, which is generated by interplay between many different nonlinear processes occurred in the MOFs, have gathered huge interests both in telecom and nontelecom applications. In this paper, designs and fabrications of HAFs and HFs, having unique dispersion properties in near infrared and visible regime, will be discussed. In particular, we focus on: 1) the dispersion control of HAFs in the 1.0- mum band, which has become of great interest stimulated by maturing ytterbium-doped fiber amplifier (YDFA) technologies, and 2) dispersion shift towards visible wavelengths (500 nm) using HFs. Finally, we will briefly discuss future developments of the MOFs for short-wavelength applications.

Estimation of Waviness of the Longitudinal Road Profile from Straightedge Measurement

The paper presents a method for estimation of power spectral density (PSD) parameters (unevenness index and waviness) of a longitudinal road profile in the short wavelength band from data collected by straightedge (SE) measurement (distance of contact points of the lath and maximum clearance from the straightedge between the supports). This method is based on the estimation of mean values of straightedge measurement parameters established by simulated artificial profiles of different waviness levels from 1.5 to 3.5. The method was validated on a series of simulated profiles with different waviness values and on some in-service road profile records. The method provides a reliable estimate of waviness as a complement to the currently used straightedge indicators based on the mean maximum clearance under the straightedge. Thus, mutual conversion between SE- and PSD-unevenness indicators is possible.

Comparison of Thermal Infrared Emissivities Retrieved With the Two-Lid Box and the TES Methods With Laboratory Spectra

Knowledge of surface emissivity in the thermal infrared (TIR) region is critical for determining the land surface temperature (LST) from remote-sensing measurements. If emissivity is not well determined, it can cause a significant systematic error in obtaining the LST. The main aim of this paper is to compare different methods for measuring accurate land surface emissivity in the field, namely, the box method and the temperature and emissivity separation (TES) algorithm. Field emissivities were compared with soil spectra from laboratory measurements. Emissivities were measured for the bands of a multispectral radiometer CE312-2 with effective wavelengths at 8.4, 8.7, 9.1, 10.6, and 11.3 mum, similar to the Advanced Spaceborne Thermal Emission and Reflection Radiometer TIR bands, and a wide channel 8-13 mum. The measurements were made at two sites in New Mexico: the White Sands National Monument and an open shrub land in the Jornada Experimental Range. The measurements show that for both sites the emissivities derived with the Box method agree with those derived with the TES algorithm for the 10.6 and 11.3 mum bands. However, the emissivities for the shorter wavelength bands are higher when derived with the Box method than those with the TES algorithm, with differences ranging from 2% to 7%. The field emissivities agree within 2% with the laboratory spectrum for the 8-13-, 11.3-, and 10.6-mum bands. However, the field and laboratory measurements in general differ from 2.4% to 9% for the shorter wavelength bands, with the larger value most likely caused by variations in soil moisture.

Intramolecular photoinduced proton transfer in 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones with different electron-withdrawing N-substituents

Fluorescence spectra of N-substituted 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones consist of two bands, the long-wavelength band with anomalous Stokes shift, which corresponds to the emission of the product of intramolecular photoinduced proton transfer, and the short-wavelength band belonging to the form in which proton transfer does not occur. It is assumed that there is equilibrium between two planar rotamers in the ground state: one with the N-H…N hydrogen bond in which the intramolecular photoinduced proton transfer occurs and the other with the N-H…O bond, which does not experience hydrogen transfer. According to ab initio quantum-chemical calculations, the potential energy of proton transfer in the first excited singlet state has a potential barrier of 2.1–26.8 kJ/mol depending on the electron-withdrawing ability of the substituent on the amino group.

Observation of Stimulated Emission in Short Wavelength Band from Silica-Based Superstructure Films

We report Ultraviolet (UV)-induced visible light luminescence in artificial-lattice thin films of ion-doped silica glass (silica superstructure thin films). The film was composed of periodic nanometer layers of germanium-doped silica (Ge: SiO2), titanium-doped silica (Ti: SiO2), and tin-doped silica (Sn: SiO2). The thickness of each layer was between 10 and 30nm. Despite the small thickness of the film (few microns), a relatively bright luminescence of white light was observed, along with cathode-ray luminescence in the superstructure film. In addition, irradiation of the superstructure film with UV light led to light amplification by stimulated emission at 405nm. The experimental results suggest the potential application of silica superstructure thin films as optical amplifiers.

Violation of Vavilov’s law upon excitation of luminescence of uranyl solutions at the short-wavelength absorption band (200–240 nm)

It was found that in the UV spectral region (200–240 nm) where intense absorption bands of the UO22+ ion are located, excitation of its luminescence in solutions is not observed, thereby contradicting Vavilov’s law about independence of luminescence quantum yields from the excitation light wavelength. The violation of Vavilov’s law is explained in terms of nonradiative deactivation processes as the result of photoinduced electron transfer to the uranyl ion with its reduction to the pentavalent state and the subsequent disproportionation reaction to form uranium(IV). The presence of uranium(IV) ions during UV irradiation of uranyl solutions was proved by the chemiluminescent method.

Experimental investigation of the slow light and superluminal effects in high-quality three-dimensional colloidal photonic crystals

We investigated by time-of-flight technique the slow light and superluminal effects in high-quality three-dimensional （3D） colloidal photonic crystals （PCs） with opal structure, fabricated by pressure controlled isothermal vertical deposition technique. They produce high transmission in the pass bands and sharp band edges due to low volume density of defects and dislocations. The variation of the group velocity across the short-wavelength band edge was measured. Group velocity as low as 0.43c was observed near the band edge while group velocity as high as 1.34c was found in the band gap. We have also simulated the transportation of ultrashort pulses through 3D PCs and compared the simulation results with the experimental ones.

Computational study of steric and spectroscopic characteristics of bi-chromophoric cyanine dyes: Comparison with experimental data

The spectral and energetic characteristics of four bi-chromophoric cyanine dyes (BCDs) which possess angles between chromophores 180°, 150°, 120°and 90°, were studied using quantum chemical calculations in comparison with experimental data. It was demonstrated that for BCD with 180°, 150° and 90° trans–trans isomers possess the lowest energy, while for BCD with 120° the trans–trans and cis–trans isomers have comparable energies and in the temperature range from 273 K up to 373 K both isomers of this dye are present. It was also demonstrated that the splitting of the spectra of cyanine dyes with two chromophores (BCD) was determined by two effects: the dipole–dipole chromophore interaction and the electron tunneling through the central heterocycle. Both effects depend on the central heterocycle structure, which on the one hand determines the distance between the chromophores, thus determining the value of the dipole–dipole interaction, and on the other hand the degree of π-conjugation in the central heterocycle determines the probability of electron tunneling. The central heterocycle structure determines relative orientation of the chromophore dipoles, as well, thus determining the intensities of the short-wavelength and long-wavelength bands in the BCD absorption spectra.

Luminescence studies of thallium co-doped KBr:Eu2+ powder phosphors

In this study, photoluminescence (PL) and photostimulated luminescence (PSL) properties in KBr:Eu2+, Tl+ powder phosphors are reported. PL emission spectra of these Tl+ co-doped KBr:Eu2+ phosphors show four overlapping bands around 310, 325, 360 and 375 nm in addition to the characteristic of Eu2+ ions at 420 nm. These additional short wavelength bands were attributed to centres involving Tl+ ions. The decrease in PSL intensity of γ-irradiated KBr:Eu2+, Tl+ powder phosphors with Tl+ concentration and the absence of thallium emission bands in PSL were attributed to the efficient electron trapping by Tl+ ions during irradiation.

Description of a New Method for Retrieving the Aerosol Optical Thickness from Satellite Remotely Sensed Imagery Using the Maximum Contrast Value and Darkest Pixel Approach

Satellite sensors have provided new datasets for monitoring regional and urban air quality. Satellite sensors provide comprehensive geospatial information on air quality with both qualitative remotely sensed imagery and quantitative data, such as aerosol optical depth which is the basic unknown parameter for any atmospheric correction method in the pre-processing of satellite imagery. This article presents a new method for retrieving aerosol optical thickness directly from satellite remotely sensed imagery for short wavelength bands in which atmospheric scattering is the dominant contribution to the at-satellite recorded signal. The method is based on the determination of the aerosol optical thickness through the application of the contrast tool (maximum contrast value), the radiative transfer calculations and the ‘tracking’ of the suitable darkest pixel in the scene. The proposed method that needs no a-priori information has been applied to LANDSAT-5 TM, LANDSAT-7 ETM+, SPOT-5 and IKONOS data of two different geographical areas: West London and Cyprus. The retrieved aerosol optical thickness values show high correlations with in-situ visibility data acquired during the satellite overpass. Indeed, for the West London area a logarithmic regression was fitted for relating the determined aerosol optical thickness with the in-situ visibility values. A high correlation coefficient (r2= 0.82; p= 0.2) was found. Plots obtained from Tanre et al. (1979, 1990) and Forster (1984) were reproduced and estimates for these areas were generated with the proposed method so as to compare the results. The author's results show good agreement with Forster's aerosol optical thickness vs. visibility results and a small deviation from Tanre's model estimates.

Lichtexposition bei vitreoretinaler Chirurgie

Light can cause phototoxic retinal damage. The aim of this study was to evaluate the risk of retinal hazard by endoilluminators during vitreoretinal surgery. The spectra, radiance, and irradiance of six light sources with different associated fibre optics (20 G, 23 G, standard collimated, wide-angle diffuse) were measured and compared with thresholds published by international standardisation committees. The spectra of the endoilluminators differed significantly in the short wavelength band. The maximum radiance ranged from 15 mW to 190 mW and the calculated irradiance from 36 mW/cm2 to 1,130 mW/cm2 (distance 5 mm) and from 9 mW/cm2 to 376 mW/cm2 (distance 10 mm). Compared with published thresholds for surgery, time limits ranging from 0.7 min to 264 min (distance 5 mm) and 2.7 min to 1,052 min (distance 10 mm) seem to be safe. Light systems used for vitreoretinal surgery differ considerably in spectra, radiance, and irradiance; these differences have an impact on the maximum tolerable exposure times during surgery.