Broad Spectral Interval Research Articles

Role of arsenic vapor pressure in transformation of InAs quantum dots during overgrowth by a GaAs capping layer

The results of a study of the optical properties of self-assembled InAs/GaAs (001) quantum dots (QDs) overgrown under different arsenic pressures, obtained using photoluminescence (PL) and PL excitation spectroscopy, are presented. If the arsenic pressure during overgrowth is low (2.5·10−6 Pa), a series of pronounced QD-related peaks is observed in the 77-K PL spectrum over a 200-meV broad spectral interval with the brightest one located at 1.37 eV. With increasing arsenic pressure, the PL spectrum becomes smoother and is red-shifted to 1.16 eV at 1·10−5 Pa and to 1.26 eV at 3·10−5 Pa. We explain this behavior in terms of enhanced QD decomposition, the mechanism of which is strongly dependent on the arsenic deficiency or excess during the overgrowth process, determined by the arsenic pressure. Supported by the analysis of the wetting layer luminescence, we also conclude that the total indium content in the QD layer decreases with decreasing arsenic pressure during the overgrowth. This study reveals an essential role of the arsenic pressure in the overgrowth of InAs QDs.

Increased identification rate of scrap metal using Laser Induced Breakdown Spectroscopy Echelle spectra

In this paper we address the application of an Echelle spectrometer for metal sorting tasks using laser induced breakdown spectroscopy in the industrial environment. Echelle spectrometers guarantee the simultaneous detection of broad spectral intervals with sufficient spectral resolution, which is highly desirable for LIBS applications. Until today, this benefit was overshadowed by the reduced speed of detection which typically could not exceed 10 frames per second. In this paper we present a newly developed high speed classification setup for scrap metal sorting, equipped with an Echelle spectrometer, capable of classifying 25 samples per second using a single burst double pulse excitation, a fast EMCCD camera and an externally triggered, chopper based timing. Different multivariate discriminatory techniques (PCA, PLS-DA) are applied and compared to gain maximum use of the spectra created by this setup. A simultaneous discrimination of 10 classes was done with >90% correctness for most classes and an analysis of possible sources of error unique for non-laboratory experiments is presented.

Retrieving cloud geometrical extents from MIPAS/ENVISAT measurements with a 2-D tomographic approach

Clouds represent a critical factor in regulating the Earth's atmosphere and its energy balance. Satellite instruments can measure the energy balance and global atmospheric properties only through an accurate knowledge of the vertical profile of cloudiness, which is as yet one of the key shortages in atmospheric science. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on-board the ENVISAT satellite, designed to infer the amount of atmospheric trace-gases, demonstrated also sensitivity to the radiation emitted from clouds. In order to model the effect of the geometrical extent of a cloud on MIPAS measurements, we developed a retrieval model capable to simulate cloud effects on broad spectral intervals accounting for the two-dimensional (2-D) variability of the atmosphere in the satellite orbit plane. The 2-D analysis revealed a sensitivity of MIPAS spectra to both the vertical and horizontal extents and the position of clouds along the instrument line of sight. One-dimensional models were found to underestimate Cloud Top Height (CTH) by approximating clouds as an infinite horizontal layer with a finite vertical extents. With the 2-D approach, we showed it is possible, for optically thin Polar Stratospheric Clouds (PSCs), to retrieve both CTH and horizontal dimension by analyzing simultaneously all the limb observations that come across the cloud with their field of view. For a selected case study we found a very good agreement for both PSC CTH and horizontal extents retrieved from MIPAS measurements and those retrieved from coincident CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarisation) measurements.

Raman and optical spectroscopy characteristics of Se-doped Bi 12SiO 20 crystals

Crystals of BSO doped with Se in two different concentrations (BSO:Se(I) and BSO:Se(II)) were grown by the Czochralski method. It is established that doping with Se is accompanied with preferential absorption of Fe from the melt. According to the in-depth chemical analysis, it is assumed that the doping Se ions enter the tetrahedral positions by means of the substitution of 3Si 4+ by (Se 6+ + 2Fe 3+) ions. The measured absorption spectrum of the low-concentration Se-doped crystal (BSO:Se(I)) confirms such a conclusion as the absorption coefficient is increased in a broad spectral interval (1.4–3.1 eV) – an effect typical for all Fe-doped Bi 12SiO 20 crystals. The polarized Raman spectra of BSO:SeI show that the doping-induced lattice distortions are small. The IR spectrum of the BSO:SeI crystal yields indications for local lowering of the symmetry of the Fe-occupied tetrahedral positions. Doping with Se at high concentration (BSO:Se(II)) leads to occasional second phase inclusions and to a downshift of practically all modes in the Raman spectrum by 2–5.5 cm −1. It is concluded that the doping with Se at high concentrations follows the same mechanism as that at low concentrations but the introduced lattice distortions are more significant, and lead to an enlargement of the unit cell while preserving the overall cubic symmetry.

Intrinsic detection efficiency of superconducting nanowire single-photon detectors with different thicknesses

We evaluate experimentally the intrinsic detection efficiency (IDE) of superconducting NbN nanowire single-photon detectors in the range of wire thicknesses from 4 to 12 nm. The study is performed in the broad spectral interval between near-ultraviolet (wavelength 400 nm) and near-infrared (wavelength 2000 nm) light with plane waves at normal incidence. For visible light the IDE of the thinnest detectors reaches 70%. We use numerically computed absorptance of the nanowire-structures for the analysis of the experimental data. Variations in the detection efficiency with both the wire thickness and the wavelength evidence the red boundary of the hot-spot photon-detection mechanism. We explain the detection at larger wavelengths invoking thermal excitation of magnetic Pearl vortices over the potential barrier at the edges of the wire.

Spectral Variable Selection for Partial Least Squares Calibration Applied to Authentication and Quantification of Extra Virgin Olive Oils Using Fourier Transform Raman Spectroscopy

The limits of quantitative multivariate assays for the analysis of extra virgin olive oil samples from various Greek sites adulterated by sunflower oil have been evaluated based on their Fourier transform (FT) Raman spectra. Different strategies for wavelength selection were tested for calculating optimal partial least squares (PLS) models. Compared to the full spectrum methods previously applied, the optimum standard error of prediction (SEP) for the sunflower oil concentrations in spiked olive oil samples could be significantly reduced. One efficient approach (PMMS, pair-wise minima and maxima selection) used a special variable selection strategy based on a pair-wise consideration of significant respective minima and maxima of PLS regression vectors, calculated for broad spectral intervals and a low number of PLS factors. PMMS provided robust calibration models with a small number of variables. On the other hand, the Tabu search strategy recently published (search process guided by restrictions leading to Tabu list) achieved lower SEP values but at the cost of extensive computing time when searching for a global minimum and less robust calibration models. Robustness was tested by using packages of ten and twenty randomly selected samples within cross-validation for calculating independent prediction values. The best SEP values for a one year's harvest with a total number of 66 Cretian samples were obtained by such spectral variable optimized PLS calibration models using leave-20-out cross-validation (values between 0.5 and 0.7% by weight). For the more complex population of olive oil samples from all over Greece (total number of 92 samples), results were between 0.7 and 0.9% by weight with a cross-validation sample package size of 20. Notably, the calibration method with Tabu variable selection has been shown to be a valid chemometric approach by which a single model can be applied with a low SEP of 1.4% for olive oil samples across three different harvest years.

Tunable wavelength-selective waveguiding of photoluminescence in Si-rich silica optical wedges

We report accurate tuning of spectral peaks built up upon guiding of broadband light by annealed Si-rich silica layers with varying optical thickness. The optical wedges are characterized with various methods (photoluminescence, Raman spectroscopy, visible absorption, and x-ray photoelectron spectroscopy) showing that their properties change smoothly from point to point along the sample surface. As measured in the conventional transverse detection geometry, the annealed (1100 °C) Si-rich silica material exhibits well-known broad photoluminescence (PL) with maximum at ∼800 nm conventionally connected with Si nanocrystals. In the waveguiding detection geometry, the guiding of PL light by the silica layer containing Si nanocrystals is wavelength-dependent, and the detected spectrum exhibits narrow (down to 100 cm−1) and linearly polarized spectral peaks. As the central observation of the present work, these spectral peaks can be accurately tuned over the whole PL spectral region by moving the optical wedges across the laser beam. For our samples, the tuning coefficient is ∼150 cm−1/mm. It is shown that tuning of the transmission peaks is caused by change of both refractive index (from 1.6 to 1.9) and thickness (from 1.4 to 2.0 μm) of the prepared optical wedges. The propagation loss for the guided light is found to increase for shorter wavelengths and for materials with larger Si concentration. The propagation losses reliably below 0.5 cm−1 are observed in the broad spectral interval in sample areas with smaller Si concentrations.

Retrieval and characterization of ozone profiles from solar infrared spectra at the Jungfraujoch

Vertical distributions of ozone from June 1996 to November 2000 have been retrieved from high‐resolution Fourier transform infrared (FTIR) solar absorption spectra recorded at the primary Network for Detection of Stratospheric Change station of the Jungfraujoch in the Swiss Alps (46.5°N, 8°E, 3580 m above sea level (asl). The retrievals were performed using the Optimal Estimation Method (OEM), both in a narrow spectral interval (1002.567–1003.2 cm−1) and in a broad spectral interval (1000.0–1005.0 cm−1) in the O3 9.6‐μm band. A thorough characterization of the retrievals has been performed following the lines of OEM, including an information content analysis, a study of the correlations between retrieved instrumental parameters and retrieved ozone concentrations, and an evaluation of the O3 profile error budget. It is demonstrated that the information content is significantly higher for spectra in the broad microwindow, resulting in higher vertical resolutions, on the order of 8 km, of the retrieved profiles extending up to 40 km, and less correlations between retrieved parameters. An independent statistical verification of the retrieval results and their characterization has been performed by comparison of the FTIR ozone profiles with independent measurements. These are the ozone profile measurements from balloon soundings at Payerne, from the microwave radiometer at Bern and the lidar at Observatoire de Haute‐Provence (OHP), and the total column data from the Dobson spectrophotometer at Arosa. Applying the optimum retrieval procedure in the broad spectral interval, an excellent agreement has been found between the FTIR O3 profile data and the correlative data. The largest offset of the FTIR data in comparison with the correlative data is found with respect to the lidar data in the 24‐ to 40‐km layer, and is on the order of 5%. No systematic biases have been found in the troposphere, neither in the upper troposphere‐lower stratosphere (UTLS) up to 18 km. The dispersion of the relative differences between the data sets, if any, is never larger than half of the natural ozone variability.

Authentication and Quantification of Extra Virgin Olive Oils by Attenuated Total Reflectance Infrared Spectroscopy Using Silver Halide Fiber Probes and Partial Least-Squares Calibration

Infrared attenuated total reflectance spectroscopy has been assessed for the analysis of extra virgin olive oil samples from various Mediterranean sites and their adulteration by sunflower oil. In this study two different silver halide fiber-optic probes were separately tested for the mid-infrared spectroscopic measurement of pure olive oil samples and these same oils adulterated with sunflower oil. One fiber-optic probe contained an exchangeable U-shaped section of the silver halide fiber, whereas the second probe consisted of a fiber-coupled diamond crystal, which performed slightly less well than the whole fiber probe. The optimum standard error of prediction for the sunflower oil concentrations in spiked olive oil samples, obtained by optimized partial least-squares (PLS) calibration models and leave-one-out cross-validation, was 1.2% by weight with the use of a special variable selection strategy based on a pairwise consideration of significant respective minima and maxima of the optimum PLS regression vector, calculated for broad spectral intervals. Calibration robustness was proven by also using packages of 10 randomly selected samples within a further cross-validation for calculating independent prediction values. The implications for product monitoring are discussed.

A Liquid Crystal Broad Band Stokes-Meter

A device for the determination of the complete polarization state of light in a broad spectral interval is presented. Two tunable achromatic phase retarders with planar aligned nematic liquid crystals are used to modulate the polarization without moving components.

A study of preflare situations using spectral data on fluxes of solar radio emission in the period from 1970 to 1994

We present the results of observations performed at “Zimenki” facility at six radio frequencies over a broad spectral interval in the period that covers more than two 11-year cycles of the solar activity. In the analysis we have determined the laws of the spectral-time development of the preflare situations for the proton and non-proton events, which differ by the duration and range of spectral composition of the phenomena. The phenomenon of the “directivity” of precursor radiation is studied depending on the location of sources on the solar disc.

Theoretical analysis of infrared spectrum of 9,10-anthraquinone molecule

For the molecule of 9,10-anthraquinone, vibration frequencies and intensities have been calculated, and a theoretical spectral curve has been plotted, giving an adequate reflection of experimental manifestations over a broad spectral interval. This study made it possible to obtain a correct system of parameters for a model of the 9,10-anthraquinone molecule. Theoretical analysis has provided a basis for distinguishing vibration frequencies, active in the spectrum, that are characteristic with respect to frequency and intensity.

Photosynthetically active components of global solar radiation: Measurements and model computations

A three-year(1980–82) record of global solar radiation measurements made at Corvallis, Oregon, U.S.A. (44°34′N, 123°14′W; 65.5 m) has been utilized to determine the distribution of radiant energy in broad spectral intervals in the visible (0.385–0.695 μm) region of the spectrum which is often referred to as the photosynthetically active radiation or PAR band. The mean values of the ratios borne by the daily irradiation in the spectral intervals 0.385–0.530 μm and 0.630–0.695 μm, which cover most of the action spectra of plant photochemistry, to the daily total (0.295–2.8 μm) global irradiation are found to be 0.222 (standard deviation: 0.030) and 0.086 (standard deviation: 0.013) respectively; the corresponding ratios in the PAR band and in the spectral interval 0.530–0.630 km are 0.457 (standard deviation: 0.029) and 0.149 (standard deviation: 0.021). These irradiation ratios exhibit seasonal variations, attributable to changes in local air mass climatology, and dependence on sky conditions, with slightly larger values being observed under “cloudy” skies. The mean value of 0.457 that we have obtained for the irradiation ratio in the PAR band compares favorably with the values reported in the literature for different locations over a wide geographical area. There is reasonable correspondence between the observed irradiation ratios and the computed values in a cloud-free model of a turbid atmosphere.. The sensitivity of the broad-band spectral energy distribution of the global solar radiation to changes in atmospheric turbidity, precipitable water, ozone and surface albedo has been examined in the light of these model computations.

Determination of fluorescence quantum efficiency and laser emission cross sections of neodymium crystals: Application to KNdP<inf>4</inf>O<inf>12</inf>

The stimulated emission cross sections σ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> of neodymium crystals at the laser wavelengths (1.05 and 1.32 μm) are useful parameters to predict their laser performance. The determination of σ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> has traditionally been a delicate experimental task. We describe here a method which relies upon the measurement of the branching ratios of the radiative transitions from the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3/2</inf> doublet to the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> I multiplets, and the calibration is provided by an absorption measurement concerning a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9/2</inf> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3/2</inf> transition. The precise determination of the spectrometer system response over a broad spectral interval is not required. This method also leads to the value of the fluorescence quantum efficiency η of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3/2</inf> level. It is applied here to potassium-neodymium tetrametaphosphate, KNd P <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> .