Spectrally Dependent Research Articles

Spectrally-dependent positive and negative photo-plastic effects in CdZnTe and CdZnTeSe

We studied the Vickers microhardness HV0.025 of CdZnTe and CdZnTeSe samples upon illumination with quasi-monochromatic light with wavelengths ranging between 750 and 1540 nm. We observed that light with a wavelength close to or above the bandgap of the samples results in a positive photo-plastic effect (hardening), and light in the spectral range well below the band gap can lead both to positive or negative effects depending on the parameters of deep levels in the bandgap. We performed measurements of spectrally dependent photoconductivity and compared them with HV0.025. Based on the analysis, we propose a complex qualitative model explaining the spectral dependence of HV0.025 by the charging and neutralization of deep levels by optical transitions between the valence and conduction bands and between the bands and deep levels in the band gap. Tuning of parameters of deep levels could be used to control the sign and magnitude of the photo-plastic effect.

Spectrally dependent linear depolarization and lidar ratios for nonspherical smoke aerosols

We use the numerically exact T-matrix method to model light scattering and absorption by aged smoke aerosols at lidar wavelengths ranging from 355 to 1064 nm assuming the aerosols to be smooth spheroids or Chebyshev particles. We show that the unique spectral dependence of the linear depolarization ratio (LDR) and extinction-to-backscatter ratio (or lidar ratio, LR) measured recently for stratospheric Canadian wildfire smoke can be reproduced by a range of model morphologies, a range of spectrally dependent particle refractive indices, and a range of particle sizes. For these particles, the imaginary part of the refractive index is always less than (or close to) 0.035, and the corresponding real part always falls in the range [1.35, 1.65]. The measured spectral LDRs and LRs could be produced by nearly-spherical oblate spheroids or Chebyshev particles whose shapes resemble those of oblate spheroids. Their volume-equivalent effective radii should be large enough (reff = 0.3 µm or greater) to produce the observed enhanced LDRs. Our study demonstrates the usefulness of triple-wavelength LDR measurements as providing additional size information for a more definitive characterization of the particle morphology and composition. Non-zero LDR values indicate the presence of nonspherical aerosols and are highly sensitive to particle shapes and sizes. On the other hand, the LR is a strong function of absorption and is very responsive to changes in the particle refractive index.

The role of aerosol layer height in quantifying aerosol absorption from ultraviolet satellite observations

Abstract. The purpose of this study is to demonstrate the role of aerosol layer height (ALH) in quantifying the single scattering albedo (SSA) from ultraviolet satellite observations for biomass burning aerosols. In the first experiment, we retrieve SSA by minimizing the near-ultraviolet (near-UV) absorbing aerosol index (UVAI) difference between observed values and those simulated by a radiative transfer model. With the recently released S-5P TROPOMI ALH product constraining forward simulations, a significant gap in the retrieved SSA (0.25) is found between radiative transfer simulations with spectral flat aerosols and those with strong spectrally dependent aerosols, implying that inappropriate assumptions regarding aerosol absorption spectral dependence may cause severe misinterpretations of the aerosol absorption. In the second part of this paper, we propose an alternative method to retrieve SSA based on a long-term record of co-located satellite and ground-based measurements using the support vector regression (SVR) approach. This empirical method is free from the uncertainties due to the imperfection of a priori assumptions on aerosol microphysics seen in the first experiment. We present the potential capabilities of SVR using several fire events that have occurred in recent years. For all cases, the difference between SVR-retrieved SSA and AERONET are generally within ±0.05, and over half of the samples are within ±0.03. The results are encouraging, although in the current phase the model tends to overestimate the SSA for relatively absorbing cases and fails to predict SSA for some extreme situations. The spatial contrast in SSA retrieved by radiative transfer simulations is significantly higher than that retrieved by SVR, and the latter better agrees with SSA from MERRA-2 reanalysis. In the future, more sophisticated feature selection procedures and kernel functions should be taken into consideration to improve the SVR model accuracy. Moreover, the high-resolution TROPOMI UVAI and co-located ALH products will guide us to more reliable training data sets and more powerful algorithms to quantify aerosol absorption from UVAI records.

Analysis of the multi-spectral energy bundle method: finding the number of sub-bundles for the minimum computational time

The multi-spectral energy bundle (MSB) is a method proposed to reduce the computational time of Monte Carlo simulations in media with spectrally dependent properties. In this method the typical energy bundles of the Monte Carlo are treated as multi-spectral by assuming that they are composed of a set of monochromatic sub-bundles. The number of sub-bundles in each bundle affects the computational time in such a way that there exists an optimum number of sub-bundles that leads to the minimum time for the computation. However, due to the stochastic nature of the Monte Carlo, the optimum number of sub-bundles is not easy to be exactly found. This paper proposes a methodology which allows to accurately determine the number of sub-bundles for the minimum computational time, and apply the proposed methodology to 25 cases consisting of a one-dimensional slab of gas composed of H $$_2$$ O and non-emitting/absorbing species, where the FSK was used to consider spectral dependence of properties. The results showed that for the majority of the considered cases, 3 sub-bundles lead to either the minimum or a computational time quite close to the minimum, and always provides reduction of computational time. Thus, 3 sub-bundles is indicated for the analyzed class of problems.

SYMMETRY OF THEIBEXRIBBON OF ENHANCED ENERGETIC NEUTRAL ATOM (ENA) FLUX

The circular ribbon of enhanced energetic neutral atom (ENA) emission observed by the Interstellar Boundary Explorer (IBEX) mission remains a critical signature for understanding the interaction between the heliosphere and the interstellar medium. We study the symmetry of the ribbon flux and find strong, spectrally dependent reflection symmetry throughout the energy range 0.7-4.3?keV. The distribution of ENA flux around the ribbon is predominantly unimodal at 0.7 and 1.1?keV, distinctly bimodal at 2.7 and 4.3?keV, and a mixture of both at 1.7?keV. The bimodal flux distribution consists of partially opposing bilateral flux lobes, located at highest and lowest heliographic latitude extents of the ribbon. The vector between the ribbon center and heliospheric nose (which defines the so-called BV plane) appears to play an organizing role in the spectral dependence of the symmetry axis locations as well as asymmetric contributions to the ribbon flux. The symmetry planes at 2.7 and 4.3?keV, derived by projecting the symmetry axes to a great circle in the sky, are equivalent to tilting the heliographic equatorial plane to the ribbon center, suggesting a global heliospheric ordering. The presence and energy dependence of symmetric unilateral and bilateral flux distributions suggest strong spectral filtration from processes encountered by an ion along its journey from the source plasma to its eventual detection at IBEX.

Characterization of coupled nonlinear spatiospectral phase following an ultrafast self-focusing interaction

We use collinear spatially resolved spectral interferometery to characterize the nonlinear phase changes experienced by an intense ultrashort pulse propagating in glass. The measurement yields the spectrally dependent wavefront, allowing us to measure the spatial and chromatic aberrations of the nonlinearly induced lens. For these conditions, we find that while the shape of the spatial wavefront follows the beam profile as expected, the spectral dependence of the lensing power is determined by the self-phase modulation. The simultaneous measurement of the nonlinear spatiospectral phase demonstrates how the nonlinear spectral phase is coupled to self-focusing.

Ultrafast photoluminescence of nanocrystalline diamond films

AbstractWe report on ultrafast photoluminescence of nanocrystalline diamond membranes and films on silicon and fused silica glass measured by femtosecond upconversion technique. The nanodiamond samples were prepared by microwave plasma enhanced CVD technique. Photoluminescence with very fast non‐exponential decay with the effective time of about 1 ps was observed in the investigated spectral interval 1.8–2.8 eV. The decay was spectrally dependent being faster at higher photon energies. We have found that the photoluminescence dynamics are affected by temperature and ambient pressure. We interpret the photoluminescence dynamics in terms of charge‐carrier relaxation via the continuum of the nanocrystal surface‐related energy states lying energetically below the diamond band‐gap energy. The results can be explained by the spectral dependence of the density of these states and its changes due to ambient conditions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Monte Carlo implementation to solve radiation heat transfer in non-uniform media with spectrally dependent properties

This paper presents the application of the Monte Carlo method to solve the radiative heat exchange in non-homogeneous, non-isothermal gases with spectrally dependent properties. Among others models, the absorption-line blackbody (ALB) distribution function, originally defined and derived for the spectral line-based weighted-sum-of-gray-gases (SLW) model, allows an immediate, simple implementation of the Monte Carlo method to account the spectral dependence of the radiative properties. This work shows how the Monte Carlo method can be combined to the ALB distribution function, and provides results for heat transfer in a mixture of water vapor, carbon dioxide and nitrogen that have satisfactory agreement with the SLW method and with line-by-line integration. Finally, the solution technique is employed to solve two examples aiming at demonstrating the effect of the absorbing species concentration on the thermal radiative exchanges. The method is of great interest for the computation of radiative transfer in combustion systems where the chemical species concentration and the temperature are not uniform.

Spectral dependence of visible light absorption by carbonaceous particles emitted from coal combustion

Optical characteristics of particles that absorb visible light are needed to model their effects on atmospheric radiation. Light absorption by particles emitted from low‐technology coal combustion has exhibited a strong spectral dependence. I investigate various explanations for this phenomenon and conclude that a spectrally dependent imaginary refractive index is the most plausible. Following previous work on the structure of amorphous carbon, I propose that both the magnitude and spectral dependence of light absorption are controlled by the size of graphitic clusters within the material, and can be described using the optical band‐gap theory. This hypothesis is an alternative to the current measurement divisions of light‐absorbing “black carbon” and non‐absorbing “organic carbon,” and offers an explanation for preferential absorption at blue wavelengths that may extend to ultraviolet wavelengths.

The Spectrally Dependent Monotic Component in the Decreasing-Loudness Aftereffect: Implications for Dynamic Auditory Localization

Listeners exposed to a tone increasing in intensity report an aftereffect of decreasing loudness in a steady tone heard afterward. In the present study, the spectral dependence of the monotic decreasing-loudness aftereffect (adapting and testing 1 ear) was compared with (a) the spectral dependence of the interotic decreasing-loudness aftereffect (adapting 1 ear and testing the other ear) and (b) a non-adaptation control condition. The purpose was to test the hypothesis that the decreasing-loudness aftereffect may concern the sensory processing associated with dynamic localization. The hypothesis is based on two premises: (a) dynamic localization requires monaural sensory processing, and (b) sensory processing is reflected in spectral selectivity. Hence, the hypothesis would be supported if the monotic aftereffect were more spectrally dependent and stronger than the interotic aftereffect; A. H. Reinhardt-Rutland (1998) showed that the hypothesis is supported with regard to the related increasing-loudness aftereffect. Two listeners were exposed to a 1-kHz adapting stimulus. From responses of “growing softer” or “growing louder” to test stimuli changing in intensity, nulls were calculated; test carrier frequencies ranged from 0.5 kHz to 2 kHz. Confirming the hypothesis, the monotic aftereffect peaked at around the 1-kHz test carrier frequency. In contrast, the interotic aftereffect showed little evidence of spectrally dependent peaking. Except when test and adaptation carrier frequencies differed markedly, the interotic aftereffect was smaller than the monotic aftereffect.

Spectral Dependence and Wavelength Selectivity in Heterogeneous Photocatalysis. I. Experimental Evidence from the Photocatalyzed Transformation of Phenols

The photocatalyzed transformation of phenol (PhOH) and 4-chlorophenol (ClPhOH) in air-equilibrated TiO2 dispersions has been used to examine the spectral behavior of the quantum yields Φ (and photonic efficiencies, η) of loss of these two substrates at various wavelengths. Contrary to the band model of semiconductors and conventional wisdom which predict rapid thermalization of photogenerated charge carriers to the lowest energy levels in their respective conduction and valence band states, and consequently to spectrally independent quantum yields, experimental results demonstrate that in fact the quantum yields of loss of PhOH and ClPhOH are spectrally dependent, displaying well resolved band structures at 3.13, 3.21, 3.32, 3.60, 3.81, 4.28 and 4.57 eV for phenol and at 3.16, 3.25, 3.41, 3.59, 3.70 and 4.15 eV for 4-chlorophenol. These energies correlate with absorption and emission band energies of direct and indirect band-to-band transitions reported earlier by Serpone et al. (J. Phys. Chem., 1995, 99,...

Increasing-loudness aftereffect following decreasing-intensity adaptation: spectral dependence in interotic and monotic testing.

Listening to decreasing intensity leads to illusory increasing loudness afterwards. Evidence suggests that this increasing-loudness aftereffect may have a sensory component concerned with dynamic localisation. This was tested by comparing the spectral dependence of monotic aftereffect (adapting and testing one ear) with the spectral dependence of interotic aftereffect (adapting one ear and testing the other ear). Existence of the proposed component implies that monotic aftereffect should be more spectrally dependent than interotic aftereffect. Three listeners were exposed to a 1 kHz adapting stimulus. From responses of "growing softer" or "growing louder" to test stimuli changing in intensity, nulls were calculated; test carrier frequencies ranged from 0.5 kHz to 2 kHz. Confirming the hypothesis, monotic aftereffect was about three times as strong as interotic aftereffect for the 1 kHz test carrier frequency, while monotic and interotic aftereffects were comparable in magnitude for test carrier frequencies below about 0.8 kHz and above about 1.2 kHz. The latter residual aftereffects are attributed to cognitive processing, perhaps concerning response bias. Sensitivity did not vary systematically across conditions; this is consistent with evidence that changing intensity entails mainly direct processing. The results cannot be attributed to the loudness adaptation elicited by steady stimuli.

Changing-loudness aftereffects: slope of response functions and spectral dependence.

Aftereffects of azimuthal auditory motion may have two components. A sensory component is inferred from strong aftereffects, because they are spectrally dependent and have shallower response functions than those for non-adaptation. Neither property applies to weak aftereffects, suggesting a cognitive component. Two experiments determined whether changing-loudness aftereffects (CLA) might be understood similarly. In a single-interval forced-choice procedure, listeners responded "growing softer" or "growing louder" to test stimuli changing in intensity. In Exp. 1, adapting and test stimuli were diotic and had the same 1-kHz sinusoidal carrier. Although response functions following adaptation were displaced from response functions for non-adaptation-indicating CLA-their slopes were broadly similar. In Exp. 2, stimuli were monotic; adapting frequency was 1 kHz and test frequencies were between 0.5 and 2.0 kHz. CLA was present in most adaptation conditions, but was strongest when the test frequency was 1.0 kHz; functions' slopes again evinced no systematic variation. The two-component hypothesis for CLA is supported by spectral dependence alone. It is argued that the slope of response functions is due to the nulling procedures for measuring auditory aftereffects. The slope depends on whether the adapted property is processed by "direct" and "indirect" mechanisms; aftereffects tap "direct" mechanisms alone, which may affect sensitivity during measurement.

Asian dust aerosol: Optical effect on satellite ocean color signal and a scheme of its correction

The paper first exhibits the influence of the Asian dust aerosol (KOSA) on a coastal zone color scanner (CZCS) image which records erroneously low or negative satellite‐derived water‐leaving radiance especially in a shorter wavelength region. This suggests the presence of spectrally dependent absorption which was disregarded in the past atmospheric correction algorithms. On the basis of the analysis of the scene, a semiempirical optical model of the Asian dust aerosol that relates aerosol single scattering albedo (ωA) to the spectral ratio of aerosol optical thickness between 550 nm and 670 nm is developed. Then, as a modification to a standard CZCS atmospheric correction algorithm (NASA standard algorithm), a scheme which estimates pixel‐wise aerosol optical thickness, and in turn ωA, is proposed. The assumption of constant normalized water‐leaving radiance at 550 nm is adopted together with a model of aerosol scattering phase function. The scheme is combined to the standard algorithm, performing atmospheric correction just the same as the standard version with a fixed Angstrom coefficient except in the case where the presence of Asian dust aerosol is detected by the lowered satellite‐derived Angstrom exponent. Some of the model parameter values are determined so that the scheme does not produce any spatial discontinuity with the standard scheme. The algorithm was tested against the Japanese Asian dust CZCS scene with parameter values of the spectral dependency of ωA, first statistically determined and second optimized for selected pixels. Analysis suggests that the parameter values depend on the assumed Angstrom coefficient for standard algorithm, at the same time defining the spatial extent of the area to apply the Asian dust scheme. The algorithm was also tested for a Saharan dust scene, showing the relevance of the scheme but with different parameter setting. Finally, the algorithm was applied to a data set of 25 CZCS scenes to produce a monthly composite of pigment concentration for April 1981. Through these analyses, the modified algorithm is considered robust in the sense that it operates most compatibly with the standard algorithm yet performs adaptively in response to the magnitude of the dust effect.

Sound generated aerodynamically revisited: large-scale structures in a turbulent jet as a source of sound

Lighthill’s formulation of the aerodynamic sound problem (Lighthill,Proc. R. Soc. Lond. A 211, 564 (1952)) is here considered as fundamental to the sound generated by real turbulent jets. For convenience, the aerodynamic sound integral is recast, via Michalke & Fuchs [J.Fluid Mech. 70, 179 (1975)), into a form involving the pressure fluctuations. It is first conjectured that the large-scale coherent structures in the turbulent jet, whose existence is now well recognized, would be responsible for the spectrally dependent highly oriented radiation patterns in the aerodynamic sound field. Accordingly, only contributions that arise from the coherent structures are retained in the aerodynamic sound integral. The neglected fine-grained turbulence as far as the sound field is concerned is thought otherwise to contribute to the broadband, nearly isotropic radiation. The present source description follows Mankbadi & Liu [Phil.Trans. R. Soc. Lond. A 298, 541 (1981)), but suitably modified to include an ensemble ofn= 0 axisymmetric andn= 1 spiral modes in the relevant Strouhal number range. The coherent structures interact with the mean flow and the finegrained turbulence as an ensemble through energy exchanges dictated by rates according to their individual spectral characteristics. Because such coherent structures are relatively ‘weak’ in a real, developing turbulent jet, their mutual interactions are neglected as a first approximation. The sound sources, in a stationary coordinate system and evaluated at the appropriate retarded time, give rise to an equivalent streamwise distribution of line radiators after performance of the azimuthal and radial integrations in the aerodynamic sound integral. The streamwise oscillation of the equivalent sources is determined by an axial interference function strongly influenced by the wavenumber of each individual mode whereas the streamwise growth and decay of the source envelope is determined primarily by the coherent structure amplitude whose spectral dependence is also strong. The streamwise net imbalance of the source contribution, reflected by the axial integration in the aerodynamic sound integral, gives rise to the far sound field. It is found that in general, the radiation is primarily in the direction of the jet exhaust; the radiation patterns of then= 0 modes resembling those of longitudinal quadrupoles and those of then= 1 modes resembling those of lateral quadrupoles. However, then= 0 modes tend to peak at Strouhal numbers less than those of then= 1 modes. The superposition gives a directional-spectral behaviour that strikingly resembles that of observations: lower frequency sound radiates preferentially in the forward direction and as the frequency increases, the peak radiation moves towards the lateral directions; it is also found that contributions to the high-frequency sound come from coherent structures that peak nearer the nozzle lip, whereas contributions to the low-frequency sound come from such structures that peak further downstream in the jet. The calculated spectral shapes are narrower than observations by typically a deficit of 4—7 dB per octave on both the high and low frequency sides and this is most likely attributable to the nearly isotropic radiation caused by the broad-band fine-grained turbulence whose direct contribution to the sound field is not accounted for. For the same reason, the calculated aerodynamic sound field has a large deficit compared with observations in the vicinity of the 90- degree region. The dominant contributions to the radiation come from the so-called shear noise in the forward arc, whereas both the shear and self-noise of the coherent structures become equally insignificant to the same order in the 90-degree region. Although the source distribution within the jet is calculated for an identically incompressible fluid, it is used in a limited sense to study the effect of jet exit velocity on the peak radiation frequency in the forward direction: it is found that the peak value offd/a0, wherefis the frequency,dthe jet nozzle diameter anda0the ambient sound speed, take on a value of about 0.30 independently of the jet velocity and this compares favourably with an observational value of about 0.20. In general, the angular distribution of the peak frequency due to coherent structures radiation compared favourably with observations. Compressibility effects that somewhat limit the amplification of coherent structures, as well as the effects of higher azimuthal modes whose radiation would peak at higher frequencies and larger lateral directions, warrant further study in the light of the present considerations. The present work, however, has already shown that the consequences of Lighthill's formulation of the aerodynamic sound problem agree with major features of observations and that this is brought about by taking into account as sources the growing and decaying largescale coherent eddies whose development within the turbulent jet and whose radiational properties are all strongly dependent upon their spectral contents.