Backscattering Efficiency Research Articles

Antenna-mediated back-scattering efficiency in infrared near-field microscopy

We evaluate the efficiency of back-scattering, eta(B), from a standard cantilevered AFM probe contacting a flat sample, and also the back-scattering phase. Both quantities are spectroscopically determined over a broad 9-12 mum wavelength range by coherent frequency-comb Fourier-transform spectroscopy (c-FTIR). While Fresnel reflectivity contributes a key factor with the SiC Reststrahlen edge at 975 cm(-1)as previously documented, we observe spectral effects ascribable to antenna resonances involving the shaft, cantilever, and sample. Most conspicuous is strong (eta(B) = 13%), resonant back-scattering at 955 cm(-1), a frequency that suggests the involvement of surface-phonon-polariton excitation, when the tip probes the area near a SiC/Au boundary. The probe's antenna properties are elucidated by numerically simulating the near fields, the fields in the radiation zone, and the far-field scattering distributions. The simulations are performed for a realistic tip/sample configuration with a three-orders-of-magnitude scale variation. The results suggest a standing-surface-plasmon-polariton pattern along the shaft, as well as far-field antenna lobes that change with the sample's dielectric properties.

CloudSat spaceborne 94 GHz radar bright bands in the melting layer: An attenuation‐driven upside‐down lidar analog

The CloudSat satellite supports a W‐band (94 GHz) cloud profiling radar. At this 3.2 mm wavelength, ground‐based measurements of rainfall associated with melting snowflakes do not show the radar reflectivity peak that is characteristic of bright band measurements at longer (Rayleigh scattering‐dominated) wavelengths. Nonetheless, examination of downward‐looking CloudSat returns in precipitation often indicate an obvious signal peak in the melting region. Through melting layer microphysical and scattering model simulations, we demonstrate that this downward‐viewing radar feature is analogous to the lidar bright band observed from the ground in that it owes its existence to strong attenuation. In the upward‐looking lidar case, the strong attenuation comes from large low‐density snowflakes. In the downward‐looking 94 GHz radar case, it is due to the effects of the greater refractive index of water particles compared to ice: it is comparable to an upside‐down lidar bright band. A W‐band radar dark band, which contributes to the visibility of the bright band, is shown to be due to attenuation in the snowfall. For comparison, the bright and dark bands for an upward viewing lidar are also modeled: the latter is simulated by a reduction in light backscattering efficiency of ice‐containing raindrops.

Contribution of water-leaving radiances to multiangle, multispectral polarimetric observations over the open ocean: bio-optical model results for case 1 waters

Multiangle, multispectral photopolarimetry of atmosphere-ocean systems provides the fullest set of remote sensing information possible on the scattering properties of aerosols and on the color of the ocean. Recent studies have shown that inverting such data allows for the potential of separating the retrieval of aerosol properties from ocean color monitoring in the visible part of the spectrum. However, the data in these studies were limited to those principal plane observations where the polarization of water-leaving radiances could be ignored. Examining similar potentials for off-principal plane observations requires the ability to assess realistic variations in both the reflectance for and bidirectionality of polarized water-leaving radiances for such viewing geometries. We provide hydrosol models for use in underwater light scattering computations to study such variations. The model consists of two components whose refractive indices resemble those of detritus-minerallike and planktonlike particles, whose size distributions are constrained by underwater light linear polarization signatures, and whose mixing ratios change as a function of particulate backscattering efficiency. Multiple scattering computations show that these models are capable of reproducing realistic underwater light albedos for wavelengths ranging from 400 to 600 nm, and for chlorophyll a concentrations ranging from 0.03 to 3.0 mg/m(3). Numerical results for spaceborne observations of the reflectance for total and polarized water-leaving radiances are provided as a function of polar angles, and the change in these reflectances with wavelength, chlorophyll a concentration, and hydrosol model are discussed in detail for case 1 (open ocean) waters.

Using inherent optical properties to investigate biogeochemical dynamics in a tropical macrotidal coastal system

The variability in the inherent optical properties along an estuary‐coast‐ocean continuum in tropical Australia has been studied. The study area, the Fitzroy Estuary and Keppel Bay system, is a shallow coastal environment (depth < 30 m) with highly turbid waters in the estuary and blue oceanic waters in the bay and subject to macrotides. Biogeochemical and inherent optical properties (IOPs) were sampled in the near‐surface layer spatially and across the tidal phase during the dry season. These determinations included continuous measurements of spectral absorption, scattering and backscattering coefficients, together with discrete measurements of spectral absorption coefficients of phytoplankton, nonalgal particles and colored dissolved organic matter, and concentrations of phytoplankton pigments and suspended matter. Because of a large variability in the characteristics of the water components on short spatial and temporal scales, we observe a large variability in the associated optical properties. From the estuary to the bay, particle scattering and dissolved absorption decreased by 2 orders of magnitude, and nonalgal particle absorption decreased by 3 orders of magnitude. We also observed a strong variability in particle single scattering albedo and backscattering efficiency (by a factor of 6) and in specific IOPs (IOPs normalized by the relevant constituent concentration) such as suspended matter‐specific particle scattering and chlorophyll‐specific phytoplankton absorption. Superimposed on this strong spatial variability is the effect of the semidiurnal tide, which affects the spatial distribution of all measured properties. These results emphasize the need for spatially and temporally adjusted algorithms for remote sensing in complex coastal systems.

Reduced backscattering in potassium-doped nanotubes:Ab initioand semiempirical simulations

By means of combined ab initio and semiempirical simulations, the charge transmission properties of potassium- and nitrogen-doped nanotubes are compared. The backscattering efficiency of adsorbed potassium is shown to be much weaker, owing to a shallower impurity potential well that weakly traps the induced quasibound states. The difference in conductance and elastic mean free paths is further investigated for nanotubes with lengths of several tens of micrometers and random distribution of dopants. Our results provide clear criteria for favoring doping by physisorption instead of chemical substitutions.

Durable, Ultraluminous Structure for Incandescent, High‐Power White‐LED

A new particle‐ and resin‐free system composed of transparent, binary single crystals entangled with each other continuously has been successfully developed for a light source for illumination. These single crystals exist in a semi‐coherent state like a block copolymer; one of these single crystals exhibits effective photoluminescence and the other plays an important role as a light guide of excitation and emitting lights. This material can have low backscattering and high quantum efficiency, resulting in twice the emission light of conventional particle‐dispersed systems. Furthermore, this system shows outstanding durability, because of its thermostructural properties. This concept will be expanded into wide combinations to create incandescent systems in the future.

Thermal infrared spectroscopy to investigate the composition of mercury – The MERTIS instrument on BepiColombo

Telescopic measurements have shown that the surface composition of Mercury can be derived only indirectly from VIS-NIR-spectra: the spectral slopes and the absence of any feature in the range from 0.5 to 1 μm indicate feldspar as main component. [Hapke, B., Danielson, G.E., Jr., Klaasen, K., Wilson, L. Photometric observations of Mercury from Mariner 10. J. Geophys. Res. 80, 2431–2443, 1975; Hapke, B. Interpretations of optical observations of Mercury and the moon. Physics of the Earth and Planetary Interiors, vol. 15, no. 2–3, pp. 264–274, 1977.] suggested based on the analysis of albedo and color relationships on Mariner 10 images that very low contents of FeO (at about 3–6 wt%) and TiO 2 (<1 wt%) could be present in the mercurian regolith. Feldspars are expected to compose the bulk of Mercury’s crust. In the NIR-range, however, pure feldspars have no specific spectral signature. In the Thermal Infrared (TIR), however, feldspars can be detected–and specified–by means of their diagnostic spectral signatures: Christiansen frequency, reststrahlen band, and transparency feature. In addition, pyroxenes and most other minerals can be detected and specified in this spectral range. Thermal infrared spectroscopy operating in the range between 7 and 14 μm will enable valuable mineral identification of feldspars and low-iron species that are expected to be prevailing on Mercury’s surface. Performing TIR measurements may make it possible to identify spectral features associated with the high radar backscattering efficiency of putative minerals and to differentiate between the proposed compositions (water ice, sulfur, and cold silicate glasses) for the high-latitude volatiles, something that cannot be done by ground based observing or near-infrared spectroscopy. In addition, TIR measurements will allow studying areas of geologic importance where composition can give significant new understanding to relative age of tectonic features. The MERTIS instrument, a TIR imaging spectrometer based on our suggestions, onboard of ESA’s BepiColombo mission will fulfil all the requirements needed to improve our knowledge about Mercury’s surface composition.

Light scattering by marine algae: two-layer spherical and nonspherical models

Light scattering properties of algae-like particles are modeled using the T-matrix for coated scatterers. Two basic geometries have been considered: off-centered coated spheres and centered spheroids. Extinction, scattering and absorption efficiencies, plus scattering in the backward plane, are compared to simpler models like homogeneous (Mie) and coated (Aden–Kerker) models. The anomalous diffraction approximation (ADA), of widespread use in the oceanographic light-scattering community, has also been used as a first approximation, for both homogeneous and coated spheres. T-matrix calculations show that some light scattering values, such as extinction and scattering efficiencies, have little dependence on particle shape, thus reinforcing the view that simpler (Mie, Aden–Kerker) models can be applied to infer refractive index (RI) data from absorption curves. The backscattering efficiency, on the other hand, is quite sensitive to shape. This calls into question the use of light scattering techniques where the phase function plays a pivotal role, and can help explain the observed discrepancy between theoretical and experimental values of the backscattering coefficient in observed in oceanic studies.

Light scattering by a sphere with variable optical properties of the intermediate layer

A two-parameter model of a scattering spherical particle with a layer of a variable thickness (the first parameter), inside which the refractive index is specified by an arbitrary continuous function (the second parameter), is proposed. An algorithm for calculating the extinction and backscattering efficiency factors with the help of a developed piecewise-hyperbolic approximation of the scattering coefficients is presented. A correct choice of the parameters allowed us to obtain good agreement between the experimental and calculated data on the extinction and backscattering efficiency for typical polydisperse systems of particles of irregular shape.

Polarization-dependent optical second-harmonic imaging of a rat-tail tendon.

Using scanning confocal microscopy, we measure the backscattered second harmonic signal generated by a 100 fs laser in rat-tail tendon collagen. Damage to the sample is avoided by using a continuous scanning technique, rather than measuring the signal at discrete points. The second harmonic signal varies by about a factor of 2 across a single cross section of the rat-tail tendon fascicle. The signal intensity depends both on the collagen organization and the backscattering efficiency. This implies that we cannot use intensity measurements alone to characterize collagen structure. However, we can infer structural information from the polarization dependence of the second harmonic signal. Axial and transverse scans for different linear polarization angles of the input beam show that second harmonic generation (SHG) in the rat-tail tendon depends strongly on the polarization of the input laser beam. We develop an analytical model for the SHG as a function of the polarization angle in the rat-tail tendon. We apply this model in determining the orientation of collagen fibrils in the fascicle and the ratio gamma between the two independent elements of the second-order nonlinear susceptibility tensor. There is a good fit between our model and the measured data.

Atmospheric correction for airborne sensors: Comment on a scheme used for CASI

In a recent work by Gould and Arnone [Remote Sens. Environ. 61 (1997) 290] the compact airborne spectrographic imager (CASI) was used to map the absorption and scattering properties of a coastal environment. The methodology used in this work is also cited in a number of more recent studies. The authors found large discrepancies between the parameters estimated from measurements (AC9) and from a bio-optical model by Carder et al. [J. Geophys. Res. 104 (1999) 5403]. As a result, they adjusted the backscattering efficiency parameter of the model. We found, however, that the discrepancies may actually be due, at least in part, to an erroneous atmospheric correction used for the airborne data. Because the removal of the surface-reflected sky radiance was neglected, the estimated water-leaving radiance was erroneously augmented and used as the input of the bio-optical model. This caused a scattering overestimate and an absorption underestimate. It is suggested that the surface reflection, unlike for land applications, should always be removed to obtain the water signal for use with such bio-optical models.

Light backscattering by hexagonal ice crystals

Backscattering of light by hexagonal ice columns and plates has been calculated by means of a ray-tracing code. It is shown that backscattering by the hexagonal ice cylinders at their arbitrary orientations is caused by a peculiar corner-reflector effect. A gigantic peak of backscattering is found at the angle of about 32.5° between the principal axis of a particle and the incidence direction for both hexagonal ice columns and plates. This peak is explained by multiple total internal reflections inside the crystals that take place for a part of incident rays. The obtained results on backscattering efficiency allow one to calculate backscattering by an ensemble of the hexagonal ice cylinders of various sizes, shapes and orientations. Slant lidar remote sensing of cirrus clouds for discrimination between aligned columns and plates is suggested as an application of the results obtained.

Bio‐optical properties of oceanic waters: A reappraisal

The apparent optical properties (AOPs) of oceanic case 1 waters were previously analyzed [Morel, 1988] and statistically related to the chlorophyll concentration ([Chl]) used as a global index describing the trophic conditions of water bodies. From these empirical relationships a bio‐optical model of the upper layer was developed. With objectives and structure similar to those of the previous study the present reappraisal utilizes AOPs determined during recent Joint Global Ocean Flux Study cruises, namely, spectral attenuation for downward irradiance Kd(λ) and irradiance reflectance R(λ). This revision also benefits from improved knowledge of inherent optical properties (IOPs), namely, pure water absorption coefficients and particle scattering and absorption coefficients, and from better pigment quantification (via a systematic use of high‐performance liquid chromatography). Nonlinear trends, already observed between optical properties and algal biomass, are fully confirmed, yet with numerical differences. The previous Kd(λ) model, and subsequently the R(λ) model, is modified to account for these new relationships. The R(λ) values predicted as a function of [Chl] and the predicted ratios of reflectances at two wavelengths, which are commonly used in ocean color algorithms, compare well with field values (not used when developing the reflectance model). This good agreement means that semianalytical ocean color algorithms can be successfully applied to satellite data. Going further into purely analytical approaches, ideally based on radiative transfer computations combined with a suite of relationships between the IOPs and [Chl], remains presently problematic, especially because of the insufficient knowledge of the phase function and backscattering efficiency of oceanic particles.

Light scattering by viral suspensions

Viruses represent one of the most abundant, ocean‐borne particle types and have significant potential for affecting optical backscattering. Experiments addressing the light‐scattering properties of viruses have heretofore not been conducted. Here we report the results of laboratory experiments in which the volume‐scattering functions of several bacterial viruses (bacteriophages) were measured at varying concentrations with a laser light‐scattering photometer using a He‐Ne and/or Argon ion laser (632.8 and 514.0 nm, respectively). Four bacterial viruses of varying size were examined, including the coliphages MS‐2 (capsid size 25–30 nm) and T‐4 (capsid size ∼100 nm), and marine phages isolated from Saco Bay, Maine (designation Y‐1, capsid size 50–80 nm) and Boothbay Harbor, Maine (designation C‐2, capsid size ∼110 nm). Volume‐scattering functions (VSFs) were fitted with the Beardsley‐Zaneveld function and then integrated in the backward direction to calculate backscattering cross section. This was compared to the virus geometric cross section as determined by transmission electron microscopy and flow‐field fractionation. Typical backscattering efficiencies varied from 20 × 10−6 to 1,000 × 10−6 . Data on particle size and backscattering efficiencies were incorporated into Mie scattering calcula‐tions to estimate refractive index of viruses. The median relative refractive index of the four viruses was ∼1.06. Results presented here suggest that viruses, while highly abundant in the sea, are not a major source of backscattering.

Application of the Generalized Multipole Technique (GMT) to High-Frequency Electromagnetic Scattering from Perfectly Conducting and Dielectric Bodies of Revolution

The generalized multipole technique (GMT) is utilized to calculate rigorously the full-vector electromagnetic (EM) fields scattered from electrically large, perfectly conducting (PC) and penetrable bodies of revolution, of smooth but otherwise arbitrary geometry. Analytically tractable, multiple spherical multipole (MSM) equivalent sources, involving spherical Hankel functions (SHFs) with their origins embedded within the surface and suitably located along the axis of rotational symmetry, are employed as eigenfunctions for the representation of the fields in the exterior domain of the scattering problem. The salient feature of the proposed formalism is that the induced fields in the interior of dielectric volumes are expressed as a superposition of appropriate set of equivalent multiple spherical vector wavefunctions involving spherical Bessel functions (SBFs) of the first kind and located in the interior of the object to be treated to take advantage of rotational symmetry. Numerical results for the differential scattering cross-section (DSCS) patterns as well as the extinction, scattering, absorption, and backscattering efficiencies are presented for a wide range of 3-D electrically extended geometries which, to the best of our knowledge, have not been hitherto considered by either analytical solutions or any other integral, differential, and/or hybrid numerical methods.

Chemical and optical properties of marine boundary layer aerosol particles of the mid‐Pacific in relation to sources and meteorological transport

Incorporating the direct effect of tropospheric aerosol on climate into global climate models involves coupling the optical properties of the aerosol with its physical and chemical properties. This coupling is strengthened if the optical, physical, and chemical properties of the individual aerosol components are known as well as how these properties depend on the air mass source and synoptic scale meteorology. To relate properties of the aerosol components to air mass sources over a wide range of meteorological conditions, two long latitudinal cruises were conducted in the central Pacific Ocean from 55°N to 70°S. Submicron non‐sea‐salt (nss) SO4= aerosol averaged about 35 to 40% of the submicron ionic mass as analyzed by ion chromatography and 6% of the total ionic mass, while supermicron nss SO4= aerosol contributed about 1% to the total ionic mass. About 1% of the remaining total ionic mass was composed of methanesulfonate and 90% was sea salt. Ionic mass fractions of nss SO4= aerosol were highest in regions having the longest marine boundary layer residence times or the largest source of marine or continental gas phase precursors. The calculated scattering by nss SO4= aerosol was highest in these same regions due to the dependence of scattering on particle size and the concentration of nss SO4= in the submicron size range. The calculated scattering by submicron sea salt was similar to that of the nss SO4= aerosol, indicating that its contribution to scattering in the marine boundary layer can be significant or even dominant depending on its mass concentration. Mass scattering efficiencies for nss SO4= at 30% RH ranged from 4.3 to 7.5 m2 g−1 and for submicron sea salt from 3.5 to 7.7 m2 g−1. Mass backscattering efficiencies for nss SO4= ranged from 0.41 to 0.58 m2 g−1 and for submicron sea salt from 0.33 to 0.63 m2 g−1. These values fall within the same range as others reported previously for the marine atmosphere.

Splitting of high-Q Mie modes induced by light backscattering in silica microspheres

We have observed that very high-Q Mie resonances in silica microspheres are split into doublets. This splitting is attributed to internal backscattering that couples the two degenerate whispering-gallery modes propagating in opposite directions along the sphere equator. We have studied this doublet structure by high-resolution spectroscopy. Time-decay measurements have also been performed and show a beat note corresponding to the coupling rate between the clockwise and counterclockwise modes. A simple model of coupled oscillators describes our data well, and the backscattering efficiency that we measure is consistent with what is observed in optical fibers.

Coherent launch-site atmospheric wind sounder: theory and experiment

The coherent launch-site atmospheric wind sounder (CLAWS) is a lidar atmospheric wind sensor designed to measure the winds above space launch facilities to an altitude of 20 km. In our development studies, lidar sensor requirements are defined, a system to meet those requirements is defined and built, and the concept is evaluated, with recommendations for the most feasible and cost-effective lidar system for use as an input to a guidance and control system for missile or spacecraft launches. The ability of CLAWS to meet NASA goals for increased safety and launch/mission flexibility is evaluated in a field test program at Kennedy Space Center (KSC) in which we investigate maximum detection range, refractive turbulence, and aerosol backscattering efficiency. The Nd:YAG coherent lidar operating at 1.06 µm with 1-J energy per pulse is able to make real-time measurements of the three-dimensional wind field at KSC to an altitude of 26 km, in good agreement with our performance simulations. It also shows the height and thickness of the volcanic layer caused by the volcanic eruption of Mount Pinatubo in the Philippines.

Light backscattering efficiency and related properties of some phytoplankters

By using a set-up that combines an integrating sphere with a spectroradiometer LI-1800 UW, the backscattering properties of nine different phytoplankters grown in culture have been determined experimentally for the wavelengths domain ν = 400 up to 850 nm. Simultaneously, the absorption and attenuation properties, as well as the size distribution function, have been measured. This set of measurements allowed the spectral values of refractive index, and subsequently the volume scattering functions (VSF) of the cells, to be derived, by operating a scattering model previously developed for spherical and homogeneous cells. The backscattering properties, measured within a restricted angular domain (approximately between 132 and 174°), have been compared to theoretical predictions. Although there appear some discrepancies between experimental and predicted values (probably due to experimental errors as well as deviations of actual cells from computational hypotheses), the overall agreement is good; in particular the observed interspecific variations of backscattering values, as well as the backscattering spectral variation typical of each species, are well accounted for by theory. Using the computed VSF, the measured backscattering properties can be converted (assuming spherical and homogeneous cells) into efficiency factors for backscattering ( Q bb ) . Thhe spectral behavior of Q bb appears to be radically different from that for total scattering Q b. For small cells, Q (λ) is practically constant over the spectrum, whereas Q b(λ) varies approximately according to a power law (λ −2). As the cell size increases, Q bb conversely, becomes increasingly featured, whilst Q b becomes spectrally flat. The chlorophyll-specific backscattering coefficients ( b b ∗ appear highly variable and span nearly two orders of magnitude. The chlorophyll-specific absorption and scattering coefficients, a ∗ and b ∗ , are mainly ruled by the interspecific variations in cellssize ( D) and intracellular pigment concentration ( C i) (actually by the variations of the product DC i). Though b b ∗ is involved in the modelling of the diffuse reflectance of waters, the impact of its actual variation is greatly limited because typical b b ∗ values, even at their maximum (10 −3 m 2 mg −1), are very low. This result confirms that living algae have a negligible influence on the backscattering process by oceanic waters; other particles (bacteria, detritus, etc.) associated with algae are mainly responsible for this process.

Optical properties of photosynthetic picoplankton in different physiological states as affected by growth irradiance

A phycocyanin-rich cyanobacterium belonging to the genus Synechocystis has been adapted and grown under differing irradiances (PAR), ranging from 16 to 1450 μE m −2 s −1, and differing spectral compositions (“white”, “blue” and “green”). Chlorophyll-specific as well as carbon-specific spectral absorption and scattering coefficients were determined for all conditions. Due to drastic changes in chlorophyll and phycocyanin content per cell in response to the radiative level imposed to the culture, these coefficients undergo extreme variations, in a range wider than the inter-specific range already reported for eucaryotic algae. The optical dimensionless efficiency factors have been computed and used to calculate the bulk refractive index (in the range 1.05–1.06 with respect to the index of water). The optical properties of this picoplanktonic species is typical of “small” optical particles, with a scattering efficiency increasing towards the blue, and a backscattering efficiency increasing towards the red end of the spectrum. Superimposed on this pattern are features associated with the presence of pigments, including the phycocyanin signature. Although the cellular pigment concentrations are high (particularly at low irradiances), the package effect remains negligible. Thus Synechocystis is well suited for harvesting light, even if the presence of biliprotein appears to be useless in regards to the spectral quality of the light available in the deep layers of the euphotic zone.