Simple Optical Model Research Articles

Optical properties of Finnish lakes estimated with simple bio-optical models and water quality monitoring data

The aim of this study was to estimate the distributions of spectral diffuse attenuation coefficient, attenuation depth and subsurface reflectance of Finnish lakes. In addition, the optimum empirical water quality interpretation algorithms employing reflectance ratios were investigated for the needs of remote sensing. Estimations of the optical properties were based on simple optical models and measured concentrations of optically active substances (the sum of chlorophyll a and phaeophytin a, total suspended solids and coloured dissolved organic matter (CDOM)) at 1670 monitoring stations representing 1113 lakes. The models were parameterized using optical data from 10 lakes. The mean diffuse attenuation coefficient in PAR was 3.5 m−1 and the location of the maximum attenuation depth was in the range 564–714 nm. The simulated reflectance spectra showed a shift of the maximum value to longer wavelengths as trophic status changed from oligotrophic to hyper-eutrophic. High CDOM concentrations decrease the estimation accuracy of chlorophyll a from reflectance spectra using empirical algorithms, particularly in oligotrophic lakes. The models described can be used in studying light availability for photosynthesis at different depths, in the simulation of water temperatures, in estimating how different management alternatives affect light attenuation and Secchi depth, and in various remote sensing applications.

Determination of volume, shape and refractive index of individual blood platelets

Light scattering patterns (LSP) of blood platelets were theoretically and experimentally analyzed. We used spicular spheroids as a model for the platelets with pseudopodia. The discrete dipole approximation was employed to simulate light scattering from an individual spicular spheroid constructed from a homogeneous oblate spheroid and 14 rectilinear parallelepipeds rising from the cell centre. These parallelepipeds have a weak effect on the LSP over the measured angular range. Therefore, a homogeneous oblate spheroid was taken as a simplified optical model for platelets. Using the T-matrix method, we computed the LSP over a range of volumes, aspect ratios and refractive indices. Measured LSPs of individual platelets were compared one by one with the theoretical set and the best fit was taken to characterize the measured platelets, resulting in distributions of volume, aspect ratio and refractive index.

Spectroellipsometric assessment of HfO 2 thin films

Present paper looks into the possibilities and limitations of near ultraviolet–visible range spectroscopic ellipsometry in investigating HfO 2 thin films (thickness < 7 nm). The “high k” dielectric films were produced by Atomic Layer Deposition—ALD, sputtering, and Metalo-Organic Chemical Vapour Deposition—MOCVD, on silicon and on silicon/silicon dioxide structures. Using a simple optical model (Cauchy dispersion, with an Urbach absorption tail), suitable for the optical range investigated, we extract the thickness of the layers and their optical constants. Results related to the optical properties show the important impact made by the initial surface and the growth/deposition procedure. It is also shown that for the case of ALD HfO 2 films grown on RTO oxides a significant increase in the absorption coefficient is recorded in the 4.7–5.15 eV range; this can be linked with the formation of defects related to oxygen vacancies. Subsequent anneal cycles performed in oxygen reveal that changes do occur both at the transition layer level, and in the structure of the HfO 2 film, for which an increase in the absorption is recorded.

Internal Tides in the Southwestern Japan/East Sea

Abstract This paper investigates the internal tidal energy distribution in the southwestern Japan/East Sea using vertical round-trip travel time (τ) data from 23 pressure-sensor-equipped inverted echo sounders (PIES). The τ records are analyzed by bandpass filtering to separate time-dependent variability of the semidiurnal and diurnal bands. The semidiurnal internal tides exhibit a horizontal beam pattern of high energy, propagating into the open basin. They originate from a restricted portion of the shelf break where the Korea Strait enters the Ulleung Basin. The generation appears to occur at ∼200-m water depth near 35.5°–35.7°N and 130°–131°E, where the slope of bottom topography matches that of the wave characteristics, coinciding with the location where the semidiurnal barotropic cross-slope tidal currents are strongest. Maximum vertical displacement of the thermocline interpreted as a long-wave first baroclinic mode from the measured τ is about 25 m near the generation region. Annual and monthly variations of the propagation patterns and generation energy levels are observed, and these are closely associated with changes in the mesoscale circulation and stratification. Eastward (westward) refraction is observed when a warm (cold) eddy crosses the path of internal tide propagation. Moreover, when the generation region is invaded by cold eddies that spoil the match between shelf break and thermocline depth, the internal tidal energy level decreases by a factor of about 2. A simple geometric optics model is proposed to explain the observed horizontal refraction of the beam of semidiurnal internal tides in which stratification and current shear play essential roles. In contrast, diurnal internal tides are observed to be trapped along the continental slope region around 36°N.

Manufacturing tolerances for UV LIGA using SU-8 resist

UV LIGA involves the exposure of SU-8 negative resist, using a UV mask aligner, to produce high aspect ratio pillars or microchannels as part of the manufacturing process for microsystems. This has been made possible by the widespread use of a UV sensitive resist SU-8. Many papers have been written on the Fresnel diffraction theory of exposure, some key properties of SU-8 and prototype applications, but little attention has been paid to the variation in parameters that affect the tolerances and hence the reproducibility with which component dimensions can be maintained. It will be shown that the accuracy and precision, with which a feature on a mask can be reproduced throughout a thick resist structure, will depend on key parameters in the set-up of the mask aligner, the material properties of the SU-8 resist and the thickness of the resist structure. A simplified optical exposure model is used to predict the dimensions and wall taper angles of resist pillars or microchannels and their variation with changes in these parameters.

Simple Optical Models for Diagnosing Surface–Atmosphere Shortwave Interactions

Abstract A technique is developed for diagnosing effective surface and atmospheric optical properties from climate model shortwave flux diagnostics. These properties can be used to distinguish the contributions of surface and atmospheric optical property changes to shortwave flux changes at the surface and top of the atmosphere. In addition to the four standard shortwave flux diagnostics (upward, downward, surface, and top of atmosphere), the technique makes use of surface-down and top-up fluxes over a zero-albedo surface obtained from an auxiliary online shortwave calculation. The simple model optical properties, when constructed from the time-mean fluxes, are effective optical properties, useful for predicting the time-mean response to optical property changes. The technique is tested against auxiliary online shortwave calculations at four validation albedos and shown to predict the monthly mean surface absorption with an rms error of less than 2% over the globe. The reasons for the accuracy of the technique are explored. Less accurate techniques that make use of existing shortwave diagnostics are presented and compared.

A simple empirical optical model for simulating light attenuation variability in a partially mixed estuary

Representation of the subsurface light field is a crucial component of pelagic ecosystem and water quality models. Modeling the light field in estuaries is a particularly complicated problem due to the significant influence of high concentrations of dissolved and particulate matter that are derived from both terrestrial and estuarine sources. The goal of this study was to develop a relatively simple but effective way to model light attenuation variability in a turbuid estuary (Chesapeake Bay, United States) in a coupled physical-biological model. We adopted a simple, nonspectral empirical approach. Surface water quality data (salinity was used as a proxy of chromophoric dissolved organic matter [CDOM]) and light measurements from the Chesapeake Bay Program were used to determine the absorption coefficients in a linear attenuation model using regression methods. This model predicts Kc (specific attenuation due to phytoplankton/chlorophylla [chla]), Kt (specific attenuation due to total suspended solids), and Ks (a function of specific attenuation coefficients of CDOM in relation to salinity). The Bay-wide fitted relation between the light attenuation coefficient and water quality concentrations gives generally good estimates of total light attenuation, Kd. The direct inclusion of salinity in the relationship has one disadvantage: it can yield negative values for Kd at high salinities. We developed two separate models for two different salinity regimes. This approach, in addition to solving the negative Kd problem, also accounts for some changes in specific light absorption by chla, seston (nonphytoplankton particulate matter), and CDOM that apparently occur in different salinity regimes in Chesapeake Bay. The resulting model predicts the statistical characteristics (i.e., the mean and variance) of Kd quite accurately in most regions of Chesapeake Bay. We also discuss in this paper the feasibility and caveats of using Kd converted from Secchi depth in the empirical method.

Why do dusk-active cockchafers detect polarization in the green? The polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset

In the retina of dusk-active European cockchafers, Melolontha melolontha, the linear polarization of downwelling light (skylight or light from the tree canopy) is detected by photoreceptors in upward-pointing ommatidia with maximal sensitivity at 520 nm in the green portion of the spectrum. To date no attempt has been made to answer the question of why these beetles detect polarization in the green. Here we present an atmospheric optical and receptor-physiological explanation of why longer wavelengths are advantageous for the perception of polarization of downwelling light under canopies illuminated by the setting sun. Our explanation focuses on illumination situations during sunset in canopied optical environments, because cockchafers are active at sunset and fly predominantly under canopies during their swarming, feeding, and mating periods. Using three simple atmospheric optical models, we computed the degree of linear polarization, the linearly polarized intensity of downwelling light, the quantum catch, and quantum catch difference between polarization detectors with orthogonal microvilli under canopies illuminated by the setting sun as functions of wavelength and solar zenith angle. Based upon these computations, we show that the green sensitivity of polarization detectors in M. melolontha is tuned to the high polarized intensity of downwelling light in the green under canopies during sunset, an optimal compromise between simultaneous maximization of the quantum catch and the quantum catch difference. We also briefly discuss how green-sensitive polarization detectors can function efficiently enough during the pre-feeding and egg-laying flights of cockchafers, which always occur prior to sunset and under the sky.

Beam divergence measurements of InGaN∕GaN micro-array light-emitting diodes using confocal microscopy

The recent development of high-density, two-dimensional arrays of micrometer-sized InGaN∕GaN light-emitting diodes (micro-LEDs) with potential applications from scientific instrumentation to microdisplays has created an urgent need for controlled manipulation of the light output from these devices. With directed light output these devices can be used in situations where collimated beams or light focused onto several thousand matrix points is desired. In order to do this effectively, the emission characteristics of the devices must be fully understood and characterized. Here we utilize confocal microscopy to directly determine the emission characteristics and angular beam divergences from the individual micro-LED elements. The technique is applied to both top (into air) and bottom (through substrate) emission in arrays of green (540nm), blue (470nm), and UV (370nm) micro-LED devices, at distances of up to 50μm from the emission plane. The results are consistent with simple optical modeling of the expected beam profiles.

Comment on “Formation of nanocrystallites governed by the initial stress in the ultrathin hydrogenated amorphous silicon films” [J. Appl. Phys. 90, 1067 (2001)

A recent article suggested that “paracrystalline” silicon could be detected in very thin hydrogenated amorphous silicon films by using spectroscopic ellipsometry. We show that the important features of the ellipsometry data can be reproduced using a simple optical model of amorphous Si:H, with no assumptions about unusual structures or thickness-dependent optical properties.

SiO 2/Si(100) interface characterization using infrared spectroscopy: estimation of substoichiometry and strain

The transverse optical (TO) and longitudinal optical (LO) phonons of the asymmetric stretch of the O in SiOSi bridging bond in thermal SiO 2 show red shift with decreasing oxide thickness. These shifts are primarily due to interface effects like strain and substoichiometric silicon oxides (suboxides). A method to isolate the contributions of strain and suboxide concentration towards the observed shifts is proposed. The procedure, which utilizes simple optical model and effective medium approximation, allows estimation of the average strain and suboxide concentration in films of different thickness. Analysis of infrared spectra shows that suboxide-rich layers in low temperature dry and wet oxides are confined to a distance of ∼1.5 nm from the interface; wet oxidation may generate lower stress near the interface.

Strain and substoichiometry at theSi(100)/SiO2interface

Interfacial strain and substoichiometric silicon oxides are the two principal causes that result in the redshift of the transverse and longitudinal optical phonons of the asymmetric stretch of O in the Si-O-Si bridging bond of thermal ${\mathrm{SiO}}_{2}$ with decreasing oxide thickness. Analyses to comprehend these effects, therefore, require a consideration of both strain and interfacial substoichiometry. A method to isolate the contributions of strain and substoichiometry towards the observed shifts is proposed. The procedure, which utilizes a simple optical model and an effective medium approximation, allows an estimation of the average strain and substiochiometry in films of different thicknesses. Analyses of oxides formed at two different temperatures (550 and 700 \ifmmode^\circ\else\textdegree\fi{}C) with dry and wet oxygen reveal how process conditions affect the interface properties.

Diffuse reflection of ceramics coated with dielectric thin films.

We have studied the diffuse reflection properties of ceramics in the presence of dielectric thin films on the surface. A simple optical model was proposed in which interference effects in a thin film were considered for light scattered out of a ceramic in various directions. Measurements were performed on angle-resolved reflection spectra of a thin-film-coated alumina ceramic in the case of normal incidence. They showed that the presence of the thin film on the ceramic's surface modified the angular distributions of scattered radiation from that of a bare ceramic, which suggested a way to tailor the scattering properties of a diffuse reflector as needed.

Secondary electron spectra for fast proton impact on gaseous and liquid water

Secondary electron spectra for proton impact on water in the vapor and liquid phase are calculated and compared. Fast but non-relativistic protons are considered (∼0.3–10 MeV) where the first Born approximation is well justified. For both phases the response of the valence electrons is modeled according to the theory of Bethe by using available optical data, while the binary-encounter approximation was used for the K-shell electrons. In particular, for the vapor, photo-ionization data were used to construct the continuous optical-oscillator strength of the molecule and to evaluate the leading term of Bethe’s asymptotic expansion. The contribution of subsequent terms was established semi-empirically, allowing for the high-energy free-electron limit. For the liquid, the energy and wave number dependence dielectric-response function, which accounts for long-range effects in the condensed phase, was employed. Optical data and the impulse approximation were used to construct the full Bethe surface of liquid water and calculate cross-sections. The comparison was also extended to more simple (semi-) classical and optical models.

Translation of colloidal particles next to a flat plate using evanescent waves

Abstract A modeling and experimental study was performed to evaluate the ability of an evanescent wave, produced by the total internal reflection of a continuous wave laser beam at an optical interface, to translate colloidal-sized particles in water. Using a simple ray optics model, it was predicted that polystyrene spheres with a diameter much larger than the incident wavelength would be pushed in the direction of wave propagation, pulled toward the interface at which the internal reflection occurred, and rotated about an axis parallel to the interface. Experimental validation of the model was then performed using an argon ion laser beam that was focused down to a beam radius of 40 μm. Laser powers ranging between 0.02 and 0.7 W produced translational velocities up to 1.0 μm s −1 on a 5 μm radius polystyrene sphere in water above a glass plate. At the lower laser powers (less than 0.2 W), the measured velocities were within a factor of approximately two of the model predictions. At higher powers, the measured velocities became less sensitive to changes in the incident power, which was not predicted by the model. The cause of this deviation is currently unknown, although one possible factor is heating produced by absorption.

Turbidity in the southern Irish Sea

This paper presents new in situ optical and associated measurements from 85 stations in the central and southern Irish Sea. There is a strong linear relationship between the irradiance reflectance R A in the orange–red part of the spectrum (580–680 nm) and the diffuse attenuation coefficient, K, for white light: K=0.05+0.26 R A , where K is in m −1 and R A has been corrected to just above-surface reflectance and expressed as a percentage. The significance of this result is that this particular reflectance can be measured by the advanced very high resolution radiometer on board the NOAA series of satellites. In principle, therefore, cloud cover permitting, the transparency of the Irish Sea to sunlight, can be mapped from space. This result is shown to be consistent with a simple optical model in which light scattering is principally by mineral suspended solids, and light absorption is by water, mineral suspended solids and chlorophyll. Best fit between model and observations is achieved with a specific scattering coefficient of 0.5 m 2 g −1. The measurements were made during four cruises, at different times of year and across the range of turbidity found in the Irish Sea. The geographical distribution of suspended sediments confirms the presence, previously inferred from satellite imagery, of two separate turbidity maxima, one off Wicklow Head, the other off Anglesey. These correspond to the areas of strongest tidal currents. Yellow substance was found in highest concentration in a band along the Irish coast. Chlorophyll concentrations were generally low during these cruises. A residual problem is that a direct comparison of in situ reflectance and satellite measured reflectance possible on one of the cruises shows a serious discrepancy, although on average there appears to be a good agreement between satellite and in situ reflectance.

Magnetic field effects on the 1 083 nm atomic line of helium

The structure of the excited $2^{3}$S and $2^{3}$P triplet states of $^{3}$He and $^{4}$He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1083 nm $2^{3}$S-$2^{3}$P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the $2^{3}$S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at $B\sim$0.1 T, a field for which the usual optical methods could not operate.

High optical anisotropy in thin films of polyfluorene and its affect on the outcoupling of light in typical polymer light emitting diode structures

The anisotropic optical response of thin films of the conjugated polymer PF2/6am5 (polyfluorene) is investigated using spectroscopic ellipsometry. The influence of the highly dispersive and anisotropic character of refractive index, found in this work for the π–π* transition region, on the outcoupling efficiency of light emitting devices is calculated by applying a simple ray optics model. Polymer light emitting diodes are constructed using PF2/6am4 and PtOEP doped PF2/6am4 (0.5%, 2%, and 4%) as an active layer and their external quantum efficiency (EQE) measured. Electroluminescence spectra of the doped devices are shifted to red wavelengths. The effect of transferring the emission away from high dispersive wavelengths on the outcoupling efficiency is determined and internal quantum efficiency (IQE) values for all devises are calculated. It is shown, that the increase in IQE by doping with PtOEP of up to 80% is completely accountable to the higher photoluminescence quantum yield of the dye. Accounting properly for the anisotropic optic properties of thin spun cast polymer films shows that the simple 2n2 relationship between IQE and EQE yields a large overestimate of the former.

Experimental and Theoretical Studies of the Si(100)/SiO2 Interface Formed by Wet and Dry Oxidation

ABSTRACTInterfacial strain and substoichiometric silicon oxides are the two principal causes that result in the redshift of the transverse and longitudinal optical phonons of the asymmetric stretch of O in the Si-O-Si bridging bond of thermal SiO2 with decreasing oxide thickness. Analyses to comprehend these effects, therefore, require consideration of both strain and interfacial substoichiometry. A method to isolate the contributions of strain and suboxide content towards the observed shifts is proposed. The procedure, which utilizes simple optical model and effective medium approximation, allows estimation of the average strain and suboxide concentration in films of different thickness. Analyses of oxides formed at two different temperatures (550 and 700°C) with dry and wet oxygen reveal how process conditions affect the interface properties.

6Heinteraction with protons

Experimental data for ${}^{6}\mathrm{He}{+}^{1}\mathrm{H}$ elastic scattering and the two-neutron transfer reaction, measured recently at the Joint Institute for Nuclear Research in Dubna, Russia, were analyzed by coupled channels calculations using a dineutron model of ${}^{6}\mathrm{He}.$ The results of the analysis reveal that all the processes induced by ${}^{6}\mathrm{He}$ on a hydrogen target are coupled. This analysis shows that the couplings to the ${}^{6}\mathrm{He}\ensuremath{\rightarrow}\ensuremath{\alpha}+2n$ breakup channels can be included in a simple optical model calculation by means of a polarization potential that is repulsive and whose strength is about 25% of the single-folding potential. Predictions for the ${p+}^{6}\mathrm{He}$ inelastic scattering to the ${2}^{+}$ resonance are made. A good description of the two-neutron transfer reaction data is obtained when the triton transfer process is coherently added to the dineutron transfer. The spectroscopic amplitude of the $t+t$ component of the ${}^{6}\mathrm{He}$ ground-state wave function is found to have the opposite sign to that of $\ensuremath{\alpha}+2n$ with a magnitude close to that predicted by a microscopic four-cluster model of ${}^{6}\mathrm{He}.$