Michael Sydor

Optical scattering and backscattering by organic and inorganic particulates in U.S. coastal waters

We present the results of a study of optical scattering and backscattering of particulates for three coastal sites that represent a wide range of optical properties that are found in U.S. near-shore waters. The 6000 scattering and backscattering spectra collected for this study can be well approximated by a power-law function of wavelength. The power-law exponent for particulate scattering changes dramatically from site to site (and within each site) compared with particulate backscattering where all the spectra, except possibly the very clearest waters, cluster around a single wavelength power-law exponent of -0.94. The particulate backscattering-to-scattering ratio (the backscattering ratio) displays a wide range in wavelength dependence. This result is not consistent with scattering models that describe the bulk composition of water as a uniform mix of homogeneous spherical particles with a Junge-like power-law distribution over all particle sizes. Simultaneous particulate organic matter (POM) and particulate inorganic matter (PIM) measurements are available for some of our optical measurements, and site-averaged POM and PIM mass-specific cross sections for scattering and backscattering can be derived. Cross sections for organic and inorganic material differ at each site, and the relative contribution of organic and inorganic material to scattering and backscattering depends differently at each site on the relative amount of material that is present.

Effect of suspended particulate and dissolved organic matter on remote sensing of coastal and riverine waters

We use remote sensing reflectance (RSR) together with the inherent optical properties of suspended particulates to determine the backscattering ratio b(b)/b for coastal waters. We examine the wavelength dependence of b(b)(lambda) and f(lambda)/Q(lambda) and establish the conditions when C(lambda) in RSR(lambda) approximately or = C(lambda)b(b)(lambda)/a(lambda) can be treated as a constant. We found that for case 2 waters, RSR was insensitive to the natural fluctuations in particle-size distributions. The cross-sectional area of the suspended particulate per unit volume, x(g), showed an excellent correlation with the volume scattering coefficient.

Differential photoreflectance from a high-mobility and highly luminescent two-dimensional electron gas

We present a differential modulation technique which is effective in extracting the photoreflectance from GaAs/AlxGa1−xAs samples exhibiting excessive room‐temperature photoluminescence. Using the technique, we obtain the photoreflectance from the triangular potential‐well region of a high‐electron‐mobility transistor. Surprisingly, the signal from the potential well can be extracted from underneath two layers of heavily doped material, making differential photoreflectance useful in detection of two‐dimensional electron gas in high‐electron‐mobility transistors cladded with doped protective GaAs caps which usually obliterate the PR from the buried interfaces. We isolate absorptionlike photoreflectance peaks at 1.447 and 1.472 eV and show that their energies and the ratio of their amplitudes agree with the 2DEG theory for asymmetric triangular potential well.

Photoreflectance of AlxGa1−xAs and AlxGa1−xAs/GaAs interfaces and high‐electron‐mobility transistors

Photoreflectance is used to measure AlxGa1−xAs composition, and to determine carrier concentrations in Si‐doped AlGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AlGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AlGaAs band edges precisely. A similar broad signal from modulation‐doped heterostructures is apparently associated with samples that show the presence of two‐dimensional electron gas.

Photoreflectance and the electric fields in a GaAs depletion region

We present results which may resolve the recently reported discrepancy between the experimental and the calculated electric fields in the depletion region of undoped GaAs. The photoreflectance theory reportedly underestimates the electric field by nearly a factor of 2. We have found that changes in photoreflectance with laser pump penetration reveal the full character of the electric field over the entire depletion zone. It is often assumed that the built‐in surface potential produces a uniform electric field throughout a thin (100 nm) undoped layer of GaAs grown on top of a heavily doped energy pinning underlayer. Instead, it appears that the heavily doped underlayer provides a potential step at the interface. The step is separated from the surface depletion zone by a region of low electric field which is characteristic of the low fields found in thick, undoped GaAs with (2–4)×1014/cm3 of unintentional impurities.

Uniqueness in remote sensing of the inherent optical properties of ocean water

We examine the problem of uniqueness in the relationship between the remote-sensing reflectance (Rrs) and the inherent optical properties (IOPs) of ocean water. The results point to the fact that diffuse reflectance of plane irradiance from ocean water is inherently ambiguous. Furthermore, in the 400 < lambda < 750 nm region of the spectrum, Rrs(lambda) also suffers from ambiguity caused by the similarity in wavelength dependence of the coefficients of absorption by particulate matter and of absorption by colored dissolved organic matter. The absorption coefficients have overlapping exponential responses, which lead to the fact that more than one combination of IOPs can produce nearly the same Rrs spectrum. This ambiguity in absorption parameters demands that we identify the regions of the Rrs spectrum where we can isolate the effects that are due only to scattering by particulates and to absorption by pure water. The results indicate that the spectral shape of the absorption coefficient of phytoplankton, a(ph)(lambda), cannot be derived from a multiparameter fit to Rrs(lambda). However, the magnitude and the spectral dependence of the absorption coefficient can be estimated from the difference between the measured Rrs(lambda) and the best fit to Rrs(lambda) in terms of IOPs that exclude a(ph)(lambda).

Remote-sensing technique for determination of the volume absorption coefficient of turbid water

We use remote-sensing reflectance from particulate R(rs) to determine the volume absorption coefficient a of turbid water in the 400 < lambda < 700-nm spectral region. The calculated and measured values of a(lambda) show good agreement for 0.5 < a < 10 (m(-1)). To determine R(rs) from a particulate, we needed to make corrections for remote-sensing reflectance owing to surface roughness S(rs). We determined the average spectral distribution of S(rs) from the difference in total remote-sensing reflectance measured with and without polarization. The spectral shape of S(rs) showed an excellent fit to theoretical formulas for glare based on Rayleigh and aerosol scattering from the atmosphere.

DIFFERENTIAL PHOTOREFLECTANCE FROM MODULATION-DOPED HETEROJUNCTIONS

We present a new photoreflectance (PR) technique for studies of layered electronic materials. Using the technique, we isolate the reflected signal from buried interfaces and extract the signature from the two‐dimensional electron gas in high electron mobility transistor. We compare the results from the technique with conventional PR measurements performed on sequentially etched samples. The results indicate that the new technique picks out the PR from buried interfaces and provides data on the in situ electric fields at the interfaces.

Photoreflectance measurements of unintentional impurity concentrations in undoped GaAs

Modulated photoreflectance is used to measure the unintentional impurity concentrations in undoped epitaxial GaAs. A photoreflectance signal above the band gap spreads with the unintentional impurity concentrations and shows well‐defined Franz–Keldysh peaks whose separation provide a good measure of the current carrier concentrations. In samples less than 3 μm thick, a photoreflectance signal at the band edge contains a substrate‐epilayer interface effect which precludes the analysis of the data by using the customary third derivative functional fits for low electric fields.

Transports in the Duluth-Superior Harbor

A hydrodynamic model and a water quality model for the Duluth-Superior harbor are developed and verified by using data on currents, water levels, and water quality parameters. The water quality model is subsequently used to simulate the transport of particulates resuspended by ship traffic and the dispersal of dissolved material spilled into the harbor at a major industrial site. Resuspension of bottom sediments by ship traffic is an important secondary source of harbor turbidity. Suspended solids in ship resuspension plumes range from 10 to 50 mg/L, five times the usual concentration of suspended solids in the harbor. An estimated 105 kg of material is resuspended per passage of a ship. The resuspended material is coarse and settles rapidly, thus only 0.1% of the material flows out directly into Lake Superior. Most of the material resuspended by ship traffic is redistributed to the low turbulence areas within the shipping channels. It was found that a spill of dissolved pollutant in the inner harbor would take from 8 to 21 days to reach Lake Superior at peak concentrations. The peak concentration of pollutant at the lake entries depends on seiche amplitude and ranges from 0.05 to 0.1% of the initial concentration at the spill site. At a 0.1% concentration level, the spill would extend over 30% of the harbor, and would remain in the harbor for 30 to 40 days.