We consider the flow of a viscous liquid along an unknown topography. A new strategy is presented to reconstruct the topography and the free surface shape from one component of the free surface velocity only. In contrast to the classical approach in inverse problems based on optimization theory we derive an ordinary differential equation which can be solved directly to obtain the inverse solution. This is achieved by averaging the Navier–Stokes equation and coupling the function parameterizing the flow domain with the free surface velocity. Even though we consider nonlinear systems including inertia and surface tension, the inverse problem can be solved analytically with a Fourier series approach. We test our method on a variety of benchmark problems and show that the analytical solution can be applied to reconstruct the flow domain from noisy input data. It is also demonstrated that the asymptotic approach agrees very well with numerical results of the Navier–Stokes equation. The results are finally confirmed with an experimental study where we measure the free surface velocity for the film flow over a trench and compare the reconstructed topography with the measured one.

Bulk crystals of CdGeAs2 have been characterized using photoluminescence (PL), optical absorption, Hall effect, and electron paramagnetic resonance (EPR) techniques. An absorption band near 5.5 microns at room temperature is observed in all of the p-type samples we have studied. A correlation between the magnitude of this optical absorption and the excess hole concentration at room temperature is established. Also, an EPR signal is found to correlate with the strength of this absorption band. PL data are consistent with an increased concentration of shallow acceptors being present in high-absorption samples. From the EPR data, we suggest that a model for the paramagnetic defect associated with the absorption at 5.5 microns may be an acceptor on an anion site.

We compared the relative risks of mother-to-child transmission of Toxoplasma gondii and clinical manifestations due to congenital toxoplasmosis associated with intensive prenatal treatment in Lyon and Austria, short term treatment in 51% of Dutch women, and no treatment in Danish women. For each cohort, relative risks were standardized for gestation at seroconversion. In total, 856 mother–child pairs were studied: 549 in Lyon, 133 in Austria, 123 in Denmark and 51 in The Netherlands. The relative risk for mother-to-child transmission compared to Lyon was 1·24 (95% CI: 0·88, 1·59) in Austria; 0·59 (0·41, 0·81) in Denmark; and 0·65 (0·37, 1·01) in The Netherlands. Relative risks for clinical manifestations compared with Lyon (adjusted for follow-up to age 3 years) were: Austria 0·19 (0·04, 0·51); Denmark 0·60 (0·13, 1·08); and The Netherlands 1·46 (0·51, 2·72). There was no clear evidence that the risk of transmission or of clinical manifestations was lowest in centres with the most intensive prenatal treatment.

Imaging techniques often suffer from distortion effects. Former methods of reducing these distortions have been based either on improving the imaging technique (i.e., to avoid distortions) or on the use of reference samples (i.e., to determine the distortion field by imaging of a known structure. We present a novel method of correcting image distortion by evaluating the imaged position changes due to two small sample position shifts. The algorithm allows us to calculate a vector field, which enables us to determine the “undistorted” position of any point of the image. The presented method has very low presuppositions about the sample, requires no reference samples, and is applicable to any type of image distortion. In addition to the presentation of the method's theoretical basis and a description of the computational method, we present corrected secondary ion mass spectroscopy (SIMS) images of a regular structure (a copper grid) as well as a stochastic distribution (sodium impurities) to show the results of empirical data.

The optical and electrical properties of chrome-doped CdGeAs2 (CGA), an important non-linear optical material, are reported. CGA, a chalcopyrite semiconductor of the pseudo- III-V type, is a close ternary analog to GaAs, possessing significant differences. To date, the electrical and optical properties of as-grown undoped CGA have been controlled by a somewhat shallow dominant residual acceptor which it is the source of significant undesirable optical absorption. Highly transparent semi-insulating CGA should be attainable using compensation and counterdoping schemes similar to those used for GaAs. However, identifying suitable deep and shallow n-type and p-type dopants will require extensive empirical studies. As a starting point of survey to find deep levels, the properties of CGA:Cr have been investigated. Cr is a reasonable choice as it has been used extensively to provide a deep level in GaAs. Thermal Admittance Spectroscopy was used to examine the electrically active levels in this material. These measurements were correlated with temperature dependent Hall effect measurements, and IR absorption measurements. SIMS analysis was utilized to estimate the Cr concentration as the segregation coefficient for Cr in CGA has not been reported.. The results show that there is a p-type level introduced into the band gap at about 0.16 eV above the valence band, a value nominally 50% deeper than that of the native acceptor. The background doping as measured by Capacitance-Voltage measurements was determined to be 8 × 1016 cm−3 near the surface, and 1.0 × 1017 cm−3 in the bulk. These results are compared to similar measurements in undoped material.

Bulk ZnGeP2 (ZGP) crystals grown for high-power tunable mid-infrared laser systems contain large concentrations of three native defects. Using EPR, these three defects have been identified as the Zn vacancy (an acceptor), the phosphorus vacancy (a donor), and the germanium-on-zinc antisite (also a donor). We have studied the wavelength dependence of the photoinduced changes in the EPR signal intensity of the three defect centers using 633 nm and 1064 nm light. We observe a significant increase in the EPR signal under 633 nm light. The 633-nm light also produces an increase in the defect-related 1-R.m absorption band, and we have used a spectrophotometer to measure the spectral shape of the photoinduced change in absorption in this near-IR region. The 633-nm wavelength produces paramagnetic forms of both donor centers, while 1064 nm light only produces the EPR-active center. Time decays of the photoinduced EPR signals have been measured for each of the donors.

Zinc germanium diphosphide (ZnGeP2) is a nonlinear optical material used in mid-infrared optical parametric oscillators. The near-infrared photoluminescence (PL) from single crystals of bulk ZnGeP2 has been studied as a function of excitation power, wavelength, temperature, and polarization. At 5 K, a broad PL band extending from 0.7 µm to beyond 1 µm is typically observed. Two distinct emissions with different polarization, power, and temperature behaviors have been resolved. These bands have peaks in intensity near 1.6 eV and 1.4 eV. The relative intensities of these two bands were found to correlate with the presence of phosphorus vacancies, as determined by electron paramagnetic resonance (EPR). A resonance in the intensity of the 1.6-eV band occurs when pumping into a level ∼90 meV below the minimum conduction band. This level is tentatively assigned to the shallow state.

As-grown crystals of ZnGeP2 are highly compensated and contain significant concentrations of donors and acceptors. The dominant acceptor in ZnGeP2 is believed to be the zinc vacancy. This center is paramagnetic in its normal singly ionized state, and gives rise to an electron paramagnetic resonance (EPR) signal characterized by a resolved primary hyperfine interaction with two equivalent phosphorus nuclei adjacent to the vacancy. The present investigation has focused on electron-nuclear double resonance (ENDOR) measurements of additional hyperfine interactions which are not resolved in the regular EPR spectra. Principal values and principal axes directions for four additional phosphorus nuclei are determined from the ENDOR angular dependence. These parameters support the zinc-vacancy assignment for the acceptor and they provide an experimental check of wave functions generated in future computational modeling efforts.

Raman scattering experiments on high quality ZnGeP2 single crystals grown by the seeded horizontal dynamic gradient technique have been carried out. Polarized Raman spectra were obtained in the backscattering geometry at both room and low temperatures for several crystal orientations and compared with group theoretical predictions. Raman spectra from as-grown and annealed samples display distinctive differences which were explored by utilizing two different excitation wavelengths: 514.3 nm and 632.8 nm; the observed differences are attributed to a surface interdiffuasion effect.

Electron paramagnetic resonance (EPR) has been used to monitor native defects, both acceptors and donors, in ZnGeP2 crystals grown by the horizontal gradient freeze technique. These active centers include singly ionized zinc vacancies (Vzn-), neutral phosphorus vacancies (VP0), and neutral phosphorus antisite defects (PGe0). The concentration of Vzn− acceptors correlates with the near-infrared optical absorption present in all ZnGeP2 crystals. A photoluminescence band near 1.4 eV is shown to be polarized and is attributed to donor-acceptor-pair (DAP) recombination. Preliminary time-decay measurements support this assignment. Observation of the EPR spectrum of Mn2+ is also reported.

Using contactless electroreflectance at 300K we have characterized the complete potential profile of a pseudomorphic 0.98 μm InGaAs/GaAs/GaAlAs graded index of refraction separate confinement heterostructure laser fabricated by molecular beam epitaxy. Signals were detected from all three relevant portions of the sample. Comparison of the observed transitions from the InGaAs single quantum well (SQW) section with an envelope function calculation (including the effects of strain) made it possible to evaluate the In composition and width of SQW. These values were in good agreement with the intended parameters. The energy of 11H, the fundamental conduction to heavy-hole level (which is closely related to the lasing frequency) can easily be determined to less than ± 1 nm at 300K. From the period of the observed Franz-Keldysh oscillations from the graded GaAlAs region it was possible to directly evaluate the built-in electric field in the structure.

The temperature and polarization dependence of the optical absorption in ZnGeP2 at two micrometers is reported for the first time over the temperature range from 10K to 300K. The radiation was normally incident upon the face of a cubic sample which contained the c-axis. The absorption of o-rays (E parallel to c), and erays (E perpendicular to c) was determined. It was found that the e-ray absorption coefficient was always significantly larger than the o-ray absorption coefficient and that it had a less significant temperature dependence. For example, the ratio of e-ray to o-ray absorption coefficient was approximately two at 300K and five at 77K. Correspondingly the o-ray absorption coefficients were reduced upon cooling to 77K by a factor of 2.5, while the e-ray absorption coefficients were reduced only slightly (10%-20%). These results indicate that for Type I optical parametric oscillators (OPOs) which use an oray pump beam, that performance may be improved by cooling the crystal.

Three InAs/GaAs single quantum wells of 2, 3, and 4 monolayer thickness were characterized using optical and structural techniques. The results of high-resolution transmission electron (HRTEM) microscopy and optical studies which combine absorption, photoluminescence (PL), photoreflectance and cathodoluminescence are presented.

Using the polarization modulated absorptance technique we observed two absorption features in our samples at 77 K. On the basis of their polarization properties and comparison with an envelope function calculation, these structures are assigned to transitions between the confined heavy-hole and confined and unconfined electron levels. Photoreflectance spectra of the 3- monolayer sample in 77-300 K range show only the fundamental quantum well transition. The temperature dependence of this transition is approximately linear with a slope of 2.2·10−4 eV/K which is significantly lower than in both constituent materials.

Comparison to the absorption data reveals that the PL spectra are affected by the carrier diffusion and therefore do not provide direct measure of the exciton density of states. Therefore, photoluminescence results alone do not provide unequivocal information about the fundamental transition energy or the interface quality in quantum wells.

The HRTEM images indicate that while the interfaces of the 2-monolayer sample are smooth and the well thickness is uniform, the 4-monolayer sample has uneven interfaces and contains domains of 2, 3, and 4 monolayers. In agreement with these observations, absorption features broaden with the increased well width. Scanning cathodoluminescence images of the 2- monolayer sample present no evidence of dislocations, which is consistent with the HRTEM observations.

A tumour of the singular nerve was found on examination of the temporal bones of a child who died 13 months after meningitis. The tumour consisted of a main mass with the appearance of an acoustic neuroma but close by and not connected were some nests of tumour cells inside the vestibule. This very unusual finding raises questions of the aetiology of this tumour which may have a bearing on the aetiology of other tumours of the VIIIth. nerve.