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A contact-free optical technique is developed to enable a spatially resolved measurement of minority carrier diffusion length and the associated mobility-lifetime (Î¼Ï ) product in bulk semiconductor materials. A scanning electron microscope is used in combination with an internal optical microscope and imaging charge-coupled device (CCD) to image the bulk luminescence from minority carrier recombination associated with one-dimensional excess carrier generation. Using a Greenâ s function to model steady-state minority carrier diffusion in a three-dimensional half space, non-linear least squares analysis is then applied to extract values of carrier diffusion length and surface recombination velocity. The approach enables measurement of spatial variations in the Î¼Ï product with a high degree of spatial resolution.

The acoustical performance of a submerged linear array of spherical shells in shallow water is examined by combining the T-Matrix method of solving for multiple acoustic interactions among separate bodies, with a model for the ocean as a fluid layer over a half-space of a distinct fluid. The system of source and reflected waves is analyzed by standard approximate contour integration techniques, valid for the far field of the array. Calculations using the results show the effects of shallow water on array performance., Supported in part by Antisubmarine Warfare group of NPS.

Bi-orthogonality relationships are established for a vertically heterogeneous porous media in contact with a fluid, a solid, a second porous medium, and a free surface. Fraser's bi-orthgonality relation for propagation of Rayleigh-Lamb modes in a plate with traction free eurfaces is shown to be a special case of the bi-orthogonality relations derived herein in which the medium can be thought of as a porous slab with zero porosity.

Application of a time dependent. nonlocal radiation boundary condition. used in conjunction with the finite difference technique is applied to the acoustic problem of scattering from an infinite array of baffled. fluid loaded membranes. The new boundary operator is compared with the second order Engquist and Majda boundary operator, and with several fluid/structure decoupling approximation techniques in the determination of scattering amplitudes.

The scattering of a time harmonic acoustic wave by an army of N identical baffied membranes is considered in the limit as N ----+ x. The method of matched asymptotic expansions is used to construct an inner and outer expansion: the inner expansion contains the physics of the local periodic structure and the outer expansion has the proper behavior in the far-field. The niatching process is used to determine a set of unknown coefficients in the analysis and the result is a far-field pattern for the structure which blends the physics of both expansions. In particular. the far-field pattern is highly localized about the Dragg or mode angles of the inner representation. The maximum value of this function is directly proportional to the reftection coefficient determined numerically from the inner probleni. Several nunicrical exan1ples arc presented illustrating these features. Moreover, it is observed that the a. '. -lyrnptotic theory agrees quite well with the exact answer (obtained by using a finite difference scheme) when N = 3.

The acoustical performance of a submerged linear array of spherical transducers is examined by combining the T-Matrix method of solving for multiple acoustic interactions among separate bodies with a model for the transducers as thin spherical elastic shells. This approach solves the fully coupled problem of the response of the array to internal forcing. The results show that the assumptions giving rise to the Chebyshev criteria for optimal arrays of point sources appear to apply well even for large spheres at low frequencies. However, at frequencies near or above the lowest resonant frequency the directional pattern may be degraded, depending on the material of the shells. Keywords: Spherical harmonies, Elastic shell, Sonar array.