Abstract

Bistatic- and nonspecular-scattering cross-section measurements of CO₂ laser radiation from roughened metal and dielectric surfaces were made and compared with predictions given by the facet (tangent-plane approximation) model. The incident radiation was linearly polarized perpendicular to the incident plane. The scattered polarization state was analyzed along two directions, perpendicular (HH) and parallel (HV) to the scattering plane. A scattering apparatus was designed especially for this work. This apparatus allowed us to make polarization-dependent bistatic measurements over the entire hemisphere, which to the best of our knowledge has not been done before. The average slopes and radii of curvature of the roughened surfaces utilized in this study were determined with a mechanical profilometer from surface tracings. The (σ₀)_HH nulls predicted by the facet model have been verified at steep incident angles for both metallic and dielectric surfaces. Agreement is generally better for metals than for dielectrics. At shallower angles measurements diverge from theoretical predictions. Our data indicate that the departure from the predictions of the facet model is most likely associated with surface curvature. A number of calculations of polarization-dependent scattering amplitudes from metal and dielectric cylinders of radius r as a function of r/λ have been performed. These calculations suggest that, even for surfaces with large radii, rapid amplitude and phase variations are responsible for the onset of depolarization at shallower incident and scattering angles, which leads to the disagreement with the tangent-plane model predictions. These calculations also clearly demonstrate why the facet model is a better approximation, in the region of validity, for metals than for dielectrics. The role of curvature in determining the operable regions of the tangent-plane model is further illuminated through a series of scattering measurements from metal wires with radii of curvature of the order of λ. The experimental measurements are described in detail, and data for both roughened metals and dielectrics are presented for several scattering aspect and depression angles.

© 1992 Optical Society of America

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