Near-field diffraction by two slits in a black screen

Measurements were made with microwaves of the near-field diffraction pattern formed by two parallel slits in a highly absorbing ('black') screen. The first purpose was to examine the validity of superposing the fields of the two separate slits, as would follow from the standard Kirchhoff or Rayleigh-Sommerfeld theories. An earlier paper showed that, with a conducting screen, there is a departure from exact superposition of 7%, on average, when the polarization is parallel to the slits and 25% when it is perpendicular. The new experiments, which were done at normal incidence with slit widths of 0.5 lambda and separation 1.5 lambda, show that, with a black screen, there is a departure from exact superposition of 11% when the polarization is parallel, and 15% when it is perpendicular. Each slit generates a field behind the screen, but, unlike the case of the conducting screen, the source of the field cannot be considered to be merely the slit itself; it is as if the wave field generated by each of the two slits is affected by whether the other is open or closed. The width of the maximum immediately behind a single slit depends strongly on the polarization, an observation that has implications for resolution in, for example, the making of mad;ks using near-field optics. All scalar theories of diffraction by a black screen with translational symmetry imply that the patterns of electric field associated with two different polarizations in the incident wave are related: one is the spatial derivative of the other. The fact that this theoretical relation is obeyed by the measurements to a close approximation shows that scalar theory is applicable, as an approximation, for the imperfect 'black' screen used. The relation would not be obeyed for a conducting screen.