Near–field diffraction by two slits in a black screen

Abstract

Measurements were made with microwaves of the near–field diffraction pattern formed by two parallel slits in a highly absorbing (‘blac’) 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 0.5λ and separation 1.5λ, 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 masks 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.