Anomalies of dielectric-coated gratings

Experimental results at millimeter wavelengths on dielectric-coated gratings are reported that show sharp anomalous dips in the transmission intensity for both P and S polarizations (i.e., incident electric field parallel and perpendicular to the slits, respectively). These dips almost disappear when either the dielectric is removed or the slits are randomly spaced. Next, a theory is developed for the gratings of finite extent and nonuniform spacings. Theoretical results, although approximate, agree remarkably well with the experiment.