Highly directed transmission of multipole radiation by an interface

Electromagnetic multipole radiation can have a highly directed peak in its angular intensity distribution when transmitted through an interface (xy-plane) with a medium that has a higher index of refraction. We show that the possible occurrence of such a peak depends on the order of the multipole. It is also shown that the peak intensity is determined uniquely by the intensity of the multipole radiation in the xy-plane in the absence of the interface. This peak occurs at the slightest difference in index of refraction of the two media, and when the indices of refraction are almost equal, the peak appears just below the interface in the denser medium, and it has an intensity of four times the intensity in the xy-plane of the unbounded multipole.