Generation of transition radiation in the form of electromagnetic surface waves by electron bunches

Abstract

The generation of transition radiation in the form of electromagnetic surface waves by a nonrelativistic electron bunch as it crosses the vacuum-semiconductor interface or a thin semiconductor plate in vacuum is investigated. A study is made of a bunch that has the shape of an ellipsoid of revolution, with a uniform charge density distribution over its volume, and moves along the normal to the interface. When the energy dissipation in the semiconductor is taken into account, the spectrum of the transition radiation emitted in the form of surface waves comprises a peak whose width is comparable to its mean frequency. It is shown that, in each of the two cases under consideration, the generation efficiency, defined as the ratio of the radiated energy to the kinetic energy of the bunch electrons, is maximum for a bunch of certain dimensions. The dependence of the radiated energy and of the generation efficiency on the thickness of a thin semiconductor plate is investigated for given bunch dimensions. It is found that the corresponding dependences have a maximum, which can be explained as being due the competition between the two effects: as the plate thickness increases, on the one hand, the region where the radiation is generated becomes larger, so that the radiation power increases, and, on the other hand, the dissipative energy losses become higher.