Non-radiative optics: interaction between electrons and near-optical fields

Studies on the Cerenkov effect show that a free electron moving at uniform velocity in a dielectric medium can emit light only when its velocity v is greater than the phase velocity c/n of the emitted light. This result led to the belief that emission of light by a free electron always needs relativistic electrons. This is not exact. The electromagnetic field generated by an electron can be thought of as being made up of evanescent wave whatever may be its velocity. It is correct that a part of evanescent waves can be converted into propagating waves by refraction processes only when the Cerenkov condition is fulfilled. However, even when that condition is not fulfilled (v<c/n), a part of the non-radiative field of the electron can also be converted into propagating waves by using optical processes other than refraction. By using subwavelength scattering centres the Cerenkov condition can be overpassed exactly as the Rayleigh criterion is overpassed in near-field microscopy. Some applications of this effect are discussed.