Focusing infrared edge and synchrotron radiation with a large numerical aperture

Abstract

For a large numerical aperture, the focusing of long-wavelength edge and synchrotron radiation from an electron is modeled. The central obscuration of Schwarzschild optics is also considered. For a numerical aperture of 0.65, typical of an infrared microscope, the maximum energy density of the focused electric field is reduced by ∼50%, compared to paraxial focusing calculations that do not consider a central obscuration. With a large numerical aperture, some light rays strike the focal plane at grazing incidence, producing longitudinally polarized radiation. For a numerical aperture of one, focused edge and synchrotron radiation produce similar spots of longitudinally polarized radiation, whose diameter is about one-half wavelength. The small size and large radiation field of these spots may be useful in experiments that require localized radiation with polarization perpendicular to the sample surface.