Improving Focusing Properties of Gradient Annular Metal Gratings of Terahertz Range

The paper presents the results of studies to improve the focusing properties of gradient ring metal terahertz gratings. The ring gradient gratings consist of concentric metal rings, the distances between which vary in the radial direction. Gratings with the width of the rings and the distance between them less than half the wavelength are considered. Such ring structures located on flat substrates have the properties of lenses and spherical mirrors. They can be used as output mirrors for terahertz lasers. In this case, the laser radiation acquires azimuthal polarization. The focusing properties of such gratings can reduce diffraction losses in the cavity and partially compensate for the divergence of the laser beam. In a number of cases an increase in focusing properties of gradient ring gratings is required. To this end, it has been proposed to perform sectioning of gradient gratings according to the principle of Fresnel lenses. The paper discusses variants for such sectioning and presents the results of modeling the properties of sectioned gradient ring gratings.