Metasurfaces for wave conversions

Abstract

Since ancient times people were fascinated by science. In the beginning, the discoveries were a part of learning the nature phenomenon and the way the world works. Electromagnetic waves represented by the visible spectrum were one of the wonders which attracted philosophers and motivated them to learn and explore. Such wave transformations as reflection from the mirror and focusing of light into a beam by a parabolic mirror were described a long ago. Later on, more "unnatural" discoveries were made: People were trying to create structures which would behave in previously unknown ways. Could anyone imagine that at some time the scientist will be able to break the ordinary geometrical way of light reflection and re-direct it in any direction? This dissertation is devoted to wave transformations by metasurfaces including metasurface-inspired structures. Metasurfaces are two-dimensional artificial materials created as arrangements of subwavelength inclusions. The inclusions are engineered to manipulate electromagnetic waves in the desired manner. The development of this area brings exciting opportunities to enhance known devices and to create new ones. The first part of the thesis introduces the reader into the world of two-dimensional structures developed to date and the place of metasurfaces in it (or the other way around, the place of two-dimensional structures in metasurfaces). The second and third parts are dedicated to two types of wave manipulations in reflection regime: propagating-to-propagating waves transformation and surface-to-propagating waves conversion. The former consists of two segments. The first contains a theoretical review of propagating waves transformations using metasurfaces described by the generalized reflection law. A novel reflector antenna with a subwavelength focal distance is presented as an example of implementation of the method. The second segment introduces a novel approach to synthesis of metasurface-inspired gratings for perfect propagating wave transformation. This method allows reaching unprecedented conversion efficiency in practical realizations. The latter part at first presents a theoretical review of known structures which perform surface-to-propagating wave conversion (leaky-wave antennas) and their operation principle. Further, a novel concept for conversion with maximized efficiency is introduced for different types of propagating waves including the polarization transformation case. The results are of fundamental importance for further development and enhancement of leaky-wave antennas.