Analysis and design of advanced radiating devices based on leaky waves

In recent years, millimeter-wave and THz applications of electromagnetics, such as near-field focusing, medical and security imaging, wireless power and information transfer, just to mention but a few, have gained much attention in the area of information and communication technology (ICT) due to their potentially high impact on social and technological development. On the one hand, the need of highly directive radiating devices with wideband or tunable radiating features in the far-field region at millimeter-wave or THz frequencies has recently boosted the research activity in the design of flexible, cost-effective, and low-profile radiators. On the other hand, the `chimera' of point-to-point wireless communications is to transfer the largest amount of information at the farthest distance, possibly with the slightest waste of power and resilience to obstacles. These needs call for devices capable of radiating focused beams in the near-field region (to minimize diffraction and thus waste of power) with as larger as possible cover distance at high frequencies (to maximize the available bandwidth and thus the information content). In this context, leaky-wave theory is an elegant and effective formalism, which allows for describing in a common fashion guiding and radiating phenomena in both the near- and the far-field region at any frequency. For this reason, this PhD thesis deals with the rigorous investigation (in diverse cases supported by experiments) of innovative radiating systems under the leaky-wave perspective. The proposed devices are designed to outperform the state of the art as concerns the bandwidth and reconfigurable performance of THz devices operating in the far-field (i.e., antennas) and to further enhance the focusing behavior and depth of field of microwave, millimeter-, and submillimeter-wave devices operating in the near field (i.e., launchers). Specifically, an innovative leaky-wave approach has been proposed for the analysis and design of planar, low-cost, and easy-to-implement devices working in the millimeter-wave frequency range to strongly improve the figure of merit (FoM) given by the product of the antenna gain and fractional bandwidth. This FoM enhancement could play a fundamental role for the implementation of these devices in advanced terrestrial and satellite communication systems. Moreover, the use of a relatively new and promising material, such as graphene, has been considered for the design of THz leaky-wave antennas. In particular, since ohmic losses in graphene notably hinder the efficiency of graphene-based THz devices, an innovative antenna based on hybrid metal–graphene metasurface has been proposed to mitigate this issue, achieving very interesting results in terms of reconfigurability, efficiency, and radiating capabilities. In addition to the intriguing capabilities offered by leaky waves in far-field applications, interesting focusing properties have been observed in the near-field region. In particular, it is shown that leaky waves can profitably be used to generate limited-diffraction field distributions by means of planar, single-feeder, and cost-effective radiators at millimeter-wave frequencies. The latter, commonly referred to as Bessel-beam launchers, can be implemented with either a compact or an electrically large structure thus featuring either a narrow or a wideband behavior, respectively. These devices are able to generate Bessel beams, which possess: i) a nondiffractive character, i.e., the capability to maintain constant their transverse section for several wavelengths from the radiating aperture; ii) a high resolution, namely a transverse spot size having linear dimensions on a wavelength scale; iii) a self-healing character, i.e., the capability to reconstruct themselves if an obstacle is put along the line-of-sight. These intriguing properties are usually preserved up to a finite distance, which is called nondiffractive range. By exploiting leaky-wave theory, the possibility to extend the nondiffractive range up to 50 times the radiator diameter has been demonstrated (typical values of the nondiffractive range at microwave and millimeter-wave frequencies usually do not exceed three or four time the radiating-aperture diameter), further increasing the potential of these radiating devices from a practical viewpoint. Still, in the context of Bessel-beam launchers, this PhD thesis also investigates, designs, and experimentally validates original and promising focusing systems working from microwave to THz frequencies. The performance of these devices is also predicted and experimentally validated in a wireless-power-transfer scenario by generating radiative near-field links with higher performance with respect to typical inductive and far-field links in terms of working distance and efficiency, respectively. As concerns the THz frequency range, this PhD thesis analyzes, design, and experimentally validates original THz detectors based on different kinds of photoconductive antennas and transistors. The obtained promising results pave the way for the implementation of future, fast, and accurate medical and security imaging systems.