# Your search: "author:Capolino, Filippo"

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## Scholarly Works (209 results)

With the proliferation of high-speed communications and energy-aware electronic systems worldwide, efficient electromagnetic (EM) radiation sources remain amongst the most critical components. The desired properties of EM oscillators include a precise oscillation frequency, high output power, and high power-generation efficiency to preserve energy and battery life, besides low phase noise and small form factors. Boosting the performance of microwave and optical sources in terms of enhanced efficiency and coherence is a fundamental goal and a very active research area. This dissertation focuses on a new class of EM devices and sources, whose architecture relies on dispersion engineering principles exploiting a new operational condition: the so-called exceptional points of degeneracy (EPD).

An exceptional point of degeneracy (EPD) in the state space of a dynamical system refers to condition at which two, or more, physical eigenstates coalesce into one, and bifurcate as a parameter is varied. In this dissertation, a comprehensive investigation of the use of EPDs in microwaves and optics is undertaken, stimulated by their remarkable physical properties.

First, a coupled transmission line theory is established which provides access to novel dispersion engineering concepts at radio frequencies. Moreover, experimental observation of the degenerate band edge (DBE), which is a fourth order EPD, is also demonstrated in metallic waveguides as well as microstrip lines at microwaves even in the presence of loss and tolerances. Furthermore, new paradigms for coherent light-matter interaction are proposed, resulting in a low-threshold degenerate band edge lasers. Such lasers eliminate the need of cavity mirrors for light confinement, and possess a mode-selection scheme that ensures a single frequency lasing.

We further harness the notions of EPDs to conceive highly-efficient electron-beam-driven oscillators. In traveling wave tubes and oscillators, an electron beam is synchronized with the EM radiation in a slow-wave structure causing amplification. A super synchronous regime is established when the electron beam transfers energy to multiple degenerate waves supported by the structure causing strong amplification. As a result, we demonstrate a design of an electron-beam-driven degenerate band edge oscillator capable of generating mega-watts of microwave power with efficiency up to 60% and compatible with realistic experimental setup.

With wide adaption of 5G technology just around the corner, and ever-increasing shift of daily life and activities into the mobile world, every facet of an RF communication system becomes a critical component in its design, with oscillator being one of the most important ones. Oscillators in today’s world must have an impeccable properties such as stable oscillation frequency, high efficiency, low power consumption, as well as a small form factor to fit into the ever-shrinking but yet more densely packed devices. In our modern mobile devices, it is important for an oscillator to be as low-power consuming as possible; long battery life being one of the biggest selling points for any mobile device. This dissertation’s main focus is on a new class of electromagnetic/RF devices, primarily oscillators, which rely on dispersion engineering, and more precisely on phenomena known as Degenerate Band Edge (DBE). When electromagnetic propagation eigenmodes of a waveguide coalesce into one in the state space of a system, a special point in the parameter space of the waveguide is formed and is known as an Exceptional Point of Degeneracy (EPD). Degenerate band edge (DBE) refers to a special case where an infinitely long and lossless system supports four propagating eigenmodes that, given proper operational condition, can coalesce. In practice, since we cannot have an infinitely long and lossless system, operation of a device near this condition can lead to unique and beneficial properties such as enhanced quality factor and giant gain enchantment, both properties being an integral part of highly efficient and stable oscillator. First, a relatively simple, double-ladder, lumped element circuit is introduced that can support DBE, which will serve as the basis of a first variation of DBE oscillator. Comparison to a well-known single-ladder oscillator shows that a double-ladder oscillator exhibits all around better performance despite its relatively more complex structure. Then, the idea of DBE oscillator is further explored and is actually realized in a microstrip technology. We demonstrate its measured performance and compare it to simulations, showing that the behavior is very predictable and controllable due to the DBE phenomenon. Finally, the high-Q structure is used as a pulse generation device, where it is first fed with energy until it reaches steady-state and then this energy is quickly extracted to produce a narrow in time pulse.

A reconfigurable reflectarray antenna with beam steering is presented in the thesis. The reflectarray system includes a reconfigurable reflecting surface and a feed antenna. The reflecting surface of the reflectarray antenna is composed of a periodic arrangement of unit cell elements where each cell has an embedded varactor for reconfigurability. The capacitance of varactors varies with the applied reverse bias voltage and it is used to control the phase distribution on the reflecting surface for realizing a reconfigurable system. By tuning the capacitance of the varactors, a reflected-phase tuning range of 300◦ is achieved by the proposed reflectarray element. The feed antenna of the reflectarray system is designed using a microstrip patch antenna with air gap included to increase the gain. The feed antenna has maximum gain of 9.8 dB and the main beam is set to be directed at the reflecting surface center. A mathematical method for feed radiation model is presented to analyze the aperture efficiency and directivity of the reflectarray system. The far-field radiation pattern result is obtained from a full-wave simulation using the commercial software CST Studio Suite. We demonstrate that the reflectarray system generates beam radiation along three different directions, as a proof of beam steering capability. The Full-wave result of the far-field simulations show that the maximum directivity is 18.5 dB which agrees favorably with the theoretical analysis.

Rapid development in wireless communication systems at microwave and millimeter-wave frequencies increases the demand for more efficient wideband and high-gain electromagnetic radiator capable of supporting the high-speed data transfer and mitigate the free-space path loss. Fabry-Pérot cavity (FPC) antennas are a potential candidate as radiators since they exhibit useful radiation characteristics, such as high-directivity of broadside beam with high radiation efficiency from a low-profile structure, and feature simplicity of design and low-cost fabrication. This two-dimensional (2-D) class of leaky-wave antennas (LWAs) are conventionally constructed with a thin frequency selective surface (FSS) (metallic patches or holes in a metallic screen), forming a partially reflected surface (PRS), placed half of a wavelength from a ground plane and excited with a single source.

This dissertation focuses on the modal and radiation analysis of FPC antennas formed by an electrically thick PRS (single or multiple metal-dielectric layers) optimized for wideband radiation. A novel set of formulas is analytically derived, which relates the leaky-wave parameters (phase and attenuation constants of the traverse complex wavenumber) to the PRS reflection coefficient and input admittance. Additionally, we derived a new leaky-wave based power formula that is capable of describing the far-field behavior for any FPC antenna formed by an arbitrary thick PRS. The formulas are validated with different examples of wideband FPC antennas constructed using single-layer and multiple-layers of PRS structures. Moreover, we show how to efficiently radiate circularly polarized (CP) waves from a wideband FPC antenna excited by a single CP source. Furthermore, a study was carried out to further enhance the broadside gain with a sparse array implemented as a primary excitation source.

Additionally, we have investigated the radiation performance of an extremely thin (100th of a wavelength) 2-D leaky-wave antenna constructed from a high impedance surface (HIS) directly excited to work as an antenna. Physical insight is provided by analyzing the radiation mechanism of this class of HIS antennas and prove that radiation is in part related to a TM-like leaky-wave supported by the structure in the vicinity of its magnetic resonance. Wideband broadside radiation, in addition to a beam steering capability, was demonstrated. Finally, a HIS antenna with a differential feeding network was designed and fabricated for K-band wireless systems.

Periodic structures have been utilized in many novel active devices due to their unique properties such as the presence of electromagnetic band-edges and band-gaps. Dispersion diagrams are associated to such structures with a unit cell repeated periodically and show the relation between frequency and periodic state phase shift or the eigen-states of the system. The dispersion characteristics of periodic structures can be engineered to exhibit exceptional modal characteristics, which can be exploited to design novel devices with improved features and enhanced performance. Distinct sorts of degeneracy may exist in periodic schemes where the eigen-states of the system coalesce and form a single degenerate periodic eigen-state. These phenomena can be classified as exceptional points of degeneracy (EPD) and is shown to have some fascinating features which make them desirable for a wide variety of applications including oscillators, amplifiers, lasers, and pulse compressors which are widely used in IoT scenarios. A special and well-known category of degeneracies in periodic structures is recognized as the fourth order degenerate band edge (DBE) where four periodic eigenstates coincide at the edge of the Brillouin zone. In this work we put the focus on two other significant types of EPDs which are the stationary inflection point (SIP, with third order degeneracy) and the sixth order degenerate band edge (6DBE). In particular, the 6DBE characteristics are expected to be noteworthy in enhancement of the Q-factor and in lowering the oscillation threshold in finite length resonators with gain compared to conventional uniform or homogeneous structures. Furthermore, a novel amplification regime based on special dispersion characteristics of an SIP, also called “frozen mode regime”, is proposed leading to a higher gain and larger gain-bandwidth product compared to conventional Pierce-type traveling wave tubes. Moreover, a novel design of a periodically coupled three-way microstrip waveguide is presented utilizing PT-glide symmetry by proper balancing of the loss/gain featuring third order EPDs. This latter concept could be advantageous in diverse applications including but not limited to distributed amplifiers and radiating arrays of antennas. The first physical realization and experimental demonstration of an exceptional point of sixth order (6DBE) in a triple ladder (three-way) microwave waveguide is provided; the unique properties of the 6DBE may be exploited in designing innovative high-Q resonators, oscillators, filters, and pulse shaping devices. An in-depth investigation of the modes and dispersion relations based on the state vectors and transfer matrix formalism is offered where we have also provided the analytical framework for the Puiseux fractional power series expansion of the system’s eigenvalues around the degeneracy condition that will be used to approximate the dispersion relation, density of states (DoS), and other important parameters. The analytical framework and physical concepts established in this thesis may be applied to a variety of structure designs and applications featuring degeneracy conditions of different orders and can be a useful tool in designing and evaluating novel passive and active devices.

Operation mechanism of many novel sensors is based on the detection of splitting of resonant frequencies. Recently emerged topic known as exceptional points of degeneracy (EPD) paves the way to engineer structures where they exhibit highly sensitive frequency splitting phenomenon. An EPD is a special point in the system parameter space at which at least two eigenmodes coalesce in both their eigenvalues and eigenvectors. The unique properties of higher order EPD can provide means of enhancing the frequency shift as they would increase the effect of perturbation which should lead to higher sensitivity. In this dissertation, we propose a circuit comprised of three RLC coupled resonators with balanced loss and gain that exhibits a third order EPD and investigate the conditions for a third order EPD to occur in such circuit. We validate the existence of the EPD by examining the behavior of the system at that special point. We finally illustrate the time domain response of such system operating at regular frequency (not an EPD frequency) and also at a third order EPD frequency where we show the quadratic growth of the system eigenstates in time.