The escalating demand for high-performance, low-profile transceivers in 5G/6G networking and broadband satellite internet necessitates innovative wireless communication hardware. Metasurface-based antennas, artificial surfaces engineered to manipulate electromagnetic waves, offer a promising solution. Leaky-wave antennas (LWAs) are particularly attractive due to their high directivity, low profile, simple feeding, and inherent beam-scanning capabilities. However, traditional LWAs have limited control over radiation parameters like aperture size, scan angle, and polarization state. This limitation hinders their ability to meet the growing demands for data capacity, channel diversity, and energy efficiency in modern wireless infrastructure. Metamaterials and metasurfaces offer a pathway to overcome these limitations. These artificial structures, composed of subwavelength meta-atoms or meta-units with engineered electromagnetic responses, allow for unprecedented manipulation of fields and waves. Previous research has explored LWAs using Huygens' metasurfaces, cascaded impedance sheets, and arrays of resonant dipoles, each offering different tradeoffs in beamforming degrees of freedom (DoFs) and design complexity. This research explores the emerging field of "nonlocal" metasurfaces, which harness mutual coupling among meta-units to fully customize electromagnetic wave interactions. This work leverages the principles of quasi-bound states in the continuum (q-BICs) to design advanced nonlocal leaky-wave metasurface (LWM) antennas with fully controlled radiation features. The q-BIC framework, originating from spatial symmetries, provides a rational design scheme that links the four DoFs of a monochromatic wave (amplitude, phase, orientation, and polarization ellipticity) to four independent geometric DoFs within the meta-unit. This approach contrasts with conventional methods that often require extensive numerical optimization or look-up tables. The research demonstrates three LWM antenna prototypes showcasing advanced functionalities for improved wireless communication.

Leaky-wave antennas (LWAs) have been extensively studied and utilized in various applications, including wireless communication, imaging, radar detection, and remote sensing. Traditional LWAs, however, possess limited degrees of freedom in controlling radiation characteristics. Recent advancements in metamaterials and metasurfaces have led to the development of more sophisticated LWA designs. These designs incorporate various approaches, such as the use of Huygens' metasurfaces to achieve independent control over amplitude, phase, and polarization, cascaded tensorial impedance sheets for simultaneous control, and discrete arrays of non-interacting resonant dipoles for simpler designs. The field of nonlocal metasurfaces, exploiting mutual coupling among meta-units, offers a promising avenue for advanced control. Bound states in the continuum (BICs), particularly symmetry-protected BICs, have garnered significant attention due to their ability to confine resonant modes despite the availability of radiation channels. Breaking these symmetries leads to quasi-BICs (q-BICs), enabling precise control over wavefront, polarization, and emission spectra. This concept has been successfully implemented in integrated photonic LWMs. This paper extends the q-BIC framework to the micro/millimeter-wave spectrum, leveraging the advantages of printed circuit board (PCB) manufacturing for increased flexibility and the integration of advanced wave launchers.

The study utilizes a parallel plate waveguide (PPWG) perforated by a staggered array of rectangular slots as a platform to demonstrate the principles of q-BIC-based LWM antennas. The shape and orientation of the slots are tailored to control the energy leakage in a controlled manner. The parameters A (amplitude), φ (phase), ψ (longitude of polarization), and χ (latitude of polarization) are independently controlled by manipulating the slot geometry. The control of radiation magnitude is achieved by introducing a period-doubling symmetry-breaking perturbation, transforming an artificial BIC (a-BIC) into a leaky q-BIC with a finite radiative Q-factor. The degree of asymmetry in each meta-unit is individually controlled to pattern the amplitude profile. Control of the linear polarization state is achieved by rotating the slots, breaking the mirror symmetries. Control of the aperture phase is implemented by introducing different perturbation strengths to the upper and lower rows of the meta-unit, allowing independent control of in-phase and quadrature components. A set of design equations relates the four DoFs of the radiated wave to the geometric DoFs (perturbation strength and rotation angles) of the meta-unit. These equations allow for rational design of LWMs with various slot topologies and physical implementations. Three prototypes are designed and fabricated: a SIMO LWM lens focusing to two orthogonally polarized spots, a MIMO lens focusing to different locations depending on the excited port, and a multi-beam LWM antenna with dual-polarized far-field beam shaping. Full-wave numerical simulations using COMSOL and ANSYS HFSS are performed for design validation and performance prediction. Experimental measurements using a near-field planar scanner and a far-field measurement setup are conducted to verify the functionality of the prototypes. The simulations considered factors such as lossy dielectric substrates and realistic feeding networks. Fabrication was done in-house using LPKF prototyping systems on Rogers RO3003 substrates.

The research successfully demonstrated the feasibility and advantages of using q-BICs for designing advanced LWM antennas at micro/millimeter-wave frequencies. The key findings include: 1. **Pointwise Control of Radiation:** The researchers achieved independent and precise control over the amplitude, phase, and polarization state of the radiated fields by strategically breaking the symmetries in the meta-unit design. This control was experimentally validated. 2. **Single-Input Multi-Output (SIMO) Focusing:** A SIMO LWM antenna prototype was successfully demonstrated, generating two spatially separated near-field focal spots with orthogonal circular polarizations from a single input. Both simulation and experimental results showed good agreement, with observed focal spots located near the designed locations. Frequency scanning of the focal spots was also achieved. 3. **Multi-Input Multi-Output (MIMO) Focusing:** A MIMO LWM antenna prototype was demonstrated, achieving focusing to different near-field locations when excited from different ports. Simulation and experimental results showed distinct focal spots for each port excitation. The inter-channel isolation (signal-to-crosstalk ratio) was measured, indicating satisfactory performance although it suggested that improvements in launcher-to-metasurface transitions could further enhance isolation. 4. **Dual-Polarized Multi-Beam Antenna:** A multi-beam LWM antenna capable of generating multiple independent beams with customized directions and polarizations was designed and experimentally verified. The measured radiation patterns closely matched the simulation results in terms of realized gains and beam directions. Although the radiation efficiency was 44%, this can be significantly improved by optimizing design parameters. The research demonstrated a design methodology based on q-BICs that is significantly simpler and more rational compared to conventional metasurface design techniques, avoiding the need for cumbersome numerical optimizations or extensive look-up tables. The proposed design equations provide a direct mapping between the desired radiation characteristics and the geometric parameters of the meta-unit, which allows for the design of more sophisticated meta-units without substantial changes to the design process.

The findings address the research question by demonstrating the capability of nonlocal leaky-wave metasurface antennas to achieve unprecedented control over radiated fields. The results show that the q-BIC framework enables a rational design approach that overcomes the limitations of traditional LWA designs. The successful realization of SIMO and MIMO focusing, along with dual-polarized multi-beam capabilities, showcases the significant potential of this technology for advanced wireless communication systems. The achieved level of control over radiation parameters (amplitude, phase, and polarization) opens opportunities for high-throughput smart radio environments, improved satellite communication, and advanced radar systems. The demonstrated flexibility and design simplicity of the q-BIC approach offer a significant advantage over conventional methods, potentially accelerating the development and deployment of next-generation wireless technologies.

This work successfully introduced a novel class of LWM antennas based on q-BICs, demonstrating the ability to achieve full control over amplitude, phase, and polarization of radiated fields. The simplicity and rationality of the proposed design method, coupled with the experimental validation of advanced functionalities, showcase the potential of this technology for next-generation wireless systems. Future work could focus on improving efficiency, expanding to two-dimensional aperture engineering, employing multiple orthogonal q-BIC modes, and exploring applications in the terahertz regime.

While the study demonstrated impressive results, some limitations should be noted. The prototypes were quasi-1D, consisting of seven repeated columns of meta-units, which may have introduced edge effects affecting performance. The use of a simplified model for preliminary simulations of the SIMO and MIMO LWMs could have contributed to discrepancies between simulated and measured results. The radiation efficiency of the multi-beam LWM was 44%, although this could be improved with design optimization. Furthermore, the accuracy of the design formulae is highest near broadside, and it may be necessary to incorporate higher-order multipole terms to achieve accurate results at oblique angles.