Spatial Transform Design and Application of Gradient Index Lens Antennas

Abstract

The relentless evolution toward sixth-generation wireless systems has created an urgent demand for advanced technologies capable of precise wavefront manipulation at millimeter-wave and terahertz frequencies. While gradient-index lenses represent a promising passive platform for this purpose, their practical application is constrained by foundational challenges in achieving functional versatility and manufacturability. This dissertation addresses these critical gaps by introducing novel design methodologies and demonstrating their efficacy in a complete, high-performance system.

First, this work presents a truncated Maxwell fish-eye meta-lens to overcome the functional limitations of classical gradient-index architectures. This strategy resolves the critical issue of boundary-confined dual foci inherent in generalized Maxwell fish-eye lenses, enabling the generation of high-performance, steerable dual beams in the far field. The design is enhanced by an integrated Klopfenstein taper anti-reflection layer, which suppresses reflection losses and improves beam separation. An experimental prototype operating at 28 GHz validates this approach, achieving an exceptional measured null depth below -50 dB between the beams.

Second, to mitigate the fabrication barrier posed by the high refractive index requirements of conventional transformation optics, we introduce a new design paradigm termed approximately quasi-conformal mapping. By strategically relaxing the strict local angle-preserving constraints of quasi-conformal mapping, the approximately quasi-conformal mapping method significantly reduces the maximum required permittivity of transformation optics-based devices—for instance, from ~6.0 to 2.8 in a comparative case—without a substantial compromise in optical performance. The power of this method is demonstrated through the design and experimental validation of a near-omnidirectional scanning meta-lens at 240 GHz, fabricated on a silicon wafer using deep reactive-ion etching.

Finally, bridging the gap between component innovation and practical application, this thesis presents an integrated, multi-beam mmWave energy harvesting system. This system synergistically combines a modified Luneburg lens, designed using the approximately quasi-conformal mapping principle, with a 58-port dual-polarized antenna array and a novel, simplified DC power-combining network. The fully integrated prototype achieves a maximum realized gain of 27 dBi over a 130° field of view and demonstrates the ability to harvest over 4 mW of power from two ambient sources to power a low-consumption electronic chip successfully.

Collectively, this dissertation establishes a comprehensive framework for designing and realizing next-generation gradient-index meta-lenses. The developed methodologies and system-level demonstration provide a tangible pathway toward advanced applications in wireless communication, high-resolution sensing, and sustainable power for the Internet of Things.

Date of Award	2 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwai Man LUK (Supervisor)