Planar Antennas and Lenses for Terahertz Source Integration

Abstract

The terahertz frequency band shows promise in various applications, yet the limited output power from terahertz sources has hindered the development of practical terahertz applications. Therefore, to preserve the precious terahertz power, the design of efficient antennas and lenses becomes crucial. Traditional antennas and lenses are often bulky, lossy, and expensive. Hence, there is an urgent need to explore planar solutions for highly efficient antennas and lenses tailored specifically to the terahertz range. These solutions should be cost-effective, capable of significantly reducing the device profile, supporting various beam shapes, and minimizing additional losses. Furthermore, these solutions should have the ability to integrate with sources for system compactness. To this end, this thesis focuses on innovative approaches to design terahertz planar antennas and lenses with integrated sources. In a first approach derived from microwave technologies, planar metallic antennas are integrated with a terahertz resonant-tunneling diode, with extended bias lines allowing the propagation of surface waves. Instead of eliminating these surface waves, which might lead to energy waste and compromised efficiency, planar metallic antennas are employed to effectively control the propagating surface wave, resulting in distinct radiation characteristics. Two types of planar metallic antennas are explored: one approach involves a series-fed patch array antenna designed to enhance radiation performance, while the other approach utilizes a log-spiral antenna for circularlypolarized radiation. In a second approach derived from optical technologies, planar lenses can be realized through various phase control techniques, resulting in specific output phase distributions for different applications. For a metasurface, modifying the geometry of a unit cell allows control of the phase response. Metasurface lenses can achieve various types of beams based on the designed phase profiles. As an alternative, adjusting the density of air holes etched into a silicon substrate controls the effective refractive index, and therefore controls the phase response. Effective medium lens can achieve a uniform output phase distribution for high-gain performance. Likewise, adjusting the height of 3-D printed dielectric pillars controls the phase response. A 3-D printed lens with random pillar heights can achieve a random output phase distribution and uncorrelated radiation patterns across the frequency range. As a non-planar design, this type of lens offers distinct radiation patterns that can be utilized for direction-of-arrival estimation. The antennas and lenses proposed in this doctoral thesis offer cost-effective, highly efficient, low-profile, and source-integrable solutions, which are expected to support practical terahertz applications in sensing and communications.