Highly efficient terahertz metasurfaces
Creators: name: AKO, Rajour Tanyi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400899
subject: Electrical engineering not elsewhere classified
classificationCode: 401605
subject: Functional materials

description: Terahertz (THz) radiation, spanning 0.1 - 10 THz on the electromagnetic (EM) spectrum is gaining huge popularity due to its effectiveness at conditions where microwaves and optical radiations have strong limitations. It can enable higher data transfer rates of up to 1 terabits/s, that can support ultra-fast communication network beyond 5G. Moreover, with a low energy and unique wavelength, terahertz can penetrate through dry fabrics, foggy and dusty environment, and thus is fundament for functionalities, such as, standoff detection for safe screening, and faster and broadband ranging in vehicle-to-vehicle or satellite-to-satellite communication. Its unique wavelength is also widely tapped to probe vibrational modes in a wide range of biological and chemical molecules, otherwise not possible with visible light. Therefore, with growing trend in demand for miniaturization and huge data traffic flow, ultra-thin, more compact and efficient optoelectronic devices, such as, polarizers, lenses, absorbers, reflect arrays, modulators, switches, sensors, waveguides/connectors, etc, are highly desired.
 
The fabrication methods used to realize such devices are either limited or those often used induce more loses that poses a challenge to the advancement in terahertz technology. More so, the inherent attenuation in bulk natural materials, ohmic losses in metals, radiation losses in polymer and semiconductors, applied as dielectric layers or as resonators, require proper investigation.
 
This thesis explores the methods and limitations of fabrication techniques for realizing these terahertz devices. New techniques are proposed where focus is placed on the use of cyclic olefin co-polymer (COC), that has not seen wide application, due to fabrication limitations.
 
In the thesis COC is used in single layer devices, a reflective linear-to-linear polarization converter employing COC dielectric and subwavelength metallic resonators, is presented, highlighting a record conversion efficiency of 102%, that is stable to 45⁰ incidence angle. More so, a novel eutectic bonding fabrication technique is employed to realize a reflective half-wave and a reflective quarter-wave, polarization conversion devices using high resistive dielectric resonators on a gold metallic ground plane, showcasing a conversion efficiency of 53% and 49%, respectively, when excited by a linearly-polarized wave at 45⁰ incidence angle.
 
The usefulness of multilayer devices with COC spacers is also demonstrated in a three-layer, ultra-thin transmissive linear-to-linear polarization converter that exhibit a record operational band width of 133%, from 0.2 - 1 THz, at normal incidence. And another ultra-thin three-layer quarter-wave metasurface with a 3-dB axial ratio relative bandwidth of 53.3%, and conversion efficiency beyond 70.2%, spanning 205 to 340 GHz, is presented.
 
In all the study, finite element simulations using CST Microwave studio software package is used to optimize the model before fabrication.  Terahertz time domain spectroscopy and vector network analyser are used to study the Electromagnetic response on the fabricated sample.

The thesis offers a novel approach in realizing functional, free-standing and highly efficient terahertz devices, and the potential for a new paradigm of establishing devices in other frequency ranges as well.