Tunable Terahertz Components on Substrateless Silicon Platform

Abstract

The integration and portability of terahertz systems is vital for practical applications in sensing and communications. In recent years, a substrateless silicon platform based on effective-medium-cladded waveguides has emerged as a promising pathway for realising diverse active and passive components monolithically combined for terahertz integrated systems. This platform is realised using high-resistivity silicon that has exceptionally low loss to terahertz waves. On the basis of the effective medium concept, a subwavelength hole array created into such a silicon slab results in a homogeneous material with a refractive index lower than that of solid silicon. As a consequence, controlling the array configuration yields a high contrast of refractive indices between a solid silicon waveguide core and cladding made of effective medium, while entailing design flexibility for various components. Inheriting characteristics from its constituting high-resistivity silicon, the platform has demonstrated remarkably low attenuation in broad bandwidth. Within this platform, diverse passive components can be created, such as filters, waveguide crossings, 2D horn antennas, or frequency- and polarisationdivision multiplexers. However, this platform is yet still constrained by the absence of tunable components, thereby limiting its potential for a broader range of applications. This thesis focuses on the development of efficient tunable terahertz components on the substrateless silicon platform for applications requiring sensing, switching, and modulation. One contribution involves, a series of disk resonators that are integrated onto the substrateless silicon platform to achieve a high-Q factor. Photoexcitation of selected areas of the silicon-based platform populates free carriers, resulting in the switchability of the resonance. Leveraging this photoexcitation effect, integrated disk resonators are employed to introduce switching functionality. The proposed switch achieves low insertion loss and directional switching capabilities. Moreover, the switch is monolithically integrated with a Luneburg lens on the same platform to function as a beam switching antenna. In terahertz communications, achieving efficient control over the modulator of terahertz waves is beneficial. We have proposed the integration of a photonic crystal cavity and a dipole resonator into the substrateless silicon platform to create an optically tunable terahertz modulator. This modulator has shown enhanced modulation depth with high efficiency and low required optical power. The proposed tunable components on the substrateless silicon platform showcase the potential to serve various functions, including sensing, switching, wave routing, and modulation. This contributes to a promising pathway for the development of future terahertz integrated systems.