From basic to advanced: design, fabrication and characterization of functional Terahertz devices

Abstract

Terahertz (THz) radiation is an emerging research field with a broad range of potential applications in cross-disciplinary fields spanning material science, pharmaceutical industry, security and safety for screening drugs and explosives, wireless communication in some windows of the atmosphere that allows covering the “last mile”. This frequency range is located inside the electromagnetic spectrum between the microwaves and infrared optics ranges. However, the absence of powerful sources and sensitive detectors have generated the wording “THz gap”. This gap is being progressively filled and now THz sources and detectors with significant enhanced performance becoming available. In this scenario, the ability to control the properties of THz waves is a primary issue. As in optics, also in the THz regime novel devices able to control some features of the radiation like polarizers, beam splitters, filters, lens, amplitude and phase modulator, waveplates, diffractive optics, and so on are needed. However, in order to give a real boost to the progress of the THz frequency range, these components should be realized at an affordable cost. For these reasons, the driving concept of the work presented in this research activity is the carry out the entire cycle of development of these devices, starting from their design and its optimization, following their fabrication by means of low cost techniques and finally characterizing them in the THz operating frequency range. Chapter 1 introduces the main features of THz radiation, describing the systems for generating and detecting the THz waves, its application in various fields and a detailed study of the potential market. Moreover, a survey on the properties of various materials to be used as substrates is reported. After a critical evaluation, the best trade-off material is identified in the low cost, low-loss, and mechanically stable cyclo-olefin polymer Zeonor®, manufactured by Zeon Co. This material is available in thin flexible foils from 13 μm to 188 μm, thick rigid plates and pellets. In order to achieve our aim, in this dissertation all the proposed components were designed and fabricated on thin flexible foils having thickness of 40, 100 and 188 μm. In chapter 2, we demonstrate wire grid polarizers with extremely low insertion losses and high extinction ratio fabricated on 40 and 100 μm thick Zeonor® substrates by means of standard photolithography techniques. The general design and rules are illustrated by conducting a systematic parametric study on the relevant geometrical parameters. Moreover, one of the proposed polarizers is characterized in two different bending configurations maintaining its high-performances. Chapter 3 and Chapter 4 focus on two novel types of bandpass filters constituted by an aluminium layer opportunely patterned on the available Zeonor® substrates. In particular, the filters proposed in Chapter 3 are based on the Frequency Selective Surface (FSS) cross-shaped apertures design. For this class of filters, an extensively numerical study and experimental characterization both at normal and oblique incidence are presented. Moreover, the influence of the polarization angle of the impinging THz wave on the devices spectral response is discussed. As in case of the polarizer, the performance of the filters is evaluated in bending conditions as well. The investigation of these filtering components reveals the manifestation of a secondary effect, known as guided mode resonance (GMR) that shows up in presence of a substrate of finite thickness. This interesting concept is the core of Chapter 4. There, we report the fabrication and characterization, also at oblique incidence, of bandpass filters based on this concept that exhibit high transmittance and high quality factor. For all considered filters, their spectral response lays in the two THz communication windows. Thanks to the flexible substrate employed for their realization, the proposed polarizers and filters can be fabricated with large area electronics and/or roll-to-roll techniques. Finally, in Chapter 5, we present a low-cost, easy-to-fabricate, and high-quality mechanical tuneable Fabry-Perot filters, made by a simple stacking procedure of Zeonor® thin films. The spacing between successive Zeonor®/air layers is readily controlled by using a simple bi-adhesive tape. The general design and rules are reported. All the presented devices, exhibiting outstanding performances, can open the route for a novel class of flexible and conformal devices operating in the THz frequency range.