High-speed terahertz signal control and read-out for future terahertz communication systems

Abstract

The realisation of next-generation terahertz (THz) communication systems necessitates fast and efficient THz signal control and readout, enabling the encoding of data streams onto THz-frequency signals and their subsequent decoding. This project aims to design, develop, and demonstrate THz detectors and modulators, and to study the physical principles of their THz interactions and optimise their performance based on experimental parameters. Chapter 1 provides a brief overview of state-of-the-art terahertz technology, reviewing various sources, modulators, detectors, and measurement systems operating in the THz range. Chapters 2 and 3 present devices that modulate the amplitude of THz radiation. The development of efficient, high-speed modulators with large modulation depth is essential for effectively control of terahertz radiation. This challenge is often addressed using metamaterials - artificial sub-wavelength optical structures engineered to resonate at the desired terahertz frequency. Metamaterial-based devices exploiting graphene as an active tuneable element have proven to be a highly effective solution for THz modulation. However, while graphene conductivity can be tuned over a wide range, it cannot be reduced to zero because of the gapless nature of graphene, limiting the maximum achievable modulation depth for single-layer metamaterial modulators. In Chapter 2, we demonstrate two novel solutions to circumvent this limitation. Firstly, we excite the modulator from the back of the substrate, and secondly, we incorporate air gaps into the graphene patches. This results in a ground-breaking THz modulator based on graphene-metal metamaterials, operating at 2.0 - 2.5 THz, demonstrating a 99.01% amplitude modulation and a depth of modulation of 99.99% at 2.15 THz, with a reconfiguration speed exceeding 30 MHz. Chapter 3 presents top-gate devices that operate with a lower bias voltage and higher modulation speed compared to back-gate devices. In back-gate devices, the gate voltage is applied across a p-doped silicon substrate and a 300 nm SiO2 insulating layer. The gate voltage required (approximately 10 V - 100 V) is too high for high-speed communication applications. Using a thinner insulating layer, the bias voltage needed to achieve a high modulation depth can be reduced. Another motivating factor in developing top-gate modulators is to increase the modulation speed. The operating speed is limited by the RC constant of the device, and improving the operating speed requires removing the resistance of a 550 µm p-doped silicon. The experimental data was obtained on three different graphene-metal metamaterial terahertz modulators. One of them was fabricated using UCAM CVD-grown graphene and operates at a centre frequency of 1.9 THz, the other was fabricated using graphene from a commercial supplier (ACS Material) and was designed for a centre frequency of 1.5 THz. Both devices achieve amplitude modulation in excess of 3 dB, respectively, with a modulation speed estimated above 800 MHz. Chapter 4 presents metasurface-based photoelectric tuneable-step (PETS) detectors. This work focuses on integrating PETS detectors as part of a metamaterial to enhance device performance. A device achieves a responsivity of 2.7 - 3.5 A/W, corresponding to a remarkable external quantum efficiency of 2.1% (with zero source-drain bias), with an optical NEP estimated at 0.4 pW/√ Hz at 1.9 THz. This represents a 33-fold improvement in external current responsivity compared to the first demonstrated PETS detector, which had 77 mA/W external responsivity. Furthermore, the output resistance of the device is about an order of magnitude lower, with two-contact resistance around 400-510 Ω, compared to 4-5 kΩ for previous single antenna PETS devices. Chapter 5 summarises the key results and provides an overview of potential future research directions.