Switchable THz Reflectors (PDF)

THz technology which covers the frequency range between 100 GHz and a few THz is placed between optical technology and a high speed electronics. To date, the best method of generation and detection of the THz waves combines both microwave and optical techniques. Ultrashort optical pulses from a femtosecond laser are used to excite short but broad-band transient currents in the photoconductive micro-strip antenna. Within the last two decades, THz technology has experienced tremendous development. Currently a state of the art THz spectrometer based on an ultrafast Ti:Sapphire laser is already well developed and is a commercially available product. But still, due to the high cost of the THz system there are only a few real commercial applications of THz waves. Therefore, alternatives to the most expensive component of the spectrometer, the Ti:Sapphire laser, are needed. Passive components, which are necessary to establish a mature technology are also missing. Switchable mirrors, modulators, filters and switches are essential for the future THz communication and still need to be developed.

In this Thesis a novel concept of a switchable narrow-band THz reflector is presented. The development process included a number of tasks ranging from improvement of existing THz spectrometers, introduction of novel algorithms for THz spectroscopy, exploration of new material systems to the design and practical implementation of the switchable mirrors. All these tasks that often had to be performed in parallel are described in the following chapters. Chapter 2 describes the state-of-the-art THz spectrometers based on the Ti:sapphire fs laser. This pulsed and broad-band THz sectrometer is a very convenient tool for material characterisation, but other examples of its applications are here also discussed. The pioneering contribution to the THz technology presented in this Chapter is the introduction of novel THz spectrometers based on less expensive and more stable pulsed lasers lasing at 1550 nm and 1060 nm which are the telecommunication standard and industry standard, respectively. Communication using electromagnetic waves is usually carried out using narrow-band continuous wave radiation rather than pulsed and broad-band waves. Therefore in Chapter 3 an alternative method for the generation and detection of continuous wave THz radiation based on the photomixing phenomenon is described. Typically, such cw spectrometers radiating a single frequency offer a much higher resolution than pulsed systems and a more accurate tool for the characterisation of the THz devices dedicated for telecommunication systems. Unfortunately, the photomixing phenomenon is less efficient and such systems require impractical bolometeric detectors or very sensitive antennas with interdigitated structures. Due to the fragility of these highly efficient antennas, there is no commercially available cw THz system so far. Here, the world first all room-temperature cw THz spectrometer based on cheap and robust antennas without any fragile structures is developed. Prior to the design and practical implementation of novel THz devices an accurate material characterisation has to be performed. In Chapter 4 novel and the most accurate data processing algorithms for pulsed spectroscopy of single and multi-layer structures is presented. Chapte 5 is concentrated on the characterisation of different liquid crystal types from one rod-like family in the THz range. This very interesting material is commonly used in the optical range, but in the far infrared its properties are poorly explored. Finally, an ultimate goal of this Thesis is presented in Chapter 6, which is a novel concept for switchable THz reflectors based on one of the investigated liquid crystals.