Terahertz (THz) waves constitute radiation with frequencies around 10^(12) Hz, lying between the microwave and infrared regions. For a long time, the THz range has been one of the less explored areas of research in contemporary physics, owing to the fact that physical principles used in both the higher and the lower frequency ranges no longer work in the THz range, a phenomenon known as the "Terahertz gap". Although progress over the recent decades has considerably reduced the Terahertz gap, the technology in the THz region is not yet sufficiently developed for practical applications because of a lack of efficient, inexpensive, and easy-to-use sources and detectors operating in this range.

Nowadays, THz science is an actively developing research area, with still many open questions. THz radiation could have numerous technological applications, ranging from non-damaging methods of visualising cancerous tissue in medicine over the detection of hidden weapons and illegal drugs in security applications to ultrafast data communication beyond the era of 5G wireless networks. This showcases the wide range of opportunities that THz science provides and highlights the need for the development of devices operating in the THz range and for research on the physics of THz interaction with various structures and materials, including semiconductor nanostructures and quantum systems.

This is the topic that this thesis is dedicated to. In this work, I demonstrate a highly sensitive THz detector based on a new physical principle of operation which was called the "in-plane photoelectric effect", show the realisation of the Talbot effect in the THz range for focusing of THz radiation using waveguides, and describe the work towards developing devices bridging the terahertz and visible/near-infrared frequency ranges, such as THz-to-Optics interfaces based on quantum dots.

In chapter 1, a brief overview of state-of-the-art terahertz technology is given. A number of sources, detectors, and measurement systems operating in the THz range are reviewed.

To carry out the subsequent measurements, an experimental setup is required that makes it possible to study electrically contacted samples at liquid helium temperatures with simultaneous optical and terahertz access. For this purpose, I designed and constructed a unique system, described in chapter 2, which solves this ambitious task using cryogenically compatible terahertz waveguides. The waveguide system has a high transmission and exhibits a number of advantages compared to free-space setups. It enables the determination of the frequency of the THz source and allows accurate measurements of the power, as well as of the spatial distribution of the intensity and electric field polarisation directly at the position of the sample.

As part of the characterisation of the experimental setup, the THz mode profile (i.e., the spatial intensity distribution) at the end of the THz waveguide system was measured. The results revealed highly focused radiation with a complicated pattern. Its exact origin was unknown, since a comprehensive theory describing propagation after the end of cylindrical waveguides was lacking in literature. To explain the observed mode profiles, I developed a ray-optical theory of cylindrical multimode waveguides, described in chapter 3. It predicts the beam profile both within the waveguide and in the free space after its end. Remarkably, this theory also predicts that a waveguide itself can be used to focus radiation, without any additional lenses or parabolic mirrors. I show that the waveguide can be understood as an interferometric system and that its output beam profile is a "two-dimensional interferogram" of the input beam. It contains information about the input electric field, such as its direction of polarisation and the angles of angular misalignment of the source.

To check the predicted focusing effect, I fabricated waveguides of the geometrical sizes expected to yield optimal focusing. Measurements of the beam propagation at the waveguide end showed the expected pattern, confirming the theory and proving the focusing effect of multimode cylindrical waveguides. At terahertz frequencies, this effect combines the advantage of low losses of multimode waveguides with the ability of free-space setups to focus radiation to a tight spot. Physically, the phenomenon is related to the Talbot or self-imaging effect, and the constructed waveguided experimental setup represents its first practical realisation in circular waveguides at THz frequencies.

In chapter 4, I describe the design, fabrication, and measurements of a THz detector based on a two-dimensional electron gas (2DEG). It is based on a novel antenna-coupled, dual-gated device architecture. After simulation of the antenna design, the fabrication procedure is developed and successfully processed samples are demonstrated. Then the journey of searching for and finding a THz response is described. After a number of improvements of the setup, I demonstrate a highly efficient direct detector of THz radiation, that shows a giant photocurrent and photovoltage response.

The origin of the THz photoresponse was unclear. The electron transport in the 2DEG was analysed in a variety of aspects and additionally characterised using supporting measurements of the Hall effect and Shubnikov-de Haas oscillations in a magnetic field. The experimentally measured THz response was compared with existing theories, and it was found that they could not explain the observed effect. For example, the demonstrated THz detector exhibits a response more than an order of magnitude larger than expected from the common interpretation by the classical plasmonic mixing mechanism. I found that the observed phenomenon is due to a new, quantum-mechanical effect, which I called the "in-plane photoelectric effect". This phenomenon, which is described in chapter 5, is observed under conditions where the conventional, three-dimensional photoelectric effect cannot be observed and has a number of crucial features that make it superior to the three-dimensional photoelectric effect. In collaboration with a theorist, a quantitative theory was developed, which describes the measured data very well. The experimental results demonstrate the great potential of the in-plane photoelectric effect, which makes it possible to create a new class of highly sensitive, ultrafast THz detectors. Theoretically, the effect is expected to provide efficient THz detection across the entire THz range.

In chapter 6, I describe the work carried out in parallel towards studying the interaction of THz radiation with optically active semiconductor quantum dots. These small, quasi-zero-dimensional objects are often called artificial atoms and can be used as a "box" to store one or several electrons. The spin of the captured electrons represents a unit of quantum information – a qubit. Thus, quantum dots can be used as a physical implementation of quantum memory. In the area of quantum computation, qubits in quantum dots are initialised, manipulated and read out using infrared or visible light. However, to date, there are no reports where all-optically controlled quantum states were manipulated by THz photons. If THz photons could be used to switch optically written and read states of a quantum dot, this would represent a quantum interface between THz and optics – a breakthrough in the field of quantum computing and THz technology. This challenging task requires careful experimental preparation to ensure optimal coupling of the dots to both THz and optical (visible/near-infrared) photons. For this purpose, various material systems and different growth methods of quantum dots and quantum dot molecules (coupled pairs of quantum dots) have been studied together with a molecular beam epitaxy grower and checked for their suitability for this task. In addition, I have proposed and simulated a bandstructure that aims to enable all-optical readout of the quantum states over a wide range of both positive and negative applied electric fields.

Finally, in chapter 7 the main results are summarised, and an overview of potential future research directions is given.