An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection

Many mid- and far-infrared semiconductor photodetectors rely on a photonic response, when the photon energy is large enough to excite and extract electrons due to optical transitions. Toward the terahertz range with photon energies of a few milli–electron volts, classical mechanisms are used instead. This is the case in two-dimensional electron systems, where terahertz detection is dominated by plasmonic mixing and by scattering-based thermal phenomena. Here, we report on the observation of a quantum, collision-free phenomenon that yields a giant photoresponse at terahertz frequencies (1.9 THz), more than 10-fold as large as expected from plasmonic mixing. We artificially create an electrically tunable potential step within a degenerate two-dimensional electron gas. When exposed to terahertz radiation, electrons absorb photons and generate a large photocurrent under zero source-drain bias. The observed phenomenon, which we call the “in-plane photoelectric effect,” provides an opportunity for efficient direct detection across the entire terahertz range.George and Lilian Schiff Studentship, Schiff Foundation, University of Cambridge Honorary Vice-Chancellor’s Award, Cambridge Trust, University of Cambridg

Metamaterial/graphene active terahertz modulators

Within the last years there has been a tremendous thrust into research and technology in the THz spectral region (broadly defined as 0.1-10 THz) mainly driven by the unique potential where this radiation finds applications in, such as imaging, spectroscopy and communication. In all these fields a fast, integrated and versatile platform for modulating light is required. Metamaterial/graphene devices fulfill all these requirements as their subwavelength nature lends itself naturally to strong light-matter interaction, and therefore highly efficient and miniaturized devices. Graphene's unique properties, e.g. the large carrier concentration modulation, provide a large degree of compatibility with several architectures which can be exploited in a range of modulation or detection schemes. Finally, metamaterial/graphene devices realize a fast, versatile platform, which can be easily scaled to other frequencies, and adapted into amplitude, frequency, polarization and phase modulators, as well as integrated detectors, for the next generation of wireless-communication

An Antenna-Coupled Dual-Gated Electron Channel as Direct Detector of 2 THz Radiation

An antenna-coupled dual-gated two-dimensional electron gas (2DEG) based on a GaAs-AlGaAs heterostructure shows a pronounced response to 2 THz radiation. The device is shown to be a direct detector, and its photoresponse arises without any source-drain bias. The detection is based on a novel mechanism that yields a substantially stronger photoresponse than predicted by the classical plasma-wave self-mixing and other mechanisms

Interaction of Terahertz Radiation with Semiconductor Nanostructures and Quantum Systems

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.George and Lilian Schiff Studentship, Schiff Foundation, University of Cambridge Honorary Vice-Chancellor's Award, Cambridge Trust, University of Cambridge Semiconductor Physics Group, Cavendish Laboratory, University of Cambridg

Cylindrical Multimode Waveguides as Focusing Interferometric Systems.

Delivery and focusing of radiation requires a variety of optical elements such as waveguides and mirrors or lenses. Heretofore, they were used separately, the former for radiation delivery, the latter for focusing. Here, we show that cylindrical multimode waveguides can both deliver and simultaneously focus radiation, without any external lenses or parabolic mirrors. We develop an analytical, ray-optical model to describe radiation propagation within and after the end of cylindrical multimode waveguides and demonstrate the focusing effect theoretically and experimentally at terahertz frequencies. In the focused spot, located at a distance of several millimeters to a few centimeters away from the waveguide end, typical for focal lengths in optical setups, we achieve a more than 8.4× higher intensity than the cross-sectional average intensity and compress the half-maximum spot area of the incident beam by a factor of >15. Our results represent the first practical realization of a focusing system consisting of only a single cylindrical multimode waveguide, that delivers radiation from one focused spot into another focused spot in free space, with focal distances that are much larger than both the radiation wavelength and the waveguide radius. The results enable design and optimization of cylindrical waveguide-containing systems and demonstrate a precise optical characterization method for cylindrical structures and objects

Photoelectric tunable-step terahertz detectors: a study on optimal antenna parameters, speed, and temperature performance

Field effect transistors have shown promising performance as terahertz (THz) detectors over the past few decades. Recently, a quantum phenomenon, the in-plane photoelectric effect, was discovered as a novel detection mechanism in gated two-dimensional electron gases (2DEGs), and devices based on this effect, photoelectric tunable-step (PETS) THz detectors, have been proposed as sensitive THz detectors. Here, we demonstrate a PETS THz detector based on GaAs/AlGaAs heterojunction using a dipole antenna. We investigate the dependence of the in-plane photoelectric effect on parameters including the dimensions and the operating temperature of the device. Two figures of merit within the 2DEG, the maximum electric field and the radiation-induced ac-potential difference, are simulated to determine the optimal design of the PETS detector antenna. We identify the optimal antenna gap size, metal thickness, and 2DEG depth, and demonstrate the first PETS detector with a symmetric dipole antenna, which shows high-speed detection of 1.9 THz radiation with a strong photoresponse. Our findings deepen the understanding of the in-plane photoelectric effect and provide a universal guidance for the design of future PETS THz detectors.China Scholarship Council and Cambridge Trust: CSC Cambridge International Scholarship (Ran Chen) Trinity College Cambridge: Junior Research Fellowship (Wladislaw Michailow

A Terahertz Chiral Metamaterial modulator

Active control of chirality in artificial media such as metamaterials is fundamental in many scientific areas, ranging from research into fundamental optical phenomena to the investigation of novel materials, spectroscopy, and imaging. Precise control of the light polarization states has great importance for light‐matter interaction in chemistry and biology, as media with diverse chiral properties react differently to the incoming polarization of light. In this work an active double layer metamaterial device based on vertically stacked ring resonators is realized by integrating electrostatically tunable graphene as an active element. The device is characterized with a THz time domain spectroscopic system demonstrating an all‐electrical control of circular dichroism and optical activity at ≈2 THz, reporting a tunable ellipticity of 0.55–0.98 and >20° rotation of the plane polarization, respectively, by modifying the conductivity of graphene. Further integration with a narrow frequency quantum cascade laser emitting at ≈1.9 THz, in a crossed polarizer experimental arrangement, realizes an active amplitude modulator, hence highlighting the versatility of this approach. These results represent an important milestone for the investigation of novel concepts in optics and in several applications in the THz range, such as wireless communications and spectroscopy