PROFESSOR PAVLOS LAGOUDAKIS
Network of Excellence: Nanophotonics for Energy Efficiency: 2010-2014
The Nanophotonics for Energy Efficiency aims to create a virtual centre of excellence to reorient and focus nanophotonics research towards the challenges in energy efficient applications. The network clusters nanophotonic laboratories and research groups in Europe combining their expertise in the development of disruptive approaches to lighting and solar cell technology. The consortium consolidates know-how and resources of 9 different institutions in 6 European countries with complimentary research and development expertise, integrating more than 130 scientists, engineers, technicians and managers in nanophotonics.
Project partners:
1. Institut de Ciències Fotòniques, Spain
2. Technische Universitaet Dresden, Germany
3. University of Southampton, UK
4. Commissariat Energie Atomique, France
5. Laboratorio Europeo Di Spettroscopie Non Linear, Italy
6. Consejo Superior De Investigaciones Cientificas, Spain
7. Bilkent University, Turkey
8. Kungl Tekniska Högskolan, Sweden
9. Universitat Politecnica De Catalunya, Spain
ITN-Icarus:2010-2013
Our key aim is to develop hybrid semiconductor materials that support entirely new optical states. In many cases, such excitations arise from a direct hybridization of organic and inorganic states using a confined cavity-photon to ‘mediate’ the excitations. Such systems will necessarily have optical and electronic properties that cannot be found in either material system alone. We believe that this will permit us to develop materials having entirely new applications in photonics, electro-optics and photovoltaics. Our methodology is distinctly different to work performed on the development of semiconductor materials for optoelectronics. Here, the vast majority of research has centered on the development of technologies based largely on organic or inorganic semiconductors. This division of effort has established two parallel yet largely non-interacting research communities. The establishment of ICARUS is in direct response to this division of effort, with the central goal of our network being methods to control energy transfer in hybrid semiconductor systems, driven by the desire to create new types of light emitting devices (LEDs), lasers, photodetectors, and photovoltaics.
Project partners:
1. University of Sheffield, Physics and Astronomy
2. Foundation for Research and Technology Hellas, Institute of Electronic Structure and Laser
3. IBM Ltd., Zurich Research Lab
4. Scuola Normale Superiore di Pisa, Classe di Scienze
5. Politecnico di Milano, Physics Department
6. Imperial College, Department of Physics
7. Ludwig Maximilians University of Munich, Department of Physics
9. University of Southampton, Department of Physics
ITN-Spinoptronics:2009-2013
Spin-optronics is a new emerging research area, which combines studies of spin and optical polari-sation effects in solids with the ultimate goal of creating quantum optoelectronic devices. This is an interdisciplinary research field at the crossroads of fundamental physics of quantum-mechanical spin, optoelectronics, and nano-technology. Most of the recent developments in spin-optronics and prospects for future applications are based on the last decade’s spectacular progress in nanotech-nology. Quantum confinement of the particle motion in one or more directions drastically changes the spin-orbit interaction and correspondingly all spin properties. All three main themes of the Network research activities – growth & technology, spectroscopy and theory - are concentrated on novel spin and light polarisation effects in nanostructures, taking advantage of con-finement of not only charges and spins, but also photons.
Project partners:
1. University Blaise Pascal, LASMEA
2. CNRS&University Joseph Fourier, Institut Neel
3. INSA-CNRS-UPS, LPCNO
4. University of Sheffield, Dept of Physics and Astronomy
5. University of Southampton, School of Physics and Astronomy
6. University of Exeter, School of Physics
7. Toshiba
8. Ioffe Institute, Center of Nanoheterostructure
9. Universidad Autónoma de Madrid, Física de Materiales y Física Teórica de la Materia Condensada
10. Dortmund University, Experimental Physics
ITN-CLERMONT-4:2009-2013
The primary goal of the CLERMONT4 network is to facilitate the exploitation of the breakthroughs in polaritonics which occurred in 2006-2008 by leading European industrial groups. To exploit the huge potential of polaritonics, we are planning to educate and train a new generation of physicists and device engineers able to conduct research and its application in this new area and to implement the ambitious theoretical concepts of polariton devices in practice. We shall focus on realisation of four prototypes of polariton devices: electrically pumped polariton lasers, micron size optical parametric oscillators, optical logic gates and cavity-based emitters of entangled photonic pairs. These devices would open a new époque in optoelectronics bringing quantum coherent effects into the everyday life. In order to realise these goals we have built a consortium of academic teams which have already given to Europe an enormous lead in the international competition with American and Japanese groups to realize practical polariton devices. Furthermore, we bring these academic teams together with an outstanding group of industrial partners capable of effectively driving through the translation of emerging promising new physical demonstrations into devices.
Project partners:
1. University of Rome II, Physics department
2. Centre National de la Recherche Scientifique, Délégation de Languedoc Roussillon
3. Universite Paris VI, Ecole Normale Supérieure
4. University of Sheffield, Physics and Astronomy Department
5. University of Southampton, Physics and Astronomy Department
6. University of Cambridge, Cavendish Laboratory
7. Durham University, Department of Physics
8. Universidad Autónoma de Madrid, Depts. Física de Materiales y Física Teórica de la Materia Condensada
9. Foundation for Research and Technology - Hellas, Institute of Electronic Structure & Laser
10. Ecole Polytechnique Fédérale de Lausanne, Institut de Photonique et d’Electronique Quantiques
CURRENT EPSRC SUPPORT
Engineering polariton non-linearity in organic and hybrid-semiconductor microcavities (EP/G063494/1)
Strongly-coupled microcavities are a fascinating system for the exploration of the fundamental physics of the interactions between light and matter. Under such circumstances, the emissive states in such microcavities are termed 'polaritons', and can be described as an admixture between an exciton and a confined cavity photon. The optical properties of polaritons can be very different from their constituent parts (excitons and cavity photons), and thus there is a significant opportunity to explore new fundamental processes, and develop new types of devices that may find applications as low-threshold lasers, optical-amplifiers and high-speed optical switches.
At present, the majority of work done on the strong-coupling regime in microcavities has centred on structures that contain inorganic semiconductors (either III-V, II-VI or GaN based materials). We have however pioneered the study of strong-coupled microcavities containing organic (carbon-based) semiconductors, which are anticipated permit new effects to be engineered. Despite the importance of organic-semiconductors in a range of optoelectronic devices (LEDs, photovoltaics, FETs, lasers etc) relatively little is understood regarding the microscopic processes that occur in strongly-coupled organic microcavities.
Development of a basic understanding of non-linear processes and properties of organic-semiconductors in strongly-coupled microcavities will thus be a key area that we will address in this project. Key components of the research include studies the interactions between organic-polaritons and vibrational modes of the molecular semiconductor and the generation of organic exciton-polaritons at high density following electrical injection of carriers. We will also explore the fabrication and optical properties of 'hybrid-semiconductor' microcavities and devices (containing organic and inorganic semiconductors), and will study optically-driven energy-transfer between the different types of excitation using both linear and ultra-fast measurements.
We are confident that our work will provide new fundamental insights into the optical properties of organic-polaritons (including relaxation and condensation), the transfer of excitations between different semiconductor materials via a cavity photon over large distances (> 100 nm) and the generation of new electrically-driven polariton devices. We believe that we are in an excellent position to undertake such an ambitious programme of research due to our world-leading expertise in strongly coupled organic semiconductor microcavities (Sheffield), and two-colour ultra-fast spectroscopy of microcavities (Southampton).
Femtosecond semiconductor lasers (EP/G059268/1)
The aim of this proposal is to demonstrate for the first time a semiconductor laser emitting transform-limited optical pulses of less than 200 fs duration in a diffraction-limited beam. This achievement will open the way for the development of truly compact ultrafast optical systems. Our device is a surface-emitting laser, optically pumped using the cheap and rugged technology developed for diode-pumped solid state lasers, with perfect beam quality enforced by an extended cavity. It emits a periodic train of ultrashort pulses at a repetition rate of a few GHz using the optical Stark effect passive mode-locking technique introduced by the Southampton group. Recent proof-of-principle experiments have shown that these lasers can generate stable 260-fs pulse trains. We have shown, moreover, by modelling and by experiment, that the optical Stark mechanism can shorten pulses down to durations around 70 fs, comparable with the quantum well carrier-carrier scattering time. Our proposal is to build on these world-leading results with a systematic exploration of the physics of lasers operating in this regime. The key is to grow quantum well gain and saturable absorber mirror structures in which dispersion, filtering and the placing of the quantum wells under the laser mode are controlled to tight tolerances. We shall achieve this using molecular beam epitaxy to realise structure designs that are developed with the aid of rigorous numerical modelling of the optical Stark pulse-forming mechanism. We shall also use femtosecond pump and probe spectroscopy to determine the dynamical behaviour of our structures in this regime directly. For these pioneering studies, the compressively-strained InGaAs/GaAs quantum well system operating around 1 micron is most suitable; and this is where we shall work; however, the devices that we develop can in principle in future be realised in other material systems in different wavelength regions. We shall also make a first study of incorporating quantum dot gain and absorber material into optical Stark mode-locked lasers, aiming to exploit the intrinsically fast carrier dynamics of these structures. In summary, this proposal aims to shrink femtosecond technology from shoebox-size to credit-card size, and in the process explore a regime of ultrafast semiconductor dynamics that has never before now been exploited to produce light pulses.
Spin currents and superfluidity of microcavity polaritons (E/F026455/1)
The overall goal of the project is to detect experimentally exciton-polariton superfluids and spin currents in microcavities and to develop a full quantum theory of exciton-polariton superfluidity. The fingerprints of polariton superfluidity will be searched for in spatially- and directionally-resolved optical measure-ments with spectral, temporal and polarization-detection, with or without application of external magnetic fields, on improved quality strain free microcavity samples. We shall look for conventional and superfluid polariton spin currents in the regime of the optical spin Hall effect. We expect theoretically important differences between polariton and conventional superfluids caused by a peculiar dispersion and spin structure of exciton-polaritons. We aim to study theoretically and experimentally the polarization dynamics of both resonantly and non-resonantly excited polariton condensates to reveal the specifics of polariton superfluid-ity and search for new effects including the optical spin-Hall effect and the spin analogue of the Meissner effect.
Actively manipulating electronic excitations in nanocrystals (EP/F013876/1)
Colloidal nanocrystals made of semiconductor materials resemble fluorescent beads that are only a few nanometres in diameter. Their optical emission properties can be tuned from ultraviolet to infrared wavelengths by suitably choosing the material and adjusting their size and shape. To date, nanocrystals have been exploited in areas ranging from genomic and proteomic bio-assays, cell-staining and high-throughput screening, where they serve as fluorescence markers and more applications have been envisaged in LEDs, lasers, optical switches, photovoltaics, data storage devices, catalysis, drug delivery and other biomedical assays. Compared to self-assembled quantum dots made by molecular beam epitaxy, colloidal nanocrystals can be produced by comparatively simple and inexpensive solution methods, and are freely suspended in a solvent or matrix, while retaining a high optical and electronic stability. The precisely controlled size and shape of nanocrystals, such as in quantum dots, rods or even tetrapods, renders them promising building blocks for nanoscience and nanotechnology. Furthermore, shape control in the synthesis of colloidal nanocrystals offers unprecedented abilities to tune the interaction of solid state quantum structures with the environment, opening up the possibility of performing nanoscale manipulations of the optical and electronic properties. This 'First Grant' proposal aims for key experimental studies on the fundamental properties of colloidal nanocrystals. The overall plan is to develop novel applications based on the active manipulation of the optoelectronic properties of nanocrystals and on self-assembly methods for their alignment in large array device configurations. The ultimate applications range from electric-field nanosensors, single photon tunable sources to optical memory elements and all optical parallel processing.
DOCTOR T. G. M. FREEGARDE
Laser trapping, cooling and sensing of atoms and molecules with nanostructured surfaces (EP/E058949/1)
The study and manipulation of atoms and molecules has until recently nearly always been performed upon mobile and energetic species. Yet, as in so many fields, measurement and manipulation could be performed with far greater precision and finesse if the subject were confined and immobilized. Despite many techniques which focus on the slowest species, most measurements and virtually all reactions of atoms and molecules are performed on thermal distributions. The consequences for fundamental studies, processing and sensing are a finite interaction time and a moving, randomly orientated sample. Laser tweezers and Doppler cooling techniques use the radiation pressure exerted by a stream of photons to slow and capture a limited range of atoms. Pinned down and virtually stationary, the atoms can be examined and manipulated like never before. Deterministic quantum mechanics dominates their behaviour; collisions are reversible; and molecules can be formed and then broken with exquisite remote control. Even in these early days, a wide range of technological exploitations has been proposed, from metrology to sensing and information processing. Unfortunately, only single, tiny traps are usually possible, and, because the cooling process only works with a limited range of species, most atoms and all molecules that enter the trap retain enough kinetic energy to leave shortly after. We propose to use nanofabrication techniques, developed in Southampton, to produce arrays of concave mirrors whose foci, when illuminated with a laser, will each become a tiny trap. Such arrays offer to store many more species than a single trap, and each trap can easily be distinguished under a microscope. Confining species within a few wavelengths of a surface allows new interactions and techniques that increase the trap strength and enhance the sensitivity with which the species may be detected and observed. The extent and proximity of the surface also presents exciting new mechanisms for cooling a far wider range of species than previously possible. This research will investigate a range of trapping and cooling geometries, with the ultimate aim of extending to molecules the control currently limited to atomic samples of just a small number of elements.
Cavity-mediated cooling using nanostructured surfaces (EP/E039839/1)
Cavity-mediated cooling has emerged as the only general technique with the potential to cool molecular species down to the microkelvin temperatures needed for quantum coherence and degeneracy. The EuroQUAM CMMC project will link leading theoreticians and experimentalists, including the technique's inventors and experimental pioneers, to develop it into a truly practical technique, reinforcing European leadership in this field. Four major experiments will explore a spectrum of complementary configurations and cavity-mediated cooling will be applied to molecules for the first time; a comprehensive theoretical programme will meanwhile examine the underlying mechanisms and identify the optimal route to practicality. The close connections between theory and experiment, and between pathfinding and underpinning studies, will allow each to guide and inform the others, ensuring that cavity-mediated cooling is swiftly developed as a broad enabling technology for new realms of quantum coherent molecular physics and chemistry. The Southampton component will address, both experimentally and theoretically, fundamental aspects of the cooling process that result from the retarded interaction of a trapped molecule with its reflection in a single mirror, and developments of this prototype scheme that exploit nanostructured mirror arrays that can be produced in our fabrication facilities, and which show both geometric and plasmonic resonances. Our particular aims are hence to understand and explore the most basic version of cavity-mediated cooling, and to develop new implementations suitable for nanoscale integration as a future technology.
Optical cooling and coherent manipulation of atoms and molecules (GR/S71132/01)
The optical manipulation of molecules requires new schemes of coherent manipulation to overcome the complications introduced by their rovibrational level structure. Broadband interactions are needed to access multiple levels, and spontaneous emission must be minimized to reduce population migration. Starting with atomic rubidium in a magneto-optical trap, and progressing to photo-associated rubidium molecules, we shall explore various schemes of coherent manipulation, including deflection, amplification of Doppler cooling, interferometric cooling and 'algorithmic' cooling using our proposed momentum-state quantum computer. The optical manipulation is in each case is achieved via stimulated Raman transitions using a phase-controllable modulated c.w. laser source. These experimental studies will complement on-going theoretical analyses, and will ultimately be combined with advanced dipole-force trap geometries. This research thus explores a broad range of novel manipulation schemes that will allow the advantages of laser cooling and trapping to be extended to molecules of scientific and technological interest. The ultimate applications range from fundamental physics and reaction chemistry, to directed chemical synthesis, nanofabrication and quantum information.
PROFESSOR A. KAVOKIN
CURRENT EPSRC SUPPORT
Polaritonics for Quantum Technology Applications (EP/K007173/1)
This project is aimed at development of several revolutionary new device concepts within the new interdisciplinary research area of Polaritonics, which studies interaction of light with electronic excitations in crystals. PI of this project leads an international collaborative effort in Polaritonics since 2000 coordinating three subsequent European network projects which involve over 15 universities. This collaboration has already yielded a number of spectacular discoveries including the Bose-Einstein condensation of mixed light-matter quasiparticles, features of superfluidity in crystals, spin multistability, ultralow threshold polariton lasing. Development of Polaritonics is led by theory, which predicts new effects, designes new structures and proposes new experiments. This project proposes a program of theoretical research tightly linked with an international experimental effort and aimed at bridging the gap between fundamental discoveries and device applications in Polaritonics. We shall develop and realise and experimentally test several device concepts including the Aharonov-Bohm polariton interferometer and the vertical cavity coherent terahertz light source based on a polariton laser. Fabrication of these devices would manifest a technology breakthrough in opto-electronics with quantum coherent effects brought into everyday life. The new quantum light sources and logic gates are important for realisation of optical computers and enhancement of capacities of optical communication lines. Compact sources of coherent terahertz radiation have their applications in medicine (skin cancer cure), environment protection, industrial sensing, security. The Southampton university will coordinate a multinational research effort in Polaritonics, host leading experts in Polaritonics for short visits, organise a series of international workshops and closely collaborate with several industrial companies including Hitachi, Sharp, Toshiba.
Spin currents and superfluidity of microcavity polaritons (EP/F026455/1)
The overall goal of the project is to detect experimentally exciton-polariton superfluids and spin currents in microcavities and to develop a full quantum theory of exciton-polariton superfluidity. The fingerprints of polariton superfluidity will be searched for in spatially- and directionally-resolved optical measure-ments with spectral, temporal and polarization-detection, with or without application of external magnetic fields, on improved quality strain free microcavity samples. We shall look for conventional and superfluid polariton spin currents in the regime of the optical spin Hall effect. We expect theoretically important differences between polariton and conventional superfluids caused by a peculiar dispersion and spin structure of exciton-polaritons. We aim to study theoretically and experimentally the polarization dynamics of both resonantly and non-resonantly excited polariton condensates to reveal the specifics of polariton superfluidity and search for new effects including the optical spin-Hall effect and the spin analogue of the Meissner effect.
Quadron-polariton: a new semiconductor quasiparticle (EP/F011393/1)
Contrary to real particles whose basic properties (mass, charge, spin) are stable and may be determined once and forever, the properties of quasiparticles propagating in crystals can be tuned by a proper design of crystal structures and different external factors (heating, illumination of the crystal by light, application of electric or magnetic fields). This allows for creation of new kinds of quasiparticles by relatively simple means. This project proposes a new quasiparticle which has never been observed nor theoretically described before. We call it quadron-polariton. Quadron because it is composed by four elementary quasiparticles (three electrons and one hole) and polariton because it is created by light passing its polarization to the crystal. The quadron-polaritons are expected to have truly remarkable properties. They carry a negative charge equal to two electron charges. They have an effective mass which is approximately 10000 times lighter than the free electron mass. They are spatially extended over a few wave-lengths of light. Finally, they have an integer spin and obey the statistics of Bose-Einstein therefore. That is why, we expect that quadron-polaritons should be able to form a superfluid, i.e. roughly speaking, to unify their energies and velocities. Theoretically, because of the very light mass of quadron-polaritons, their condensation may happen even at room temperature in specially designed crystal structures. The superfluid of quadron-polaritons would be charged, that is why its creation is expected by us to lead to the phenomenon of superconductivity (circulation of current without any applied voltage). Observation of light-induced superconductivity would be an extraordinary discovery also having a considerable economic effect. We are going to pave the way towards this observation by complex theoretical and experimental studies of quadron-polaritons in ultrathin artificial crystal structures.
DOCTOR H. ULBRICHT
CURRENT EPSRC SUPPORT
Nonclassicalities and Quantum Control at the Nanoscale (EP/J014664/1)
While the quantum behaviour of atomic-scale objects is no surprise, it would be absolutely arresting to find the same weird features being displayed by much more macroscopic objects. As quantum physics underpins much of our everyday technology, the importance of stretching its domain of applicability can hardly be overemphasized. For example larger systems (easier to control), could play a key role in quantum information processing.
Recently, a number of new methods have become available to probe the quantum nature, in other words the "nonclassicality", of nanoscale objects. One of the foremost is the interference of freely moving objects in which one of the Co-Is is an expert. Another is an early idea by the PI to probe superpositions with confined (stationary) nanoscale objects by controlling them with an auxiliary quantum system. While such schemes are yet to be realised, they have suddenly started to look quite feasible in view of a clever idea by one of the Co-Is, namely to optically levitate such objects, which largely isolates them from their environments and prevents decoherence -- a phenomenon that causes the irreversible demise of quantum features.
In the above backdrop, we propose a project that aims at coupling spins to nanoscale objects to control their quantum motion and perform complementary tests of the nonclassicality of free and trapped mesoscale objects. Theory led by the PI and a Co-I, both experts in somewhat complementary areas of quantum optics and information, will outline the appropriate strategies for the above experiments, as well as explore the exploitation of these systems for the eventual benefit of quantum information processing. As opposed to other world-wide efforts that we are aware of, we will avoid both extensive cooling and preparing high quality optical cavities. This strategy is expected to give us significant competitive advantage in probing several quantum attributes for which the above are not really necessary. An experimental Co-I in spin manipulation will enable us to levitate a spin bearing nano object and couple the spin to its motion. The presence of expert Co-Is in both interference and levitation is going to enable us to access two promising yet complementary techniques of probing the macroscopic limits of quantum mechanics with the same or similar objects. Significant milestones for levitated objects include probing the validity of the superposition principle and quantum commutation relations for these systems, single shot spin readout through their motion, their entanglement and their potential as quantum walkers and registers for quantum computation. For free objects, we plan to enhance the mass of objects in interferometry by several orders of magnitude, perform tomography of their highly nonclassical states during interferometry, as well as perform precision spin measurements through the interferometry of spin bearing nano particles. The feasibility of more challenging experiments for the future will also be explored within the project, such as a Stern-Gerlach interferometry to probe superpositions of free objects and the usage of a levitated object as a mediator for entangling spins. The ultimate ramifications of the project are expected to be in two directions: the fundamental question of whether there are any limits to the validity of quantum principles when one applies them to nanoscale objects, and the applied issue of the usage of such systems in information technology. Such research is also expected to raise public interest in science by highlighting the counterintuitive quantum behaviour of macroscopic systems.
PROFESSOR J. RUOSTEKOSKI
CURRENT EPSRC SUPPORT
Cavity optomechanics: towards sensing at the quantum limit (ep/h049568/1)
Nanostructured Photonic Metamaterials (ep/g060363/1)
Detection and dynamics of ultra-cold atoms in optical lattices (EP/F022204/1)
When atoms are cooled down to very low temperatures their thermal motion almost completely stops. The development of methods to trap and cool atoms be means of laser light and magnetic fields has provided tools to reach the lowest known temperatures in the Universe. These are within one billionth of a degree of absolute zero. At very cold temperatures the wave functions of the atoms start overlapping and they become indistinguishable. The bosonic atoms undergo the Bose-Einstein condensation, representing a new form of matter, predicted by Bose and Einstein almost a century ago. The Bose-Einstein condensates form a coherent source of atoms analogous to optical lasers; the resulting atom lasers are as different from ordinary atomic beams as optical lasers are from light bulbs. When the Bose-Einstein condensates are placed in periodic potential arrays formed by lasers, known as optical lattices, they behave like electrons in crystal lattices. However, unlike in crystal lattices, in optical lattices there are no lattice imperfections and the lattice height and the periodicity can be easily engineered. In optical lattices the atoms can behave like electrons in superconductors and could potentially be, e.g., the building block of a next generation quantum computer. The expected research outcomes are the means to observe, manipulate and control cold atoms by light, to further the basic understanding of quantum atomic gases and to influence the experimental progress with trapped atoms. The potential applications are in precision measurements, such as in the development of improved time measurements using atom clocks in satellite navigation.
Interaction of Light with Quantum Degenerate Atomic Gases and Atom Lasers (GR/A00780/01)
Cooling and trapping of dilute atomic gases by means of optical laser beams and magnetic fields has provided the first evidence of Bose-Einstein condensation with well-understood interactions. Bose-Einstein condensates form a coherent source of atoms analogous to optical lasers; the resulting atom lasers are as different from ordinary atomic beams as optical lasers are from light bulbs. Furthermore, in a cold atomic gas, with many atoms per cubic wavelength of light, the interactions between light and matter are strongly coupled. In a coherent atomic gas light may become localised; the atomic gas behaves as an optical insulator. Cold atomic gases of fermions may form bound pairs of particles analogous to superconductivity in metals. The aim is to study theoretically optical and coherence properties of cold atomic gases. The goal is to further the basic understanding of light-matter interactions and coherence properties of atom lasers. The expected outcomes are the means to observe, manipulate and control cold atoms by light and to develop more sophisticated models of atom lasers. The potential applications are in high-resolution measurements, for instance, in satellite navigation due to the high-accuracy clocks.
DOCTOR S. DE LIBERATO
CURRENT EUROPEAN SUPPORT
Bilayer Graphene Exciton Polaritons (IEF/239413WF)
The aim of this project is to develop a quantum theory capable of describing strong light-matter coupling of excitons in gapped graphene bilayers with a mid-infrared or terahertz microcavity photon mode. The mixed excitations resulting from the strong coupling, grapheme exciton polaritons (GEP), will thus be hybrid excitations, half graphene exciton, half microcavity photon. Not only graphene interacts extremely well with light (with an absorbance around 5% for a single bilayer) but excitons in graphene bilayers have been shown to be characterized by an extremely large oscillator strength, due to the 1D nature of their joint density of states. This makes of GEP an outstanding candidate for the observation of non-perturbative ultrastrong coupling regimes and for the realization of pioneering optoelectronic devices, also thanks to the outstanding transport properties of graphene.
DOCTOR V. APOSTOLOPOULOS
CURRENT EPSRC SUPPORT
Quantum Cascade amplifiers for high power Terahertz time domain spectrometry (EP/J007676/1)
Currently, most commercial THz spectrometers are time-domain spectrometers (TDS), where THz pulses are generated on antennas by a photocurrent created from a pulsed laser. The detection scheme uses a similar antenna where carriers are generated by the same pulsed laser. The advantage of this apparatus is that the detection scheme is synchronous: the receiver is only "on" when the THz electric field is incident, and this results in a very high signal-to-noise ratio of approximately 50 dB. The disadvantage is that the THz pulses have only micro-Watts of output power, thus the apparatus will only penetrate thin or transparent materials. The major competing THz technology is that of quantum cascade (QC) lasers, which generate radiation with tens of mWatts of power. However, the power advantage of QC lasers is lost by the lack of sensitive detection techniques, and hence they are not used commercially. Until now researchers have tried to combine the technologies of THz-TDS and QCs but the two geometries have proven very difficult to integrate, with antenna emitters in particular proving incompatible with integration.
However, a new geometry emerged in 2010: the so-called the lateral photo-Dember effect that can be used to generate broadband THz pulses. The effect is quite simple, relying on the different mobilities of holes and electrons in a semiconductor which create a changing dipole under photoexcitation to generate THz pulses. We believe that this effect has great potential because it is flexible and its geometry is compatible with integration and quantum cascade lasers. Using the lateral photo-Dember effect will provide an elegant means of coupling a THz pulse into the QC structure, directly, with great efficiency. We intend to exploit this effect and generate THz pulses directly on the facet of a QC cavity and amplify them in the QC waveguide. Therefore we will combine the high output power of quantum cascade lasers with the detection sensitivity and broadband nature of state-of-the-art time-domain technology. It is a game-changing approach that is, according to all indications, absolutely feasible. It is very rare to propose such a potentially high impact research route, which is at the same time such low risk!
Quantum Cascade Amplifiers
Most commercial THz spectrometers are time domain spectrometers; they exhibit a very high signal-to-noise ratio (50 dB) but quite low power (few micro-Watts). The major competing THz technology is that of quantum cascade (QC) lasers, which generate radiation with tens of mWatts of power. However, the power advantage of QC lasers is lost by the lack of sensitive detection techniques, and hence they are not used commercially. Until now researchers have tried to combine the technologies of THz time domain spectrometers and Quantum cascade lasers (QC) but the two geometries have proven very difficult to integrate, with antenna emitters in particular proving incompatible with integration.
However, a new THz emitter geometry emerged in 2010, the lateral photo-Dember effect, which can be used to generate broadband THz pulses and can provide an elegant means of coupling a THz pulse into the QC structure, directly, with great efficiency. Our task is to combine the high output power of quantum cascade lasers with the detection sensitivity and broadband nature of state-of-the-art time-domain technology.
The project is an EPSRC funded collaborative project between Physics at the University of Southampton, and Cavendish and Chemical Engineering at the University of Cambridge.
Novel Terahertz emitters.
The photo-Dember (PD) effect is a THz emission mechanism based on ultrafast carrier transport. THz radiation is produced by illumination of a semiconductor surface by an ultrafast near infrared laser with energy above the bandgap (usually Ti:S). The strong absorption of light near the surface creates a large carrier gradient of electrons and holes, which initiates a diffusion current. Because of the different mobilities, electrons and holes spatially separate on a picosecond time scale.
In the THz laboratories group we simulated the diffusion of the carriers in combination with a theoretical analysis of the emission of a dipole under a metal sheet. We have developed an emitter that works based on diffusion currents and the suppression of dipoles under a semi-infinite metallic sheet [1, 2]. We have experimentally demonstrated the emitters using semiconductor (GaAs) substrates. This mechanism indicates a novel method of generating THz radiation that is based on the diffusion current created by ultrafast radiation but also uses the inhibition of radiation due to a metal surface. This concept gives rise to design proposals for a series of emitters, which would give similar performance to a Photo-conductive antenna, which is currently the standard THz emitter. This work is funded by EPSRC for the development of a high power THz spectrometer based on PD emitters and THz lasers.
- G. Klatt, et al, “Terahertz emission from lateral photo-Dember currents,” Opt. Express 18, 4939–4947 (2010).
- M. E. Barnes, et al., "Terahertz emission by diffusion of carriers and metal-mask dipole inhibition of radiation," Opt. Express 20, 8898-8906 (2012)