POLATER · Polaritonic TeraHertz Devices

Polaritonic TeraHertz Devices

The realization of efficient terahertz (THz) radiation sources and detectors is one of the important objectives of modern applied physics. THz emitters and detectors have potential applications in biology, medicine, security and non-destructive in-depth imaging. In past decade, the public view of THz has been largely influenced by a number of articles in the press often citing its ability to image through clothing and detect explosives or weapons. However, the potential applications of THz light are far more wide ranging, and deep reaching, than this. The low photon energies associated with THz radiation (~meV) correspond to the thermal energy associated with a range of biological processes. Thus THz can provide a direct probe to reveal the fundamental properties and operation of life through, for example, the study of conformational changes in protein structures, protein hydration shells and DNA. The creation of inexpensive, reliable, scalable and portable sources and detectors of THz radiation is extremely important to its future exploitation. None of the existing THz emitters such e.g. gyrotrons, photomixing, carcinotrons and Free Electron Lasers, completely satisfy these application requirements. The unique position of the THz frequency range which is beyond the reach of conventional electronics and photonics attracts a broad range of approaches to bridging the THz gap. These approaches are based on a diverse range of physical principles. In this research programme, our consortium is undertaking research which explores the possibility of generating THz radiation in semiconductor microcavities in the regime of exciton polariton lasing, where a radiative THz transition between microcavity polariton branches becomes allowed - in the case of a special design of quantum well microcavity which provides mixing of bright and dark quantum well excitons. The originality of the method of polariton-based THz generation comes from the use of a new physical idea: that the THz photons are generated due to stimulated transitions of bosonic quasiparticles (exciton-polaritons) in a compact monolithic semiconductor heterostructure. This new basic physics allows one to formulate entirely new device concepts for both THz emitters and detectors. In general, the polaritonic approach is based on the engineering of the light-matter interaction in microcavity systems with a dense two-dimensional electron gas for the achievement of efficient THz optoelectronic emitters. The spontaneous emission in the difficult THz spectral range is extremely slow when compared to non-radiative processes, being responsible for quantum optoelectronic devices which unfortunately have low quantum efficiency. In particular, quantum cascade lasers based on intersubband transitions in doped quantum wells (please note: the only semiconductor lasers working in the THz range) currently operate at relatively low temperatures (less than 180K) with an efficiency of the order of a few percent even at cryogenic temperatures. The motivation of this research activity is to understand the physics required to obtain novel THz emitters working in the strong coupling regime between an intersubband transition of a two-dimensional electron gas and a semiconductor microcavity photon mode. The consortium members are: Durham University (UK), Haskoli Islands University (Iceland), University of Regensburg (Germany), FORTH (Crete), EPFL (Switzerland), St Petersburg Academic University (Russia) and the Ioffe Physical Technical Institute of Russian Academy of Science (Russia). The key objectives of the research, which is undertaken by seconded researchers, include: 1. Design and fabrication of planar and pillar microcavities for observation of THz emission; 2. Design and fabrication of THz emitters and lasers based on intersubband microcavity polaritons; 3. Design and fabrication of THz cavities; 4. Integration of polariton microcavities and THz cavities and experimental study of THz stimulated emission; 5. Non-linear THz emission. The project has produced a wide range of outputs which have included underpinning theoretical advances in the above areas. This has included the development of detailed theory and design of radiative THz transitions in quantum microcavities and THz intersubband polaritons; the fabrication of a set of microcavities with effective polaritonic THz radiative transition. Microcavities of various designs have been modelled, fabricated and investigated theoretically. GaAs, InAs as well as GaN structures were investigated. An analysis of the polaritonic THz active area, based on quantum wells, quantum wires and quantum dots with broken symmetry has been carried out. The team has developed and fabricated a novel type of cavity based on cylindrical Tamm plasmons. The concept of polaritonic emitters based on monolayer InAs quantum wells was introduced. Multi-quantum well structure based on InAs monolayers was fabricated and experimentally investigated. The structure demonstrates a pronounced superadiant mode that proves that such a structure can be used for development of THz emitters. Furthermore, the design of polaritonic cascade structures was developed and analysed. An important outcome is the development of the Bosonic Cascade Laser theory, where each polariton injected to the device emits several THz photons. Realization of coherent THz emitters based on polariton lasers would allow THz devices to be brought into day-to-day life, since the proposed devices are extremely compact and easily scalable allowing the creation of a matrix of emitters. Also, the FORTH team practically realized a polariton condensate switch.

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