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End date: 2019 × Start date: 2019 × DDC: 620 × WGL Institute: PDI ×

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Now showing 1 - 10 of 22
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    Crystal phase quantum well emission with digital control
    (Washington D.C. : American Chemical Society, 2017) Assali, S.; Lähnemann, J.; Vu, T.T.T.; Jöns, K.D.; Gagliano, L.; Verheijen, M.A.; Akopian, N.; Bakkers, E.P.A.M.; Haverkort, J.E.M.
    One of the major challenges in the growth of quantum well and quantum dot heterostructures is the realization of atomically sharp interfaces. Nanowires provide a new opportunity to engineer the band structure as they facilitate the controlled switching of the crystal structure between the zinc-blende (ZB) and wurtzite (WZ) phases. Such a crystal phase switching results in the formation of crystal phase quantum wells (CPQWs) and quantum dots (CPQDs). For GaP CPQWs, the inherent electric fields due to the discontinuity of the spontaneous polarization at the WZ/ZB junctions lead to the confinement of both types of charge carriers at the opposite interfaces of the WZ/ZB/WZ structure. This confinement leads to a novel type of transition across a ZB flat plate barrier. Here, we show digital tuning of the visible emission of WZ/ZB/WZ CPQWs in a GaP nanowire by changing the thickness of the ZB barrier. The energy spacing between the sharp emission lines is uniform and is defined by the addition of single ZB monolayers. The controlled growth of identical quantum wells with atomically flat interfaces at predefined positions featuring digitally tunable discrete emission energies may provide a new route to further advance entangled photons in solid state quantum systems.
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    Wavelength-tunable sources of entangled photons interfaced with atomic vapours
    (London : Nature Publishing Group, 2016) Trotta, Rinaldo; Martín-Sánchez, Javier; Wildmann, Johannes S.; Piredda, Giovanni; Reindl, Marcus; Schimpf, Christian; Zallo, Eugenio; Stroj, Sandra; Edlinger, Johannes; Rastelli, Armando
    The prospect of using the quantum nature of light for secure communication keeps spurring the search and investigation of suitable sources of entangled photons. A single semiconductor quantum dot is one of the most attractive, as it can generate indistinguishable entangled photons deterministically and is compatible with current photonic-integration technologies. However, the lack of control over the energy of the entangled photons is hampering the exploitation of dissimilar quantum dots in protocols requiring the teleportation of quantum entanglement over remote locations. Here we introduce quantum dot-based sources of polarization-entangled photons whose energy can be tuned via three-directional strain engineering without degrading the degree of entanglement of the photon pairs. As a test-bench for quantum communication, we interface quantum dots with clouds of atomic vapours, and we demonstrate slow-entangled photons from a single quantum emitter. These results pave the way towards the implementation of hybrid quantum networks where entanglement is distributed among distant parties using optoelectronic devices.
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    Interface formation of two- and three-dimensionally bonded materials in the case of GeTe–Sb2Te3 superlattices
    (Cambridge : Royal Society of Chemistry, 2015) Momand, Jamo; Wang, Ruining; Boschker, Jos E.; Verheijen, Marcel A.; Calarcob, Raffaella; Kooi, Bart J.
    GeTe–Sb2Te3 superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb2Te3, contradicting previously proposed models in the literature. For this purpose [GeTe(1 nm)–Sb2Te3(3 nm)]15 superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the film actually consists of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb2Te3 and rhombohedral GeSbTe. Moreover, it is shown by annealing the film at 400 °C, which reconfigures the superlattice into bulk rhombohedral GeSbTe, that this van der Waals layer is thermodynamically favored. These results are explained in terms of the bonding dimensionality of GeTe and Sb2Te3 and the strong tendency of these materials to intermix. The findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application.
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    Toward edges-rich MoS2 layers via chemical liquid exfoliation triggering distinctive magnetism
    (Milton Park : Taylor & Francis, 2016) Gao, Guanhui; Chen, Chi; Xie, Xiaobin; Su, Yantao; Kang, Shendong; Zhu, Guichi; Gao, Duyang; Eckert, Jürgen; Trampert, Achim; Cai, Lintao
    The magnetic function of layered molybdenum disulfide (MoS2) has been investigated via simulation, but few reliable experimental results have been explored. Herein, we developed edges-rich structural MoS2 nanosheets via liquid phase exfoliation approach, triggering exceptional ferromagnetism. The magnetic measurements revealed the clear ferromagnetic property of layered MoS2, compared to the pristine MoS2 in bulk exhibiting diamagnetism. The existence of ferromagnetism mostly was attributed to the presence of grain boundaries with abundant irregular edges confirmed by the transmission electron microscopy, magnetic force microscopy and X-ray photoelectron spectroscopy, which experimentally provided reliable evidences on irregular edges-rich states engineering ferromagnetism to clarify theoretical calculation.
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    Fano-like resonances sustained by Si doped InAsSb plasmonic resonators integrated in GaSb matrix
    (Washington, DC : Optical Society of America, 2015) Taliercio, Thierry; Guilengui, Vilianne NTsame; Cerutti, Laurent; Rodriguez, Jean-Baptiste; Barho, Franziska; Rodrigo, Maria-José Milla; Gonzalez-Posada, Fernando; Tournié, Eric; Niehle, Michael; Trampert, Achim
    By using metal-free plasmonics, we report on the excitation of Fano-like resonances in the mid-infrared where the Fano asymmetric parameter, q, varies when the dielectric environment of the plasmonic resonator changes. We use silicon doped InAsSb alloy deposited by molecular beam epitaxy on GaSb substrate to realize the plasmonic resonators exclusively based on semiconductors. We first demonstrate the possibility to realize high quality samples of embedded InAsSb plasmonic resonators into GaSb host using regrowth technique. The high crystalline quality of the deposited structure is confirmed by scanning transmission electron microscopy (STEM) observation. Second, we report Fano-like resonances associated to localized surface plasmons in both cases: uncovered and covered plasmonic resonators, demonstrating a strong line shape modification. The optical properties of the embedded structures correspond to those modeled by finite-difference time-domain (FDTD) method and by a model based on Fano-like line shape. Our results show that all-semiconductor plasmonics gives the opportunity to build new plasmonic structures with embedded resonators of highly doped semiconductor in a matrix of un-doped semiconductor for mid-IR applications.
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    High-spectral-resolution terahertz imaging with a quantum-cascade laser
    (Washington, DC : Optical Society of America, 2016) Hagelschuer, Till; Rothbart, Nick; Richter, Heiko; Wienold, Martin; Schrottke, Lutz; Grahn, Holger T.; Hübers, Heinz-Wilhelm
    We report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH3OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH3OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.
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    Real-time gas sensing based on optical feedback in a terahertz quantum-cascade laser
    (Washington, DC : Optical Society of America, 2017) Hagelschuer, Till; Wienold, Martin; Richter, Heiko; Schrottke, Lutz; Grahn, Holger T.; Hübers, Heinz-Wilhelm
    We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D2O and CH3OD in an absorption cell.
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    A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler
    (Washington, DC : Optical Society of America, 2010) Richter, H.; Greiner-Bär, M.; Pavlov, S.G.; Semenov, A.D.; Wienold, M.; Schrottke, L.; Giehler, M.; Hey, R.; Grahn, H.T.; Hübers, H.-W.
    We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.
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    Terahertz quantum-cascade lasers as high-power and wideband, gapless sources for spectroscopy
    (Washington, DC : Optical Society of America, 2017) Röben, Benjamin; Lü, Xiang; Hempel, Martin; Biermann, Klaus; Schrottke, Lutz; Grahn, Holger T.
    Terahertz (THz) quantum-cascade lasers (QCLs) are powerful radiation sources for high-resolution and high-sensitivity spectroscopy with a discrete spectrum between 2 and 5 THz as well as a continuous coverage of several GHz. However, for many applications, a radiation source with a continuous coverage of a substantially larger frequency range is required. We employed a multi-mode THz QCL operated with a fast ramped injection current, which leads to a collective tuning of equally-spaced Fabry-Pérot laser modes exceeding their separation. A continuous coverage over 72 GHz at about 4.7 THz was achieved. We demonstrate that the QCL is superior to conventional sources used in Fourier transform infrared spectroscopy in terms of the signal-to-noise ratio as well as the dynamic range by one to two orders of magnitude. Our results pave the way for versatile THz spectroscopic systems with unprecedented resolution and sensitivity across a wide frequency range.
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    Evidence for frequency comb emission from a Fabry-Pérot terahertz quantum-cascade laser
    (Washington, DC : Optical Society of America, 2014) Wienold, M.; Röben, B.; Schrottke, L.; Grahn, H.T.
    We report on a broad-band terahertz quantum-cascade laser (QCL) with a long Fabry-Pérot ridge cavity, for which the tuning range of the individual laser modes exceeds the mode spacing. While a spectral range of approximately 60 GHz (2 cm−1) is continuously covered by current and temperature tuning, the total emission range spans more than 270 GHz (9 cm−1). Within certain operating ranges, we found evidence for stable frequency comb operation of the QCL. An experimental technique is presented to characterize frequency comb operation, which is based on the self-mixing effect.