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End date: 2019 × Start date: 2010 × DDC: 620 × WGL Institute: PDI ×

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Now showing 1 - 10 of 29
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    Molecular beam epitaxy of graphene on ultra-smooth nickel: growth mode and substrate interactions
    (London : Institute of Physics, 2014) Wofford, J.M.; Oliveira Jr, M.H.; Schumann, T.; Jenichen, B.; Ramsteiner, M.; Jahn, U.; Fölsch, S.; Lopes, J.M.J.; Riechert, H.
    Graphene is grown by molecular beam epitaxy using epitaxial Ni films on MgO(111) as substrates. Raman spectroscopy and scanning tunneling microscopy reveal the graphene films to have few crystalline defects. While the layers are ultra-smooth over large areas, we find that Ni surface features lead to local non-uniformly thick graphene inclusions. The influence of the Ni surface structure on the position and morphology of these inclusions strongly suggests that multilayer graphene on Ni forms at the interface of the first complete layer and metal substrate in a growth-from-below mechanism. The interplay between Ni surface features and graphene growth behavior may facilitate the production of films with spatially resolved multilayer inclusions through engineered substrate surface morphology.
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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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    Electrical and optical properties of epitaxial binary and ternary GeTe-Sb2Te3 alloys
    (London : Nature Publishing, 2018) Boschker, Jos E.; Lü, Xiang; Bragaglia, Valeria; Wang,Ruining; Grahn, Holger T.; Calarco, Raffaella
    Phase change materials such as pseudobinary GeTe-Sb2Te3 (GST) alloys are an essential part of existing and emerging technologies. Here, we investigate the electrical and optical properties of epitaxial phase change materials: α-GeTe, Ge2Sb2Te5 (GST225), and Sb2Te3. Temperature-dependent Hall measurements reveal a reduction of the hole concentration with increasing temperature in Sb2Te3 that is attributed to lattice expansion, resulting in a non-linear increase of the resistivity that is also observed in GST225. Fourier transform infrared spectroscopy at room temperature demonstrates the presence of electronic states within the energy gap for α-GeTe and GST225. We conclude that these electronic states are due to vacancy clusters inside these two materials. The obtained results shed new light on the fundamental properties of phase change materials such as the high dielectric constant and persistent photoconductivity and have the potential to be included in device simulations.
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    Advances in group-III-nitride photodetectors
    (Sharjah [u.a.] : Bentham Open, 2010) Rivera, C.; Pereiro, J.; Navarro, A.; Muñoz, E.; Brandt, O.; Grahn, H.T.
    Group-III nitrides are considered to be a strategic technology for the development of ultraviolet photodetectors due to their remarkable properties in terms of spectral selectivity, radiation hardness, and noise. The potential advantages of these materials were initially obscured by their large density of intrinsic defects. The advances were thus associated in general with improvements in material quality. Although technology still also needs improvement, efforts are being intensified in the fabrication of advanced structures for photodetector applications. In particular, this review discusses the recent progress in group-III-nitride photodetectors, emphasizing the work reported on quantum-well-based photodetectors, the use of novel structures exploiting the effect of piezoelectric polarization-induced fields, and polarization-sensitive photodetectors. Furthermore, some ideas can be generalized to other material systems such as ZnO and their related compounds, which exhibit the same crystal structure as group-III nitrides. © Rivera et al.; Licensee Bentham Open.
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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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    Doppler-free spectroscopy with a terahertz quantum-cascade laser
    (Washington, DC : Optical Society of America, 2018) Wienold, M.; Alam, T.; Schrottke, L.; Grahn, H.T.; Hübers, H.-W.
    We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.
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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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    Acoustically driven arrayed waveguide grating
    (Washington, DC : Optical Society of America, 2015) Crespo-Poveda, A.; Hernández-Mínguez, A.; Gargallo, B.; Biermann, K.; Tahraoui, A.; Santos, P.V.; Muñoz, P.; Cantarero, A.; de Lima, M.M.
    We demonstrate compact tunable phased-array wavelength-division multiplexers driven by surface acoustic waves (SAWs) in the low GHz range. The devices comprise two couplers, which respectively split and combine the optical signal, linked by an array of single-mode waveguides (WGs). Two different layouts are presented, in which multi-mode interference couplers or free propagating regions were separately employed as couplers. The multiplexers operate on five equally distributed wavelength channels, with a spectral separation of 2 nm. A standing SAW modulates the refractive index of the arrayed WGs. Each wavelength component periodically switches paths between the output channel previously asigned by the design and the adjacent channels, at a fixed applied acoustic power. The devices were monolithically fabricated on (Al,Ga)As. A good agreement between theory and experiment is achieved.
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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.