Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading

The structural, thermodynamic and optical properties of M

Structural and optical properties of Mg_xAl_1-xH_y gradient thin films: a combinatorial approach

The structural, optical and dc electrical properties of MgxAl1-x (0.2≤x≤0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N,αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x=0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition

Thermal and electrostatic tuning of surface phonon-polaritons in LaAlO₃/SrTiO₃ heterostructures

Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 – a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.</p

Impact of LED Temperature on the Performance of LiFi Optical Wireless Communication Links

In this paper, we calculate the degradation of an Optical Wireless Communication (OWC) system caused by the junction temperature of the LED. Its throughput depends on the LED frequency response and on the received power versus the noise floor. Thus, the throughput can be calculated from the optimal modulation bandwidth, the efficiency, and the spectral density of the LED, which all are temperature dependent. For an OWC system based on LEDs, the low 3 dB bandwidth limits the achievable throughput of the system. Although, we increase the bandwidth by driving the LED at higher current density, that increases the junction temperature. We explore the effects that temperature has on the dynamic behavior of the LED and subsequently on the SNR and throughput of the OWC link. Our results can be used to optimize the throughput including the effect of self-heating nature of the LED

Impact of LED Temperature on the Performance of LiFi Optical Wireless Communication Links

In this paper, we calculate the degradation of an Optical Wireless Communication (OWC) system caused by the junction temperature of the LED. Its throughput depends on the LED frequency response and on the received power versus the noise floor. Thus, the throughput can be calculated from the optimal modulation bandwidth, the efficiency, and the spectral density of the LED, which all are temperature dependent. For an OWC system based on LEDs, the low 3 dB bandwidth limits the achievable throughput of the system. Although, we increase the bandwidth by driving the LED at higher current density, that increases the junction temperature. We explore the effects that temperature has on the dynamic behavior of the LED and subsequently on the SNR and throughput of the OWC link. Our results can be used to optimize the throughput including the effect of self-heating nature of the LED

Sensing moisture patterns using terahertz spectroscopy

We report on a novel and comprehensive method to determine leaf wetness by using machine learning algorithms applied on transmission THz time-domain data of a vast set of moisture patterns. More generally, our approach opens ways to determine properties of materials with a complex structure for which a model-based signal processing approach is not feasible

The dielectric function of MgyNiHx thin films (2 ≤ y ≤ 10)

M

Structural and optical properties of MgyNi1-yHx gradient thin films in relation to the as-deposited metallic state

Thin MgyNi1-yHx, films with a gradient in chemical composition are investigated by optical spectrophotometry, dc resistivity and X-ray diffraction measurements before and after exposure to hydrogen. In the metallic state crystalline Mg2Ni is present for 0.6 <= y <= 0.8 and coexists with amorphous Mg and/or Ni. The hydride state is mainly characterized by the presence of Mg2NiH4 around the stoichiometric [Mg]/[Ni] = 2 composition and some MgH2 on the Mg-rich side. The abrupt microstructural changes found in the as-deposited metallic state around the Mg-Mg2Ni eutectic point correlate well with the compositional dependence of the optical properties in the hydride state. We conclude that the formation of the hydride depends directly on the detailed nature of the metallic parent phase. Furthermore, we demonstrate that high-throughput compositional screening via fiber-optic spectrophotometry is useful for hydride identification. When no structural long-range order is present, this provides a new tool for the search for hydrogen storage materials. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved

Double layer formation in Mg-TM switchable mirrors (TM : Ni, Co, Fe)

The optical properties of Mg-TM (TM: Ni, Co, Fe; Mg:TM ≈ 2) thin films during hydrogenation are investigated using reflection and transmission measurements. Mg-Ni and Mg-Co show an unusual behavior upon exposure to hydrogen. The nucleation of the hydride starts at the film substrate interface and not - as intuitively expected - at the Pd covered top side of the film. Consequently, a double layer structure is formed. Reflection measurements are used to identify the loading behavior as the double layering yields to an remarkable optical black state at intermediate hydrogen concentrations. Photometric measurements on samples loaded at

Mixed PbFBr1-xIx crystals: structural and spectroscopic investigations

Mixed single PbFBr1−xIx crystals have been prepared. X-ray powder diffraction structure determinations show that all samples crystallize with the matlockite structure. However, the single crystal structure of PbFBr0.5I0.5 involves not only fractional occupation of one site corresponding to the stoichiometry, but also split positions of the Pb2+ ion. Raman spectra reveal the presence of new additional bands with respect to PbFBr and PbFI. DFT calculations of lattice vibrations for PbFI show good agreement with experimental spectra. The calculated phonon dispersion curve suggests that for the mixed crystals the centre of inversion is conserved locally. These combined results suggest the presence of domains with ordered F–Pb–Br–Br–Pb–F and F–Pb–I–I–Pb–F layers in the mixed crystals. Calculations on PbFBr0.5I0.5 show that this suggested structure is more stable than the structure consisting of the F–Pb–Br–I–Pb–F arrangement