ORCID Profile
0000-0003-2170-1183
Current Organisation
IEE (Luxembourg)
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Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2020
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 02-2021
Publisher: IEEE
Date: 10-2017
Publisher: IOP Publishing
Date: 24-11-2020
Publisher: IOP Publishing
Date: 06-10-2020
Abstract: Metasurfaces have emerged as a promising technology for the manipulation of electromagnetic waves within a thin layer. In planar ultrathin metasurfaces, there exist rigorous design methods, based on the equivalent surface impedance of patterned metallic layers on dielectric substrates. In this work, we derive a limit on bandwidth achievable in these metasurfaces, based on constraints that their meta-atoms should be passive, causal and lossless and that they should obey the time-bandwidth product rules of a single resonance structure. The results show that in addition to elementary design parameters involving variation of the surface impedance, the bandwidth is critically limited by the dielectric substrate thickness and permittivity. We then propose a synthesis method for broadband ultrathin metasurfaces, based on an LC resonance fit of the required surface impedance and experimentally verify a broadband dispersive structure at millimeter-wave frequencies. This results in a bandwidth enhancement of over 90%, relative to a reference metasurface created with the narrowband design process.
Publisher: AIP Publishing
Date: 09-2014
DOI: 10.1063/1.4893743
Abstract: Amorphous chalcogenides usually exhibit a resistivity, which increases with age following a power law ρ ∼ tα. Existing theories link this change in amorphous state resistivity to structural relaxation. Here, the impact of fundamental glass properties on resistance drift phenomena in amorphous GexTe1−x networks is studied. Employing Raman spectroscopy, the Maxwell rigidity transition from flexible to stressed rigid is determined to occur in the compositional range 0.250 & xc & 0.265. Stressed rigid glasses (x & 0.265) exhibit rather strong resistance drift, where the drift parameters increase steadily from α = 0.13 for amorphous GeTe to α = 0.29 for compositions near the stiffness threshold xc. On the other hand, the drift parameter in flexible glasses (x & 0.25) decreases with decreasing Ge content x to values as low as α = 0.05. These findings illustrate the strong impact of the stiffness threshold on resistance drift phenomena in chalcogenides.
Publisher: IEEE
Date: 10-2017
Publisher: Optica Publishing Group
Date: 2020
DOI: 10.1364/CLEOPR.2020.C7C_2
Abstract: We investigate near-infrared photodetectors based on subwavelength Ge nanoparticles. While the photodetector size guarantees a high-bandwidth device, the high quantum efficiency is possible by the localization of the optical energy.
Publisher: American Physical Society (APS)
Date: 04-06-2019
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2020
No related grants have been discovered for Andreas E. Olk.