106 research outputs found
Current-controlled light scattering and asymmetric plasmon propagation in graphene
We demonstrate that plasmons in graphene can be manipulated using a DC
current. A source-drain current lifts the forward/backward degeneracy of the
plasmons, creating two modes with different propagation properties parallel and
antiparallel to the current. We show that the propagation length of the plasmon
propagating parallel to the drift current is enhanced, while the propagation
length for the antiparallel plasmon is suppressed. We also investigate the
scattering of light off graphene due to the plasmons in a periodic dielectric
environment and we find that the plasmon resonance separates in two peaks
corresponding to the forward and backward plasmon modes. The narrower linewidth
of the forward propagating plasmon may be of interest for refractive index
sensing and the DC current control could be used for the modulation of
mid-infrared electromagnetic radiation.Comment: 5 pages, 5 figure
Optical signatures of nonlocal plasmons in graphene
We theoretically investigate under which conditions nonlocal plasmon response
in monolayer graphene can be detected. To this purpose, we study optical
scattering off graphene plasmon resonances coupled using a subwavelength
dielectric grating. We compute the graphene conductivity using the Random Phase
Approximation (RPA) obtaining a nonlocal conductivity and we calculate the
optical scattering of the graphene-grating structure. We then compare this with
the scattering amplitudes obtained if graphene is modeled by the local RPA
conductivity commonly used in the literature. We find that the graphene plasmon
wavelength calculated from the local model may deviate up to from the
more accurate nonlocal model in the small-wavelength (large-) regime. We
also find substantial differences in the scattering amplitudes obtained from
the two models. However, these differences in response are pronounced only for
small grating periods and low temperatures compared to the Fermi temperature.Comment: Accepted for publication in Physical Review B. 15 pages, 9 figure
High-sensitivity plasmonic refractive index sensing using graphene
We theoretically demonstrate a high-sensitivity, graphene-plasmon based
refractive index sensor working in the mid-infrared at room temperature. The
bulk figure of merit of our sensor reaches values above , but the key
aspect of our proposed plasmonic sensor is its surface sensitivity which we
examine in detail. We have used realistic values regarding doping level and
electron relaxation time, which is the limiting factor for the sensor
performance. Our results show quantitatively the high performance of
graphene-plasmon based refractive index sensors working in the mid-infrared.Comment: This is an author-created, un-copyedited version of an article
accepted for publication/published in 2DMaterials. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
https://doi.org/10.1088/2053-1583/aa70f
Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis
The photoredox activity of well-known RuII complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. CrIII polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a CrIII complex is very similar to that for a RuII polypyridine, and thereby validate the concept of estimating the redox potential of d3 spin-flip excited states in analogous manner as for the MLCT states of d6 compounds. Building on this insight, we use our CrIII complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts
Developing future visions for bio-plastics substituting PET – a backcasting approach
The demand for plastics far exceeds that for any other bulk material and is expected to grow further due to global economic and population growth. Packaging is by far the largest end-user segment for plastics. Interest in bioplastics is increasing as public awareness of plastic waste accumulation in natural environ- ments increases. 2,5-Furandicarboxylic acid (FDCA) is the key monomer in the production of polyethylene 2,5-furandicarboxylate (PEF), a polymer that offers a sustainable solution to replace the commonly used polymer polyethylene terephthalate (PET). A backcasting workshop with 42 experts was held to identify current barriers and challenges that block the commercialization of FDCA-based products and to outline potential pathways toward future market diffusion. Several barriers which are strongly related to techno- logical and market-related aspects are preventing the full potential of FDCA from being unlocked. FDCA products cited in the literature are versatile and cover a wide array of niche applications. In the back- casting workshop, participants described their specific – yet highly divergent – future visions for PEF. Participants with a background in FDCA production referred mostly to developments that would need to take place in the field of FDCA applications to turn their vision into reality, while participants with a background in FDCA product development tended to refer to open issues related to FDCA synthesis. The findings of this study indicate that there is a great need for intensified cross-disciplinary communication and collaboration.publishe
Impact of Bidentate Pyridyl-Mesoionic Carbene Ligands: Structural, (Spectro)Electrochemical, Photophysical, and Theoretical Investigations on Ruthenium(II) Complexes
We present here new synthetic strategies for the isolation of a series of Ru(II) complexes with pyridyl-mesoionic carbene ligands (MIC) of the 1,2,3-triazole-5-ylidene type, in which the bpy ligands (bpy = 2,2′-bipyridine) of the archetypical [Ru(bpy)3]2+ have been successively replaced by one, two, or three pyridyl-MIC ligands. Three new complexes have been isolated and investigated via NMR spectroscopy and single-crystal X-ray diffraction analysis. The incorporation of one MIC unit shifts the potential of the metal-centered oxidation about 160 mV to more cathodic potential in cyclic voltammetry, demonstrating the extraordinary σ-donor ability of the pyridyl-MIC ligand, while the π-acceptor capacities are dominated by the bpy ligand, as indicated by electron paramagnetic resonance spectroelectrochemistry (EPR-SEC). The replacement of all bpy ligands by the pyridyl-MIC ligand results in an anoidic shift of the ligand-centered reduction by 390 mV compared to the well-established [Ru(bpy)3]2+ complex. In addition, UV/vis/NIR-SEC in combination with theoretical calculations provided detailed insights into the electronic structures of the respective redox states, taking into account the total number of pyridyl-MIC ligands incorporated in the Ru(II) complexes. The luminescence quantum yield and lifetimes were determined by time-resolved absorption and emission spectroscopy. An estimation of the excited state redox potentials conclusively showed that the pyridyl-MIC ligand can tune the photoredox activity of the isolated complexes to stronger photoreductants. These observations can provide new strategies for the design of photocatalysts and photosensitizers based on MICs
Graphene plasmons: Impurities and nonlocal effects
This work analyzes how impurities and vacancies on the surface of a graphene sample affect its optical conductivity and plasmon excitations. The disorder is analyzed in the self-consistent Green’s function formulation and nonlocal effects are fully taken into account. It is shown that impurities modify the linear spectrum and give rise to an impurity band whose position and width depend on the two parameters of our model, the density and the strength of impurities. The presence of the impurity band strongly influences the electromagnetic response and the plasmon losses. Furthermore, we discuss how the impurity-band position can be obtained experimentally from the plasmon dispersion relation and discuss this in the context of sensing
Dual Purpose Lyot Coronagraph Masks for Simultaneous High-Contrast Imaging and High-Resolution Wavefront Sensing
Directly imaging Earth-sized exoplanets with a visible-light coronagraph
instrument on a space telescope will require a system that can achieve
raw contrast and maintain it for the duration of observations
(on the order of hours or more). We are designing, manufacturing, and testing
Dual Purpose Lyot coronagraph (DPLC) masks that allow for simultaneous
wavefront sensing and control using out-of-band light to maintain high contrast
in the science focal plane. Our initial design uses a tiered metallic focal
plane occulter to suppress starlight in the transmitted coronagraph channel and
a dichroic-coated substrate to reflect out-of-band light to a wavefront sensing
camera. The occulter design introduces a phase shift such that the reflected
channel is a Zernike wavefront sensor. The dichroic coating allows higher-order
wavefront errors to be detected which is especially critical for compensating
for residual drifts from an actively-controlled segmented primary mirror. A
second-generation design concept includes a metasurface to create
polarization-dependent phase shifts in the reflected beam, which has several
advantages including an extended dynamic range. We will present the focal plane
mask designs, characterization, and initial testing at NASA's High Contrast
Imaging Testbed (HCIT) facility.Comment: To appear in the Proceedings of the SPIE, Techniques and
Instrumentation for Detection of Exoplanets X
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