16 research outputs found
Excitation of localized graphene plasmons by a metallic slit
In this paper we show that graphene surface plasmons can be excited when an electromagnetic wave packet impinges on a single metal slit covered with graphene. The excitation of the plasmons localized over the slit is revealed by characteristic peaks in the absorption spectrum. It is shown that the position of the peaks can be tuned either by the graphene doping level or by the dielectric function of the material filling the slit. The whole system forms the basis for a plasmonic sensor when the slit is filled with an analyte.The authors are grateful for useful discussions with H. Crespo. The authors acknowledge support from the European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (Ref. No. 881603), and the Portuguese Foundation for Science and Technology through the Strategic Funding UID/FIS/04650/2019. Additionally, the authors acknowledge financing from FEDER and the Portuguese Foundation for Science and Technology (FCT) through Project No. POCI-01-0145-FEDER-028114
Calculation of the ground-state Stark effect in small molecules using the variational quantum eigensolver
As quantum computing approaches its first commercial implementations, quantum
simulation emerges as a potentially ground-breaking technology for several
domains, including Biology and Chemistry. However, taking advantage of quantum
algorithms in Quantum Chemistry raises a number of theoretical and practical
challenges at different levels, from the conception to its actual execution. We
go through such challenges in a case study of a quantum simulation for the
hydrogen (H2) and lithium hydride (LiH) molecules, at an actual commercially
available quantum computer, the IBM Q. The former molecule has always been a
playground for testing approximate calculation methods in Quantum Chemistry,
while the latter is just a little bit more complex, lacking the mirror symmetry
of the former. Using the Variational Quantum Eigensolver (VQE) method, we study
the molecule's ground state energy versus interatomic distance, under the
action of stationary electric fields (Stark effect). Additionally, we review
the necessary calculations of the matrix elements of the second quantization
Hamiltonian encompassing the extra terms concerning the action of electric
fields, using STO-LG type atomic orbitals to build the minimal basis sets.Comment: Soft Comput (2021
Localized polariton states in a photonic crystal intercalated by a transition metal dichalcogenide monolayer
Beyond the extensively studied microcavity polaritons, which are coupled
modes of semiconductor excitons and microcavity photons, nearly 2D
semiconductors placed in a suitable environment can support spatially localized
exciton-polariton modes. We demonstrate theoretically that two distinct types
of such modes can exist in a photonic crystal with an embedded transition metal
dichalcogenide (TMD) monolayer and derive an equation that determines their
dispersion relations. The localized modes of two types occur in the zeroth- and
first-order stop-bands of the crystal, respectively, and have substantially
different properties. The latter type of the localized modes, which appear
inside the light cone, can be described as a result of coupling of the TMD
exciton and an optical Tamm state of the TMD-intercalated photonic crystal. We
suggest an experiment for detecting these modes and simulate it numerically.Comment: 2021 Optica Publishing Group. One print or electronic copy may be
made for personal use only. Systematic reproduction and distribution,
duplication of any material in this paper for a fee or for commercial
purposes, or modifications of the content of this paper are prohibite
Multi-stacks of epitaxial GeSn self-assembled dots in Si: Structural analysis
We report on the growth and structural and morphologic characterization of stacked layers of self-assembled GeSn dots grown on Si (100) substrates by molecular beam epitaxy at low substrate temperature T = 350 °C. Samples consist of layers (from 1 up to 10) of Ge0.96Sn0.04 self-assembled dots separated by Si spacer layers, 10 nm thick. Their structural analysis was performed based on transmission electron microscopy, atomic force microscopy and Raman scattering. We found that up to 4 stacks of dots could be grown with good dot layer homogeneity, making the GeSn dots interesting candidates for optoelectronic device applications.This work was partly supported by the Portuguese Foundation for Science and Technology (FCT) through Strategic Project PEst-C/FIS/UI0607/2013 and PhD Fellowship (F. Oliveira)
Impact of Dâ‚‚O/Hâ‚‚O solvent exchange on the emission of HgTe and CdTe quantum dots: Polaron and energy transfer effects
We have studied light emission kinetics and analyzed carrier recombination channels in HgTe quantum dots
that were initially grown in H2O. When the solvent is replaced by D2O, the nonradiative recombination rate changes highlight the role of the vibrational degrees of freedom in the medium surrounding the dots, including both solvent and ligands. The contributing energy loss mechanisms have been evaluated by
developing quantitative models for the nonradiative recombination via (i) polaron states formed by strong coupling of ligand vibration modes to a surface trap state (nonresonant channel) and (ii) resonant energy transfer to vibration modes in the solvent. We conclude that channel (i) is more important than (ii) for HgTe dots in either solution. When some of these modes are removed from the relevant spectral range by the H2O to D2O replacement, the polaron effect becomes weaker and the nonradiative lifetime increases. Comparisons with CdTe quantum dots (QDs) served as a reference where the resonant energy loss (ii) a priori was not a factor, also confirmed by our experiments. The solvent exchange (H2O to D2O),
however, is found to slightly increase the overall quantum yield of CdTe samples, probably by increasing the fraction of bright dots in the ensemble. The fundamental study reported here can serve as the foundation for the design and optimization principles of narrow bandgap quantum dots aimed at applications in long wavelength colloidal materials forinfrared light emitting diodes and photodetectors.We acknowledge financial support by the grant from the Research Grants Council of the Hong Kong S.A.R., China (project CityU 11302114). MIV acknowledges financial support from the FCT (Portugal)
Raman study of insulating and conductive ZnO: (Al, Mn) thin films
Raman spectroscopy results obtained for undoped and Al- and/or Mn-doped ZnO thin films produced by RF-sputtering are reported. The effect of the doping method (either co-sputtering or ion implantation), the dopant type and its concentration on the Raman-active vibrational modes in these films were studied in detail. The results are discussed with focus on the peak shifts and broadening, and on the doping-induced relaxation of the symmetry selection rules. A particular attention is paid to the 520-530 cm-1 Raman band observed in all Mn containing samples and a simple theoretical model and arguments are presented in support of its relation to the local (gap) phonon mode produced by Mn atoms substituting Zn in the cationic sublattice of the ZnO crystal.Karlsruhe Nano Micro Facility (KNMF), a Helmholtz Research Infrastructure at KI
Exciton-polaritons in 2D dichalcogenide layers placed in a planar microcavity: tuneable interaction between two Bose-Einstein condensates
Exciton-polariton modes arising from interaction between bound excitons in monolayer thin semiconductor
sheets and photons in a Fabry-Perot microcavity are considered theoretically.We calculate the dispersion curves, mode lifetimes, Rabi splitting, and Hopfield coefficients of these structures for two nearly 2D semiconductor materials, MoS2 and WS2, and suggest that they are interesting for studying the rich physics associated with the Bose-Einstein condensation of exciton polaritons. The large exciton binding energy and dipole allowed exciton transitions, in addition to the relatively easily controllable distance between the semiconductor sheets, are the advantages of this system in comparison with traditional GaAs or CdTe based semiconductor microcavities. In particular, in order to mimic the rich physical properties of the quantum degenerate mixture of two bosonic species of dilute atomic gases with tunable interspecies interaction, we put forward a structure containing two semiconductor sheets separated by some atomic-scale distance (l) using a nearly 2D dielectric (e.g., h-BN), which offers the possibility of tuning the interaction between two exciton-polariton Bose-Enstein condensates. We show that thedynamics of this structure are ruled by two coupled Gross-Pitaevskii equations with the coupling parameter∼ l−1.CNPq (Brazil), FCT (Portugal), EC Graphene Flagship Project (Contract No. CNECTICT-
604391
Terahertz response of patterned epitaxial graphene
We study the interaction between polarized terahertz (THz) radiation and
micro-structured large-area graphene in transmission geometry. In order to efficiently
couple the radiation into the two-dimensional material, a lateral periodic patterning
of a closed graphene sheet by intercalation doping into stripes is chosen. We observe
unequal transmittance of the radiation polarized parallel and perpendicular to the
stripes. The relative contrast, partly enhanced by Fabry-Perot oscillations reaches
20 %. The effect even increases up to 50 % when removing graphene stripes in analogy
to a wire grid polarizer. The polarization dependence is analyzed in a large frequency
range from < 80 GHz to 3 THz, including the plasmon-polariton resonance. The results
are in excellent agreement with theoretical calculations based on the electronic energy
spectrum of graphene and the electrodynamics of the patterned structureThe authors thank J. Jobst for fruitful discussions. The research was performed in the
framework of the Sonderforschungsbereich 953 "Synthetic carbon allotropes", funded
by Deutsche Forschungsgemeinschaft. We acknowledge support from the EC under
Graphene Flagship (contract no. CNECT-ICT-604391)
Resonant Excitation of Confined Excitons in Nanocrystal Quantum Dots Using Surface Plasmon-Polaritons
Surface plasmon-polaritons (SPPs) in a multilayer structure
consisting
of a metallic film and one or more layers of nanocrystal (NC) quantum
dots (QDs) are studied theoretically. It is shown that there is a
resonance coupling between the plasmon-polaritons propagating along
the metal/NC-layer interface and excitons confined in the dots, which
produces a considerable effect on the optical properties of the structure
unless the dispersion of the QD size is too large. Using a transfer
matrix formalism, multilayer structures consisting of NC composite
and metallic films are considered and it is demonstrated that the
coupling extends over several layers constituting the structure. It
can be explored in order to selectively excite QDs of different size
by making a layer-by layer assembled NC planar structure and using
an attenuated total reflection (ATR) configuration for the SPP-enhanced
excitation of the dots. In particular, it opens the possibility to
control the relative intensity of light of different color, emitted
by the QDs of different size