14 research outputs found
Unusual polarization patterns in flat epitaxial ferroelectric nanoparticles
Interest in epitaxial ferroelectric nanoislands was growing rapidly in recent
years driven by their potential for devices, especially ultradense memories.
Recent advances in the "bottom- up" (self-assembly) nanometer scale techniques
have opened up the opportunities of fabricating high-quality epitaxial
ferroelectric nanoislands with extremely small thickness and lateral size on
the order of 1 nm and 20 nm, respectively. On the other hand, recent emergence
of powerful probes, such as piezoresponse force microscopy (PFM), has enabled
imaging of a local domain structure with sub-10 nm resolution. In spite of
those developments, a clear understanding of the polarization patterns in
epitaxial ferroelectric nanoislands is lacking, and some important
characteristics, like a critical lateral size for ferroelectricity, are not yet
established. Here, we perform ab-initio studies of non-electroded epitaxial
Pb(Zr0.5Ti0.5)O3 and BaTiO3 nanoislands and show the existence of novel
polarization patterns driven by the misfit strains and/or anisotropy energy.
The results allow interpretation of the data and design of the ferroelectric
nanostructures with tailored response to external field.Comment: 7 pages, 7 figure
Modulation of Negative Index Metamaterials in the Near-IR Range
Optical modulation of the effective refractive properties of a "fishnet"
metamaterial with a Ag/Si/Ag heterostructure is demonstrated in the near-IR
range and the associated fast dynamics of negative refractive index is studied
by pump-probe method. Photo excitation of the amorphous Si layer at visible
wavelength and corresponding modification of its optical parameters is found to
be responsible for the observed modulation of negative refractive index in
near-IR.Comment: 11 figures, 4 figure
Plasmonic resonances and hot spots in Ag octopods
New type of plasmonic nanoparticles - silver octopods that can be synthesized
with a variety of shapes - have been demonstrated to show versatile optical
response using the discrete dipole approximation. The octopods show a complex
behavior at optical (visible, IR) wavelengths, with three major resonances that
can be tuned up to a desired response that makes them especially attractive to
use in e.g. high-performance surface enhanced Raman (SERS) detectors. The
excited resonant modes strongly depend on the geometrical parameters of the
stars, while dependence on their orientation with respect to an incident
radiation is moderate, owing to cubic symmetry. The field "hot spots" are
formed with the local field enhancement up to 50 times compared to an incident
field. They are usually localized at the surface between the arms and may be
both "electric" and "magnetic". While the former are of primary importance for
SERS, the latter may be identified by trapping magnetic nanoparticles in their
vicinity. The results are in very good agreement with the data where available
and may be used as a type of a "shape spectroscopy" for the nanoparticles.Comment: 12 pages. 6 figure
Lattice-Induced Double-Valley Degeneracy Lifting in Magnetic Field in Graphene
We show that the recently discovered double-valley splitting of the low-lying
Landau level(s) in the Quantum Hall Effect in graphene can be explained as
perturbative orbital interaction of intra- and inter-valley microscopic orbital
currents with a magnetic field. This effect is provided by the
translational-non-invariant terms corresponding to graphene's crystallographic
honeycomb symmetry but do not exist in the relativistic theory of massless
Dirac Fermions in Quantum Electrodynamics. We discuss recent data in view of
these results
Coupled-Mode Theory of Field Enhancement in Complex Metal Nanostructures
We describe a simple yet rigorous theoretical model capable of analytical
estimation of plasmonic field enhancement in complex metal structures. We show
that one can treat the complex structures as coupled multi-pole modes with
highest enhancements obtained due to superposition of high order modes in small
particles. The model allows one to optimize the structures for the largest
possible field enhancements, which depends on the quality factor Q of the metal
and can be as high as Q^2 for two spherical particles. The "hot spot" can occur
either in the nano-gaps between the particles or near the smaller particles. We
trace the optimum field enhancement mechanism to the fact that the extended
dipole modes of larger particles act as the efficient antennas while the modes
in the gaps or near the smaller particles act as the compact sub-wavelength
cavities. We also show how easily our approach can be extended to incorporate
large numbers of particles in intricate arrangements.Comment: 23 pages, 7 figure
Ultrafast modulation of optical metamaterials
We show by pump-probe spectroscopy that the optical response of a fishnet metamaterial can be modulated on the femtosecond time scale. The modulation dynamics is dominated by pump-induced changes in the constituting dielectric medium, but the strength of modulation is dramatically enhanced through the plasmon resonance. The pump-induced spectral responses of the metamaterial provide understanding on how the resonance is modified by pump excitation. Our study suggests that metamaterials can be used as high-speed amplitude/phase modulators with terahertz-bandwidth