205 research outputs found
Low-temperature thermal conductivity in polycrystalline graphene
The low-temperature thermal conductivity in polycrystalline graphene is
theoretically studied. The contributions from three branches of acoustic
phonons are calculated by taking into account scattering on sample borders,
point defects and grain boundaries. Phonon scattering due to sample borders and
grain boundaries is shown to result in a -behaviour in the thermal
conductivity where varies between 1 and 2. This behaviour is found to
be more pronounced for nanosized grain boundaries.
PACS: 65.80.Ck, 81.05.ue, 73.43.C
Towards the grain boundary phonon scattering problem: an evidence for a low-temperature crossover
The problem of phonon scattering by grain boundaries is studied within the
wedge disclination dipole (WDD) model. It is shown that a specific q-dependence
of the phonon mean free path for biaxial WDD results in a low-temperature
crossover of the thermal conductivity, . The obtained results allow to
explain the experimentally observed deviation of from a
dependence below in and .Comment: 4 pages, 2 figures, submitted to J.Phys.:Condens.Matte
Purcell effect in Hyperbolic Metamaterial Resonators
The radiation dynamics of optical emitters can be manipulated by properly
designed material structures providing high local density of photonic states, a
phenomenon often referred to as the Purcell effect. Plasmonic nanorod
metamaterials with hyperbolic dispersion of electromagnetic modes are believed
to deliver a significant Purcell enhancement with both broadband and
non-resonant nature. Here, we have investigated finite-size cavities formed by
nanorod metamaterials and shown that the main mechanism of the Purcell effect
in these hyperbolic resonators originates from the cavity hyperbolic modes,
which in a microscopic description stem from the interacting cylindrical
surface plasmon modes of the finite number of nanorods forming the cavity. It
is found that emitters polarized perpendicular to the nanorods exhibit strong
decay rate enhancement, which is predominantly influenced by the rod length. We
demonstrate that this enhancement originates from Fabry-Perot modes of the
metamaterial cavity. The Purcell factors, delivered by those cavity modes,
reach several hundred, which is 4-5 times larger than those emerging at the
epsilon near zero transition frequencies. The effect of enhancement is less
pronounced for dipoles, polarized along the rods. Furthermore, it was shown
that the Purcell factor delivered by Fabry-Perot modes follows the dimension
parameters of the array, while the decay rate in the epsilon near-zero regime
is almost insensitive to geometry. The presented analysis shows a possibility
to engineer emitter properties in the structured metamaterials, addressing
their microscopic structure
Self-induced Torque in Hyperbolic Metamaterials
Optical forces constitute a fundamental phenomenon important in various fields of science, from astronomy to biology. Generally, intense external radiation sources are required to achieve measurable effects suitable for applications. Here we demonstrate
Optomechanical manipulation with hyperbolic metasurfaces
Auxiliary nanostructures introduce additional flexibility into optomechanical
manipulation schemes. Metamaterials and metasurfaces capable to control
electromagnetic interactions at the near-field regions are especially
beneficial for achieving improved spatial localization of particles, reducing
laser powers required for trapping, and for tailoring directivity of optical
forces. Here, optical forces acting on small particles situated next to
anisotropic substrates, are investigated. A special class of hyperbolic
metasurfaces is considered in details and is shown to be beneficial for
achieving strong optical pulling forces in a broad spectral range. Spectral
decomposition of the Green functions enables identifying contributions of
different interaction channels and underlines the importance of the hyperbolic
dispersion regime, which plays the key role in optomechanical interactions.
Homogenised model of the hyperbolic metasurface is compared to its
metal-dielectric multilayer realizations and is shown to predict the
optomechanical behaviour under certain conditions related to composition of the
top layer of the structure and its periodicity. Optomechanical metasurfaces
open a venue for future fundamental investigations and a range of practical
applications, where accurate control over mechanical motion of small objects is
required
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