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 $T^{\alpha}$-behaviour in the thermal conductivity where $\alpha$ 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, $\kappa$. The obtained results allow to explain the experimentally observed deviation of $\kappa$ from a $T^3$ dependence below $0.1K$ in $LiF$ and $NaCl$.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