Laurent Tranchant

Anomalous thermal conductivity by surface phonon-polaritons of polar nano thin films due to their asymmetric surrounding media

The surface phonon-polaritons contribution to the thermal conductivity of a nano thin film of silicon dioxide is investigated based on the Maxwell equations and the Boltzmann transport equation. It is shown that: (1) a small difference between the permittivities of the substrate and superstrate of the film can generate giant propagation lengths and therefore remarkably enhances its thermal conductivity with respect to values obtained for a freestanding one. (2) The propagation of surface phonon-polaritons is present in a broad band of frequencies and exhibits its largest propagation lengths at the frequency where the absorption of energy is minimal. (3) The increase of the thermal conductivity of the film as its thickness decreases is higher when it is deposited on potassium bromide instead of being suspended in air. The difference in the thermal conductivity for these two systems increases with increasing temperature and reducing the film thickness. A thermal conductivity as high as 2.5 W/m K is obtained...

Thermal conductivity of nano-layered systems due to surface phonon-polaritons

The effective thermal conductivity of a layered system due to the propagation of surface phonon-polaritons is studied. We analytically demonstrate that the thermal conductivity of a set of nanolayers can be described as one of a single layer with an effective permittivity, which does not ordinarily appear in nature and depends on the permittivities and thicknesses of the individual components. For a two-layer system of SiO2 and BaF2 surrounded by air, it is shown that: (i) the propagation length of surfaces phonon-polaritons can be as high as 3.3 cm for a 200 nm-thick system. (ii) The thermal conductivity of the system with total thickness of 50 nm is 3.4 W/m·K, which is twice that of a single layer of SiO2, at 500 K. Higher values are found for higher temperatures and thinner layers. The results show that an ensemble of layers provides more channels than a single layer for the propagation of surface phonon-polaritons and therefore for the enhancement of the thermal conductivity of common polar materials.

Effects of anisotropy and size of polar nano thin films on their thermal conductivity due to surface phonon-polaritons

The effects of the permittivity anisotropy and size of a nano thin film on its thermal conductivity due to surface phonon-polaritons are studied. We demonstrate that this thermal conductivity is a linear combination of the inverse first and third powers of the film thickness. For a 100-nm-thick film of α-quartz surrounded by air, the thermal conductivity along the optical axis is comparable to the phonon counterpart and equals 13 W m−1 K−1, which is 25% higher than that along the perpendicular direction, at room temperature. Higher values are found for thinner films at higher temperatures.

Focusing of surface phonon-polaritons along conical and wedge polar nanostructures

Focusing of surface phonon-polaritons propagating toward the tip of a cone and the edge of a wedge is theoretically analyzed and compared. Based on Maxwells equations, explicit expressions for the dispersion relations in each structure are determined and solved numerically for a propagation parameter driving the surface phonon-polariton energy density. For conical and wedge structures of SiO2, it is found that: (1) the cone (wedge) supports the polariton focusing only for aperture angles in the interval 18°– 68° ( 21°– 51°), and within the range of excitation frequencies from 32.1 THz (31.5 THz) to 33.9 THz (33.9 THz). In this frequency interval, the real part of the SiO2 permittivity is negative and the presence of polaritons is significant. (2) The polariton focusing efficiency of both the cone and wedge reaches its maximum values at the critical frequency fcr=33.6 THz and at different aperture angles of about αopt=45° and αopt=30°, respectively. (3) When the polaritons travel from 100 nm to 5 nm towar...

Surface Phonon Polariton Mediated Thermal Conduction of a Micrometric Glass Waveguide

Calculations of the dispersion relation and of the propagation length of surface phonon-polariton modes in a micrometric glass waveguide have been performed. The dispersion relation was solved in two ranges of frequency where SPP appear in glass. We succeeded in showing a maximal propagation length of 35 μm in a 10 μm radius waveguide with a 1 μm thick wall.

Polaritonic figure of merit of plane structures

Based on the ability of plane structures to simultaneously optimize the propagation, confinement, and energy of surface plasmon-polaritons or surface phonon-polaritons, we develop the polaritonic figure of merit Z = βRΛ2/δ, where βR, Λ and δ are the longitudinal wave vector, propagation length, and penetration depth, respectively. Explicit and analytical expressions of Z are derived for a single interface and a suspended thin film, as functions of the material permittivities and the film thickness. Higher Z are obtained for thinner films and smaller energy losses. The application of the obtained results for a SiC-air interface and a SiC thin film suspended in air shows that both structures are able to maximize the presence of polaritons at a frequency near to, but different than that at which the real part of the SiC permittivity exhibits a dip. Furthermore, using the temperature change of this dip, we show that the propagation length, confinement and energy of polaritons increases with its deepness, which provides an effective way to enhance the overall Z of polaritonic structures.

Optimized thermal conductivity enhancement of polar nanotubes due to surface phonon-polaritons

Surface phonon-polaritons (SPP) are hybrid electromagnetic waves that are the result of the coupling between photons and phonons at the interface between two different media [1,2]. Taking into account that the surface effects predominate over the volumetric ones due to the high surface area/volume ratio in nanomaterials, the energy transport by SPP is expected to be particularly important in nano-sized polar media, whose bulk thermal conductivity is usually low and decreases as the size is scaled down. The potential applications of these surface waves to improve the thermal performance of nanoscale devices have been attracting significant research efforts in the last few years [3], however the contribution of the SPP to the thermal conductivity of these materials is not well understood to date, especially at nanoscale. The objective of this work is to quantify and optimize this contribution in nanotubes of circular cross section.