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International audience; Heat conduction. in micro- and nanoscale and in ultrafast processes may deviate from the predictions of the Fourier law, due to boundary and interface scattering, the ballistic nature of the transport, and the finite relaxation time of heat carriers. The transient ballistic-diffusive heat conduction equations (BDE) were developed as an approximation to the phonon Boltzmann equation (BTE) for nanoscale heat conduction problems. In this paper, we further develop BDE for multidimensional heat conduction, including nanoscale heat source term and different boundary conditions, and compare the simulation results with those obtained from the phonon BTE and the Fourier law. The numerical solution strategies for multidimensional nanoscale heat conduction using BDE are presented. Several two-dimensional cases are simulated and compared to the results of the transient phonon BTE and the Fourier heat conduction theory. The transient BTE is solved using the discrete ordinates method with a two Gauss-Legendre quadratures. Special attention has been paid to the boundary conditions. Compared to the cases without internal heat generation, the difference between the BTE and BDE is larger for the case studied with internal heat generation due to the nature of the ballistic-diffusive approximation, but the results from BDE are still significantly better than those from the Fourier law. Thus we conclude that BDE captures the characteristics of the phonon BTE with much shorter computational time.

International audience; Heat transfer between two plates of polar materials at nanoscale distance is known to be enhanced by several orders of magnitude as compared with its far-field value. In this article, we derive accu- rate analytical expressions to quantitatively predict heat fluxes in the near-field. These analytical expressions reveal the physical mechanisms responsible for the enhancement. For two dielectric polar materials and for gaps smaller than 75nm at room temperature the heat transfer is dominated by the surface phonon polariton contribution. Between 75nm and 500 nm, the enhancement is mostly due to frustrated total internal reflection. The paper reports accurate analytical expressions for both contributions. Our analytical results highlight two differences between radiation flux at the nanoscale and in the far field: i)the heat flux spectrum depends on the gap distance, ii) the temperature dependence of the heat transfer coefficient deviates strongly from the T3 law valid for gray bodies in the far-field

In operating hot carrier solar cells, a steady-state hot carrier distribution is established in the absorber in such a way that the excess kinetic energy of carriers can be collected. A high-carrier concentration is normally favorable to the formation of a nonequilibrium hot-carrier population. A small absorber thickness is thus expected to improve the efficiency of hot carrier solar cells, but no quantitative analysis of the impact of the cell thickness on its performance has been done so far. Here, the potential of efficiency improvement using thinned absorber is investigated by simulating the absorption, heat losses, and efficiency of a hot carrier solar cell with varying absorber thickness. Efficiency improvement requires efficient light trapping to maintain absorption in ultrathin layers. Solutions are proposed to achieve strong absorption in a 25-50-nm-thick absorber, resulting in cell efficiencies that are higher than the Shockley-Queisser limit corresponding to the absorber's bandgap.

International audience; We demonstrate that almost 100% of incident photons can interact with a monolayer of scatterers in a symmetrical environment. Nearly perfect optical extinction through free-standing transparent nanorod arrays has been measured. The sharp spectral opacity window, in the form of a characteristic Fano resonance, arises from the coherent multiple scattering in the array. In addition, we show that nanorods made of absorbing material exhibit a 25-fold absorption enhancement per unit volume compared to unstructured thin film. These results open new perspectives for light management in high-Q, low volume dielectric nanostructures, with potential applications in optical systems, spectroscopy, and optomechanics. Enhancing light-matter interactions at the nanometer scale is a key for many applications in the area of pho-tonics, biophysics and material sciences [1,2]. Metallic particles are the archetype of nano-objects that can lead to strong interaction with light due to plasmonic resonances , with the drawback of intrinsic metal absorption [3]. Conversely, the weak scattering cross section of tiny dielectric nanoparticles originates from their nonresonant nature, making them inefficient for optical manipulation at the nanoscale. However, the extinction cross section can be increased by coherent multiple scattering in assemblies of nanoparticles, offering another degree of freedom to manipulate their optical response. It has been shown theoretically that it can lead to extremely sharp geometric resonances [4–9]. From the experimental point of view, arrays of resonant, metallic nanoparticles demonstrated the potential of the effect [10,11], making the localized surface plasmon resonance much narrower [12,13]. Arrays of non-resonant, dielectric nanorods should offer new possibilities for even higher quality-factor geometric resonances [14,15]. Indeed, sharp reflection resonances [7,8] and absorption enhancement [9] have been predicted for dielec-tric, sub-wavelength cylinder arrays. It has to be emphasized that the scattering properties of nanoparticles are substantially modified, or even suppressed , in the proximity of a surface [6,7,16–19]. Here, we study a free-standing array of nanorods, with a filling fraction around 0.15. In contrast with the periodic nano-structures studied extensively in the past ten years (nano-hole arrays in metal films [20,21], metal nanoparticle arrays [10–13], high-index-contrast gratings [22], guided-mode resonant structures [23,24]), resonant effects cannot be attributed to localized resonances neither to interactions between nanostructures mediated by electromagnetic

We provide the first experimental evidence of sharp resonant extinction in free-standing arrays of non-resonant dielectric nanorods. Nearly perfect optical extinction is shown for transparent material. High-resolution optical measurements (absolute transmission and reflection) of one dimensional gratings with very low fill factors have been obtained. The results can be fully explained by coherent multiple scattering in arrays of non-resonant subwavelength nanorods and are in good agreement with an analytical model.

International audience

As the importance of renewable energy sources grows, the development of highly-efficient solar cells is increasingly gaining relevance. Today, the most efficient laboratory prototypes for solar cells are based on thin film multi-layered structures. These cells have 40% solar-to-electricity conversion efficiencies, which are well below the theoretical limit of 87% and leave considerable room for improvement. However, the design of multilayered solar cells can be complicated, and their workings might fail to tolerate changes to their operational conditions, such as the cell temperature or the power of the incident sunlight. Hotcarrier solar cells (HCSC), with their simplicity of design and ability to approach limiting conversion efficiencies, provide an attractive alternative to the multi-layer approach.1 Heat dissipation occurs when a material absorbs photons with energies larger than its bandgap. To circumvent this problem, the photo-generated charge carriers have to be collected through specially designed contacts that are energy-selective. In this way, heat production can be minimized: carriers with large kinetic energies—‘hot-carriers’—reach these contacts before losing most of their energy as heat. In principle, efficiencies as high as 86% could be achieved.1 However, since hot-carriers normally transfer their kinetic energy to the material in sub-picosecond times, the collection through contacts should be fast. This could be achieved at high-injection conditions, under which the interaction between the absorbent material and the hot-carriers becomes inefficient.2 As for any solar cell design, conversion efficiency is expected to grow with the concentration of incoming light, as this increases the output voltage of the solar cell by increasing the extracted work per absorbed photon. However, an optimal coupling between the incident radiation and the solar cells will lead to the high-injection regime, where drops in the cell’s output Figure 1. Energy band diagram of a hot-carrier solar cell with bandgap Eg and voltage qV. Electron-hole pairs are photo-generated in the absorber and kept hot at a temperature of TH (TH > TC , where TC is the ambient temperature). They are subsequently extracted using energyselective contacts with a transmission range iE and an extraction energy Eext. The Fermi levels in the electrodes are n and p , and the electron and hole chemical potentials in the absorber are e and h. The difference e h D H is known as the quasi-Fermi level splitting.

We report efficient white upconversion luminescence in Yb(3+)-, Er(3+)- and Tm(3+)-doped monophasic and biphasic Y(2)BaZnO(5) phosphors under 977 nm near-infrared excitation and at low excitation power densities (down to ∼25 mW mm(-2)).

The optical properties of Yb3+ and Tm3+ co-doped Y2BaZnO5, synthesized by solid-state reaction, are investigated in detail. Three main emission bands centered around 479 nm (blue), 654 nm (red), and 796 nm (near-infrared) are observed under near-infrared laser excitation via an upconversion process. Detailed studies of the upconversion properties as a function of dopant concentrations are described and upconversion efficiencies quantified precisely. Maximum efficiencies of ∼ 1.53% in the 730-870 nm near-infrared emission range and of ∼ 0.09% in the 420-530 nm blue range are obtained. The results of power dependence studies and concentration dependent lifetime measurements are presented. This in-depth spectroscopic study allows us, for the first time, to identify the dominant processes involved in the upconversion mechanism of Yb3+, Tm3+ co-doped Y2BaZnO5 oxides.

Heating of dielectric nanoparticles by black-body radiation is investigated by using molecular-dynamics simulation. The thermal interaction with the radiation is modeled by coupling the ions with a random electric field and including a radiation reaction force. This approach shows that the heat is absorbed by the polariton mode. Its subsequent redistribution among other vibration modes strongly depends on the particle size and on temperature. We observe energy trapping in a finite subset of vibrational modes and study the relaxation pathway of (MgO)${}_{4}$ by performing a selective excitation with a deterministic force.

3 páginas, 3 figuras., Optical management is essential to increase absorption in thin photovoltaic cells. In this article, full electromagnetic simulations show that a back mirror and a one-dimensional front SiC sawtooth grating of ∼ 1 μm dimensions can significantly increase absorption in a thin layer under light concentration. A 50 nm thick GaSb active layer in the described configuration absorbs ∼ 66% of the incident solar photons above the band gap for a concentration equivalent to a numerical aperture NA = 1/(raiz de 2). This absorption represents a ∼ 76% or 26% increase over the same structure but with the grating removed or substituted by an ideal antireflection coating, respectively, We acknowledge financial support from the ANR project THRI-PV and support of the RTRA Triangle de la Physique. One of us (R.E.) acknowledges financial support from the Spanish National project “Euroinvestigación” EUI2008–03816 CUBiHOLE.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 683-685, Hot carrier solar cells allow potential efficiency close to the thermodynamical limit in ideal conditions. However, the feasability of such devices has not been clearly stated so far and only ideal cells were considered in previous studies. Here we develop a model with realistic energy selective contacts, carrier thermalization and absorptivity. The requirements in term of contact selectivity is investigated, showing that semi-selective contacts are not incompatible with high efficiencies. Candidates for absorbing material were synthesized and the required thermalization properties are obtained. Specific structures were designed to enhance absorption of concentrated sunlight in a small material thickness, allowing high carrier density in the absorber.

International audience; Antennas are widely used by electrical engineers to enhance the coupling between propagating waves and electric sources or detectors. It is thus tempting to develop an optical analog to tailor visible light emission or absorption by an atom or a molecule. This idea has been put forward recently and it has been demonstrated that both the radiative rate and the emission pattern of optical emitters can be modified by metallic nanostructures. In this Letter, we introduce the concept of impedance for a nanoantenna and for two-level systems or nanoparticles described by electric dipole moments. We show how these concepts can be used to reconcile different descriptions and also to optimize nanoantennas.

Radiative heat transfer at the nanoscale is becoming an important technological issue with the development of nano electromechanical systems (NEMS). In this article, we derive asymptotic expressions to compute near-field radiative heat transfer between two planes of silicon. We identify two physical mechanisms that give the dominant contribution at small gaps. For intrinsic and low-doped silicon, the main contribution is due to evanescent waves coming from propagating waves undergoing frustrated total internal reflections at the interfaces. For doping levels larger than N-e=10(16) cm(-3) surface mode coupling contributes to the heat transfer. Asymptotic expressions are also given in that case. In all cases, we compare analytical formulas with exact numerical calculations when varying the temperature and the doping concentration. We also give their range of validity. (C) 2010 Elsevier Ltd. All rights reserved.

International audience; In this letter, we revisit the role of surface plasmons for nanoscale radiative heat transfer between doped silicon surfaces. We derive a new accurate and closed-form expression of the radiative near-field heat transfer. We also analyse the flux and find that there is a doping level that maximizes the heat flux.

International audience; Molecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected.

Light forces on small (Rayleigh) particles are usually described as the sum of two terms: the dipolar or gradient force and the scattering or radiation pressure force. The scattering force is traditionally considered proportional to the Poynting vector, which gives the direction and magnitude of the momentum flow. However, as we will show, there is an additional nonconservative contribution to the scattering force arising in a light field with nonuniform helicity. This force is shown to be proportional to the curl of the spin angular momentum of the light field. The relevance of the spin force is illustrated in the simple case of a 2D field geometry arising in the intersection region of two standing waves, This work was supported by the Spanish MEC through the Consolider NanoLight (CSD2007-00046), FIS2005-05137 and FIS2006-11170-C02-02 projects, Microseres-CM and by the Spanish-French PICASSO program (HF2007- 0068)

We introduce a generalized definition of the impedance of a nanoantenna that can be applied to any system. We also introduce a definition of the impedance of a two level system. Using this framework, we establish a link between the electrical engineering and the quantum optics picture of light emission.

International audience; Some applications such as ultrafast detectors or high efficiency photovoltaics require absorption by thin films. However, close to the bandgap, silicon absorbs very poorly. In this letter, we show that the absorption of a 100 nm slab can be as high as 50% in the range of wavelengths 700-830 nm when using a periodic structure properly designed.

International audience; In this letter, we study the radiative heat transfer between two nanoparticles in the near field and in the far field. We find that the heat transfer is dominated by the electric dipole-dipole interaction for dielectric particles and by the magnetic dipole-dipole interaction for metallic nanoparticles. We introduce polarizabilities formulas valid for arbitrary values of the skin depth. While the heat transfer mechanism is different for metallic and dielectric nanoparticles, we show that the distance dependence is the same. However, the dependence of the heat flux on the particle radius is different.

We revisit the electromagnetic heat transfer between a metallic nanoparticle and a metallic semi-infinite substrate, commonly studied using the electric dipole approximation. For infrared and microwave frequencies, we find that the magnetic polarizability of the particle is larger than the electric one. We also find that the local density of states in the near field is dominated by the magnetic contribution. As a consequence, the power absorbed by the particle in the near field is due to dissipation by fluctuating eddy currents. These results show that a number of near-field effects involving metallic particles should be affected by the fluctuating magnetic fields., publi\'e dans Physical Review B 77 (2008), version avant review

High efficiency solutions for photovoltaics requires taking advantage of IR photons energy. Possible solutions are based on up-conversion processes which have a low cross section. We will discuss different solutions to enhance the absorption.

It is well known that Raman scattering and fluorescence can be enhanced by the presence of metallic nanoparticles. Here, we derive simple equations to analyse the influence of metallic nanoparticles on upconversion processes such as non-radiative energy transfer or excited state absorption. We compare the resulting expressions with the more familiar Raman and fluorescence cases, and find significant differences. We use numerical simulations to calculate the upconverted signal enhancement achievable by means of metallic spheres of different radii, and find particles of 100-400nm radius at infrared frequencies to be favorable. We also discuss the considerable challenges involved in using metallic particles to enhance upconversion for solar energy., Comment: Changes mostly made on the structure of the text and on the notation to improve clarity. Other changes attempt to better clarify the conditions under which the equations and simulations are valid. Main results and conclusions essentially unchanged

International audience; Absorption and emission of light due to the resonant excitation of surface waves on a grating is a well-known phenomenon. We report the first complete study of the influence of the role of angle and polarization on thermal emission by lamellar gratings.We derive the emitted Stokes vectors in any direction. We find that a source can be quasi isotropic from the point of view of the intensity but strongly anisotropic for polarized light. It follows that the degree of polarization can vary between 0 and 1, depending on directions.

Subwavelength dielectric and metallic gratings embedded in vacuum can act as highly-resonant spectral filters. We review the theoretical principles for the design of symmetric dielectric and metal gratings to conceive efficient optical filters in the mid and far infrared range, and we show how both resonance width and resonance wavelength can be tuned. We describe an original process for the fabrication of free-standing SiC gratings, and we present the first samples obtained with 10 mm period. Experimental angularly resolved transmission spectra show evidences of their filtering properties.

International audience; We firstly compare the electric and magnetic polarizabilities of a spherical nanoparticle. We then calculate the electromagnetic heat transfer between a metallic particle and a semi-infinite substrate. We show that the power absorbed by the particle in the near field is due to the magnetic interaction. We then calculate the energy transfer between two metallic nanoparticles and compare the heat dissipated by Joule effect and eddy currents. We find that the heat dissipated due to the magnetic fields is the leading contribution to the heat power. Both calculations show that a number of near-field effects involving metallic particles are affected by the fluctuating magnetic thermal fields.

International audience; Thermal emission of a doped silicon grating has been studied in the plane perpendicular to the grooves. We show how the excitation of surface plasmons produce a resonant emission weakly depending on the polarization and azimuthal angle. We analyze in detail the polarization and angular dependence of the emission out of the plane perpendicular to the grooves. Two kinds of thermal sources, directional and quasi-isotropic, are studied. They have been designed in a previous paper. We also compute the total hemispherical emissivity of these gratings. In addition we show that in applications such as radiative cooling, these sources are less efficient than other structures.

An analytical study of the optical properties of nanocylinder arrays is presented. The conditions to tune the absorption/thermal emission and extinction resonances of such systems are derived. We predict two kinds of resonances. Close to the Rayleigh anomalies, the array can generate quasimonochromatic and highly directional thermal emission/absorption. Isotropic emission/absorption can also be obtained when the material exhibits an absorption line or in the presence of localized surface plasmon/polaritons. For $s$ polarization our analysis predicts a theoretical limit of 50% of absorption. Interestingly, for $p$-polarized light, a nanocylinder array can present perfect (100%) absorption.

A multiple scattering analysis of the reflectance of a periodic array of sub-wavelength cylinders is presented. The optical properties and their dependence on wavelength, geometrical parameters and cylinder dielectric constant are analytically derived for both s- and p-polarized waves. In absence of Mie resonances and surface (plasmon) modes, and for positive cylinder polarizabilities, the reflectance presents sharp peaks close to the onset of new diffraction modes (Rayleigh frequencies). At the lowest resonance frequency, and in the absence of absorption, the wave is perfectly reflected even for vanishingly small cylinder radii.

We show that a photonic crystal film can emit coherent thermal radiation. We demonstrate the key role of leaky waves existing at the air-photonic crystal interface. The frequency and direction of emission depend on the lattice parameters. This paves the way towards the design of coherent infrared antennas.

We show that a photonic crystal film can emit coherent thermal radiation. We demonstrate the key role of leaky waves existing at the interface air-photonic crystal.

An analytical multiple scattering study of the reflectance and absorption of a periodic array of sub-wavelength cylinders is presented. The conditions for perfect reflection and resonant absorption are discussed.

We study numerically the dispersion relation and the structure of the optical modes along a chain of metallic nanoparticles. Polariton modes exists in both the radiative and non-radiative regions. Applications to nanophotonics are discussed.

We show that a slab of photonic crystal can transmit 100% of an incident beam in the forbidden gap. This phenomenon is due to the resonant excitation of leaky modes along the interfaces.

We show that a slab of a photonic crystal whose thickness is 3.5 mu m can transmit 100% of an incident beam whose frequency lies in the forbidden gap. This phenomenon is not linked to a volume defect but to the resonant coupling of leaky surface modes at both interfaces

Nous avons concu et realise une source de lumiere thermique en tungstene, dont la directivite dans le domaine infrarouge est comparable a celle d'un laser CO 2 [1]. Cette grande directivite est la signature de la coherence spatiale du champ dans le plan de la source. Ce phenomene trouve son origine dans l'excitation thermique resonnante de plasmons polaritons de surface.

Arrays of transparent dielectric nanorods are shown to produce very large local field enhancements at specific resonant conditions. These structures would lead to enhancement of molecular fluorescence signals without quenching. The resonant angular width and field enhancements are analytically derived as a function of wavelength, grating period, rod radius, and dielectric constant.

We report the design of a tungsten thermal source with extraordinarily high directivity in the near infrared, comparable to the directivity of a CO2 laser. This high directivity is the signature of the long-range correlation of the electromagnetic field in the source plane. This phenomenon is due to the resonant thermal excitation of surface-plasmon polaritons.