Marine Laroche

Extraordinary optical reflection from sub-wavelength cylinder arrays.

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.

Influence of microroughness on emissivity

In this paper we revisit the absorption and thermal emission of electromagnetic radiation by a rough surface. We use a numerical simulation of the absorptivity of a grating to explore the validity of the ray tracing approach. We show that it often predicts correctly the absorptivity and emissivity of a surface with characteristic lengths on the order of a wavelength. Recent advances in the understanding of the microscopic mechanism of thermal emission in the near field are used to discuss the data and to explain this surprising result. We also identify three different regimes depending on the ratio of the period to the wavelength: the homogenization regime, the resonance regime, and the geometrical optics regime.

Optical extinction in a single layer of nanorods.

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.

Degree of polarization of thermal light emitted by gratings supporting surface waves

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.

Asymptotic expressions describing radiative heat transfer between polar materials from the far-field regime to the nanoscale regime

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 accurate 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 75 nm at room temperature the heat transfer is dominated by the surface phonon polariton contribution. Between 75 nm 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.

Hot carrier solar cells: Controlling thermalization in ultra thin devices

From present day 40% conversion efficiencies to the thermodynamic limit (> 85%), there is still a lot of room for improvement. Hot carrier solar cells provide an attractive solution to fill this gap, by converting more efficiently the part of the incident power that is usually lost as heat. In those devices, the photogenerated carriers are not thermally equilibrated with the lattice. This occurs if the carrier thermalization pathways are saturated, either by reducing the electron-lattice interaction, or by increasing the carrier density. Antimony-based quantum well structures with 50 nm thick active material were synthesized for investigating their thermalization properties. The carrier temperature is determined as a function of the incident power density. Results indicate potential efficiency above 50% provided the incident power can be absorbed in a 50 nm thick absorber. Without specific care, reducing the absorber thickness would result in a reduced absorption and limited efficiency. Here, we propose a nanoscale structuration of the cell surface that enables strong absorption enhancement. 70 to 80 % of the incident power can be absorbed in a 50 nm thick GaSb layer. Going for high carrier density enables to lower the requirement on the cooling rate reduction. We show that using the structure described here and the thermalization rate measured on test samples, the potential efficiency is above 50%.

Hot-carrier solar cells

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.

Nearly total optical extinction in arrays of non-resonant nanorods

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.

Drilled dielectric membranes for highly resonant filtering in the infrared

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 &mgr;m period. Experimental angularly resolved transmission spectra show evidences of their filtering properties.