[en] We propose a thermal antenna that can be actively switched on and off at the microsecond scale by means of a phase transition of a metal-insulator material, the vanadium dioxide (VO₂). This thermal source is made of a periodically patterned tunable VO₂ nanolayer, which support a surface phonon-polariton in the infrared range in their crystalline phase. Using electrodes properly registered with respect to the pattern, the VO₂ phase transition can be locally triggered by ohmic heating so that the surface phonon-polariton can be diffracted by the induced grating, producing a highly directional thermal emission. Conversely, when heating less, the VO₂ layers cool down below the transition temperature, the surface phonon-polariton cannot be diffracted anymore so that thermal emission is inhibited. This switchable antenna could find broad applications in the domain of active thermal coatings or in those of infrared spectroscopy and sensing.

[en] Incandescent radiation sources are widely used as mid-infrared emitters owing to the lack of alternative for compact and low cost sources. A drawback of miniature hot systems such as membranes is their low efficiency, e.g., for battery powered systems. For targeted narrow-band applications such as gas spectroscopy, the efficiency is even lower. In this paper, we introduce design rules valid for very generic membranes demonstrating that their energy efficiency for use as incandescent infrared sources can be increased by two orders of magnitude

[en] Directional plasmon excitation and surface enhanced Raman scattering (SERS) emission were demonstrated for 1D and 2D gold nanostructure arrays deposited on a flat gold layer. The extinction spectrum of both arrays exhibits intense resonance bands that are redshifted when the incident angle is increased. Systematic extinction analysis of different grating periods revealed that this band can be assigned to a propagated surface plasmon of the flat gold surface that fulfills the Bragg condition of the arrays (Bragg mode). Directional SERS measurements demonstrated that the SERS intensity can be improved by one order of magnitude when the Bragg mode positions are matched with either the excitation or the Raman wavelengths. Hybridized numerical calculations with the finite element method and Fourier modal method also proved the presence of the Bragg mode plasmon and illustrated that the enhanced electric field of the Bragg mode is particularly localized on the nanostructures regardless of their size. (paper)

[en] Cubic crystals are often favored due to their interesting isotropic properties -such as the refractive index, the dilatation, etc-that simplify their use in laser systems. Among all Yb-doped high-symmetry materials, Yb:CaF₂ is a very attractive one for high-energy high-power lasers. However, in such systems, thermal loads become important and thermal anisotropic effects can occur even for simple cubic crystals impacting severely the laser system design. In this context, we report here, a study of thermally induced polarization anisotropy in Yb-doped CaF₂ laser crystals. Both thermal lens and thermally induced depolarization are precisely studied for different crystal orientations, namely the [110] and the [111] orientation. The study is performed with a pump/probe setup adapted to improve the signal-to-noise ratio under high power pumping. We observed both, classical-thermally induced depolarization and thermal lensing in accordance with previous work, but also, for certain configurations, atypical depolarization figures and astigmatic lenses. These results were also confronted with numerical simulations. The comparison between the [110] and the [111] oriented crystals represents a crucial study to determine the origins of these unusual effects and to allow a better understanding of thermal anisotropy in cubic laser-crystals.