Leaky mode analysis of luminescent thin films: the case of ZnO on sapphire

Zinc oxide (ZnO) epitaxial thin films grown on c-sapphire substrates by pulsed laser deposition were investigated using angle and polarization-resolved photoluminescence spectroscopy. Side-emission spectra differed significantly from surface-emission spectra in exhibiting dominant, narrow, polarization-resolved peaks. These spectral features were attributed to leaky substrate modes in the layers. Observations were first verified using transmission calculations with non-adjustable parameters, which took into account the dispersion, the anisotropy of the ZnO refractive index and the dependence on film thickness. Results were consistent with Fabry-Perot-like interference being the origin of the distinctive ZnO luminescence observed at grazing incidence angles. A second analysis, based on the source terms method, was used in order to retrieve the bulk emission properties, including the wavelength-dependent quantum yield and the emission anisotropy. While ZnO thin films were considered here, this analysis method can be extended to any luminescent thin film of similar geometry, demonstrating the potential of leaky mode analysis for probing passive and active material properties

Simple and robust analytical model for dipolar resonances in plasmonic particle-substrate systems

We revisit the theory of light-matter interactions induced by a small metallic particle near a planar surface. The resonant features of the coupled system are investigated using an approach that determines the particle permittivity that brings the system to resonance. Analytical results are presented for the case of a spherical particle near a flat substrate and compared to previous results, whereas a general recipe involving Green's function is also given. In contrast to previous work, our analytical results allow the determination of eigenfrequencies in the presence of strong temporal dispersion and include radiative damping. Furthermore, the model can be related to stationary normal modes, thereby providing physical insight and a design tool for surface-enhanced devices and sensing

Light propagation mapping in surface waveguides using nano carbon probe grown on polymer tipped optical fiber

International audienceWe report on the fabrication, simulation and use of carbon nano-probes grown on the apex of polymer-tipped optical fibers. The carbon needles are used as near-field scattering probes to image light propagation in surface waveguides. The scattered light is selectively and efficiently coupled in the supporting optical fiber through the polymeric structure which is used here as a submicronic collection optics. This combination leads to high spatial resolution and high detection efficiency

Measurement of phonon damping by nanostructures

International audienceThe understanding of phonon lifetime and scattering rates is attracting an increasing interest due to the major role of phonon in thermal and electrical conductivity which are key properties for technological applications. The infrared complex dielectric function of a crystal is determined by the harmonic characteristics of the phonon together with the intrinsic and extrinsic phonon scattering rates. In order to investigate the interplay between the phonon intrinsic scattering and the scattering of the phonon by a nanostructured surface, infrared reflectivity measurements from SiC nano-pyramids on SiC substrate have been analysed using a Kramers-Kronig conversion technique to deduce the infrared complex dielectric function. Then, the real and imaginary parts of the dielectric function were fitted simultaneously by using a theoretical model for the dielectric constant that considers frequency-dependent phonon damping at the center of the Brillouin zone. It has been found that surface nanostructuring strongly enhances the overall scattering rate of the phonon at the Brillouin zone center

Wavelength-scale light concentrator made by direct 3D laser writing of polymer metamaterials

International audienceWe report on the realization of functional infrared light concentrators based on a thick layer of air-polymer metamaterial with controlled pore size gradients. The design features an optimum gradient index profile leading to light focusing in the Fresnel zone of the structures for two selected operating wavelength domains near 5.6 and 10.4 μm. The metamaterial which consists in a thick polymer containing air holes with diameters ranging from λ/20 to λ/8 is made using a 3D lithography technique based on the two-photon polymerization of a homemade photopolymer. Infrared imaging of the structures reveals a tight focusing for both structures with a maximum local intensity increase by a factor of 2.5 for a concentrator volume of 1.5 λ3, slightly limited by the residual absorption of the selected polymer. Such porous and flat metamaterial structures offer interesting perspectives to increase infrared detector performance at the pixel level for imaging or sensing applications