Bruno Gallas

All-Dielectric Colored Metasurfaces with Silicon Mie Resonators

The photonic resonances hosted by nanostructures provide vivid colors that can be used as color filters instead of organic colors and pigments in photodetectors and printing technology. Metallic nanostructures have been widely studied due to their ability to sustain surface plasmons that resonantly interact with light. Most of the metallic nanoparticles behave as point-like electric multipoles. However, the needs of an another degree of freedom to tune the color of the photonic nanostructure together with the use of a reliable and cost-effective material are growing. Here, we report a technique to imprint colored images based on silicon nanoparticles that host low-order electric and magnetic Mie resonances. The interplay between the electric and magnetic resonances leads to a large palette of colors. This all-dielectric fabrication technique offers the advantage to use cost-effective, reliable, and sustainable materials to provide vivid color spanning the whole visible spectrum. The interest and potential of this all-dielectric printing technique are highlighted by reproducing at a micrometer scale a Mondrian painting.

Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.

Enhancement and Inhibition of Spontaneous Photon Emission by Resonant Silicon Nanoantennas

Substituting noble metals for high-index dielectrics has recently been proposed as an alternative strategy in nanophotonics to design broadband optical resonators and circumvent the ohmic losses of plasmonic materials. In this report, we demonstrate that subwavelength silicon nanoantennas can manipulate the photon emission dynamics of fluorescent molecules. In practice, it is showed that dielectric nanoantennas can both increase and decrease the local density of optical states (LDOS) at room temperature, a process that is inaccessible with noble metals at the nanoscale. Using scanning probe microscopy, we analyze quantitatively, in three dimensions, the near-field interaction between a 100 nm fluorescent nanosphere and silicon nanoantennas with diameters ranging between 170 nm and 250 nm. Associated to numerical simulations, these measurements indicate increased or decreased total spontaneous decay rates by up to 15 % and a gain in the collection efficiency of emitted photons by up to 85 %. Our study demonstrates the potential of silicon-based nanoantennas for the low-loss manipulation of solid-state emitters at the nanoscale and at room temperature.

Structural, optical, and electrical properties of epitaxial titanium oxide thin films on LaAlO3 substrate

Titanium oxide thin films were prepared by pulsed-laser deposition on LaAlO3 single crystal substrate at 700u2009°C. Pure anatase films are obtained at high oxygen pressure (10−1u2002mbar), while the rutile phase is evidenced at low oxygen pressure (10−5u2002mbar) despite a large oxygen deficiency (O/Ti=1.75). From asymmetric x-ray diffraction measurements, the in-plane epitaxial relationships be0tween the substrate and the titanium oxide phases are highlighted. Optical constants (refractive index n and extinction coefficient k) were deduced from ellipsometric measurements. The optical band gap energies of the anatase and rutile films are found to be 3.4 and 3.3 eV, respectively. Since the nearly stoichiometric anatase films are resistive (>103u2002Ωu2009cm), the large oxygen deficiency in rutile films leads to noticeable increase in the conductivity due to the Ti3+ species, which supply electrons in the conduction band. At low temperature (T<200u2002K) the resistivity of rutile films versus temperature may be explained by a var...

Infrared ellipsometric study of SiO2 films: relationship between LO mode frequency and porosity

Abstract A method for evaluating the pore volume fraction of silica films for different pore connectivities is presented. A series of SiO2 films, evaporated by electron gun with or without ion bombardment, have been studied using visible and infrared ellipsometry. The TO and LO mode frequencies are deduced from the dielectric function calculated from infrared ellipsometric data. The study of both TO and LO mode frequencies is shown to bring independent information on the film microstructure. The TO mode frequency (near 1075 cm−1) varies mainly with the density of the silica matrix. On the other hand, variation in the LO mode frequency (near 1245 cm−1) is mainly due to changes in porosity. In the case of films with pores largely connected, the evaluation of the pore volume fraction from the LO frequency is compared to that obtained from the analysis of the large water absorption band near 3300 cm−1 (H–OH and Si–OH stretching absorption band).

Evolution of the optical properties of Si nanoparticles embedded in SiO2 as function of annealing conditions

The dielectric function of Si nanoparticles embedded in silica has been determined from spectroscopic ellipsometry and photothermal deflexion spectroscopy from 0.7to6eV. The influence of crystalline fraction and diameter of the nanoparticles on their optical properties has been investigated. Above 4nm of diameter, the nanoparticles presented a dielectric function similar to that of fine grained polycrystalline Si (poly-Si) at photon energy higher than 2eV, with the well marked structures associated with the E1 and E2 critical points. In contrast, below 2eV their absorption coefficient was smaller than for poly-Si. Below 2.5nm of diameter, the dielectric function of the nanoparticles drastically changed. The magnitude of the imaginary part of the dielectric function of the nanoparticles near the position of the E1 critical point constantly decreased, whereas it increased at the position of the E2 critical point. These observations can be interpreted as the result of the transfer of the oscillator strength ...

Oriented Gold Nanorods and Gold Nanorod Chains within Smectic Liquid Crystal Topological Defects

We show that the use of oriented linear arrays of smectic A defects, the so-called smectic oily streaks, enables the orientation of gold nanorods (GNRs) for a large range of GNR diameters, ranging from 7 to 48 nm, and for various ligands. For the small GNRs it enables oriented end-to-end small chains of GNRs when the density is increased from around 2 GNRs/μm2 to around 6 GNRs/μm2. We have characterized the orientation of single GNRs by spectrophotometry and two-photon luminescence (TPL). A strongly anisotropic absorption of the composites and an on-off switching of GNR luminescence, both controlled by incident light polarization, are observed, revealing an orientation of the GNRs mostly parallel to the oily streaks. A more favorable trapping of GNRs by smectic dislocations with respect to ribbon-like defects is thus demonstrated. The dislocations appear to be localized at a specific localization, namely, the summit of rotating grain boundaries. Combining plasmonic absorption measurements, TPL measurements, and simulation of the plasmonic absorption, we show that the end-to-end GNR chains are both dimers and trimers, all parallel to each other, with a small gap between the coupled GNRs, on the order of 1.5 nm, thus associated with a large red-shift of 110 nm of the longitudinal plasmonic mode. A motion of the GNRs along the dislocations appears as a necessary ingredient for the formation of end-to-end GNR chains, the gap value being driven by the balance between the attracting van der Waals interactions and the steric repulsion between the GNRs and leading to interdigitation of the neighboring ligands. We thus obtain electromagnetic coupling of nanorods controlled by light polarization.

Optical properties of mixed TiO2 - SiO2 films, from infrared to ultraviolet

Mixed oxides are useful for obtaining the intermediate refractive indices needed in the realization of graded-index thin films. Co-evaporated TiO2-SiO2 mixtures are studied for a large range of concentrations via UV-VIS, IR ellipsometry and XPS. An understanding of the nature of these mixtures and their air exposure stability is important for further applications. At low TiO2 concentrations, Ti4+ ions are inserted into the silica tetrahedral network, as shown by the IR peak at 945 cm-1. At higher concentrations, an evolution from TiO4 tetrahedra to TiO6 octahedra is presumed. The behavior of the O1s core level peak indicates that a least two phases coexist. Comparison between concentration determined using XPS and RBS shows a deficit in TiO2 at the surface of the films, especially at high TiO2 concentrations. The evolution of the mixtures optical constants will be presented in a large wavelength range, going from IR to UV. Particular attention will be paid to the variations with respect to the frequency of the vibration modes in the IR range, to the refractive index in the transparency region, and to the extinction coefficient at he absorption threshold. In addition, AFM measurements show the variation of the grain size as a function of the TiO2 concentration.

Measurement and modelization of silica opal optical properties

We present the synthesis process and optical characterization of artificial silica opals. The specular reflection spectra are analyzed and compared to band structure calculations and finite difference time domain (FDTD) simulations. The silica optical index is a key parameter to correctly describe an opal and is usually not known and treated as a free parameter. Here we propose a method to infer the silica index, as well as the silica spheres diameter, from the reflection spectra and we validate it by comparison with two independent infrared methods for the index and, scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements for the spheres diameter.

Enhancing or inhibiting spontaneous emission from fluorescent molecules in the near-field of silicon nanoantennas (Conference Presentation)

Optical nanoantennas have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters. Using scanning probe microscopy, we analyze in three dimensions the near-field interaction between a 100 nm fluorescent nanosphere and silicon nanoantennas. Furthermore, fluorescence correlation spectroscopy measurements demonstrate that the fluorescence intensity of dye molecules can be enhanced by more than two orders of magnitude in the nanoscale gap between silicon nanodisks. These results show the potential of silicon antennas for the manipulation of solid-state emitters at the nanoscale and at room temperature.