Design of infrared optical absorber using silver nanorings array made by a top-down process.

This paper presents the numerical simulation and fabrication of a metasurface composed of silver nanorings with a split-ring gap. These nanostructures can exhibit optically-induced magnetic responses with unique possibilities to control absorption at optical frequencies. The absorption coefficient of the silver nanoring was optimized by performing a parametric study with Finite Difference Time Domain (FDTD) simulations. The absorption and scattering cross sections of the nanostructures are numerically calculated to assess the impact of the inner and outer radii, the thickness and the split-ring gap of one nanoring, as well as the periodicity factor for a group of four nanorings. This showed full control on resonance peaks and absorption enhancement in the near infrared spectral range. The experimental fabrication of this metasurface made of an array of silver nanorings is achieved by e-beam lithography and metallization. Optical characterizations are then carried out and compared to the numerical simulations. In contrast to usual microwave split-ring resonator metasurfaces reported in literature, the present study shows both the realization by a top-down process and modelling performed in the infrared frequency range.

Spherically shaped micro-structured antireflective surfaces.

An antireflecting micro-structured interface, working in the resonance domain, and made from a bi-periodic array of semi-spherical hollowing-out in a silicon substrate is presented. Its parameters such as sphere radius and position of sphere centers from the surface are optimized numerically. A simple and robust process is described allowing such kind of antireflective surfaces to be fabricated for the infrared range. Spectral and angular reflectance measurement demonstrates the efficiency of the antireflective micro-structured interface which can easily be adapted for the visible range and for photovoltaic applications by a simple homothetic modification of the micro-structure typical dimensions.

Enhanced antireflecting properties of micro-structured top-flat pyramids.

This paper aims at modeling bi-periodic micro-structured silicon surfaces exhibiting broadband antireflection properties in the infrared range using Rigorous Coupled-Wave Analysis (RCWA). These structures of pyramidal shape, which typical dimensions are smaller than the wavelength, are not in the Effective Medium Theory (EMT) validity domain. The influence of various opto-geometrical parameters such as period, depth, shape of the pattern is examined. The antireflective properties of such bi-periodic patterned surfaces are then discussed using the photonic crystal theory and photonic band diagrams description. Correlations between the density of Bloch modes, their localizations with respect to the incident medium light line and the surface reflectance are presented.

Integrated polarization rotator made of periodic asymmetric buried Ta2O5 / silica sol-gel waveguides.

A ridge waveguide technology exhibiting high polarization dependency is developed for new efficient multi-section passive polarization rotator applications. In the presented configuration, the calculated mode coupling between the waveguide sections is very efficient and allows a polarization rotation with a high extinction ratio at lambda=1,55 mum. Experimental results show efficient polarization rotation with low cross-talk levels (-16dB) and no significant excess losses between sections. However, the overall transmission efficiency is limited by propagation losses and coupling losses to standard optical fibers.

Fabry-perot multilayers for enhancing the diffraction efficiency of ion-implanted gratings.

Enhancement of the free-space diffraction efficiency of gratings made by titanium-ion implantation is demonstrated both theoretically and experimentally. Indeed, by insertion of a grating into a multilayer dielectric Fabry-Perot cavity, the diffraction efficiency can be increased to as much as 24 times that of a single grating. The sensitivity of the diffraction efficiency to the optogeometrical parameters of the grating or of the Fabry-Perot cavity is discussed. Moreover, a process for performance of a phase grating inside a Fabry-Perot cavity is described, and experimental results concerning efficiency measurements are compared with computed values for various grating periods.

Enhanced diffraction efficiency of gratings in multilayers.

Computations with the rigorous differential method show that single gratings made by ion implantation have a diffraction efficiency in the +1 transmitted order under TE illumination of only 0.78%. The insertion of such gratings into multilayer dielectric Fabry-Perot cavities leads to an enhancement of the free-space diffraction efficiency. Different designs for the multilayer are considered. An 18.8% efficiency is reached with 11-layer mirrors. This result is obtained by optimization of the thickness of the spacer of the Fabry-Perot cavity that contains the grating and centering of the wavelength of the mirrors. The dependence of optical properties of the structure on the various optogeometrical parameters of the structure is discussed.

Solarization of glass substrates during thin-film deposition.

We demonstrate that solarization occurs in glass substrates during thin-film deposition and that it induces high absorption near the surface of the substrate. Solarization has been observed especially in ion-plating deposition. We show that the solarization of the substrate is caused by electromagnetic radiation emitted from the material to be evaporated. The radiation is due to the energy losses of the heating beam of electrons (bremsstrahlung radiation). Multicomponent glasses such as BK7 are much more sensitive to solarization than fused-silica substrates. The photoinduced high absorption can be partially reversed by thermal annealing.

Substrate effects on absorption of coated surfaces.

Photothermal deflection is used for mapping the absorption of bare and coated surfaces. The same area is mapped before and after coating and also after annealing. The great importance of the substrate with respect to the total losses of the coated component is emphasized. First the influence of surface contamination of the bare substrate on the total absorption of the coated substrate is studied for BK7 and fused-silica substrates. Then the mean value of the coated-substrate absorptance is shown to be strongly dependenton the type of substrate. Experimental results show that this effect is associated with a localization of the absorption at the near surface of the substrate and at the interfaces of the film.