Loic Berthod

Periodic TiO2 Nanostructures with Improved Aspect and Line/Space Ratio Realized by Colloidal Photolithography Technique

This paper presents substantial improvements of the colloidal photolithography technique (also called microsphere lithography) with the goal of better controlling the geometry of the fabricated nano-scale structures—in this case, hexagonally arranged nanopillars—printed in a layer of directly photopatternable sol-gel TiO2. Firstly, to increase the achievable structure height the photosensitive layer underneath the microspheres is deposited on a reflective layer instead of the usual transparent substrate. Secondly, an increased width of the pillars is achieved by tilting the incident wave and using multiple exposures or substrate rotation, additionally allowing to better control the shape of the pillar’s cross section. The theoretical analysis is carried out by rigorous modelling of the photonics nanojet underneath the microspheres and by optimizing the experimental conditions. Aspect ratios (structure height/lateral structure size) greater than 2 are predicted and demonstrated experimentally for structure dimensions in the sub micrometer range, as well as line/space ratios (lateral pillar size/distance between pillars) greater than 1. These nanostructures could lead for example to materials exhibiting efficient light trapping in the visible and near-infrared range, as well as improved hydrophobic or photocatalytic properties for numerous applications in environmental and photovoltaic systems.

Efficient reversible phase mask for TiO2 submicron gratings directly printed on cylindrical surfaces

In this article we present a radial phase mask specially designed and manufactured for direct micro-structuration under UV photolithography of a cylindrical surface covered by a photoresist TiO2 film. The period of the phase mask is sub-micron (between 480 nm and 720 nm) and allows direct printing on several types of cylindrical components. With this dedicated reversible phase mask we have demonstrated the feasibility of a TiO2 grating with a period of 960 nm, printed on a SiO2 cylinder or inside a SiO2 tube of 8 mm diameter.

Dynamic Interferometry Lithography on a TiO2 Photoresist Sol-Gel for Diffracting Deflector Module

Solar electricity is one of the most promising renewable energy resources. However, the ratio module’s cost/energy produced remains a major issue for classical photovoltaic energy. Many technologies have been developed to solve this problem, by using micro- or nanostructuring on the solar cell or on the module. These kinds of structuring are often used as antireflection and light-trapping tools. In the meantime, other solar technologies are considered, such as concentration photovoltaic modules. This article presents a module combining both approaches, that is, nanostructures and concentration, in order to increase the module’s profitability. Sol-gel derived TiO2 diffraction gratings, made by dynamic interferometric lithography, are added on the top of the glass cover to deflect unused light onto the solar cell, increasing the module efficiency.

Cost-effective and large-area process for the functionalization and surface microstructuring using photo-patternable TiO2 sol-gel process and colloidal photolithography

The authors demonstrate a low cost direct process to directly print TiO 2 grating on both planar and non-conventional substrate such as cylinder components. A well collimated i-line source emitting at a 365nm wavelength illuminates a mono layer of 1μm diameter silica microspheres deposited on a photosensitive TiO 2 -based sol-gel layer. No etching process is required since this layer is directly UV photo patternable like a negative photo-resist. Furthermore this thin layer offers interesting optical properties (high refractive index) and good mechanical and chemical stability. The paper describes the photochemistry of the TiO 2 sol-gel layer process, the modeling of the electric field distribution below the spheres during the illumination process and preliminary results of TiO 2 nanopillars of 200 nm diameter.

Nano-composite magnetic material embedded on TiO2 pillars to realize magneto-optical resonant guided mode gratings

Periodic structuration of magnetic material is a way to enhance the magneto-optical behavior of optical devices like isolators. It is useful to reduce the footprint of such integrated devices or to improve their features. However, the structuration and/or integration of efficient magnetic materials on photonic platforms is still a difficult problem, because classical magneto-optical materials require an annealing temperature as high as 700°C. A novel wafer-scale approach is to incorporate that material into an already structured template through a single step deposition at low temperature. Using the dip-coating method, a magneto-optical thin film (~300nm) of CoFe2O4 nanoparticles in silica matrix prepared by sol-gel technique was coated on a 1D and 2D TiO2 subwavelength gratings. Such gratings were realized by the patterning of TiO2 films obtained by a sol-gel process. It was confirmed by Scanning Electron Microscope images that the magneto-optical composite completely occupies the voids of the 2D structuration showing a good compatibility between both materials. This composite shows a specific Faraday rotation of about 200°cm-1 at 1,5μm for 1% of volume fraction of nanoparticles. Spectral studies of the transmission and the reflection of a 1D TiO2 grating filled with the MO composite have evidenced the presence of a guided-mode optical resonance at 1,55μm. The position of this resonance was confirmed by numerical simulations, as well as its quite low efficiency. Based on simulations results, one can conclude that an increase of the grating depth is required to improve the efficiency of the resonance.

Direct sub-micron microstructuring on cylinder using TiO2 sol-gel process and radial phase mask based lithography

Design and fabrication of a high efficiency phase mask have been performed for printing submicron period gratings along 8 mm diameter glass cylinders. In this article, the authors present the radial phase mask specially designed and manufactured for a cylindrical surface micro-structuring under UV photolithography. Its period is sub-micron (480 nm < ΛPM < 720 nm). The authors describe then the phase mask based UV lithography set-up using the interference between the transmitted beams of +1 and -1 orders generated by the phase mask. Preliminary results of printed gratings on a cylinder are described on a sol-gel TiO2 thin film layer, enabling direct photo patterning on functionalized layer. The feasibility of a grating printed with a period of Λcylin = 960 nm on an 8 mm diameter cylinder with this dedicated mask has been demonstrated.