Céline Croutxe-Barghorn

Self-developing photopolymer for the fabrication of relief micro-optical elements

The use of photopolymer films to record information by optical methods is described. The final result in such thin polymer films is its surface modulation that develops under the sole action of light and does not require any chemical treatment. The relief generating process permits the fabrication of diffractive and refractive optical elements that work in transmission or in reflexion. This last mode is achieved when a metal thin film is overcoated on its surface. Low spatial frequency gratings, microlenses and micromirrors were made. Several diffractive elements using computer generated holograms were also fabricated.

Interdependence of volume shrinkage, spatial frequency, and mass transfer in relief gratings fabricated with self-processing photopolymers

Local changes in thickness occurring upon controlled exposure to light of photopolymers capable of storing optical information were investigated. The image that developed as a relief in the photosensitive layer exhibited a complete self-processing character because it did not require any chemical post-treatment. Special attention was given to the effect of the gradient of chemical composition, the volume shrinkage, the gradient of surface free energy leading to mass transfer during the generation of low frequency relief gratings. This imaging techniques provides a high flexibility as regards height and shape of the generated relief. It allows fabrication of both diffractive and refractive optical elements that come increasingly into prominence as the micro-opto-electro-mechanical field expands.

Microlens array fabrication through crosslinking photopolymerization

Growing interest in miniaturized optical components for various applications such as optical interconnection systems and telecommunication industry have led to the development of several techniques that are used in the fabrication of micro-optical elements. One approach involves the use of polymers as recording materials: these are flexible, highly transparent and cheap. The technique described in the present paper is founded on the ability of self-processing photopolymers to generate refractive microlens arrays. Spatially controlled illumination of a photosensitive layer induces an inhomogeneous photopolymerization involving a mass-transport process of reactive species and generating a relief in the photopolymer layer. The presentation focuses on the fabrication of microlens arrays through photopolymerization with the green line of an argon-ion laser. Surface tension and differential volume shrinkage turned the illuminated area into good quality lenses. The fabricated lens arrays exhibit diameters ranging from less than 20 micrometers to more than 500 micrometers and focal lengths from 100 micrometers to a few millimeters, depending on photonic, optical and physico-chemical parameters. This imaging technique is highly flexible as regards height, shape and optical properties of the lenses that are produced. By starting from the same background, one can also fabricate diffractive optical elements such as gratings and duplicate computer-generated holograms that come increasingly into prominence as the micro-opto-electro-mechanical field expands.

Photopolymerizable hybrid sol-gel glasses as holographic recording media

Photopolymerizable hybrid sol-gel are extremely interesting for optical and photonic applications. They combine the properties of glasses with the possibility of photopatterning the layer at the micrometer scale. The presented results concern the generation of volume gratings created by transmission and reflection using an interferences pattern at 514 nm. In transmission, the diffraction efficiencies were going from 30 % to 95 % (ratio of the diffracted intensity to the diffracted plus transmitted intensities) for a thickness ranging respectively from 40 μm to 100 μm and a spatial frequency of 1000 lines/mm. It corresponded to a refractive index modulation estimated between 4 and 5 x 10-3according to Kogelniks theory. Reflection gratings with fringe spacing of 0.17 or 0.39 μm were recorded in the material. In normal incidence light beams were highly diffused, whatever the wavelength in the visible range. On the contrary, in oblique incidence, light beams were transmitted through the device without being diffused. This unusual behavior is not yet explained. Applications for information storage can be expected in view of the experimental results, the ease of use and the versatility of this hybrid material.

Photopatterning of hybrid sol-gel glasses for optical and nonlinear optical applications

The present paper deals with the development of hybrid sol-gel glasses as host matrixes for molecules having quadratic NLO properties. Second order non-linearities can be observed after poling in order to orient these molecules. However, due to their small size, thermal relaxation processes lead to a decrease of the induced orientation with time. Host matrixes showing a high rigidity and corresponding to a final material presenting a reduced free volume should overcome this drawback. An interesting way is to use the rigidity conferred by an organic-inorganic sol-gel matrix that can polymerize under irradiation. Photopatterning of the layer and simultaneous copolymerization of the hybrid precursor with functionalized chromophores should enhance the second order NLO response. The results presented in this paper focus on the formulation and conditioning of laminated hybrid sol-gel layers with a thickness of 100 μm that remain crack-free under condensation and photopolymerization. Results are presented concerning the generation of 1 μm gratings obtained under spatially controlled visible illumination using interference pattern at 514 nm. Diffraction efficiencies up to 90 % are obtained for an incident intensity of 30 mW.cm-2. The next step is now the incorporation of functionalized chromophores in the hybrid sol-gel matrix leading to a study of NLO properties of the final material.

Influence of physico-chemical parameters on the surface photopatterning in hybrid sol-gel glasses

Sol-gel process is for more than one decade an interesting way to synthesize inorganic-organic composites under a soft chemistry route. Recently a process for the fabrication of self-developing diffractive optical elements has been developed. The inorganic part of the molecule was an alkoxysilane that reacted via hydrolysis and condensation reactions at room temperature, thus leading to a gel. The mild conditions under which the inorganic part of the material was prepared are compatible with the presence of an organic moiety, a methacrylate function. The organic network was produced via spatially controlled UV illumination using amplitude masks. Surface relief gratings were obtained via this photochemical step ; generation of relief resulted from mass-transfer by diffusion and gradient of surface free energy. No etching process was required. Although the hybrid sol-gel material was assumed to perform along the same lines as the one involving all-organic photopolymers, the influence of the inorganic moieties on the photopolymerization of the methacrylate function remained unknown. Characteristics of the photopolymerization process taking place in hybrid sol-gel materials were investigated by UV and FTIR spectroscopies. Besides the study of the organic network, NMR investigations led information relative to the inorganic network formation during the sol ageing. The whole results provide insights into the influence of the temperature during the photochemical step. Relief gratings generated under various conditions and for a large scale of spatial frequencies are presented. Sinusoidal to almost binary profiles were obtained in a one-step process.