Christiane Carre

A new gated system for two-photon holographic recording in the near infrared

A new two‐photon method for recording holograms is presented. It is based on the fact that a first photon produces singlet oxygen by a photosensitization process; this species oxidizes a specific trap to produce a carbonyl compound. This latter is used as a conventional polymerization initiator and produces active species after the absorption of a second photon. The feasibility of this general process is demonstrated by using Methylene Blue (photosensitizer), diphenylisobenzofuranne (singlet oxygen trap), and a mixture of acrylamides (polymerizable compound). Finally, diffraction efficiencies higher than 2% are easily achieved with such systems. The influence of different experimental parameters was studied with a view to extending the sensitivity of the system to the infrared.

Computer-originated polarizing holographic optical element recorded in photopolymerizable layers.

The photosensitive system that is used in most cases to produce holographic optical holograms is dichromated gelatin. Other materials may be used, in particular, photopolymerizable layers. In the present investigation, we set out to use the polymer developed in the Laboratoire de Photochimie Générale in Mulhouse in order to duplicate a computer-generated hologram. Our technique is intended to generate polarizing properties. We took into account the fact that no wet chemistry processing is required; grating fringe spacings are not distorted through chemical development.

Methylene blue sensitized gelatin: evidence of photodemethylation

The dependence of diffraction efficiency of methylene blue sensitized gelatin gratings vs dye concentration is explained in terms of the aggregations tate of the dye.

Photopolymerization induced materialization of the dipolar response from isolated metallic nanoparticles

We report on the characterization of the field diffracted by Au nanoparticles under optical excitation. The spatial distribution of the scattering diagram of the nanoparticles is materialized in real space through photopolymerization. These experiments find their motivations in optics where metallic nanoparticles are thought to find promising applications in plasmonics, but also in chemistry where nanometer scale polymerization mechanisms is a subject of current interest for both fundamental purposes or lithographic applications. For our experiments, the nanoparticles embedded in a photopolymerizable material are deposited on a glass substrate. The sample is then subjected to a global illumination and the field scattered by the particles enables for a local optical activation of the polymerization reaction. The nanometric sensitivity of the polymerization reaction determines the reactions transfer function and allows for a spatially controlled characterization of the field scattered by the particles. The shape of the resulting polymeric material, representing the particles spatial diffraction pattern, is subsequently characterized using Atomic Force Microscopy (AFM). For colloidal, randomly dispersed Au nanoparticles excited with linearly polarized light, the scattering induced topography is related to the dipolar response from the particle. More specifically, different components of the scattered field were identified that we assigned to the evanescent and progressive contributions of the dipoles field. Experiments in progress are aimed to study interacting particles with various shapes and sizes.

Studies of the photoreduction of methylene blue in gelatin solutions and films with a view to improving the holographic recording process at 633 nm

The photoreduction of methylene blue (MB), in methylene blue sensitized gelatin (MBG) solutions and films, was studied for the first time with time-resolved laser spectroscopy. The dependence of the lifetime of the triplet state precursor was estimated by varying the pH, and the efficiency of this photoprocess was evaluated in solid thin films. This work constitutes the first step towards improving the holographic recording speed of MBG.

Application of High Performance Photoinitiating Systems for Holographic Grating Recording

In this chapter, a compilation of different systems able to photogenerate active radicals toward polymerization reaction( type I, type II and three component photoinitiating systems) for application in holographic grating recording when associated to monomers is reviewed. In particular, the visible curable system is associated to fluorinated acrylate monomers formulation for creation of transmission gratings. The PIS efficiencies are presented in term of diffraction grating yields and compared to photopolymerization experiments. The special case of photocyclic initiating systems is described in details, its influence on the grating built up being discussed on the basis of selected mixtures using visible dyes, electron donors (e.g. amines), electron acceptors (e.g. iodonium salts) or hydrogen donors as coinitiators. The role of the photochemical properties of dye on the performance of the holographic recording material is investigated through time resolved and steady state spectroscopic studies of the PIS (e.g. nanosecond laser flash photolysis), to highlight the photochemistry underlying active radicals photogeneration. In order to get more insight into the hologram formation, grating formation curves were compared to those of monomer to polymer conversion obtained by real time Fourier transform infrared spectroscopy (RTFTIR). This work outlines the importance of the coupling between the photoinitiating system (i.e. the photochemical reactions) and the holographic resin.

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.

Sterically hindered pyridinium phenoxides as chromophores for quadratic optics

Polymers doped with non-linear optical (NLO) molecules are key materials in the elaboration of organic NLO devices. In this field, there is an ongoing need for chromophores with large dipole moments and optical non linearities. Here, we consider pyridinium phenoxides, a type of zwitterionic biphenyl-like molecule. A combination of mathematical modelling and some preliminary experimental measurements indicate that the NLO properties of these molecules depend on the twist angle existing between the two aromatic rings. In order to corroborate this structure/activity relationship, different sterically hindered pyridinium phenoxides were synthesized using the Suzuki coupling reaction involving a boronic ester and an aryl halide. We analyze the solvatochromism of the substituted zwitterions in details, determine the chemical equilibrium of protonation and perfom nonlinear optical measurements which are interpreted with the help of semi-empirical calculations.

Pulsed-force mode AFM characterization of photopatterned polymer films for holographic data storage application

Organic materials are taking a growing place in the development of new materials for data technologies thanks to the potential of molecular engineering, the flexibility of available chemical compositions, the low costs..., but also because of their unique optical and mechanical properties. In this context, photopolymers present specific advantages particularly interesting for high density optical data storage, based on the possibility of structuring their linear and nonlinear optical properties with a great facility by direct optical patterning. In order to understand and control the physico-chemical aspects of the photopatterning, means of investigation at a micro and nanoscopic scales are required. Not only the 3D imaging of the object is needed, but some structural information on the material is necessary to go further in the investigation of the involved phenomena. AFM used in Pulsed Force Mode (PFM) fulfils these requirements: the PFM mode is a non-resonant mode designed to allow approach curves to be acquired along the scanning path. It thereby provides a recording of the sample topography and extends the possibilities of the prevalent contact and intermittent-contact AFM modes to a direct and simple local characterization of adhesion and stiffness. This paper describes the principle of Pulsed Force Mode AFM and illustrates its usefulness for investigating of the photostructuration of polymer matrixes. In a first part, homogeneously irradiated films were characterized in order to demonstrate the sensibility of the PFM analysis. In particular, the PFM signal is correlated to the monomer conversion ratio that was measured by FTIR spectroscopy. In a second step, we illustrate the potential of PFM for the investigation of photopatterned films. Holographic gratings were recorded in an acrylate-based formulation and characterized by PFM. We have successfully assigned the different areas of the film that correspond to different incident intensities. Using the information recorded on homogeneous films, it is possible to obtain an estimation of the conversion of the monomer at sub-micronic scale. Such a study is of primary importance in order to understand the mechanism leading to microstructuration and thus to optimize this process in terms of resolution.

Influence of the physico-chemical and photonic parameters on mechanical and optical properties of end-of-fiber polymer tips

A flexible method of manufacturing polymer microlenses at the extremity of both single mode and multimode optical fibers has been previously developed. The procedure consists in depositing a drop of liquid photopolymerizable formulation on the cleaved fiber end and using the light emerging from the fiber to induce polymerization leading to the formation of a polymer tip. This process is highly interesting for applications in optical fiber connecting and SNOM imaging since it is fast, highly flexible (curvature radius can range from 0.2 to 100 μm) and does not require expensive equipment. Although the fabrication process leads to well-controlled geometrical structures, the mechanism of the polymer tip formation was not fully elucidated. In this work, we particularly focus on the photoinduced physico-chemical processes that occur during the lens formation. The effect of different parameters (irradiation time, light power, received energy, oxygen...) on the final properties of polymer tip (mechanical resistance, curvature diameter) was studied. The building up of the polymer tip was characterized by optical microscopy. This study allowed selecting the synthesis parameters leading to an improvement in the mechanical and optical properties of the polymer tip. From a fundamental point of view, this study appeared to be an interesting means to investigate the photostructuration of polymers at the micro- and nanoscales.