In Lippmann photography, the interference of the image with its reflection onto a mirror in contact with the photographic
emulsion allows, for each pixel of the image, the recording of Bragg gratings. Removing the mirror, processing the plate
and reading out these Bragg gratings with a white light source diffracts the very colours used for recording and thus
reproduces the images in colours. Using Lippmann photography as a data storage technique was proposed in the 1960th:
for a given pixel, and to each recording wavelength is associated one bit of data, several bits being recorded at the same
pixel. In this paper, we revisit this data storage technique and we propose and demonstrate an homodyne detection to
improve the efficiency of Lippmann data storages. The proposed homodyne geometry also presents the advantage to
simplify the architecture: the Lippmann mirror required for recording is kept in place for data retrieving. Such an
homodyne readout could also be applied to enhance the detected signals in other holographic approaches.
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.
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.
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 Kogelnik's 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.
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.
Organic materials possessing quadratic non linear optical (NLO) properties are now part of the optical material panel available for applications. Up to now, the patterning of their optical properties for device fabrication is commonly achieved by the lithographic techniques derived from the microelectronic domain. We propose here an alternative method that allows a direct patterning of the NLO properties. The objective is to manufacture quasi phase matched devices for optical frequency conversion in guided mode and in the regime of free propagation.
Noticeable refractive index modulations (difference between the refractive index of high-density areas and the index of low-density areas ~8x10-3) can be created by photostructuration of acrylic films. The light pattern created by the interference of two plane waves induces inhomogeneous polymerization and mass diffusion processes, due to concentration gradients of monomer and dye, giving rise "in situ" to a structuration of the material at the microscopic scale. As the species involved in the initiation mechanism are gradually consumed as the hologram builds up, the incident dose is determined in order to reach full completion of the reaction at the end of the recording, i.e. to obtain stable gratings. This property makes photopolymers attractive materials for number of applications, especially in holography. A great advantage of these materials over other recording systems is that no chemical or heating post-treatment is required after illumination to reveal the hologram. Diffraction efficiencies of ca 80 % were obtained at 514 nm for transmission holograms with a fringe spacings between 0.2 and 5 μm. 675 mJ/cm2 corresponding to a bleaching of the dye of 85% allows non-destructive reading at an active wavelength (green light). Miscellaneous photonic parameters (chemical composition, intensity, dose...) were tested by grating recording. Taking into account all these data, improvement of the material is possible in view of data storage applications.
Free-radical photocurable hybrid material was used for the creation of crack free holographic gratings, showing only a refractive index modulation. Organic-inorganic materials based on the sol-gel synthesis of alkoxysilanes have demonstrated their great potential for coating and optical applications. In a first step, hydrolysis and condensation reactions were led under mild conditions along the same way as in classical sol-gel glasses. Partial elaboration of the silicate backbone was thus achieved. Then, with the use of a photoinitiator, free-radical polymerization was proceeded by irradiating the material under UV or visible light. A spatially controlled photopatterning can be thus achieved to create optical elements having the hardness of a glass-like material. The presented results concern the formation of gratings created by transmission using interference pattern at 514 nm. Diffraction efficiencies up to 90 % were reached for 1000 lines/mm in transmission, corresponding to a refractive index modulation amplitude of 4.10-3 (according to the Kogelnik's theory). To elaborate non linear quadratic optical devices, hybrid sol-gel matrixes were doped with push-pull chromophores. Such a photosensitive system enables simultaneous photopatterning of the layer and poling of the NLO molecules. Experiments were realized during the photopolymerization step, in order to study the freezing of the non-centrosymetric orientation of the dispersed NLO chromophores.
Hybrid organic-inorganic materials based on the sol-gel synthesis of organically modified silicon alkoxides have demonstrated their great potential for optical applications. They offer a high versatility in terms of chemical, physical properties and macroscopic shape molding of the final component. In a first step, hydrolysis and condensation reactions are led along the same way as in classical sol-gel glasses. Partial elaboration of the silicate backbone is thus achieved. Then, free-radical polymerization is proceeded by irradiating the sample under UV or visible light. Finally, the material consists of two crosslinked inorganic and organic networks that are interpenetrated. The present paper focuses on a photolithographic process allowing the generation of relief optical elements without requiring a wet treatment to reveal the latent image. It enables a low cost, simple and quick method for the fabrication of integrated micro-optical components with a spatial frequency up to 250 l/mm. The aim of the present work is to give particular attention to the kinetic aspects of the polymerization of the organic component. The control of the C=C double bonds conversion of acrylate functionalized alkoxides in case of photopolymerization is therefore, an essential issue to tailor material properties. The study also focuses on the influence of physico-chemical parameters that govern the relief generation. Kinetics of surface corrugation point out the importance of strain relaxation, mass-transfer by flowing and organic network formation during the photolithographic process. Some illustrations of the generated diffraction gratings are given.
Different types of polymers are proposed for holographic data storage : photopolymers like PMMA where bonds form or break in the polymer network under illumination, photochromic polymers containing for example azobenzene groups (proposed for high resolution nanolithography), and photopolymerizable systems using inhomogeneous polymerization of one or more monomers for holographic data storage. The material proposed in this work enters in the last family, giving rise to thick phase holograms. The coupling betwween polymerization and diffusion processes is extensively studied in order to characterize the photoinduced microstructuration. Diffusion processes are generated by the concentration gradients due to a disappearance of dye and monomer molecules at different rates in the reactive medium. Creation of gratings with spatial frequencies ranging from 10 to 4000 lines/mm was studied. The formulations are suited to be photopolymerized by illumination around 500 nm, allowing the polymerization of thick samples (thickness of a few hundred microns) with a good optical quality. In order to obtain a reversible process and to improve the storage capicity of the matrix, the medium is doped by a photochromic molecule while the polymerization is used for the photostructuration of the host matrix. The process needs at first the creation of tubular regions corresponding to the highest refractive index of the matrix. By entering in such a fiber, light is guided in the thickness of the material. In each microfiber, bits are recorded in the second stage one after the other one. Several bits can be stored in a same fiber by wavelength multiplexing.
Photopolymers are already well known as holographic materials. The modulation of the refractive index is patterned by the distribution of the cross-linking rate which depends on the illumination conditions. More recently, it has been demonstrated that the quadratic non linear optical (NLO) properties can be patterned in photopolymerized materials doped with push-pull chromophores. In that case, one takes advantage of the huge increase of the viscosity during the polymerization process to freeze the chromophores orientation. By using adapted sequences of applied electric field to orientate the polar NLO molecules combined with appropriate illumination conditions, it is then possible to create periodic poled structures in such doped photopolymers. The technique is therefore especially adapted for the realization of quasi phase matching structures in organic materials.
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.
KEYWORDS: Nanoparticles, Particles, Gold, Polymerization, Polymers, Near field optics, Absorption, Scattering, Photopolymerization, Atomic force microscopy
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 reaction's 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 particle's 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 dipole's field. Experiments in progress are aimed to study interacting particles with various shapes and sizes.
We present novel substrates for the elaboration of organic active devices for use in integrated optics. The selected materials are based on photopolymerizable matrices doped with optically active molecules. In such organic complexes, photoinduced chemical reactions initiate the polymerization of small monomers inducing an increase in density as well as in viscosity. Since these reactions are limited to the illuminated regions, the properties of these materials are easily patterned. Indeed, a refractive index increase occurs with the densification. Thus, it is possible to create a spatial modulation of the refractive index which can be used to make optical waveguides or phase holograms. Moreover, we induce quadratic non linear optical (NLO) properties by doping the photopolymers with push-pull chromophores. For this purpose, we need a non centro-symmetric alignment of the quadratic optical chromophores. This is obtained by orienting these polar entities with an external static electric field followed by the freeze of their orientation through the viscosity increase associated with the polymerization process. The non linear properties can then be patterned by irradiating the samples through appropriate masks. Long life-time periodically poled structures obtained with NLO chromophores doped photopolymers will be presented.
Polymers doped with nonlinear optical (NLO) chromophores are attractive candidates for manufacturing optical integrated devices, especially for applications based on second order non-linearities. Such optical devices require to select accurately the areas where the NLO push-pull molecules embedded in the matrix are oriented. This is usually achieved using patterned electrodes deposited onto the samples or microlithography techniques. We suggest a novel approach to control the spatial distribution of the oriented chromophores, based on the use of photopolymerizable mixtures. The formulation developed, which consists of a triacrylate monomer and a photoinitiator, is polymerized upon a visible light irradiation. It is doped with chromophores specially synthesized for this purpose, transparent at the actinic wavelength and highly soluble in the acrylate monomer. The doped photopolymerizable films are poled with a static electric field before irradiation and the orientation of the chromophores is then frozen in the desired areas by irradiating the samples with an appropriate pattern of light. We observed the ability of the polymerized medium to slow down the chromophore relaxation compared to the unexposed solutions. The influence of the formulation composition and irradiation parameters on the temporal stability of the chromophore orientation was studied through second harmonic generation measurements. Infrared spectroscopy measurements enabled to draw a correlation with the monomer conversion degree of the polymer host.
Waveguides can be self-written in photosensitive or photopolymerizable materials. This process results from the competition of the diffraction of the incident Gaussian beam and photopolymerization which tends to increase the refractive index where the light intensity is the highest. We have investigated the condition of quasi-solitonic and chaotic waveguide propagation inside bulk photopolymerizable materials. Light has been introduced in the medium by a single mode optical fiber. The propagated light behavior can be controlled by a careful monitoring of the input intensity. Indeed, a unique uniform channel wave guide without any broadening has been obtained by polymerization using a very low beam power of 5 μW. By increasing the input power up to 100 μW, the guide becomes chaotic and multi-channel, a process which has not been studied before in these materials. Although the transmission efficiency of two fibers connected by this type of guide is weak (10%), we can nevertheless couple two guides separated by a distance of a few millimeters. Now, this allows the study of the optical and electro-optical properties of photopolymerized guides doped by nonlinear optical chromophores and possible applications in integrated optical devices.
Photopolymerizable materials are capable of recording high-efficiency volume holograms by changing the refractivity of the layer, for fringe spacing between 0.2 and 10 ?m. As the photosensitive emulsion is embedded between two glass plates, it is possible to open the sandwich after the recording and to analyze the free polymer surface using pulsed force mode of an atomic force microscope. The modulation of properties between the bright and dark fringes, photoinduced by an interference pattern are analyzed in terms of : - relief amplitude (the surface corrugation appearing after opening is due to the relaxation in surface of the constraints stored during the grating formation) ; - local variations of the mechanical polymer properties (they are related to the coupling of the spatially controlled photopolymerization with mass diffusion processes, giving rise to the microstructuration, e.g. regions with various segment densities). Taking into account all these data, improvement of the material is possible in view of applications in data storage or creation of optical diffractive elements. In particular, in the case of multiplexed gratings, it provides a means for visualizing the Young’s modulus pattern associated with each individual record and, therefore, optimizing the recording procedure.
A photopolymerizable system sensitive in the 450-550 nm range is proposed in which thick phase holograms are recorded in situ through polymerization and diffusion processes. Three applications are presented. i) Multiplexed holograms were created with a view to optical edge extraction filtering. Several slanted fringe transmission gratings were recorded in the same location with less than 0.5° changes of the slant angle and 0.1 ?m of the fringe spacing. Diffraction angles were not affected by shrinkage of the polymer layer that occurred during the successive exposures. By optimization of the recording schedule, it was possible to obtain the same fair diffraction efficiency for each grating at the expected angle, reading them simultaneously at normal incidence. ii) Taking advantage of the two glass plates laminating the emulsion, transmission and reflection gratings with fringe spacings lower than the incident wavelength were recorded with a view to fabrication of substrate-mode diffractive elements. iii) Binary amplitude elements with pixel size of 20 ?m were duplicated by imaging in the polymerizable system, giving rise to thick phase holograms. The diffraction efficiency and the quality of the reconstructed image were studied as a function of the recording conditions.
Organic materials are of increasing interest for optical devices, especially in applications based on quadratic optical non linearities. In this context, we have studied photopolymerizable mixtures doped with non linear optical push-pull chromophores. The doped photopolymerizable films are poled with a static electric field and polymerized with a spatially controled irradiation in order to freeze the orientation of the chromophores in the desired areas only. The temporal stability of the chromophore orientation is checked by second harmonic measurements. The photopolymerizable system was chosen by taking into account the possibilities of microstructuration under visible illumination. This property was demonstrated by recording permanent thick phase gratings. Simultaneously, optimization of the polymer matrix from the viewpoint of chemical composition, conditioning and functionalization of the chromophore was performed in order to improve the stability of the chromophore orientation. Formulating of the material requires a fair knowledge of the coupling between photochemical conversion, mass transport, stiffening of the polymer matrix. Photostructuration together with stable non linear optical properties allows now to consider realistic applications.
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.
Photopolymerizable materials are capable of recording high- efficiency volume holograms by changing the refractivity of the layer. An attractive feature of these media is that they allow multiple permanent holographic storage. Chemical composition, conditioning and pre-irradiation of the reactive mixture developed in the Mulhouse laboratory were optimized for the sequential recording of several permanent holograms into the same sample, with fair diffraction efficiency and without degradation of the spatial resolution. Since the species involved in the initiation mechanism are gradually consumed as the hologram builds up, the schedule (i.e., the successive exposure times and incident intensities) must be determined, to take into account the degree of conversion of the different components and reach full completion of the reaction at the end of the ultimate imagewise exposure. Examples of more than twenty multiplexed gratings or holograms of a target will be shown, the images being recorded at the same location in the polymer at ca one degree angular separation intervals. Applications for holographic data storage can be expected in view of these results, the simplicity of use (self-processing and self-fixing material) and the possibility of short single-pulse recording.
Computer generation of Diffractive Optical Elements is a method of considerable value since it allows an user encoding of complex optical functions (e.g. lens arrays). As a rule, Computer Generated Holograms (CGH's) are thin amplitude transmission elements. The duplication of such holograms on a photopolymerizable film leads to thick phase holograms with higher diffraction efficiencies. Indeed, the incident optical information is recorded as a modulation of the refractive index. A great advantage of polymerizable materials over other recording systems it that no chemical or heating post-treatment is required once the hologram was recorded. Swelling and shrinkage effects, that are a feature of wet development processes are, thus, avoided. The different methods of duplication taken into account are conventional holographic recording, contact copying and direct imaging of CGH's on polymer layers. Advantages and drawbacks of the three methods will be discussed. The characteristics of the photopolymer developed in the Mulhouse laboratory, such as low degree of optical aberrations and high diffraction efficiency, make it possible to achieve attractive and promising results.
It was shown that self-processing photopolymerizable system can be used as holographic recording media in the field of microholography. Gabor holograms were recorded with high intensity single laser pulses. The in-line set-up involved a FD pulsed YAG laser and the pulse duration was about 9 ns. A power per pulse of ca 20 mJ corresponding to an energy density at the sample of 150 mJ/cm-2 was capable of generation in the emulsion excited species and of inducing an efficient polymerization by a free radical mechanism. Incoherent preillumination of the sample by a suitable UV source was necessary to obtain this result with one laser pulse only. Images corresponding to these phase in-line holograms were reconstructed with fair resolution using an He-Ne laser, a wavelength at which the polymer material is not sensitive. Apart from this, the pairing of similar emulsions with a cw green He-Ne laser was possible. The available power density was reduced down to 0.4 mW/cm-2 and gratings diffraction efficiencies of approximately 50 percent were achieved for an exposure time of 2 minutes. In that case, the purpose was to design low- cost, unsophisticated and not overly time-consuming manipulations, notably for simple demonstrations or university experiments of holography and interferometry.
Photopolymerizable systems were developed in Mulhouse as holographic recording materials. The direct generation of images with no post-treatment nor repositioning is their most attractive feature. Such media are ideally suited for real time interferometry. Compromises for definition of the exposure parameters were established, taking into account the desired diffraction efficiency and the exposure time that could not be lower than a few tenths of a second. On the other hand, the self-processing system was optimized for the recording of off-axis holographic lenses. With such a n end in view, the aim was not to attain the shortest and easiest handling time but the highest diffraction efficiency and the lowest optical noise. The diffracted beams were sharply focused and diffraction efficiencies higher than 80 percent were obtained at 514 nm. Since the chemical composition and the method of conditioning are very flexible, they can be adapted to meet the requirements for various applications.
Recording media based on the polymerization of monomers and oligomers commonly used in the Radiation
Curing area, appear to be promising materials to record holographic patterns produced by laser sources emitting
in the visible and near infrared part of the spectrum. A polymerization takes place only in the bright areas of the
interference pattern and volume phase holograms exhibiting high diffraction efficiencies which can be viewed
immediately after exposure, are created. Several examples of monophotonic and biphotonic systems will be
discussed and presented.
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