Proceedings Volume 2998

Photosensitive Optical Materials and Devices

Mark P. Andrews
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Proceedings Volume 2998

Photosensitive Optical Materials and Devices

Mark P. Andrews
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 23 January 1997
Contents: 7 Sessions, 36 Papers, 0 Presentations
Conference: Photonics West '97 1997
Volume Number: 2998

Table of Contents

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Table of Contents

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  • Glass Photosensitivity: Devices and Device Evaluation
  • Glass Photosensitivity: Materials, Methods, and Mechanisms I
  • Glass Photosensitivity: Materials, Methods, and Mechanisms II
  • Organic Material Photosensitivity: Azo Polymers
  • Poster Session
  • Organic Material Photosensitivity: Azo Polymers
  • Chemistry and Photophysics of Organic Materials for Devices
  • Organic Photorefractive and Nonlinear Optical Materials
  • Poster Session
  • Chemistry and Photophysics of Organic Materials for Devices
Glass Photosensitivity: Devices and Device Evaluation
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Material considerations for Bragg fiber gratings
Laurence Reekie, Liang Dong
In general, fibers designed for use in telecommunications systems are not sufficiently photosensitive to allow 'strong' fiber gratings to be written. This has led to the search for other silica-based fibers which exhibit enhanced photosensitivity without the need for hydrogen-loading, the boron co-doped germania fiber being perhaps the best known example. In addition, it is sometimes necessary to exclude dopants such as germania from the core as they would interfere with other desirable properties of the fiber. Prime examples are the rare-earth doped fiber laser and erbium-doped fiber amplifier, devices which may require low concentrations of germania in order to function properly. This paper covers some of the advances that have been made in searching for new materials, the problems which have been overcome, and those which remain.
Phenomenological model of UV-induced Bragg grating growth in germanosilicate fibers
Joerg Huebner, Mikael Svalgaard, Lars Gruener-Nielsen, et al.
Germanium doped glasses show a permanent increase of the refractive index after illumination with UV light. With a UV interference pattern produced by a phasemask, permanent Bragg gratings can be induced in optical fibers and planar waveguides. The growth of these Bragg gratings shows oscillations meaning that the grating reflectivity raises, reaches a maximum, then vanishes slowly and raises again to another maximum. We have recorded the first oscillation in more than ten different non-sensitized germanosilicate fibers. In this paper we present a model of the growth behavior, relating it to the visibility of the interference pattern generated by the phasemask, showing that the coherence of the laser light used to induce the grating has a strong influence on the growth behavior of the Bragg grating. The model is in excellent agreement with the measured grating growth behavior.
Fabrication of high-rejection low-loss filters by the concatenation of broadly chirped fiber Bragg gratings
Kate Sugden, Lin Zhang, Lorna A. Everall, et al.
Single- and multiple-bandpass filters with broad, high- rejection stopbands and narrow, low-loss passbands are significant components for fiber networks. Hitherto, this combination of characteristics has proved elusive using in- fiber grating techniques. Here, we report the realization of practical single-passband filters synthesized from concatenated chirped broadband fiber gratings written holographically in a highly photosensitive fiber. Practical, low-loss (less than 2 dB), single-passband fiber Bragg grating transmission filters have been fabricated with greater than 30 dB out-of-band rejection over a 25 nm spectral width.
Single and multi-passband broadband moire filters from dual exposure of uniform-period and chirped phase masks
Kate Sugden, Lorna A. Everall, John A. R. Williams, et al.
In-fiber chirped Moire resonators are excellent filters in terms of fiber compatibility, and can potentially provide wide stopbands, narrow passbands and high rejection levels. This paper describes the realization of chirped Moire in- fiber filters written using phase mask techniques. The filters described here were fabricated both from the dual exposure of a uniform-pitch phase mask followed by a third refractive index profiling scan and from the dual exposure of chirped phase masks. These techniques produce filters that replicate the broad bandwidths associated with holographic techniques and have the advantage of the good reproducibility associated with the use of phase masks.
Glass Photosensitivity: Materials, Methods, and Mechanisms I
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Stress-dependent kinetics of UV-induced refractive index change in germanosilicate fibers
Bertrand Poumellec, Pierre Niay, Thierry Taunay
We show that kinetics of Bragg grating formation are sensitive to the external stress applied on the fiber. This leads us to revisit the model of variable reaction pathway (VAREPA model) which accounts for the experimental index change power law. Conclusions are that a power law is still obtained for low sensitivity, just the pre-exponential factor is changed. This is consistent with the experiment. However, an isotropic strain dependent VAREPA model gives rise to an equivalent sensitivity on the mean index and on the modulation which is not consistent with the experiment. It is thus not excluded that another mechanism leading to stress redistribution was active.
Experimental and theoretical study of stable negative index gratings formed at 193 nm
Liang Dong, W. F. Liu, Laurence Reekie
We have demonstrated fast formation (on the order of 1500 pulses at approximately 1 J/cm2/pulse) of fiber gratings with high negative index modulations (on the order of minus 3 multiplied by 10-4). These gratings were found to be far more stable than the gratings with positive index modulations formed at the early stage of the grating growth. We have also found that the maximum negative index modulations achieved do not depend on the pulse intensities, although the inverse of the time taken to reach the negative index modulation maximum varies linearly with the pulse intensities. This prompts us to use a three energy level system to model the photosensitivity in the boron-doped germanosilicate fiber. All the necessary parameters of the model can be determined from a single growth measurement of the average index change and the model's prediction fits well the measured index modulation growth. A complex grating decay process is also observed at elevated temperatures as predicted by the three energy level model. The thermal stability of both positive and negative index gratings in a boron-co-doped germanosilicate fiber is characterized at fixed temperatures, so that the stability of such grating can be accessed for any writing fluence.
Photosensitivity of fluoride glass planar waveguides doped with rare earth ions (Ce3+, Eu2+, Er3+)
Marc Douay, Wenxiang Xie, Pascal Bernage, et al.
Rare earth (Ce3+, Eu2+, Er3+) doped PZG (PbF2, ZnF2 GaF3) glass thin films fabricated by vapor phase deposition on CaF2 substrates have been shown to possess high photosensitivity. Efficient and permanent gratings have been written in the PZG glass by using a cw UV laser (244 nm). The kinetics of the grating growths were monitored by using a HeNe beam (633 nm) in course of grating photoinscription. The sample surfaces at the grating places were investigated through interferometric microscopy. The changes in refractive index modulation were estimated to be 6.8 multiplied by 10-3 and 3.7 multiplied by 10-3 in the Eu2+ and Ce3+ doped samples respectively. Stability of the gratings was investigated through 30 min isochronal thermal annealing experiment.
Glass Photosensitivity: Materials, Methods, and Mechanisms II
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Photodegradation of fluoride glass blue fiber laser
Alain Chandonnet, Pierre Laperle, Sophie LaRochelle, et al.
The first demonstration of blue upconversion fiber lasers in Tm-ZBLAN has generated considerable interest among laser scientists looking for all-solid-state visible sources. Although initial experiments have shown a large conversion efficiency, a good spatial beam quality and an overall simplicity of the approach, these sources have not yet appeared on the market. In an attempt to reproduce these early results, many research teams including our own have encountered unexplained and detrimental start-up effects in these lasers. We have recently shown that this behavior is the result of photochromic damage in the fluoride fibers generated by the infrared pumping source. Progressive build- up of photoinduced loss ultimately prevents operation of the device. The photoinduced absorption spectrum extends from the UV to the near infrared, with three major bands centered around 300 nm, 500 nm and 800 nm. Pump-probe experiments show that the damage mechanism depends on the Tm3+ concentration and that it follows a fourth power dependence on the pumping intensity. Further investigation has revealed that photobleaching of the defects is possible using visible and near infrared radiation. The residual absorption spectrum following photobleaching suggests that three different species of defects are created. One type of defects is related to the 800 nm band and can be permanently removed. The other two are only temporarily removed by photobleaching and reappear on a time-scale of a few minutes. In addition, thermal bleaching can completely erase the defects in certain fibers. This paper summarizes the current understanding of photoinduced phenomena occurring in Tm3+-ZBLAN fibers.
Annealing of gratings photowritten in tin-codoped germanosilicate preform plates and fibers
Marc Douay, Wenxiang Xie, Pascal Bernage, et al.
Bragg gratings were written in the cores of tin codoped germanosilicate preform plates through exposure to cw UV light (244 nm). The experiments were performed using unloaded and H2 loaded samples. The diffraction efficiencies of the holograms were measured at a probe wavelength of 633 nm. The photoinduced refractive index modulations were calculated from these data. Microscopic inspections of the plate surfaces have shown that UV exposure induced corrugations at the grating places. A phase shift interferometric microscope was used to measure the depths of the corrugations. Afterwards, both the refractive index modulations and the depths of the corrugations were measured in the course of 30 min isochronal step annealing experiments. The heating induced refractive index modulation changes were calculated from the changes in the corrugation depths by means of a strain densification model and then compared to the experimental ones. The results are compared to the ones obtained from fibers drawn from these preforms.
Investigation of defects in highly photosensitive germanosilicate thin films
Germanosilicate glasses exhibit a significant photosensitive response which has been linked to the presence of oxygen- deficient germanium point defects in the glass structure. Based on this correlation, highly photosensitive thin films have been engineered which demonstrate the largest reported ultraviolet-induced refractive index perturbations ((Delta) n) in an as-synthesized material. Our thin-film fabrication process avoids the use of hydrogen sensitizing treatments and, thus, yields stable films which retain their predisposition for large photosensitivity for over one year of storage. Understanding the nature of the defects in such films and their relationship to charge trapping and enhanced photosensitivity is of paramount importance in designing and optimizing the materials. Toward this end, our films have been studied using electron paramagnetic resonance (EPR), capacitance-voltage, and optical bleaching and absorption spectroscopies. We find experimental evidence suggesting a model in which a change in spin state and charge state of isolated paramagnetic neutral Ge dangling bonds form either diamagnetic positively or negatively charged Ge sites which are largely responsible for the charge trapping and photosensitivity in these thin films. We present experimental data and theoretical modeling to support our defect model and to show the relevance of the work.
Strong Bragg gratings in nonsensitized low-loss planar waveguides as building blocks for WDM network components
Joerg Huebner, Jean-Marc Jouanns, Jens Engholm Pedersen, et al.
The performance of WDM networks is highly dependent on the wavelength selective components within the network. When using Bragg gratings as wavelength selective elements they have to fulfill a number of criteria such as low loss, high reflectivity, nearly square filter function and high background rejection. Highly photosensitive germanium doped silica-on-silicon planar waveguides have been produced by plasma enhanced chemical vapor deposition (PECVD). Total insertion losses (including fiber to waveguide coupling losses) down to 1 dB for a 5.1 cm long waveguide have been measured at 1541 nm. A germanium content of 8.6 mol% in the core combined with the PECVD process insures high photosensitivity towards 193 nm. Twenty mm long Bragg gratings have been induced by illuminating the waveguides with 193 nm light through a phase mask. The induced gratings show up to 99.98 percent reflectivity and a background rejection better than 22 dB at plus or minus 1 nm from the Bragg wavelength. No hydrogen loading or any other kind of sensitization was necessary to produce these gratings. The spectral flatness (3 dB reflection bandwidth: 0.55 nm and 1 dB reflection bandwidth: 0.47 nm) around the Bragg wavelength and the high background rejection make these gratings well suited building blocks for multi-wavelength network components.
Hydrogen enhancement of near-UV photosensitivity of germanosilicate glass
Dmitry S. Starodubov, Eugeni M. Dianov, Sergei A. Vasiliev, et al.
The effect of hydrogen loading on near-UV photosensitivity of germanosilicate glass is studied. The enhanced bleaching of 390 nm luminescence band and 240 nm absorption band, as well as enhanced growth of 650 nm luminescence band in hydrogen loaded samples is observed. The increase of paramagnetic GeE' defects concentration and creation of hydrogen related paramagnetic H(II) centers is obtained. Long period index gratings with low insertion loss and index change as large as 4 multiplied by 10-4 are fabricated by near-UV argon laser radiation in hydrogen loaded fibers. The effect of gamma irradiation on the properties of the fabricated gratings is reported.
Modification of the properties of silica glasses by ion implantation
John L. Brebner, Louis B. Allard, Marc Verhaegen, et al.
High energy MeV ion implantation of fused silica and Ge- doped silica renders these materials photosensitive. The physical processes involved are closely related to the photosensitization of Ge-doped silica by UV irradiation but present certain characteristics that are different. We discuss the results of studied of the induced absorption and refractive index changes under different preparation conditions, annealing sequences and subsequent bleaching by ArF and KrF excimer radiation. We include the results of a study using positron annihilation spectroscopy of the defects introduced by ion implantation and subsequent annealing and bleaching.
Novel type of highly photosensitive germanium-doped silica glass: codoping with nitrogen
Christian V. Poulsen, Torben Storgaard-Larsen, Joerg Huebner, et al.
A novel type of highly photosensitive germanosilicates has been developed. By adding ammonia to the gas mixture of silane, germane and nitrous oxide in the plasma enhanced chemical vapor deposition process, nitrogen has been incorporated in the glass matrix forming germania/silica oxynitrides. UV-induced refractive index increases greater than 3 multiplied by 10-3 has been measured without any hydrogen loading. Besides the highly increased photosensitivity the nitrogen doped germanosilicates exhibit excellent mechanical properties. Dependent on the deposition and annealing parameters the glass can be deposited to have zero stress when deposited on silicon. Further more can stable glasses with up to 30 mole % germania be deposited when nitrogen in incorporated.
Ultrastrong UV-written gratings in PECVD-grown germanosilicate waveguides
David J. Moss, John Canning, Marcy Faith, et al.
We report gratings with a transmission contrast ratio of over 45 dB, written in PECVD grown germanosilicate waveguides without the aid of hydrogen loading.
Structural defect control and photosensitivity in reactively sputtered germanosilicate glass films
The optical performance of refractive index structures induced in photosensitive (PS) glasses ultimately depends on the index modulation depth attainable. In germanosilicate materials, the photosensitive response is linked to the presence of oxygen-deficient germanium point defect centers. Prior efforts to increase PS in these materials, e.g., hydrogen loading, rely on a chemical reduction of the glass structure to enhance the population of oxygen deficient centers and thus increase the saturated refractive index change. We have previously reported the development of highly photosensitive, as-deposited germanosilicate glass films through reactive atmosphere (O2Ar) sputtering from a Ge/Si alloy target. The present work details our investigation of the effect of substrate temperature during deposition on the material structure and propensity for photosensitivity. Using optical absorption/bleaching, Raman, electron paramagnetic resonance (EPR) and selective charge injection techniques we show that the predominate defect states responsible for the PS response can be varied through substrate temperature control. We find that two regimes of photosensitive behavior can be accessed which exhibit dramatically different UV-bleaching characteristics. Thus, the corresponding dispersion of the refractive index change as well as its magnitude can be controlled using our synthesis technique. Tentative defect models for the photosensitive process in materials deposited at both ambient temperature and at elevated substrate temperatures are presented.
Near-UV photosensitivity of Ge-doped silica fibers
Eugeni M. Dianov, Dmitry S. Starodubov, Sergei A. Vasiliev, et al.
We report for the first time the refractive index grating side-writing in 10 mol.% GeO2-doped fiber by near-UV light of cw Ar+-laser (333 - 364 nm). The magnitude of induced index change as large as 1.9 multiplied by 10-4 at 1.7 multiplied by 105 W/cm2 UV power density has been achieved. The observed gratings have exhibited the same temperature stability as gratings written by KrF excimer laser (248 nm).
Organic Material Photosensitivity: Azo Polymers
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Photosensitive organized organic films in the light of bound electromagnetic waves
This paper describes recent advances in the field of photochromic polymeric structures for optical data storage. In particular, we discuss photo-induced effects in supramolecular assemblies containing azobenzene molecules (e.g. Langmuir-Blodgett-Kuhn structures and ultrathin silane layers). Reorientation of azobenzenes in these structures is compared to that observed in spin-cast films. Photoisomerization and photo-induced orientation of azobenzene molecules is studied at the molecular level by means of azosilane molecules chemisorbed on silicon oxide substrates. The study of the thermal back isomerization reaction of the azobenzene molecules in these layers reveals steric hindrance at the molecular level. These ultra-thin photochromic self-assembled monolayers (SAMs) exhibit persistent dichroism upon linearly polarized light irradiation. This dichroism could be both written and erased by irradiation with light of an appropriate wavelength. In addition, the sign of this dichroism can be inverted by choosing the appropriate polarization of the irradiating light, thus showing a 'smart communication' between the light polarization and the ultra-thin photochromic layers. Photoisomerization also induces reversible changes in the optical thickness of these molecularly thin SAMs. Langmuir- Blodgett-Kuhn (LBK) multilayer assemblies of 'hairy-rod' polyglutamates with stiff main chains and flexible side chains containing photochromic azo units exhibit a highly optically anisotropic structure when the azo molecules are in the trans form, and a nearly optically isotropic structure when the azo molecules are in the cis form. The trans $ARLR cis photoisomerization of the azo molecules switches them between a highly oriented trans configuration and a bend cis configuration, thus turning the anisotropy 'on' and 'off.' In contrast to spin-cast polymer films containing azobenzene units, photoselection under polarized light irradiation does not occur in these LBK structures. The study of the electronic density distribution of these films show that a remarkable variation in the LBK structures' stability comes with a seemingly small change in the side-chain structure. All the structures described here, form thin films that can be very sensitively characterized, by using surface plasmons and guided optical waves as electromagnetic modes bound to an interface.
Dipolar cooperative motion in amorphous azo copolymers: a molecular addressing possibility
Almeria Natansohn, Paul L. Rochon, Michel Pezolet, et al.
Copolymers containing rigid azobenzene and ester side groups linked by relatively short spacers to the main chain form amorphous films. Such films can be addressed with polarized laser light to produce oriented domains. Birefringence measurements and time dependent infrared spectroscopy are used to analyze the orientation. Two series of copolymers are analyzed in comparison. The first consists of copolymers of 4'-{[2-(methacryloyloxy)ethyl] ethylamino}4-nitroazobenzene (DR1M) and 4-{[2- ((2-methyl-1-oxo-2-propenyl)oxy)ethyl]oxy}benzoate (BEM), where both the azo and the ester rigid groups have similar dipole moments. The second series consists of copolymers of DR1M with a much less polar 4-phenyl-4- {[2-(2-(2-methyl-1-oxo-2- propenyl)oxy)ethyl]oxy}benzoate (NBEM). Birefringence studies clearly suggest that in poly(DR1M-co- BEM), the BEM groups are being oriented by 'sympathy' with the DR1M groups, while in poly(DR1M-co-NBEM) no such cooperative motion is apparent. Time dependent infrared spectroscopy can analyze separately the motions of each of the rigid groups, and it clearly shows that the BEM groups are being moved by the DR1M groups, while the orientation of the NBEM groups is much lower. Thus one can envisage materials where cooperative motion is present (polar groups), or where only the polar azobenzene groups are 'molecularly addressed' by the laser, with the non-polar rigid groups remaining relatively inert. The consequences to formation of surface gratings on these copolymer films are discussed.
Photofabrication of surface relief gratings on azobenzene polymer films
Dong Yu Kim, Taek-Seung Lee, Xianyan Wang, et al.
Surface relief gratings were photofabricated on various polymer films containing azobenzene groups in the side chain or in the main chain. Formation of the surface relief gratings on the polymer was very sensitive to the polymer structures. The surface gratings were produced on the polymers containing photoisomerizable azobenzene groups. Recording of the gratings was strongly dependent on the polarization of the beams. The localized variations of magnitude and polarization of the resultant electric field in the film are essential to the formation of the surface relief gratings. Large surface modulation (greater than 6000 angstrom) and high diffraction efficiency (greater than 40% into each of plus or minus 1 order) were obtained under optimal recording conditions. The diffraction efficiency of the surface relief grating was dependent only on the total light energy incident on the film. The surface pattern resulting from straight edge diffraction mapped the negative first derivative of the intensity distribution. Fabrication of various multiple gratings on the same spot of the polymer films were demonstrated. Fourier blazed gratings were also fabricated. The resulting surface pattern was a simple superposition of all the interfering recording waves.
Photoassisted poling vs thermal poling in copolymides for second-order nonlinear optics
Jerome Chauvin, Keitaro Nakatani, Jacques A. Delaire, et al.
Thin films of new side-chain polyimides bearing disperse red one (DR1) groups have been poled by the corona technique assisted by visible irradiation at low temperatures, far below the high glass transition temperature (Tg) of these polymers. The second harmonic generation (SHG) signal reached at the end of this photoassisted poling process is higher than with thermal poling. Furthermore, by a simple method based on absorption measurements with polarized light at variable incidence, it has been shown that the degradation ratio of the chromophores is much lower with photoassisted poling than with thermal poling at a temperature near Tg.
Poster Session
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Anistrophy of the photo-induced translation diffusion of azobenzene dyes
Large-amplitude holographic surface relief gratings are optically induced by resonant visible exposure in azobenzene dye-containing polymers. We propose a simple diffusion model accounting for the essential features of surface relief formation. It accounts for intensity, grating-pitch and polarization dependencies. Understanding of this process appears essential for real-time holography applications. It opens new perspectives associated with optically controlled anisotropic diffusion.
Organic Material Photosensitivity: Azo Polymers
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Optically inscribed surface gratings in azo polymers used in resonant structures
Paul L. Rochon, Almeria Natansohn
When surface gratings are inscribed on the surface of resonant cavities such as slab waveguides, sharp peaks can be observed in the transmission and reflection spectra of the structures. The peaks in the optical characteristics occur because the gratings act as couplers for light into the waveguides at specific wavelengths and angles of incidence. It has been found that stable surface gratings can readily be inscribed on polymers containing side groups such as poly{4'-[(2- (acryloyloxy)ethyl)ethylamino]-4-nitroazobenzene}. The gratings are made by direct exposure to an interference pattern and no further processing is required. The grating properties such as orientation, grating spacing and depth are easily controlled, and multiple gratings can be inscribed on one spot. Using various combinations of waveguides and surface gratings, structures which act as line filters and resonant reflectors are made and studied.
Chemistry and Photophysics of Organic Materials for Devices
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Self-developing photopolymer for the fabrication of relief micro-optical elements
Celine Croutxe-Barghorn, Sergio Calixto, Daniel-Joseph Lougnot
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.
Charge transfer phenomenon in holographic recording material: dichromated polyacrylic acid
Gurusamy Manivannan, Roger A. Lessard, Christine Pizzocaro, et al.
Dichromated polyacrylic acid (DCPAA) has been used as real time holographic recording material. We report here the photochemical behavior of DCPAA when irradiated under conditions similar to those used for hologram recording. A photoredox process was observed whose efficiency depends on the presence of an electron donor, dimethylformamide (DMF). By ESR spectroscopy upon irradiation four different species were detected and identified: (1) chromium (V) of thermal origin (in the dark); (2) chromium (V) of photochemical origin; (3) an organic macroradical; (4) chromium (III). DMF appeared to possess a strong influence on the formation of these species.
Nonlinear refractive indices of polydiacetylene microcrystals
Hiro Matsuda, Shinji Yamada, Edward R. Van Keuren, et al.
Polydiacetylene microcrystals in a water suspension or a polymer matrix were prepared by a simple precipitation procedure in order to obtain a thin samples with good optical quality maintaining the characteristics of crystalline materials. The nonlinear refraction, n2, and nonlinear absorption, (beta) , around the exciton peak were measured by z-scan method using a tunable picosecond laser. The signs of the n2 are positive/negative at wavelengths shorter/longer than the exciton peak and strong saturable absorption, which (beta) is negative, are observed around the peak. The largest n2 was obtained to be plus 0.024 cm2/GW (Re(chi (3) equals plus 1.4 multiplied by 10-9 esu) at 640 nm for a gelatin composite film containing 24 wt percent of polydiacetylene microcrystals.
UV-cured organic polymer gain medium and its application possibility
Shoji Miyoshi, Seiichiro Hayakawa, Iwao Seo, et al.
Developing a new UV cured organic polymer (called TEMA) with dopant of Rhodamine B, its gain characteristics have been investigated for various pumping schemes such as forward and backward edge-pumpings, and side-pumping, together with fundamental optical properties, i.e., absorption spectrum, absorption coefficient and fluorescence under pumping. Using a frequency-doubled Nd:YAG (532 nm) laser beam as a pumping source, a Nd:YAG (532 nm) laser-pumped dye 580 nm laser beam as a signal wave is amplified through the organic polymer gain medium, obtaining, thus far, the maximum gain of approximately 10 dB with the dopant concentration of 20 ppm for the path length 7 cm of bulk gain medium.
Optical switching through photoisomerization: basics of the mechanism in rhodopsin and stilbene
Valentin D. Vachev, Kathleen Nojima, John H. Frederick
Vertebrate vision is based on the unique conversion of light into nerve impulses. Detailed understanding of the primary photochemical events is important with respect to potential applications in molecular optoelectronic devices. In our studies we apply modern theory, computer simulations, and experimental knowledge to develop new understanding of the photochemical processes and ultimately devise means to control their outcome. We use a combination of quantum theory and quasiclassical calculations to model excitation with laser pulses, excited state dynamics, internal conversion, and relaxation to photoproducts. In rhodopsin, the primary photochemical event involves an 11-cis to 11- trans photoisomerization. Recent time-resolved measurements provide the time-scales for disappearance of the reactant and the appearance of products. We find microscopical details of the internal conversion which are in accordance with the experimental results. This new perspective to the microscopic mechanism reveals the optimization of the electronic structure achieved in evolutionary development in nature. Cis-Stilbene is studied as a model system for which a double resonance laser excitation can be used for influencing the reaction. UV excitation leads to two photoproducts, trans-stilbene and dihydrophenanthrene (DHP). We show that IR excitation can be used to prepare ground sate cis-stilbene in an appropriate vibrational state before electronic excitation. We find that excitation of some modes leads to up to 4.5 times increase in the quantum yield of DHP or up to 4 times increase in trans-stilbene. Because of the shifted absorption spectra of the products, if such system is realized experimentally it can be used as switching bistable device.
Organic Photorefractive and Nonlinear Optical Materials
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All-optical patterning of 3D microstructures in azo polymers: toward a full control of the molecular order
Celine Fiorini, Jean-Michel Nunzi, Fabrice Charra, et al.
One challenging requirement for the design of devices for photonic applications is to achieve complete manipulation of molecular order. The great latitude and flexibility of optical means offers interesting prospects for material engineering using light-matter interactions. Efficient spatial modulation of polymer macroscopic properties is usually achieved using holographic recording of an interference pattern between intense light-waves. However, patterning with polarized monochromatic beams results only in axial order (e.g. vertical or horizontal alignment). For second-order optical nonlinear processes, a polar organization is mandatory (e.g. upwards or downwards orientation). We report here on a new, purely optical technique based on a non-classical holographic process with coherent mixing of dual-frequency fields. It enables an efficient and complete 3-dimensional spatio-tensorial control of polymer micro-structures.
Characterization of photochromic and photorefractive chromophores
Christopher R. Moylan, I. Heng McComb, Robert J. Twieg, et al.
Although photorefractive materials are those that exhibit both appreciable electro-optic and photoconductive behavior, the chromophore parameters that lead to large electro-optic coefficients have not so far led to similar photorefractive performance. Recently, it has been shown that the electro- optic contribution is one of two principal contributions to the overall photorefractive behavior, and that it is usually the smaller of the two. The larger factor, first referred to in the literature as an 'orientational enhancement,' is due to the birefringence of the chromophore: the difference in polarizability parallel to the dipole moment and perpendicular to it. A figure of merit incorporating both effects has been derived, although its determination is more difficult than that for purely electro-optic materials. The polarizability anisotropy is a function of all three components of the diagonalized polarizability tensor. Two expressions for these three parameters can be obtained by index of refraction and light scattering measurements on chromophores in solution. The third must be estimated. Results from this new characterization protocol are presented and compared with the results of photorefractive experiments on materials containing the tested chromophores. Another class of compounds has also been found to be of potential use in holographic storage, in this case write- once applications. These chromophores undergo an irreversible photochromic reaction that is triplet sensitized. A mechanism is proposed for this reaction and supporting data provided.
Poster Session
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Layer-by-layer reading of information in bacteriorhodopsin-based 3D optical memory
Elena A. Nikanorova, Vladimir B. Kotov
The possibility of layer-by-layer reading of information in photoreversible medium based on bacteriorhodopsin is considered. We propose to use for information reading the light with wavelength in the absorption spectrum of K- intermediate, which is shifted relative to the absorption spectra of B- and M-intermediates into the long wavelength region. For layer marking, the high concentration of the K- form bacteriorhodopsin molecules is produced by means of two light waves with the wavelength in the absorption spectra of B- and M-intermediates, propagating in opposite directions. The condition of high quality of marking is the sufficiently high clarification of the medium caused by light waves. The stationary as well as nonstationary regimes of marking are considered. The possibility of reproducing the temporal sequences of layer-by-layer written images in nonstationary regime of marking is demonstrated.
High-efficiency laser coupling in BaTiO3 mutually pumped phase-conjugate mirror using novel configurations
We have discovered for mutually pumped phase conjugation (MPPC) of two incoherent beams which are all remarkably insensitive to angular and lateral positional changes in either of the two input beams or in the photorefractive BaTiO3 crystal itself. By designed choosing the geometry such as the orientation between the pump beam and the crystal's c-axis the grating can be dominated and two lasers can be coupled efficiently. Three novel configurations can be formed in photorefractive crystals when the MPPC occurred. These configurations are part of a family of mutually pumped phase-conjugate mirrors (MPPCMs) all of which have similar internal beam paths within the crystal having three internal reflections. These configurations provide broad interaction, which offer with more efficient, and large angular and positional acceptance, and even high resolution and stability. We also experimentally demonstrate our discovery of a new double mutually pumped phase- conjugate mirror (DMPPCM) which phase conjugate three incident beams in pairs operating as two MPPCMs in the same crystals at the same time but in different orientations. These characteristics make these configurations particularly interesting for practical applications such as for phase locking lasers and for optical free space communications.
Photoconductivity of multicomponent films of poly-N-epoxypropylcarbazole/fullerene (C60/C70) condensate
Alexey Koval, Yury Savin, Alexander Tolmachev, et al.
Photogeneration of charge carriers is studied in poly-N- epoxypropylcarbazole (PEPC) doped by C60/C70 fullerene condensate (30/70 mol.%). Efficiency (phi) of the charge carrier photogeneration was determined using the electrophotography technique with tritium ionizer. That efficiency is shown to increase monotonously with the fullerene concentration growth. A characteristic feature of the spectral dependence (phi) (lambda) is an increase of (phi) in the absorption region of the fullerene-sensitized PEPC. The initial efficiency of the charge carrier photogeneration (phi) 0 and thermalization radius r0 are determined.
Excited-state optical storage study in a dye-doped film using four-wave mixing spectroscopy
Xinxian Bao, Chunfei Li, Hongjing Kan, et al.
Four-wave mixing spectroscopy is presented as a convenient and effective optical method for the study of excited-state optical storage properties and nonlinear mechanisms in a dye-doped polymer film. The dynamic processes of optical storage properties and the efficiency of the diffraction grating are discussed. Backward-scattering four-wave mixing and forward-scattering four-wave mixing optical configurations are presented. A simple energy-level model is used to explain the excited-state optical storage mechanism, resulting from photo-induced excited-state population grating and trans-cis isomer grating.
Photoelectroluminescent electric field intensity sensor
The possibility of using electroluminescent semiconductors of the II-VI group to be used in the construction of the electric field intensity sensors is discussed. The idea of contact-free method of the electric field intensity measurement is based on the influence of electric field on optical properties of electroluminescent ZnS:Mn and ZnS+CdS:Mn semiconductors. The sensors presented do not belong to the group of intensity sensors. These luminescent fiber optic sensors are going to be applied in the future in high voltage electric equipment.
Chemistry and Photophysics of Organic Materials for Devices
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Photosensitive optical materials: the photochemistry of some selected systems
Jean-Pierre Fouassier
Some interesting problems encountered in the design of photosensitive systems in the area of imaging science are to extend the spectral sensitivity of the photoinitiating systems to the visible light emission. The design of efficient photosensitizers and photoinitiators is partly based on a better understanding of the basic mechanism that is involved. The time resolved laser absorption spectroscopy combined with steady state experiments appears as a very convenient tool to improve the efficiency of photoinitiating systems. The present paper is mainly concerned with a discussion of the excited state processes in selected examples of efficient photosensitive systems for laser induced polymerization reactions that are of interest e.g. in holographic recording or laser imaging.