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Wolfgang Kern - One of the best experts on this subject based on the ideXlab platform.

  • selected polymers that contain aromatic ester units synthesis photoreactions and Refractive Index Modulation
    Macromolecular Chemistry and Physics, 2007
    Co-Authors: Ute Daschiel, Georg Jakopic, Thomas Höfler, Volker Schmidt, Wolfgang Kern
    Abstract:

    Photoreactive polymers bearing aliphatic and aromatic esters of 4-(N,N-diphenyl-amino)phenol in the side chain, poly[4-(N,N-diphenylamino)phenyl acrylate] (P1) and poly[4-(N,N-diphenylamino)phenyl 4-vinylbenzoate] (P2), are prepared from their monomers by free radical polymerization. In addition, poly[naphthalene-1-yl 4-vinylbenzoate] is synthesized (P3). These materials absorb UV light up to 2=350 nm. The polymers P1-P3 as well as low-molecular-weight model compounds are investigated with respect to their photoreactions under UV illumination. It is found that decarboxylation is the dominant reaction and that the photo-Fries rearrangement occurs as a minor reaction. The fully aromatic ester units in P2 are more reactive than the ester units in P1. In the polymers, decarboxylation proceeds more efficiently than in the low-molecular-weight model compounds. Upon UV illumination of the polymers, large increases of the Refractive Index are observed (Δn 450 up to +0.07). This is in agreement with the structural changes in the polymers after irradiation as calculated from group contributions to molar refraction. A preliminary investigation under 610 nm fs laser irradiation indicates that Refractive Index patterns can be produced by two-photon effects (TPA). The polymers are of potential interest for applications in the field of optical waveguides.

  • UV reactive polymers for Refractive Index Modulation based on the photo-Fries rearrangement
    Polymer, 2007
    Co-Authors: Thomas Höfler, Thomas Grießer, X. Gstrein, Gregor Trimmel, Georg Jakopic, Wolfgang Kern
    Abstract:

    Abstract In this study we report on the synthesis and characterization of photoreactive polymers which display large changes in their Refractive Index upon UV irradiation. These polymers contain aromatic ester groups which undergo a photo-Fries rearrangement from aryl esters to hydroxyketones. The polymers under investigation were poly(4-acetoxystyrene) and two derivatives of polynorbornene, poly(bicyclo[2.2.1]hept-5-ene-2-carboxylic acid, phenyl ester) and poly(bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenyl ester). The kinetics of the photoreaction was studied by FTIR spectroscopy. Ellipsometric measurements of thin films showed that the photo-Fries rearrangement causes a large increase of the Refractive indices in these polymers (between +0.03 and +0.05). The Modulation of the Refractive Index is of interest for applications in the field of optics and data storage. The photoreaction also leads to an increase in surface energy as evidenced by contact angle measurements with water and diiodomethane as test liquids.

  • Modifying the Output Characteristics of an Organic Light-Emitting Device by Refractive-Index Modulation
    Advanced Functional Materials, 2006
    Co-Authors: T. Höfler, Markus Weinberger, Stephan Rentenberger, Martin Weinberger, Wolfgang Kern, Alexander Pogantsch
    Abstract:

    In order to modify the output characteristics of organic light-emitting devices (OLEDs), the optical properties of an active layer within the device are patterned without introducing any thickness Modulation. For this purpose a new conjugated copolymer, which serves as a hole-transporting material and at the same time can be Index patterned using UV techniques, is synthesized. Poly(VC-co-VBT) (VC: N-vinylcarbazole; VBT: 4-vinylbenzyl thiocyanate) is prepared by free-radical copolymerization of VC and VBT. The material contains photoreactive thiocyanate groups that enable altering of the material's Refractive Index under UV illumination. This copolymer is employed as a patternable hole-transporting layer in multilayer OLEDs. Refractive-Index gratings in poly(VC-co-VBT) are inscribed using a holographic setup based upon a Lloyd mirror configuration. The fourth harmonic of a Nd:YAG (YAG: yttrium aluminum garnet) laser (266 nm) serves as the UV source. In this way 1D photonic structures are integrated in an OLED containing AlQ3 (tris(8-hydroxyquinoline) aluminum) as the emitting species. It is assured that only a periodical change of the Refractive Index (Δn = 0.006 at λ = 540 nm) is generated in the active material but no surface-relief gratings are generated. The patterned devices show more forward-directed out-coupling behavior than unstructured devices (increase in luminosity by a factor of five for a perpendicular viewing direction). This effect is most likely due to Bragg scattering. For these multilayer structures, optimum outcoupling was observed for grating periods Λ ∼ 390 nm.

Yasuo Tomita - One of the best experts on this subject based on the ideXlab platform.

  • red sensitive organic nanoparticle polymer composite materials for volume holographic gratings with large Refractive Index Modulation amplitudes
    Optical Materials Express, 2021
    Co-Authors: Asako Narita, Juro Oshima, Yuko Iso, Shuma Hasegawa, Yasuo Tomita
    Abstract:

    We demonstrate volume holographic recording  at a wavelength of 640 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh Refractive Index hyperbranched-polymer (HBP) organic nanoparticles. We employ a new photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find the optimum composition of the system for volume holographic recording. We show that recorded volume gratings of 0.5-µm spacing possess the saturated peak-to-mean Refractive Index Modulation amplitudes as large as 3×10−2 at a readout wavelength of 640 nm. Our results show the usefulness of photopolymerizable HBP-dispersed NPCs for volume holographic recording materials for various photonic applications including security and color holograms, and volume Bragg grating devices in head-mounted displays.

  • stoichiometric thiol to ene ratio dependences of Refractive Index Modulation and shrinkage of volume gratings recorded in photopolymerizable nanoparticle polymer composites based onstep growth polymerization
    Optical Materials Express, 2011
    Co-Authors: Eiji Hata, Yasuo Tomita
    Abstract:

    Spectroscopic, photocalorimetric and holographic measurements are conducted to investigate effects of stoichiometric thiol-to-ene ratio on the polymerization dynamics, Refractive Index Modulation, recording sensitivity and polymerization shrinkage of volume gratings recorded in silica nanoparticle-polymer composite films based on step-growth radical addition polymerization. It is found that the polymerization rate of the composite system is maximized at the stoichiometric thiol-ene composition. It is also found that while the Refractive Index Modulation and the recording sensitivity are maximized at the stoichiometric thiol-ene composition, polymerization shrinkage decreases with increasing the thiol monomer fraction. A negative correlation between gel point conversion and shrinkage is observed.

  • real time phase shift measurement during formation of a volume holographic grating in nanoparticle dispersed photopolymers
    Applied Physics Letters, 2006
    Co-Authors: Naoaki Suzuki, Yasuo Tomita
    Abstract:

    We describe the real-time measurement of the phase shift between the light interference pattern and a volume holographic grating recorded in dry photopolymers. The time evolution of the Refractive Index Modulation including its magnitude and phase shift is measured for one-component and nanoparticle-dispersed photopolymers. It is confirmed that phase shifts in nanoparticle-dispersed photopolymers depend on the magnitude of the Refractive Index of nanoparticles relative to that of polymerized monomers.

  • silica nanoparticle dispersed methacrylate photopolymers with net diffraction efficiency near 100
    Applied Optics, 2004
    Co-Authors: Naoaki Suzuki, Yasuo Tomita
    Abstract:

    We demonstrate volume holographic recording in silica-nanoparticle-dispersed methacrylate photopolymers with reduced scattering loss as low as 2%. This is made possible by use of 13-nm silica nanoparticles. As a result a net diffraction efficiency near 100% is achieved for a transmission volume hologram of 45-μm thickness. Grating buildup dynamics are measured for various nanoparticle concentrations, and the effects of nanoparticle size on Refractive-Index Modulation and polymerization shrinkage are also evaluated.

Nasser N Peyghambarian - One of the best experts on this subject based on the ideXlab platform.

  • strong bragg gratings in phosphate glass single mode fiber
    Applied Physics Letters, 2006
    Co-Authors: Jacques Albert, Axel Schulzgen, Valery L Temyanko, Seppo Honkanen, Nasser N Peyghambarian
    Abstract:

    Bragg gratings with reflectivities greater than 99% have been fabricated in phosphate glass single mode fibers by irradiating the fibers with 193nm wavelength, high intensity pulses from an ArF excimer laser through a phase mask. Thermal treatment for 1000h at temperatures up to 170°C did not degrade the gratings and resulted in an increase in grating strength. The Refractive Index Modulation amplitudes obtained exceed 10−4.

  • optical dispersion of the Refractive Index Modulation in low tg photoRefractive polymers
    Applied Physics Letters, 1997
    Co-Authors: J F Wang, Bernard Kippelen, Nasser N Peyghambarian
    Abstract:

    We measure at different wavelengths the linear and nonlinear optical properties of a new chromophore for photoRefractive applications by means of frequency-dependent ellipsometry experiments. We show that the optical dispersion of the Refractive Index Modulation responsible for the high diffraction efficiency in low Tg polymers can be described by a two-level model.

Swee Chuan Tjin - One of the best experts on this subject based on the ideXlab platform.

  • adjustable Refractive Index Modulation for a waveguide with su 8 photoresist by dual uv exposure lithography
    Applied Optics, 2006
    Co-Authors: Biow Hiem Ong, Xiaocong Yuan, Swee Chuan Tjin
    Abstract:

    We present a new fabrication technique based on a two-step UV exposure lithographic process to marginally modulate the Refractive Index in commercial SU-8 photoresist. This technique achieves Refractive Index Modulation as different regions undergo different thermal densification prior to UV-induced polymerization. A small Refractive Index contrast of 0.0008 or lower can be achieved, and this is especially useful for fabricating waveguides with a low level of propagation modes. This technique may be extended to other UV-curable epoxy photoresists and can easily be applied in the fabrication of optical elements such as optical interconnects and integrated optical sensors without the development process.

  • photothermally enabled lithography for Refractive Index Modulation in su 8 photoresist
    Optics Letters, 2006
    Co-Authors: Biow Hiem Ong, Xiaocong Yuan, Shaohua Tao, Swee Chuan Tjin
    Abstract:

    We present a new lithographic technique based on a hybrid photothermal process to modulate the Refractive Index in commercial SU-8 photoresist. Owing to a difference in cross-linking, the Refractive Index of unexposed SU-8 cross-linked by thermally induced polymerization is 0.0072 higher than that of SU-8 cross-linked by UV exposure and postbaking. Making use of this property, we fabricated two thick, flat-topped Index-modulated diffractive optical elements (DOEs) that contain different phase distributions and measured their wavefront reconstruction. The good experimental reconstructions of the Index DOEs demonstrate the potential to extend the Refractive-Index Modulation technique for the fabrication of three-dimensional optical elements without needing a development step.

Aravinda Kar - One of the best experts on this subject based on the ideXlab platform.

  • dynamic two dimensional Refractive Index Modulation for high performance acousto optic deflector
    Optics Express, 2015
    Co-Authors: Tiansi Wang, Chong Zhang, Aleksandar Aleksov, Islam A Salama, Aravinda Kar
    Abstract:

    The performance of an acousto-optic deflector is studied for two-dimensional Refractive Index that varies as periodic and sinc functions in the transverse and longitudinal directions, respectively, with respect to the direction of light propagation. Phased array piezoelectric transducers can be operated at different phase shifts to produce a two-dimensionally inhomogeneous domain of phase grating in the acousto-optic media. Also this domain can be steered at different angles by selecting the phase shift appropriately. This mechanism of dynamically tilting the Refractive Index-modulated domain enables adjusting the incident angle of light on the phase grating plane without moving the light source. So the Bragg angle of incidence can be always achieved at any acoustic frequency, and consequently, the deflector can operate under the Bragg diffraction condition at the optimum diffraction efficiency. Analytic solutions are obtained for the Bragg diffraction of plane waves based on the second order coupled mode theory, and the diffraction efficiency is found to be unity for optimal Index Modulations at certain acoustic parameters.