Noble metal colloids in a dielectric matrix material exhibit surface plasmon resonance at visible wavelengths, which is well known to lead to intense coloration effects. The absorption peak at the resonance frequency is linked to a change of the refractive index even at wavelengths only marginally affected by the absorption. We explore the potential of using this refractive index change for creating dielectric waveguides by means of a versatile photocatalytic silver deposition process. The specific planar waveguide structure consists of a thin high refractive index layer comprising the photocatalyst and the silver colloids, topped by a cladding layer, on a low refractive index glass substrate. Using grating coupling, waveguide modes guided predominantly in the cladding could be demonstrated using red light with acceptable losses down to the 10 dB/cm range. However, the change of the effective refractive index that can be achieved with silver colloids at this low attenuation is below 10-4 , only sufficient for very weak lateral guiding. The fundamental mode centered on the high refractive index layer would be more sensitive to the amount of silver colloids, but this applies to both the effective refractive index and the attenuation. A significant reduction of the bandwidth of the surface plasmon resonance peak would be required to improve this trade-off. So far, the greater potential seems to be in the integration of areas containing silver colloids for sensors or nonlinear devices with plasmonic waveguides defined by fully metallized areas.
ITO (tin doped indium oxide) coatings with a sheet resistance of 2 to 3 kΩ(square) were produced by gravure printing process
on PET and PEN foil. The printing paste consisted of ITO nanoparticles which were dispersed in a solvent by using a
surfactant. The dispersion was mixed with a binder and a photo initiator before printing. The printed films were hardened
under UV-irradiation at low temperatures (< 130°C). The sheet resistance could be decreased by heat treatment at 120°C
under forming gas atmosphere (N2/H2) to 1.5 kΩ(square). The transmission of the ITO coated PET and PEN foils is more than
80 % in the visible range.
The ITO films were directly used as the bottom electrode in an organic photodiode (OPD). The setup of the OPD
originates from the well known Tang photodiode, consisting of a stacked layer of copper phthalocyanine (p-type
material) and perylene tetracarboxylic bisbenzimidazole (n-type material). The photodiodes are characterised via current-voltage
(I-V) characteristics. The performance of the photodiodes with printed ITO on plastic substrates could be
improved by the deposition of a PEDOT/PSS layer (Baytron(R) P) on the ITO coated foils and was then comparable to the
performance of photodiodes with semi-transparent gold as anode on PET substrates. These results demonstrate the
suitability of the printed ITO layers as anode for organic photodiodes.
Multi-layer interference filters of surface modified SiO2- and TiO2-nanoparticles have been produced on polycarbonat (PC) and on scratch resistant film. AR-coatings (anti-reflex- coatings) were prepared from a dispersion of SiO2 and TiO2 colloidal nanoparticles in a hybrid inorganic organic matrix. The TiO2 particles have been synthesized by addition of HCl and H2O to a solution of titanium tetraisopropanolate in isopropanol. The SiO2 nanoparticles were synthesized by base catalyzed hydrolyses and condensation of tetraethoxysilane (TEOS) mixed with an aqueous solvent. The surface modification of the TiO2 and SiO2 nanoparticles with 3-glycidoxypropyltrimethoxysilane (GPTS) avoids the agglomeration of the nanoparticles and covalent bonds between the matrix and nanoparticles are formed during a photopolymerization step. The nanoparticles were dispersed in a hydrolyzed GPTS matrix and UVI Cygacure 6974 was added as a photoinitiator. The (lambda) /4 ((lambda) equals 550 nm) films of TiO2 and SiO2 sols were prepared on PC and on scratch resistant coated PC by spin-coating techniques. The refractive index of the SiO2/TiO2-coatings can be adjusted from 1.47 up to 1.94 depending on the SiO2 or TiO2 concentration. By the combination of index matching and photopolymerization, a low temperature interference filter can be realized for plastic substrates with reflection of 0.5% at 550 nm. The coatings show excellent adhesion to the substrates in the cross hatch test and promising mechanical properties.
A new technique to produce a radial gradient in the refractive index (r-GRIN) in organic-inorganic nanocomposite materials using sol-gel techniques in combination with electrophoretically induced concentration profiles of oxide nanoparticles is presented. The composite material is based on methacryloxylpropyl trimethoxysilane (MPTS), zirconium n- propoxide (ZR), methacrylic acid (MA) and tetraethyleneglycol dimethacrylate (TEGDMA). Irgacure 184 was used as a photosensitive initiator. The surface of these particles is enriched with MA which is linked by a chelating complex to the initial ZR component. The TEGDMA component is used to introduce more flexibility in the organic inorganic network and to reduce polymerization stresses. Electric charges on the ZrO2 nanoparticle surface force the particles to diffuse in the gel state by electric fields employed by appropriate electrodes in presence of an electric field. The movement and interdiffusion of the Zr-nanoparticles in the matrix were measured by zeta-potential measurements and by photon- correlation spectroscopy (PCS). In the performed experiments, a radial electric field amplitude of 200 V/cm was used and held for 5 hours keeping the material in the gel state. The variation of refractive index in real time was measured by Mach-Zehnder interferometry. After the electrophoretic process, a polymerization step was carried out to immobilize the (Delta) n gradient. (Delta) n was measured by ellipsometry and the value of 0.07 was obtained for a sample of 1 cm in diameter. The form of the concentration profile and hence from the index profile was detected by EDX (energy dispersive x-ray analysis) measurements.
Micro Fresnel lenses were prepared from photosensitive organic-inorganic nanocomposites of the Ormocer-type by irradiation of films of 10 micrometers in thickness with an interference pattern of two laser beams, mixing a planar reference wavefront and a spherical object wavefront. To monitor the polymerization behavior of the organic groups during irradiation under real time conditions, a characterization method was developed to study changes in optical thickness nd by measuring changes of index of refraction and shrinkage during polymerization. Three-dimensional Fresnel structures were obtained by removing the unpolymerized areas by a solvent. Variation of index of refraction can also be obtained by diffusion of monomers in regions of higher light intensity. A model, similar to the Colburn- Haines model, is presented.
A sol-gel derived photochromic glass coating of about 1.5 micrometers in thickness is synthesized by infiltration of Ag+ into a predried Na-Al-B-Si gel layer. The formation of small Ag colloids is initiated by a soft heat treatment and the colloids are converted by a HCl vapor treatment into AgCl crystallites of about 40 nm in diameter. The coating darkens by UV irradiation, forming Ag crystallites of about 5 nm in size. This process is completely reversible at 400 degree(s)C and no decay is observed in numerous cycles. An amplitude hologram which acts as a diffraction grating with a grating period of about 10 micrometers can be produced within this layer by a two-wave mixing experiment with laser light of 351 nm wavelength. The diffraction efficiency of this grating is determined to 0.11%. After thermal erasing the diffraction pattern can be rewritten and erased repeatedly without any loss of efficiency.
A composite materials based on organically modified Si alkoxides and modified alkoxides from Zr were prepared. The index of refraction of the system was tailored by the variation of the modified Zr component. Channel waveguides were prepared by a laserwriting technique based on the ability to photopolymerize this material. To decrease optical loss caused by effects of the waveguide surface and to be independent of the used substrate, a buffer layer and a protective layer of the same material of lower index of refraction was combined with a waveguiding film and optical losses were determined.
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