There is much interest in nonlinear absorbing chromophores for applications in photonics, nanophotonics and biophotonics. We have performed studies of dispersion of the nonlinear absorption cross sections and the refractive nonlinearities of organic and organometallic nonlinear chromophores using the technique of Z-scan, with a tunable amplified femtosecond laser system. Z-scan is less sensitive than the popular technique of two-photon induced fluorescence but has advantages of being suitable for non-fluorescent substances and providing information on both absorptive and refractive nonlinearities. We have analysed the experimental results in terms of simple models and using the Kramers-Kronig transformation as shown in this paper for Coumarine 307 and an organometallic dendrimer. The dispersion curves are often dominated by two-photon resonances but inclusion of other nonlinear mechanisms seems to be necessary for better understanding of their features.
DNA is a polyelectrolyte capable of forming thin films with interesting optical properties. We investigated refractive
indices and optical anisotropy of films of the native, sodium ion-based DNA (Na-DNA) and DNA bearing the
cetyltrimethylammonium ion (DNA-CTMA) using a prism coupler technique. The light polarization direction was
either parallel (nTE) or perpendicular to the surface plane of the films (nTM). The index values and the birefringence of
DNA films vary considerably depending on the type of the counter-ion, the film fabrication method and the relative
humidity (RH) of the environment. A high negative birefringence in films of Na-DNA, nTE-nTM = -0.03 at an RH ~ 55
%, was measured in solution-cast films, indicating that the optically anisotropic DNA molecules are aligned in the plane
parallel to the film surface. Refractive indices of DNA-CTMA thin films were smaller and more isotropic than those for
films of Na-DNA polymer. The prism coupler reflectance curves showed a hysteresis of the index values when the RH
of a DNA-CTMA film environment varied. Polarization microscopy studies showed liquid-crystalline textures at the
edges of Na-DNA and DNA-CTMA films.
Third-order nonlinear optical properties were investigated for the system incorporating the Disperse Red 1 dye and the deoxyribonucleic acid - cetyltrimethylammonium complex (DNA-CTMA). The interaction of the dye with the DNA chains does not lead to major changes of the nonlinear optical effects due to the DR1 dye. Polarization dependences of the nonlinear absorption in this system reveal, however, some indications that the interactions may lead to ordering of the dye molecules against the DNA chains.
DNA is emerging as a novel exciting photonic polymer material due to its unique double-helix structure and the ability to
act as a host capable to be aligned itself and capable of inducing orientation of nonlinear optical (NLO) chromophores.
Physical and optical properties of DNA are remarkably modified with the alteration of the nucleic acid counter-ions. We
determined optical properties of salmon-derived DNA and DNA complexed with cetyltrimethyl-ammonium (CTMA)
surfactant in solutions and films. Absorption coefficients derived for an average nucleotide formula weight indicated
DNA of high purity. Prism coupling measurements showed a large birefringence in refractive indices in the direction
parallel and perpendicular to the surface plane of the films indicating anisotropic alignment of DNA molecules. Almost
isotropic refractive indices were measured in DNA-CTMA films indicating disorder in orientation of DNA-CTMA
molecules in the films. Doping with about 5 wt% Disperse Red 1 (DR1) essentially did not change this very weak
birefringence in the DNA-CTMA films. Optical properties of DNA films were sensitive to environmental humidity while
the DNA-CTMA films were less susceptible to it. The Z-scan technique using femtosecond pulsed laser system was
employed to determine the NLO properties of DNA in solutions in the 530-1300 nm wavelength range.
We investigated linear optical and second-order nonlinear optical (NLO) properties of films of urethane-urea copolymer (UU2) functionalised with a high concentration of an azobenzene chromophore. The polymer films on ITO-coated substrate were corona poled to induce a noncentrosymmetric organization of chromophore dipoles and data on the second harmonic generated with the laser beam (the fundamental wavelength 1053 nm, 6 ps/pulse, 20 Hz repetition rate) was acquired as a function of time and temperature. Second harmonic generation (SHG) was used to monitor in situ the polar alignment and relaxation of orientation of the side-chain Disperse Red-like chromophore molecules in the films poled at room temperature and high above the glass transition temperature (Tg 140-150oC). The deff coefficient was determined from the Maker-fringe method and corrected for absorption. A strong second harmonic effect with a fast relaxation was observed in "cold" (room temperature) poling experiments. A large second-order resonantly enhanced optical nonlinearity (d33 of the order of 200 pm/V) was obtained in high temperature poling. A strong and stable nonlinearity has persisted for years after the films were high-temperature poled.
Linear and nonlinear optical properties of a new soluble polymer bearing distyrylbenzene chromophore, the alkoxy-sulphono-substituted p-phenylenevinylene oligomer (MTPV-ORSO) attached as a side-chain to the methyl methacrylate backbone are reported. This chromophore was developed for introduction into single-mode nonlinear optical polymer fibres. The molecular second-order and third-order nonlinearities were estimated with quantum chemical calculations (MOPAC). The side-chain polymer shows an optical absorption maximum at 326 nm. The polymer was incorporated into the core of the fibre preform with a procedure which led to a step index profile, as measured with a preform profiler, suitable for a single-mode optical fibre. Second harmonic of the fundamental 1200 nm wave was measured in the corona poled polymer films giving the second-order susceptibility d33 = 0.8 pm/V for the polymer containing about 1.3 wt% of the side-chain chromophore and 0.1 pm/V in a guest-host system containing 0.24 wt% of the chromophore in the fibre preform. The coherence length of the side-chain polymer, equal to 28 μm at 1200 nm, was evaluated from the dispersion of the linear refractive indices. Degenerate four-wave mixing (DFWM) with amplified femtosecond pulses was also applied to measure the nonlinear refractive index of the co-polymer at 800 nm.
Time-resolved degenerate four-wave mixing (DFWM) experiments performed on films of triphenylamino-phenylene vinylene (TPA-PPV) copolymer show the modulus of the nonlinear refractive index to be (2.1#0.4) x10-13 cm2/W at 800 nm. The polymer was synthesized in the Hoerner-type polycondensation reaction. The films were characterized by optical absorption spectra, molecular weight and glass transition temperature measurements. The linear refractive index measurements performed with a prism coupler indicate that the annealed polymer films are isotropic. The films showed waveguiding of light. The DFWM experiments were performed in the forward BOXCARS geometry with simultaneous monitoring of the phase-matched and the non-phase-matched signals. This allowed measuring the nonlinearity of sub-micrometer thick films even in the presence of signals from a thick glass substrate. A cubic power dependence of the diffracted signal vs. pump intensity was observed as expected for the Kerr-type electronic nonlinearity. The signals showed a strong instantaneous response followed by a slow decay with the time constant 9#2 ps. Z-scan measurements showed the presence of two-photon absorption in the polymer.
Third-order optically nonlinear polymeric material built of rigid-rod molecules are often insoluble and transmit visible light poorly. We report on optical and waveguiding properties of a new soluble derivative of PPV: a π- conjugated polymer DFP-PDPV. We found that solvents used for processing of the DFP-PDPV polymer may influence the film optical properties. Changes in absorption spectra, birefringence, optical attenuation and the nonlinear refractive index were observed in the films made by spin coating and solution cast using different solvents.
The films are birefringent and dispersive. Refractive indices vary from 1.76 to 1.63 for the TE polarisation of the incident
light and from 1.70 to 1.60 for the TM polarisation for wavelengths from 476.5 nm to 1.55μ, respectively. The birefringence is an order ofmagnitude lower than that in films ofunsubstituted PPV.
DFP-PDPV is suitable for fabrication of optical waveguides. Low loss waveguiding layers could be prepared. Propagation losses were measured at 632.8nm, 8lOnm and 1064nm. The losses decrease at longer wavelengths reaching the level of 1 dB/cm at 1.06μ. The waveguiding properties of DFP-PDPV films depend on the solvents used in processing of the polymer. We investigated thin films of DFP-PDPV for their potential for nonlinear waveguide applications. The modulus of nonlinear refractive index |n2| in the range (0.9-1.5) x10-14 cm2/W was measured in DFP-PDPV films at 8OOnm using a femtosecond degenerate four wave mixing (DFWM) technique. These studies supplement the results of nonlinear optical studies of this polymer in solution using the Z-scan technique described in Proceedings of SPIE 3473, 79-90 (1998).
Organically modified silicates (ormosils) derived from acid catalyzed sol-gel processing of a mixture of phenyl triethoxysilane (PhTES), methyl triethoxysilane (MTES) and tetraethyl orthosilicate (TEOS) are being assessed for photonics applications. PhTES and MTES introduce phenyl (C6H5-) and methyl (CH3-) groups onto the silica backbone, respectively increasing and decreasing the refractive index, so that the refractive index can be tuned from 1.40 to 1.55 (at 633 nm) by varying the composition. Films are deposited onto silicon or glass substrates and dried under nitrogen at or below 200 degree(s)C to remove residual water and ethanol; we present refractive index and absorption data before and after drying for several compositions. Although the triethoxysilane (i.e. MTES and PhTES) content should be maximized to inhibit cracking caused by drying-induced stress, some TEOS appears to be beneficial for mechanical stability. Crack-free films dried at 200 degree(s)C have been produced with thickness up to 15 micrometers .
The techniques of closed and open aperture Z-scan as well as that of phase-matched and non-phase-matched degenerate four- wave mixing (DFWM) were used to investigate nonlinear optical properties of two poly(p-phenylenevinylenes) substituted with the phenoxyphenyl and fluorophenyl groups at the vinylene position. The polymers studied were poly[1,4-phenylene-1,2- di(4-phenoxyphenyl)vinylene] (DPOP-PPV) and poly[4,4'-biphenyl-diyl-1,2-di(4- fluorophenyl)vinylene] (DFP-PDPV). Spectroscopic studies showed a blue shift of absorption spectra of both polymers in comparison to that of poly(p-phenylenevinylene) (PPV). Z-scan experiments for solutions of the polymers in 1,4-dioxane and DFWM experiments for thin films of the polymers were carried out at 800 nm with amplified 100 femtosecond pulses. The results of both techniques are in good agreement, indicating that the nonlinear refractive index n2 is on the order of 10-14 cm2/W for the substituted polymers. The sign of the real and imaginary parts of n2 is positive. The polymers possess relatively low values of two-photon absorption merit factors at 800 nm. The nonlinear properties of the substituted polymers were compared to the results obtained on unsubstituted PPV films.
We show that the chemical and physical environment used to synthesise and process poly(p-phenylenevinylene) (PPV) by the soluble precursor route in the matrix of poly(N-vinylpyrrolidone) (PVP) has a remarkable effect on the effective conjugation length of the resulting polymer, which directly affects the third order nonlinearity. A peculiar bathochromic shift of the position of the PPV absorption maximum, an increase of absorption coefficients and the anisotropy of refractive indices upon conversion have been observed. A large third-order optical nonlinearity (In2I>lO cm2/W) at 800 nm can be achieved in the composites. The nonlinearity of the composites depends on the PPV content and the conversion conditions.
The PPV-PVP system is interesting from the point of view of its good waveguiding properties in the thin film planar waveguide structures.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.