Second-harmonic generation in poled films of nonlinear optical polymer composites

The second-harmonic generation (SHG) has been measured for studying the molecular ordering and its relaxation process in poled states of two different nonlinear optical (NLO) polymer composites.p-Nitroaniline (pNA) and the azo dye Disperse Orange 3 (DO3) are uniformly dispersed in photopolymer matrices followed by poling. Two different composites made-up of 1 wt% of pNA and 2.5 wt% of DO3 are poled at 200 MV m^-1 and 80 MV m^-1, respectively. One of the second-order NLO coefficients,d ₃₃, was found to be 0.27 pm V^-1 for the pNA-doped film and 0.20 pm V^-1 for the DO3-doped film. While for the latter the ratiod ₃₃/d ₃₃ (3.1) is consistent with the value of 3 in the low-field approximation, for the former the value of 3.5 indicates that higher-order effects of the poling field play a significant role in the NLO process. It is suggested that the SHG relaxation dynamics is also closely related to the UV absorption of the NLO molecules in a photopolymer matrix.     

Modal dispersion phase-matched frequency doubling in composite planar waveguide using ion-exchanged glass and optically nonlinear poled polymer

We report on the fabrication and characterization of composite planar waveguides for frequency doubling. Modal dispersion phase matching is obtained for TM₀ fundamental and (TM₁, TM₂) harmonic wavelength. The experimental conversion efficiency was η=0.45% with an SH wavelength of 0.516 μm for TM₀^ω–TM₂^2ω mode conversion. The phase-matching condition can be fulfilled by adjusting the diffusion time of ion-exchange and the thickness of the polymer layer.     

Second-harmonic generation in periodically poled nonlinear polymer waveguides

We demonstrate quasi-phase-matched (QPM) second-harmonic generation (SHG) at the optical communication wavelengths with side-chain polymer waveguides. A ridge waveguide structure is designed to support fundamental mode guiding at both the pump and the second harmonics, leading to a high field overlap integral of the guided modes. The nonlinearity contrast in the +/0 type QPM waveguide is maximized under a QPM poling electrode width of nearly half the coherence length. Using these configurations, we record a normalized SHG efficiency of 2.2% W(-1) cm(-2) in the polymer waveguide.     

极化聚合物薄膜中二阶非线性系数的测定和稳定性研究

详细介绍了极化聚合物薄膜中二阶非线性系数的测定方法，讨论了这种体系中进行时间稳定性研究的有关问题。估计了ＤＡＮＳ掺杂的这种聚合物中从取向状态到无序分布的表观活化能，讨论了最优入射θｍ在弛豫过程中的变化幅度与材料的时间稳定性的关系。     

Third harmonic generation in copolymer films with optically nonlinear N-substituted 4-nitroazobenzene in the side and main polymer chains

The third-order nonlinear optical susceptibility χ⁽³⁾(3ω; ω, ω, ω) of two types of copolymer films containing optically nonlinear N-substituted nitroazobenzene in the side (methacrylic series copolymers) and main (copolyimidoamido esters) polymer chains is investigated using the third harmonic generation method at a wavelength of 1.064 μm. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 379–382. Original Russian Text Copyright ? 2002 by Lebedeva, Loretsyan, Ivanova, Romashkova, Lukoshkin, Kudryavtsev.     

Measurement of the linear electrooptic effect in poled polymer thin films with coplanar electrodes

The linear electrooptic (EO) effect of poled polymer thin film is measured in the coplanar-electrode structure. The optical geometry of the coplanar-electrode thin-film setup is analysed theoretically to find a relationship between the linear optical parameters and the EO modulation intensity. EO modulated intensity is measured as a function of the direction of incident light polarization relative to the macroscopic optic axis, also as a function of optical bias determined by quarter-wave plate. Comparison of experimental measurements with the theoretical analysis shows good agreement.     

Enhancement of photoinduced polarization rotation in azobenzene polymer films

In this paper we demonstrated enhancement of photoinduced polarization rotation for improved transmission of laser light at low input intensities through a crossed polarized system. It is achieved by utilizing two azobenzene doped polymer thin films in tandem. As the input beam propagates through the films, its polarization rotation induced by each film is additive. We obtained polarization rotation of as much as 24° resulting in enhanced transmission compared to a single-film approach. In addition, this novel design is promising for use as a broadband nonlinear transmission system.     

Preparation and characterization of poled nanocrystal and polymer composite PZT/PC films

Pb(Zr_0.52Ti_0.48)O₃ (PZT) nanocrystals and transparent polycarbonate (PC) composite thin films with useful properties for ferroelectric, piezoelectric and electro-optic devices were prepared by a spin-coating technique. Ultra-fine PZT (∼40–50 nm) nanocrystals with pure perovskite tetragonal phase were synthesized by a hydrothermal method. The structure and morphology of the composite thin films were studied by means of X-ray diffraction and scanning electron microscopy. To obtain the optimum electro-optic properties of poled composite films, the poling condition under an external electric field was optimized through the dielectric properties of PZT and PC polymer and effective field intensity theory. The electro-optic coefficient of the poled PZT/PC composite film is estimated to be 30.5 pm/V. The transparency spectra were measured and the optical band gaps of the unpoled, poled at 145 °C and poled at 165 °C composite thin films are estimated to be 4.26 eV, 4.21 eV and 4.18 eV, respectively. The measured dielectric constants of PZT/PC are in good agreement with the calculated values for the composite with a very small PZT volume fraction, based on the Onsager effective-field theory. This offers a reliable and indirect way to predict the dielectric constant of nanocrystals. PACS 81.40.Tv; 78.66.Sq; 78.66.Vs     

Self-induced nonlinear polarization change of femtosecond laser pulse propagation in an optically isotropic glass

The self-induced polarization change of femtosecond laser pulses passing through an isotropic optical glass is investigated. It is shown that supercontinuum generations from femtosecond lasers tend to be identical for the incident beam with different polarizations ranging from a large-ellipticity to a linear state when the incident laser power increases. Theoretical analysis reveals that a transient intensity-dependent birefringence is induced in the glass sample by the laser pulse itself, and the accumulated nonlinear phase shift leads to a change in polarization ellipticity. Simulation results are consistent with the experimental observations. PACS 42.50.Md; 42.65.Jx; 42.81.Gs; 77.22.Ej     

偶氮液晶聚合物薄膜的二维偏振全息记录

利用偏振全息记录的方法在一种含偶氮侧链的液晶聚合物薄膜中写入了二维偏振光栅.实验采用两束正交偏振的532 nm线偏光作为写入光,在样品同一点上分别记录了相互垂直的两个一维偏振光栅,继而对所构成的二维偏振光栅的特性进行了研究.实验结果表明：二维光栅的衍射效率比一维光栅低,其偏振特性是两个一维光栅元特性的叠加;二维光栅衍射效率对入射光偏振态的依赖性和光栅的偏振转换性质来源于材料中线双折射和圆双折射的共同作用. 关键词： 偏振全息 二维光栅 偶氮液晶聚合物     

Investigations of the E-O behavior in the poled polymer waveguide

The electro-optic (E-O) properties of the slab polymer waveguide induced by electric poling have been studied experimentally. The thin film waveguide was prepared by a synthesized polymer polypelargonamide and poled by applying voltage between the electrodes. I–V curves and E-O characteristics of the polymer films were in-situ measured during poling. In order to evaluate the molecular dipole orientation under the poling field, the absorption spectra of this film was observed by FTIR. The E-O performance of the polymer slab film was also estimated by the Mach–Zehnder interferometer.     

Measurement of polarization dependence of nonlinear susceptibility responsible for Rayleigh-wing and Brillouin scattering

By using a simple optical geometry based on backward light scattering and employing a Sandercock-type tandem Fabry-Perot interferometer, we measure both the linear and the circular polarization dependences of Rayleigh-wing and Brillouin scattering in a sample of liquid-crystal 4-n-pentyl-4'-cyanobiphenyl. Observed polarization dependences are consistent with the third-order nonlinear susceptibilities, taking into account the traceless symmetric scattering tensor for Rayleigh-wing scattering and the isotropic scattering tensor for Brillouin scattering.     

Second-order nonlinear optical properties of poled coumaromethacrylate copolymers

Second-order nonlinear optical properties of newly designed and synthesized coumaromethacrylate side-chain polymers are reported. The optimum poling conditions were determined experimentally. The optimum poling temperature for these side-chain polymers is well above the glass transition temperature. The second harmonic coefficient of films poled by coronaonset at elevated temperature and the linear electro-optic coefficient of films poled by contact electrodes were measured. The stabilized value of the second harmonic coefficient, d ₃₃, at 1064nm fundamental wavelength was found to be 13 pm/V. The linear electro-optic coefficient, r ₃₃, exhibits strong dispersion ranging from 2 to 12pm/V in the wavelength range 477 to 1115 nm.     

Heterocharge accumulation and relaxation and nonlinear transfer in polymer films

St. Petersburg State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 15–19, April, 1994.     

Electro-optical microscopy: mapping nonlinear polymer films with micrometric resolution

The first demonstration, to the best of our knowledge, of electro-optical microscopy is presented and applied to a polymer-based optical device. A confocal transmission microscope with interferometric homodyne detection is implemented to measure Pockels phase shifts with micrometric spatial resolution and an accuracy level down to 4 x 10(-7) rad. This technique is applied to poled polymer films in which noncentrosymmetric molecular orientation is preliminarily achieved in the sample plane between transverse planar electrodes. The electro-optic mapping of this structure exhibits nonuniform and asymmetric patterns of the nonlinear response that are characteristic of the poling spatial inhomogeneity as confirmed by second-harmonic generation microscopy.     

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.     

Giant quadratic electro-optical effect during polarization switching in ultrathin ferroelectric polymer films

The low-frequency quadratic electro-optical effect with a maximum electro-optical coefficient of g = 8 × 10^?19 m²/V² (i.e., four orders of magnitude greater than the standard high-frequency value) has been studied in thin films of ferroelectric polymer PVDF(70%)-TrFE(30%). The observed effect is related to the process of spontaneous polarization switching, during which the electron oscillators of C-F and C-H dipole groups rotate to become parallel to the applied field. As a result, the ellipsoid of the refractive index exhibits narrowing in the direction perpendicular to the field. The field dependence of the electro-optical coefficient g correlates with that of the apparent dielectric permittivity, which can be introduced under the condition of ferroelectric polarization switching. The observed electro-optical effect strongly decreases when the frequency increases up to several hundred hertz. The temperature dependence of the effect exhibits clearly pronounced hysteresis in the region of the ferroelectric phase transition.     

Thresholds for nonlinear recording of fluorescent centers in chromone-doped polymer films

Nonlinear recording of stable fluorescent microstructures in PMMA films doped with photosensitive compound 2-(furan-2-yl)-3-(thiophen-2-carbonyl)-chromen-4-one is investigated. Energy and power density thresholds for recording stable fluorescent marks and for film destruction and photobleaching of the fluorescent marks by laser pulses were measured in wide spectral (from 530 to 620 nm) and temporal (from 80 to 7 ns) ranges. The evidences of the sequential two-step excitation mechanism for the studied dye are presented. It was shown that picosecond pulse duration range is preferable for two-photon data recording. Quasi-Gaussian and donut shaped fluorescent marks with almost the same cross-section were recorded by femtosecond pulses with rather small difference in energies.     

Interaction of light waves in active nonlinear and periodically poled nonlinear crystals

The results of classical and quantum studies of the laser-radiation self-frequency conversion processes in periodically poled active nonlinear crystals are overviewed. The theoretical and experimental results of studying quasi-phase-matched self-frequency doubling and summation of the laser and pump frequencies in an active nonlinear periodically poled Nd:Mg:LiNbO₃ crystal are presented. The possibility of producing frequency-and polarization-entangled states and the sub-Poisson field statistics through a consecutive nonlinear optical frequency conversion in periodically poled nonlinear crystals is considered.     

Photon energy dependence of the spin polarization

Spin polarization in semiconductors was investigated by pump-probe measurements, where the transmissions of the samples were monitored with probe pulses with same and opposite circular polarizations. The spin polarization as a function of the pump-probe delay was estimated, and the polarization was found to decay in a sub-ns timescale. It was also found that the polarization depends strongly on photon energy of the pump beam. The pumping energy dependence of the spin polarization was discussed based on the inter-band transition probabilities.