Nonlinear Optical Side-chain Polymer Research Articles

Nonlinear optical side‐chain polymers post‐functionalized with high‐β chromophores exhibiting large electro‐optic property

AbstractElectro‐optic side‐chain polymers have been synthesized by the post‐functionalization of methacrylate isocyanate polymers with novel phenyl vinylene thiophene vinylene bridge (FTC) nonlinear optical chromophores. For this application, FTC‐based chromophores were modified in their electronic donor structure, exhibiting much larger molecular hyperpolarizabilities compared with the benchmark FTC. Of these new chromophores, absorption spectra, hyper‐Rayleigh scattering experiment, and thermal analysis were carried out to confirm availability as effective nonlinear optical units for electro‐optic side‐chain polymers. The electro‐optic coefficients (r33) of obtained polymers were investigated in the process of in situ poling by monitoring the temperature, current flow, poling field, and electro‐optic signal. Compared with the nonsubstituted analogue, benxyloxy modified FTC chromophore significantly achieved higher nonlinear optical property, exhibiting molecular hyperpolarizability at 1.9 μm of 4600 × 10−30 esu and an r33 value of 150 pm/V at the wavelength of 1.31 μm. Synthesized electro‐optic polymers showed high glass transition temperature (Tg), so that the temporal stability examination exhibited >78% of the electro‐optic intensity remaining at 85 °C over 500 h. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Ellipsometric study of the poling effect on nonlinear-optical side-chain polymers containing disperse red 1

The purpose of this work is to investigate the effect of corona poling on the optical functions of the nonlinear-optical (NLO) polymers. We measured the complex refractive indices of NLO polymers deposited on glasses before and after corona poling using spectroscopic ellipsometry at room temperature at various incidence angles. In addition, we observed the absorption spectral change using ultraviolet-visible spectroscopy after corona poling, which is attributed to the chromophore orientation. In order to make NLO polymers, we attached a NLO active dye and photocrosslinkable moieties as side chains of poly(4-hydroxystyrene), deposited the polymer thin films on glass substrates by spin-coating, and corona-poled it along the surface-normal direction to align the dipoles of the chromophores along the electric field direction. In detail, PSDR1-25 designates a poly(4-hydroxystyrene) where 25% of hydroxyl groups in the side chain are substituted by disperse red 1 (DR1) and PSDR1-50 means the same except for 50% substitution. PSDR1 has the second-order NLO property, which is expected to show an acentric ordering by poling electric field. We used a parametric optical constant model to estimate the complex refractive index of the polymers. We found that the poling induced the uniaxial property of the complex refractive index of the polymers by aligning the chromophores along the poling direction. With increased poling voltage, we found that the oscillator strength of the major peak near the 2.4eV (designated as Ea) peak decreases in the surface-parallel direction (x), whereas that of the surface-normal direction (z) increases. Moreover, a detailed analysis of the dielectric functions using the standard critical point model showed that the poling induced a new peak Ec (∼2.6eV) between dominant Ea (∼2.3eV) and weak Eb (∼2.9eV) peaks for PSDR1-50 polymers. With increased poling voltage, the Ec peak strength increased compared to those of Ea and Eb peaks. We tentatively attribute Ec to lower symmetry, possibly Stark effect, induced in the chromophores by the corona poling.

Low temperature relaxations and effects on poling efficiencies of dendronized nonlinear optical side-chain polymers

Low temperature relaxations in a dendronized nonlinear optical (NLO) side-chain polymer were found to take place at more than 20 °C below the glass transition temperature. Relaxations of localized mobilities, removed from long range relaxations responsible for chromophore aggregation, are shown to offer new gateways for optimized acentric ordering of the chromophores. Supreme electro-optical (EO) activity was achieved by electrical poling close to the critical temperatures of localized mobilities identified as dendronized NLO side-chain relaxations. This study features, in particular, one new instrumental approach to relaxation studies of thin spin coated NLO polymer films; the shear-modulation force microscopy (SM-FM) method. Originating from scanning force microscopy (SFM), the SM-FM method grants access to the detection of low temperature relaxations in constrained thin NLO films not obtainable by conventional means.

Dielectric investigation of photo-induced chromophore degradation in nonlinear optical side-chain polymer electrets

Organic materials with noncentrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photooxidation or photoreduction reactions. Reversible isomerization is the basis for a variety of applications, such as photoinduced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of nonreversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures.

Synthesis of photoconducting nonlinear optical side-chain polymers containing carbazole derivatives

Two kinds of multifunctional monomers, 2-methyl-acrylic acid 6-[3-[2-cyano-2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester (M1) and 2-methyl-acrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester (M2) were synthesized to possess both photoconductivity and nonlinear optical (NLO) properties. Photoconducting NLO polymers (P1, P2) were obtained by the copolymerization of these multifunctional monomers with methylmethacrylate. Charge-transporting monomer, 2-methyl-acrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester (M3) was also synthesized and copolymerized with M1 or M2 to give the photoconducting NLO polymers (P3, P4) with enhanced photoconductivity. The successful polymerization was identified by spectroscopic means and gel permeation chromatography analysis. These polymers were very soluble in common solvents and high optical quality films could be easily fabricated by spin coating. Second harmonic generation (SHG) measurement confirmed P1 and P2 possessed high NLO activity. From the preliminary measurement of photoconductivity, it was found that P1 possessed distinct photoconductivity. It was also found that P3 and P4 showed much higher photoconductivity than P1, presumably due to the enhanced charge transporting property of M3 structure.

Synthesis of Nonlinear Optical Side-Chain Polymers Containing the Carbazolyldinitrophenylhydrazone Moiety by Polymer Reaction

Side-chain polymer containing the carbazolyldinitrophenylhydrazone moiety was synthesized for nonlinear optical (NLO) applications. Because free radical polymerization resulted in molecular weight limitation due to the chain transfer nature of the NLO active chromophore, to prepare a high molecular weight polymer, a polymer reaction was chosen to functionalize a preformed aldehyde prepolymer. This prepolymer (P1) was synthesized by the reaction of 6-(3-formyl-carbazol-9-yl)-hexyl methacrylate (M1) with methyl methacrylate (MMA). Successful polymerization was confirmed by 1H and 13C NMR spectroscopy and the molecular weight of the polymer was determined by gel permeation chromatography analysis. Polymer reaction of P1 with 2,4-dinitrophenyl hydrazine was carried out to prepare the NLO polymer (P2) and the reaction was monitored by NMR spectroscopy. This NLO polymer was soluble in common solvent and high optical quality thin film could be easily prepared for NLO activity measurement. The electro-optic coefficient (r33) of the polymer was determined 1.4 pm V−1 at poling field of 80 V μm−1.

Temperature-domain analysis of primary and secondary dielectric relaxation phenomena in a nonlinear optical side-chain polymer

The investigation of dipole relaxation processes in nonlinear optical (NLO) polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or α relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function ε̃(T)=ε′(T)−iε″(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time τα(t) is numerically determined from the dielectric loss ε″(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong α and the weak β relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a γ-relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the β relaxation.

A Novel Class of Nonlinear Optical Side-Chain Polymer: Polyquinolines with Large Second-Order Nonlinearity and Thermal Stability

A novel second-order nonlinear optical (NLO) polymer, side-chain polyquinoline, is developed. The synthesized NLO side-chain polyquinolines possess high glass transition temperature (Tg), good thermal stability, and excellent processibility. The polymers exhibit also large electro-optical coefficient (r33) values (up to 29 pm/V measured at 0.63 μm, and 16 pm/V at 0.83 μm), and temporal stability of dipole alignment at 100 °C for over 500 h.

Polymeric electro-optic Mach–Zehnder switches

Polymeric 2×2 electro-optic switches based on the balanced bridge, or Mach–Zehnder, design are presented. Two poled nonlinear optical side-chain polymers based on the dye Disperse Red 1, one with a glass transition temperature Tg of 131 °C and the other with a Tg of 228 °C, were used as the active layers in these devices. The switch designs incorporated traveling-wave electrodes in the phase shift sections and, for the lower Tg polymer device, electrically adjustable 3 dB couplers. At the 1.32 μm operating wavelength, extinction ratios of at least 28 dB were achieved in all four states of the lower Tg switch with a propagation loss of 1.6 dB/cm and a small signal 3 dB modulation bandwidth of over 2 GHz.

Nonlinear optical side-chain polymer with high thermal stability and its pyroelectric thermal analysis

Stable polymers with nonlinear optical side groups were synthesized from maleic anhydride copolymers and the azo dye Disperse Red 1 via esterification. After electrode poling under 135 V/μm at 185 °C, thermal stabilities were measured by pyroelectric and electro-optic thermal analysis and compared to a guest/host polymer of polymethylmethacrylate and the same dye. The pyroelectrically detected dipole orientation and the electro-optic activity of the side-chain polymers decreased only above 150 °C, and electro-optic r33 coefficients of up to 6 pm/V were achieved at 780 nm even without optimizing the dye content.

Dielectric relaxation measurements on nonlinear side-chain polymers

Dynamic dielectric relaxation measurements in the frequency range from 0.001 Hz to 5 kHz have been performed on an amorphous nonlinear optical side-chain polymer, containing the active molecule 4-dimethylamino-4′-nitrostilbene as pendant group. The results yielded the relaxation times of the group in the temperature region from 20° below to 40° above the glass-transition temperature of the polymer. The temperature dependence could be described well by the Williams-Landel-Ferry (WLF) equation. It is shown that the relaxation times have a distribution, which could be fit to the form of Cole and Cole with a parameter β=0.55. Consequently a substantial part of the active molecules will not be aligned when the polymer is subjected to a poling procedure.