A new effect in second harmonic generation by poled nematic films (original) (raw)
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The Journal of Physical Chemistry, 1995
photomirocyanines have rather high first-order hyperpolarizabilities, this photo-orientation leads to secondirarmonic generation (SHG). The second-order susceptibility dg: of PMMA films doped with Gnitro-l',3',3'trimethylspiro[2f/-1-benzopyran-2,21indolise1(25Vo Ww) has been measured at 10 pm/V. If a PMMA film doped with spiropyran and preoriented by corona poling at 90 oC is irradiated in the UV, an SHG signal of photomerocyanine is also observed as a result of a memory effect of the orientation of spiropyran molecules. The photoswitching of this nonlinear optical property can be inverted by visible light irradiation. Only paltially reveisible switching is observed because of a gradual light-induced disorientation of photomerocyanine dipoles.
It is shorvn that photoinduced ring opening of a spiropyran dissolreC in a P\lMA matrix leads to an asymmerric orieniation of photomerocyanine molecules uhen the irrac:a:io:r is performed under a static (dt) electric field. With the second harmonic generation (SHG1 re;i.:rique. it has been possible to 6emonstrate thlt both static field and photoinduced reactiLrn ha"e a s1'nergistic influence on phoroorientarion. The mechanism of photoorientirtion implies a dissin::::l:i; ring opening of spiropyran molecules which tends to line up the giirnt dipoles ol photomeroclan:::e *ilh the electric field during the irradiation process and a subsequent rotation of dipoles in the stal:c ei:ctrlc field.
Second-harmonic generation in a novel crosslinked pyroelectric liquid crystal polymer (PLCP)
Liquid Crystals, 1998
We report second-harmonic generation in samples with a pyroelectric liquid crystal polymer (PLCP) prepared from the ferroelectric liquid crystalline acrylate monomer 4-[(R)-(-)-2-(10-acryloyloxydecyl)oxy]-3-nitrophenyl 4-[4-(11-acryloyloxyundecyloxy) phenyl ]benzoate (A2c) by in situ photopolymerization. The relation between SHG e ciency, spontaneous polarization and amount of chiral substance in this and previously synthesized PLCP materials is discussed. Electro-optical properties of the monomeric and crosslinked A2c are investigated. In the crosslinked state, the molecular mobility is found to be very low, supporting the fact that the degree of polar order in the polymer is high. In contrast to the case of a poled electret NLO material, the polarization of this material is not`frozen-in', but an intrinsic thermodynamic property.
Second-harmonic light generation in pyroelectric liquid-crystal polymers
Journal of the Optical Society of America B, 1998
Pyroelectric liquid-crystal polymers were prepared by photopolymerization of binary mixtures of two monomers that exhibit a smectic C* phase: A2c, 4Љ-͕(R)-(Ϫ)-2-[(10-acryloyloxy)decyl]oxy͖-3-nitrophenyl 4-{4Ј-[(11-acryloyloxy)undecyloxy]phenyl͖benzoate and A1b, 4Љ-((R)-(ϩ)-2-octyloxy)-3Љ-nitro phenyl 4-(4Ј-͓(11acryloyloxy)undecyloxy]phenyl) benzoate. Both liquid-crystal monomers have a NO 2 substituent to enhance the nonlinear optical properties, and one of the monomers, A2c, permits polymerization to a cross-linked polymer. During the polymerization an electric field of approximately 25-50 V/m was applied over the ferroelectric liquid-crystal cells. All cases of polymers formed from the chiral smectic C* phase showed a secondharmonic-generated signal with no external field present, indicating that polar order became fixed. The orientation dependence of the second-harmonic-generated intensity was similar to that of the ferroelectric liquid-crystal monomer; however, some changes were observed that might be due to changes in the dielectric axes of the system. The highest d 16 and d 23 coefficients were found to be in the range 0.65-0.8 pm/V and differed depending on the detailed preparation of the sample. Experimental results of several polarization combinations of the pump and frequency-doubled light are presented and discussed.
Journal of Applied Polymer Science, 2014
A new, asymmetrical zinc phthalocyanine (aZnPc)-functional photocurable copolymer was prepared by the combination of atom transfer radical polymerization and copper (I)-catalyzed azide-alkyne cyclo-addition (CuAAC) click reaction and used as polymer matrix of polymer dispersed liquid crystal (PDLC) film. For this purpose, aZnPc was prepared through statistical condensation of 4-tert-butylphthalonitrile and 4-pent-4-ynyloxyphthalonitrile. Double CuAAC click reaction between azido-functional poly(methyl methacrylate-co-2-(2-bromoisobutyryloxy)-ethyl methacrylate), terminal alkynyl-substituted aZnPc, and 4-ethynyl-N,Ndimethyl aniline yielded photocurable aZnPc-functional copolymer. Thereby, synthesized copolymer was crosslinked in the presence of liquid crystalline mesogen 4 0-(octyloxy)-4-biphenylcarbonitrile by ultraviolet irradiation using benzophenone as initiator and ethylene glycol dimethacrylate as difunctional crosslinker. Thermal and optical properties of PDLC film were investigated by using differential scanning calorimetry and polarized optical microscopy. Smectic A liquid crystal mesophases were observed in both PDLC film and its mesogenic component 4 0-(octyloxy)-4-biphenylcarbonitrile. V
Time-dependent nonlinear optical properties of pyroelectric liquid crystalline polymers
Macromolecular Symposia, 1999
Highly oriented pyroelectric liquid-crystalline polymers were prepared by photopolymerization under the influence of a static electric field from binary mixtures of two acrylate monomers exhibiting chiral smectic C mesomorphism. Both monomers contained nitro groups to yield second order nonlinear optical properties (second harmonic generation) and one of the monomers had two functional groups to yield a crosslinked polymer. The room temperature second order nonlinear susceptibility of the polymers showed during the first two hours a 10 % decrease after which it remained constant during the next 48 days. At elevated temperatures there was a significant difference in the nonlinear optical properties over time between crosslinked and uncrosslinked polymers. The uncrosslinked polymer showed a pronounced loss of second order nonlinear optical activity with time at 238°C. The crosslinked polymer showed a much smaller and basically a temperature independent decrease rate in the second order nonlinear optical properties at all the ageing temperatures (23-130OC). Both the loss in mesogen order parameter, very evident for the uncrosslinked polymer, and conformational changes occurring within the mesogens (p mechanism), may account for the observations made.
Macromolecules, 2007
We performed second harmonic generation (SHG) measurements in the nematic liquid crystal formed from polar aromatic polyester which comprises 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid in a molar ratio of 73/27. The polymer is commercially available and called Vectra. The shear-oriented nematic liquid crystal of Vectra shows strong SHG mostly along the n-director. Since no electric poling is made, the observed SHG activity indicates that the polar ordering is spontaneously formed in the nematic liquid crystal. From detailed analyses of the SHG intensity profiles measured with combinations of polarizer and analyzer directions to the n-director, the nematic liquid crystal is found to possess the C s packing symmetry; in other words, it is not uniaxial but biaxial, and the polarization arises in the symmetry plane. To search the origin of the spontaneous polarization, we prepared a series of Vectra polymer with various degrees of polymerization (DP) and two chemically modified Vectras. Of interest is that the DP of polymer significantly affects the polar structure. When DP is decreased, the SHG activity in the nematic liquid crystal disappears completely. Since the dipole moment increases with DP, the huge dipole moment of polymer is considered to be responsible for the polar ordering. In one of the two chemically modified Vectras, terephthalic acid (TA) and biphenol (BP) in an equimolar content are introduced. As expected from the dissipation of head-tail character in constituent polymer, the nematic liquid crystals lose completely the SHG activity only with an incorporation of 5 mol % of TA and BP. In another modified Vectra including 3-hydroxybenzoic acid (m-HBA), the head-tail character is not lost, but their liquid crystallinity is destabilized because of kink conformation induced by the m-HBA unit. In this system, the SHG is invariably observed for the nematic liquid crystals, but as one of the significant effects, polar biaxial nematic liquid crystal of Vectra is altered to the polar uniaxial one by the introduction of m-HBA above 5 mol %.
Japanese Journal of Applied Physics, 1990
Near-edge X-ray absorption fine structure is used to investigate the temperature dependence of molecular orientation of semifluorinated liquid crystalline (SF-LC) mesogens, which are attached to the modified isoprene backbone of (i) a poly(1,2-isoprene) homopolymer and (ii) a diblock copolymer consisting of polystyrene and poly(1,2-isoprene) blocks. Our experiments reveal the existence of two temperature regions in which the surface orientation of the SF-LC mesogens changes abruptly, but even 30 K above the highest such temperature region the surface orientation does not become isotropic. The lower temperature surface transition for both homopolymer and block copolymer occurs close to the temperature of the bulk homopolymer smectic-B to smectic-A transition and well above the bulk smectic-B to smectic-A transition in the block copolymer. It seems to be controlled exclusively by the ordering phenomena originating from the surface. In contrast, the change in the surface organization of the SF-LC mesogens at higher temperatures can be associated with the bulk LC transition from the smectic-A to the isotropic phase.
Chemistry of Materials, 1990
performed on other conjugated polymers have shown complex dependence of the signal intensity on electrode potential sweep rates. Future work will describe the magnetic properties of poly-I and the dependence of its ESR signal on temperature. Conclusions Poly(N-3-thenylphthalimide), poly-I, was electrochemically synthesized, and the resulting polymeric films exhibited electrochemical and electrochromic reversibility and enhanced porosity. The bulky electron-withdrawing substituent at the 3-position widens the bandgap and favors the presence of localized charges, which, in turn, negatively affects the conductivity of the resulting polymer. Acknowledgment. Our program on polymer-modified electrodes is supported by the US Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences. Our synthesis of conducting polymers is supported by the Texas Advanced Research Program. We are grateful to Dr. Michael Schmerling for assistance in the SEM measurements, to Norman Williams for the conductivity measurements, and to Robert Meegan for advice on the construction of the ESR cell. Poly I (homopolymer), 124605-41-0; (I)(3methylthiophene) (copolymer), 124605-40-9. Registry No.