Photoinduced Birefringence Research Articles

Photoinduced birefringence at low temperatures in Langmuir–Blodgett films of azobenzene-functionalized copolymers

The temperature dependence has been investigated for the photoinduced birefringence in Langmuir–Blodgett (LB) films from the azocopolymer 4-[ N-ethyl- N-(2-hydroxyethyl)]amino-2′-chloro-4′-nitroazobenzene (MMA-DR13) mixed with cadmium stearate. The build-up and relaxation of the birefringence in the range from 20 to 296 K were fitted with a Kohlrausch–Williams–Watts (KWW) function, with a β-value of 0.78–0.98 for the build-up and 0.18–0.27 for the decay. This is consistent with a distribution of time constants for the kinetics of the birefringence processes. The maximum birefringence increased with increasing temperature up to 120 K because the free volume fluctuation also increased with temperature. Above 120 K, the birefringence decreased with temperature as thermal diffusion dominates. In the latter range of temperature, an Arrhenius behavior is inferred for both build-up and decay of birefringence. In each case two activation energies were obtained: 0.8 and 5 kJ/mol for the build-up and 10 and 30 kJ/mol for the decay. The energies for the build-up are much lower than those associated with motion of the polymer chain, which means that the dynamics is governed by the orientation of the chromophores. For the decay, local motion of lateral groups of the polymer chains becomes important as the activation energies are within the range of γ-relaxation energies.

Azobenzene-modified poly(l-glutamic acid) (AZOPLGA): its conformational and photodynamic properties.

Azobenzene-modified poly(l-glutamic acid) (AZOPLGA) polymers with 22 and 35 mol % of azo chromophores in the side chains have been synthesized by condensing 4-methoxy-4'-aminoazobenzene and poly(l-glutamic acid). These polymers have been characterized by NMR, FT-IR, and UV-visible spectroscopic techniques. The conformational features of the polymer backbone chains in the films that were cast from the polymer solutions prepared in different solvents have been investigated by circular dichroism spectroscopy. Experimental data suggested that the thermal cis-trans relaxation and photoinduced birefringence, which are related to the azo chromophores in the side chains of polymer, are not affected by the conformations of polymer backbones. However, the modulations of the surface relief gratings, the result of photoinduced mass transport process, recorded on these polymers are sensitive to polymer main chain conformation, as well as the degree of functionalization.

Optical damage resistance in Zn:Nd:LiNbO 3 laser crystals

A series of Zn, Nd co-doped LiNbO 3 crystals were grown by Czochralski technique with 0.5 mol% of Nd 2O 3 and with 0, 4.0, and 7.0 mol% of ZnO, respectively. Their optical damage resistance was characterized by measurement of the photo-induced birefringence change. The optical damage resistance of Zn (7.0 mol%):Nd:LiNbO 3 was much higher than that of Nd:LiNbO 3. The ultraviolet-visible (UV-Vis) absorption spectra and the infrared (IR) transmission spectra of Nd:Zn:LiNbO 3 crystals were measured and investigated. The defects were discussed in this paper to explain the optical damage resistance in the Zn:Nd:LiNbO 3 crystals.

Photochromism and holographic recording in polymer film containing chiral azo molecules derived from amino acid

Abstract A kind of chiral azo molecule derived from amino acid, N-[4-(4-octyloxyphenylazo)benzoyl]- l -glutamic acid (C8-Azo- l -Glu), was synthesized and the photochromism, photoinduced birefringence, and holographic recording in C8-Azo- l -Glu doped poly(methyl methacrylate) (PMMA) films were studied. C8-Azo- l -Glu underwent a reversible trans–cis–trans isomerization in the polymer matrix. The photoinduced birefringence was investigated at various intensities of Ar laser (488 nm) beam. A reversible hologram was recorded in this media and the dependence of the first order diffraction efficiency on the recording beam intensities was also presented.

Photo-orientation phenomena in photosensitive chiral nematic copolymers

Photo-orientational phenomena have been studied for two comb-shaped cholesteric copolyacrylates containing azobenzene side groups. Copolymer I contains nematogenic phenyl benzoate groups and photosensitive chiral menthyl-containing azobenzene side groups. Copolymer II is composed of nematogenic phenyl benzoate groups, photosensitive cyanoazobenzene groups and chiral photochromic benzylidene- p -methan-3-one fragments. Under the action of polarized Ar+ laser light (488 nm), orientation of the side groups of the copolymers takes place, and this orientation is perpendicular to the vector of the electric field of the incident light. This process shows a co-operative character; that is, it involves both photosensitive azobenzene and phenyl benzoate groups. The kinetics of growth of the photoinduced orientational order parameter were studied as a function of film thickness, incident light intensity, and preliminary UV irradiation. For the planar oriented films of the copolymers, irradiation with polarized light leads to the development of photoinduced birefringence δ n ; maximum values of δ n reach 0.01. The photo-optical properties of copolymers I and II are compared. Such materials may be used for ‘dual’ data recording by varying the helix pitch, selective light reflection maximum, and photoinduced birefringence or linear dichroism.

Temperature Dependence of Photoinduced Birefringence in Polystyrene Doped with Disperse Red-1

A theoretical model on the basis of the free-volume concept is presented explaining the temperature dependence of photoinduced birefringence in polystyrene films that contain Disperse Red-1. Birefringence increases with temperature up to 180 K as the free volume for isomerization increases, and then decreases as thermally activated processes dominate. The fast process of birefringence decay has a time constant that increases with temperature at low temperatures, due to the change in photoisomerization. Temperature dependence of the birefringence amplitude measured at 300 s. The amplitude increases with temperature up to 180 K and then decreases. The solid line is for guiding the eye.

Response function for the characterization of photo-induced anisotropy in azobenzene containing polymers

A response function is derived for the description of photo-induced birefringence and dichroism in case of materials where the underlying process is photo-isomerization. Our result explains the usefulness of the theoretical formulae derived earlier by Kakichashvili for photo-anisotropic materials of another kind, which are widely used in polarization holographic models. The expression proposed by us depends on the exposure time for a given intensity, and describes the saturation of the anisotropy with time and intensity. The material constants introduced in our approach are expressed by proper microscopic quantities. The results are verified by numerical simulation and also by experiments.

Comparison of the birefringence in an azobenzene-side-chain copolymer induced by pulsed and continuous-wave irradiation

A comparative study of photoinduced birefringence in azobenzene-side-chain copolymers under continuous-wave (cw) and pulsed irradiation is presented. Absolute refractive index changes were determined by the attenuated total reflection spectroscopy. We clearly demonstrated that pulsed, similarly to cw, irradiation induces a three-dimensional reorientation of azobenzene chromophores. For the same light dose, pulsed irradiation using ns pulses of appropriate pulse energy led to higher values of birefringence, which can be attributed to a cooperative motion of azobenzene side chains and thermal effects induced by the laser pulses.

Numerical analysis of the contribution of the transverse asymmetry in the photo-induced index change profile to the birefringence of optical fiber

The birefringence of an optical fiber resulting from an asymmetry of the index profile is numerically evaluated using a finite-element method with a full-vectorial formulation. Such asymmetric index profiles, assumed to vary exponentially across the fiber core, could be induced during the writing of fiber Bragg gratings with UV side-exposure techniques. The results reveal that the birefringence is a quadratic function of the effective index change. An asymmetry coefficient near 0.4 /spl mu/m/sup -1/ maximizes the birefringence. The calculated photo-induced birefringence is negligible if the index change is lower than 5/spl times/10/sup -4/. However, the birefringence can reach 5/spl times/10/sup -6/ for large values of index change. The numerical method presented could be applied to the modeling of other asymmetric index profiles.

Photochemically induced birefringence in polyurethanes containing donor‐acceptor azobenzenes as photoresponsive moieties

AbstractThree kinds of photoresponsive polyurethanes containing slightly different azobenzene moieties [i.e., disperse red 19 (DR19), 2‐position‐substituted DR19, and 2,2′‐position‐substituted DR19] were prepared, and their photoinduced birefringence behavior was examined. These polyurethanes showed nearly the same molecular weights, glass transition temperatures, viscosities, and absorption coefficients at the wavelength of the light used to induce the photoresponse. Irradiation of these polymer films with a linearly polarized Ar+ laser induced birefringence due to reorientation of the azobenzene moieties through multiple trans‐cis‐trans isomerization cycles. Despite the slight difference in the chemical structure of the azobenzene moiety, a large difference in the growth behavior of the photoinduced birefringence was observed among these polymers. Because of the small differences in the characteristics of these polymers, we concluded that the isomerization behavior of the azobenzene moiety was the main factor for the results obtained in this study. In addition, the saturated value of the induced birefringence differed to a great extent among these polymers. It was suggested that such a phenomenon could be related to the reorientation behavior induced by the introduction of two methyl groups. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 17–22, 2002

High-efficiency holographic volume index gratings in DR1-dye-doped poly(methyl methacrylate)

Holographic volume index gratings with high diffraction efficiency (greater than 80%) are recorded in a thick sample of Disperse Red 1-doped poly(methyl methacrylate) with red light (647 nm), which is far away from the absorption peak (488 nm) of the material. Measurements of photoinduced birefringence and polarization holography recording confirm that the azo-dye reorientation mechanism is responsible for the grating formation. Energy coupling between the two writing beams is observed even when the incident beams have equal intensity.

Multi-photon-absorption-induced birefringent grating in azobenzene-doped polymethyl methacrylate optical fibres

Birefringent gratings induced by absorbing multi-photons at 532 nm in azobenzene doped polymethyl methacrylate (azo-PMMA) fibres were investigated. The maximum photo-induced birefringence observed from the fibre is about 3.0 × 10−5. The two-photon absorption coefficient β of the azo-PMMA at 532 nm obtained by the Z-scan method is 10.2 cm GW−1, and the birefringence response time of the azobenzene is 10 μs.

Influence of the Backbone on Photoinduced Birefringence in a Poly(malonic ester) Containing p-Cyanoazobenzene

The influence of the backbone on the photoinduced birefringence was investigated in a poly(malonic ester) containing a p-cyanoazobenzene group as a side-chain. The photoinduced birefringence was successively measured in the fifteen cycles of the recording, relaxation, and erasing processes. The linearly polarized light, with a polarization direction of 45° relative to the analyzer, was used as the pumping beam during the first five cycles, and the same pumping beam at negative 45° was used during the next 10 cycles. When the polarization direction of the pumping beam was changed perpendicularly, a very interesting birefringence curve was observed in the recording and relaxation processes, which could be well explained by our model considering the reorientation of the backbone caused by the elastic force between the backbone and side-chain. Consequently, we confirmed the fact that the relaxation behavior of the photoinduced birefringence is closely associated with the reorientation of the backbone.

Permanent polarization gratings in photosensitive Langmuir–Blodgett films for polarimetric applications

The study of a diffractive device for polarimetric applications is reported. The device consists of a diffraction grating recorded by two interfering opposite circularly polarized beams and is based on a Langmuir–Blodgett film of an azo-compound material. This material provides long-time stability and high photoinduced birefringence that makes the grating highly diffraction efficient. We show how polarization gratings permanently stored in these films are suitable for applications as photopolarimeter components.

Ultrafast optical nonlinearity of titanylphthalocyanine films

The optical properties of titanylphthalocyanine (TiOPc) thin film fabricated by physical jet deposition technique were investigated by using absorption, femtosecond (fs) optical Kerr effect (OKE) and fs pump–probe techniques. The third-order nonlinear optical susceptibility of TiOPc film was measured to be 3×10 −10 esu. The OKE response of TiOPc films shows a decay process with subpicosecond time constant, demonstrating that the excited state of TiOPc molecule has little contribution to the photo-induced birefringence. From the fs pump–probe experiment, a complicated decay behavior with three decay components was observed, which originates from the contribution of exciton–exciton annihilation process, the enhanced intersystem crossing process and the relaxation of the triplet state.

Properties of polarized laser-induced birefringent gratings in azobenzene-doped poly(methyl methecrylate) optical fibers

Birefringent gratings induced by 532 nm polarized laser in an azobenzene-doped polymethyl methacrylate (PMMA) fibers and blocks were investigated. Significant photo-induced birefringence with about 3.0×10 −5 has been observed from the fiber. It was found that there is an intensity threshold, which is 0.29 mW/cm 2 at 532 nm. The birefringence decayed gradually when the writing laser turned off. So the dynamic process of the birefringence was studied. Both the rise and the decay processes of the birefringence include an initial “fast” response mode and a subsequent “slow” response mode. Both the fast response and the slow response become faster with increasing irradiating laser intensity. The birefringence was induced by the combination of the nonlinear absorption, linear absorption and thermal effect.

Temperature dependence of photoinduced birefringence in mixed Langmuir–Blodgett (LB) films of azobenzene-containing polymers

The temperature dependence of photoinduced birefringence was investigated for mixed Langmuir–Blodgett (LB) films from the homopolymer poly[4′-[[2-(methacryloyloxy)ethyl]ethyl-amino]-2-chloro-4-nitroazobenzene] (HPDR13) and cadmium stearate (CdSt) and from the copolymer 4-[N-ethyl-N-(2-hydroxyethyl)]amino-2′-chloro-4′-nitroazobenzene (MMA-DR13) and CdSt. Birefringence was achieved by impinging a linearly polarized light on the LB films. The maximum birefringence achieved decreased with temperature as thermal relaxation of the chromophores was facilitated. The buildup curves for birefringence were fitted with biexponential functions representing distinctly different mechanisms with time constants. The first, fast process is thermally activated and may be represented by an Arrhenius process. The decay of birefringence after switching off the laser source was described by a Kohlraush–Williams–Watts (KWW) function, consistent with a distribution of relaxation times for the polymer system. Activation energies were obtained from Arrhenius plots of the rate constant of the exponential functions and KWW function, which showed that the buildup of birefringence was very similar for the two polymer systems. The decay, however, was slower for the LB film from MMA-DR13/CdSt.

Photoinduced Birefringence and Second Harmonic Generation in AN Alternating Polymer

Photoinduced birefringence and second-harmonic generation have been studied in a - alternating polymer[poly(disilanylene-2,2'-bipyridine-5,5'-diyl)ruthenium complex (PDSBpy-Ru)]. The ruthenium complexes showed photoconductivity caused by metal-to-ligand charge-transfer excitation. The second-order optical nonlinearity was resulted from natural coordination of the PDSBpy-Ru molecules to the surface S i -O-H groups on the glass substrate. Also Photoinduced birefringence ( n ∼10 4 ) was observed.

Photoinduced Molecular Reorientation and Relaxation in a Liquid Crystalline Polymer

In this paper we present a study of photoinduced effects, including molecular relaxation in a side chain liquid crystalline polymer. In particular, we have studied relaxation phenomena using Ellipsometry and Photoinduced Birefringence in pump-probe experiments, which were carried out as a function of temperature, optical pump intensity and sample preparation and history. Two specific areas were investigated in detail: the frustration effect of the trans-cis photoisomerization transition on the nematic phase, and hence the photoinduced isothermal transition from perturbed nematic to a totally frustrated (isotropic) phase; and the approach of the glass transition, in such a fragile polymeric glass as the one we studied.

High-finesse large band Fabry-Perot fibre filter with superimposed chirped Bragg gratings

Filters with 100 GHz free spectral range (FSR) and finesse up to 16 are demonstrated over the 1525 to 1583 nm band. The photoinduced birefringence results in a polarisation-dependent spectral response. Finesse and FSR uniformity are addressed.