不同基体刚性偶氮化合物光致光学各向异性及其唯象模拟

在不同刚性的聚合物基体聚甲基丙烯酸甲酯（PMMA）、聚乙烯咔唑（PVK）和聚苯乙烯（PS）薄膜基体中掺入相同质量分数（均为0．003）的分散红13（DRl3）偶氮染料。以线偏振连续Ar^＋激光（514nm）作为控制光,连续He-Ne激光（632．8nm）作为信号光考察了具有相同厚度的三种样品在相同控制光功率（15mW）下的光致双折射效应。实验发现刚性较大的聚乙烯咔唑基体样品的双折射效应最小,刚性最小的聚苯乙烯基体样品的双折射效应最大。在偶氮生色团光致异构的四能级模型基础上,建立一新的唯象的模型。在模型中引入描述聚合物基体刚性的参量s,通过数值计算描述了不同基体刚性参量s对偶氮生色团光致异构取向的影响。计算结果表明,基体的刚性越大,样品的光致双折射效应越小,定性地解释了实验现象。     

Relationship between segmental anisotropy in polarizability and stationary ultrasonically induced birefringence in polymer solutions

Stationary ultrasonically induced birefringence in various polymer solutions was measured in order to investigate its relation to the anisotropy in polarizability of a polymer chain. From the concentration dependence of stationary ultrasonically induced birefringence in polystyrene–toluene, polycarbonate–chloroform and polybutadiene–toluene solutions, the intrinsic values of the birefringence were obtained. A linear relationship between the intrinsic value of the birefringence per segment and the segmental anisotropy in polarizability was obtained. In aqueous solutions of polyelectrolytes, sodium polystyrenesulfonate and tetramethylammonium polystyrenesulfonate, the birefringence decreased with ionic strength by the addition of the salts. The stationary birefringence per ultrasonic intensity for all polymer solutions investigated here decreased with increasing frequency.     

Mechanisms of optical anisotropy induction in azopolymer films

The process of optical anisotropy indiction in azopolymer films upon irradiation by polarized UV light is studied by means of UV and IR birefringence spectroscopy. A substantial dependence of the character of the relaxation of the induced anisotropy on the exposure time to the exciting light is revealed: whereas the process of anisotropy induction is reversible at small exposure times, at large exposure times the induced anisotropy virtually does not relax and even increases after the irradiation is terminated. A difference between kinetic curves of dichroism and birefringence induction is shown: at exposure times corresponding to the saturation of the induced dichroism a slow increase in the birefringence curve is observed. Results obtained are explained in terms of the assumption of partial ordering of macrochain fragments in irradiated films. The assumption is substantiated by measurements of polarized IR spectra. Institute of Physics of the National Academy of Sciences of Ukraine, 46, Nauka Ave., Kiev, 252650, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 121–125, January–February, 1998.     

Pump induced birefringence in dual-polarization fiber grating lasers

Birefringence is critical in dual-polarization fiber-laser-based fiber-optic sensing systems, as it directly determines the beat frequency between the two polarizations. A study of pump induced birefringence in dualpolarization fiber lasers is presented here, which shows that the pump induced birefringence is a result of the interplay among pump induced refractive index change, laser dynamics, and anisotropy inside fiber lasers.For erbium-doped fiber lasers, pumping at 1480 nm is better than pumping at 980 nm in lower pump induced birefringence. Moreover, injection at 532 nm for an adequately long enough time can permanently reduce anisotropy and, hence, reduce pump induced birefringence.     

不同生色团对聚合物薄膜全光开关效应的影响

在聚甲基丙烯酸甲酯(PMMA)中掺人不同的偶氮染料并制成薄膜样品,以调制的线偏振Ar^ 激光(514nrn,CW)作为控制光,He-Ne激光(632．8nm,CW)为信号光,研究了样品的光致双折射效应及全光开关效应。通过对样品的响应特性和全光开关效应进行比较,发现电子推拉势较强的生色团分子在控制光作用下,可以产生较大的光致双折射效应,但光响应速度较慢,其光致双折射效应以光致异构取向为主要机制;而对于具有较弱电子推拉势的生色团分子,其光致双折射效应则主要来自光致异构烧孔机制,虽然其光致双折射效应小,但光响应速度快。     

Two-Pass Method for Studying Induced Birefringence Effects

We consider a two-pass method for studying the effects of induced reciprocal and nonreciprocal birefringence. In the case of complex anisotropy of the studied sample, methods for isolating reciprocal and nonreciprocal induced effects are proposed. The results of an experimental study of the effect of nonreciprocal magneto-optical linear birefringence (difference between the phase velocities of counterpropagating waves with identical linear polarizations) in crystals of lithium iodate and quartz are presented. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 536–544, June 2008.     

Investigation of the mode splitting induced by electro-optic birefringence in a vertical-cavity surface-emitting laser by polarized electroluminescence

The mode splitting induced by electro-optic birefringence in a P-I-N InGaAs/GaAs/A1GaAs vertical-cavity surface- emitting laser （VCSEL） has been studied by polarized electroluminescence （EL） at room temperature. The polarized EL spectra with E｜｜[110] and E ｜｜ [150] directions, are extracted for different injected currents. The mode splitting of the two orthogonal polarized modes for a VCSEL device is determined, and its value increases linearly with the increasing injected current due to electro-optic birefringence; This article demonstrates that the polarized EL is a powerful tool to study the mode splitting and polarization anisotropy of a VCSEL device.     

Photo-induced anisotropy in ZnO/PVA nanocomposites prepared by modified electrochemical method in PMA matrix

Substantial photo-induced optical anisotropy was discovered in ZnO/PVA nanocomposites under the influence of external bicolor laser illumination. Zinc oxide nanoparticles were synthesized by electrolysis of a sodium chloride aqueous medium including poly-methacrylic acid (PMA) in a cell system having a soluble zinc anode. The structural analysis of the ZnO powder samples has been carried out by X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM). The polyvinyl alcohol (PVA) embedded ZnO films obtained from the powder samples possess larger grain sizes than those in powder form. The films were prepared from the same polymer matrix but elaborated with two different PVA contents which are respectively 15% and 30%. The photoinduced anisotropy was identified by using two bicolor Er: glass laser beams incident at different angles. Substantial influence of the technological processes on the embedded nanoparticle sizes and related birefringence was explored. The process of laser induced anisotropy shows an occurrence of birefringence saturation.     

Photoinduced dichroism and birefringence in films of Mordant Pure Yellow/poly(vinyl alcohol): simultaneous real-time investigations at two wavelengths

Photoinduced anisotropy in films of azo-dye Mordant Pure Yellow (MPY) dissolved in a polyvinyl alcohol matrix, is investigated. The photodichroism and photoinduced birefringence are measured in real time for two wavelengths, 488 nm (in the absorption band) and 633 nm (outside it), on excitation with an Ar⁺ laser beam. It is shown that strong dichroism is induced without changing the average optical density of the film, and the kinetic curves of the birefringence and the dichroism are substantially different. The conclusion is made that the photoprocesses of the dye molecules cause a reconstruction of the matrix.     

Physical characterisation of 4′-butyl-4-heptyl- bicyclohexyl-4-carbonitrile

We report measurements of birefringence, bend elastic constant, principal dielectric constants and conductivities as functions of temperature in the nematic phase of 4′-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). The compound exhibits a small birefringence and a large negative dielectric anisotropy. Surprisingly, it shows a negative conductivity anisotropy, which probably owes its origin to the structural details of the molecules. The orientational order parameters (S) are estimated from both birefringence and dielectric anisotropy, and they agree well in the entire nematic range. The experimental results are compared with those on CCH7, which has a similar core structure but with a longitudinal dipole moment, in place of the transverse one of CCN-47.     

Ultrafast laser-induced phase transitions in tellurium

We present experimental investigations of ultrafast phase transitions in tellurium following excitation by an intense femtosecond laser pulse. Femtosecond time-resolved polarization-sensitive microscopy is used to monitor the temporal evolution of optical anisotropy (birefringence) of the irradiated material. The decay of optical anisotropy associated with the loss of order in crystalline tellurium is fluence-dependent and occurs within 0.5–3 ps.     

Depth-resolved multiphoton polarization microscopy by third-harmonic generation

We achieve depth-resolved polarization microscopy by measuring third-harmonic generation induced by a tightly focused circularly polarized beam. In crystals exhibiting strong birefringence this signal is dominated by positively phase-matched third-harmonic generation. This process occurs in only optically anisotropic media, in which the birefringence compensates for the phase mismatch between the fundamental and the third harmonic induced by dispersion. Both the intensity and the polarization of the emitted signal provide information on the local optical anisotropy. We demonstrate the technique by imaging biogenic crystals in sea urchin larval spicules.     

偶氮类有机材料光致双折射的测量

研究了2种偶氮聚合物薄膜的光致双折射效应,并探讨了泵浦光偏振态和光强对光致双折射的影响.采用远离共振区的He-Ne光(633nm)作为探测光,用Ar+激光(488nm)作为泵浦光,通过测量相关参量得到了样品的光致双折射值.实验结果表明:改变泵浦光的偏振态可以控制光致双折射值和探测光的透过信号强度,在0°～45°范围内,泵浦光光强存在最佳值,此时偶氮薄膜实验样品具有最大的双折射值.     

Ultraviolet true zero-order wave plate made of birefringent porous silica

Porous Si made from (110) Si wafers exhibits strong in-plane optical anisotropy (birefringence) in the visible and near-IR ranges. Oxidation of the birefringent porous Si results in the formation of birefringent porous silica. We demonstrate that the degree of the birefringence of porous silica can be controlled by the oxidation condition, and very small birefringence can be achieved. The smallest anisotropy of the refractive index (Δn) is 0.001, which is about ten times smaller than that of quartz. The small birefringence allows us to produce true zero-order wave plates operating in the UV range.     

Photoinduced linear and/or circular birefringences from light propagation through amorphous or smectic azopolymer films

The self-induced rotation of the azimuth of elliptically polarized light passing through birefringent azopolymer thin films is investigated. The experiments were carried out on thin films of the amorphous p(DR1M-co-MMA) and p(DR1M) azopolymer samples and of the p(6MAN) derivative in its glassy and liquid-crystalline phases. In fact, using various controlled input light ellipticities, linear birefringence (LB) and/or circular birefringence (CB) measurements were performed separately and in conjunction with polarization analyses of the transmitted pump beam. According to a general theoretical analysis based on Jones’ matrix formalism, it is thus shown that the induced rotation angle through the films depends mainly on the ellipticity of the input light, on the generated LB level and, to a lesser extent, on the CB photoinduced in the liquid-crystalline phase. In the latter case, it is concluded that irradiation with circularly polarized light does induce a chiral arrangement in the polymer film, although the photosensitive chromophores do not contain any optically active group. Received: 22 April 2002 / Revised version: 28 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/5684-8402; E-mail: csouri@morgane.lsmc.u-bordeaux.fr     

Compensation of the photoelastic birefringence of a polymer by doping with an anisotropic molecule

We report on a method to compensate the photoelastic birefringence of a polymer. In this method, a rod-like molecule that has a polarizability anisotropy was chosen and doped in a polymer. We demonstrated this method by compensating the negative photoelastic birefringence of poly(methylmethacrylate) at a wavelength of 633 nm. Homogeneous doping with 2.2 wt. % of trans-stilbene almost eliminated the photoelastic birefringence of the polymer. The photoelastic coefficient of the synthesized zero-photoelastic birefringence polymer was 0.057×10^-12 Pa^-1. We found that the photoelastic birefringence of poly(methylmethacrylate) was compensated by the motion of trans-stilbene in the polymer by the analysis of the infrared absorption spectrum. PACS 42.70.-a; 42.70.Jk; 78.30.Jw     

Gyroscopic behavior exhibited by the optical Kerr effect in bimetallic Au–Pt nanoparticles suspended in ethanol

The modification in the third-order nonlinear optical response exhibited by rotating bimetallic Au–Pt nanoparticles in an ethanol solution was analyzed. The samples were prepared by a sol–gel processing route. The anisotropy associated to the elemental composition of the nanoparticles was confirmed by high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements. The size of the nanoparticles varies in the range from 9 to 13 nm, with an average size of 11 nm. Changes in the spatial orientation of the nanomaterials automatically generated a variation in their plasmonic response evaluated by UV–Vis spectroscopy. A two-wave mixing experiment was conducted to explore an induced birefringence at 532 nm wavelength with nanosecond pulses interacting with the samples. A strong optical Kerr effect was identified to be the main responsible effect for the third-order nonlinear optical phenomenon exhibited by the nanoparticles. It was estimated that the rotation of inhomogeneous nanostructures can provide a remarkable change in the participation of different surface plasmon resonances, if they correspond to multimetallic nanoparticles. Potential applications for developing low-dimensional gyroscopic systems can be contemplated.     

Intrinsic birefringence in (11N)-oriented quantum wells of cubic crystals

The effect of natural birefringence in (11N) oriented Quantum Wells (QWs) due to the in-plane optical anisotropy is investigated. The birefringence is more pronounced, the stronger the anisotropy of hole effective masses and larger the splitting of the heavy- and light-hole sub-bands. The magnitude of the effect is maximum in (110)-oriented QWs. An order-of-magnitude estimation of the effect in QWs of GaAs and of ZnSe is given.     

Twisted optical fibers sustaining propagation of optical vortices

The mode structure (the orbital number l = 1) of a few-mode weakly guiding optical fiber with high linear birefringence and a regular twist of its anisotropy axes is investigated. It is shown that, for certain values of twist pitch, the modes with l = 1 are almost pure linearly polarized optical vortices in the local coordinate system associated with the anisotropy axes. The range of values of twist pitch in which twisted fibers sustain propagation of linearly polarized optical vortices is determined numerically.     

A comparison of two theories of acoustoelasticity

The presence of a plane stress field causes small changes in the phase velocities of orthogonally polarized SH waves. The (small) difference in phase velocities (birefringence) can be used for non-destructive stress measurement. However, material anisotropy can affect phase velocity to the same extent as stress.Two theories have been developed which account for the effect of both stress and anisotropy. The theory of Iwashimizu and Kubomura assumes isotropy in the third-order elastic moduli and anisotropy in second-order moduli. A different approach was taken by Okada, who assumed the existence of a matrix analogous to the index of the refraction matrix in optics.In this paper, we generalize the theory of Iwashimizu and Kubomura by retaining anisotropy in third-order moduli. We show how Okada's theory can be made to agree with this more general theory.We also compare the predictions of the various theories with birefringence data obtained from uniaxial tension tests on 2024-T351 aluminium specimens. Both the Okada theory and the theory of Iwashimizu and Kubomura gave good agreement with experiment.