Photoinduced Anisotropy Research Articles

Photo-induced phenomena in GeS4 glasses

GeS4 bulk glasses were prepared by the melt-quench technique and the samples were irradiated by 532-nm linearly polarized light. After the laser treatment, the photo-induced changes of the samples were investigated by UV-1601 spectrophotometer and optical second-order nonlinear tester. The results show that the transmittance of the samples around 532 nm obviously decreases and Bragg reflector forms, which is due to the production of photon-generated carriers. With the increase of laser pulse energy or the extension of irradiation duration, the Bragg reflector increases and gradually tends to be stable. These can be ascribed to the excitationcapture process of the carriers. After irradiation, the relaxation phenomenon results from the release of part of the absorbed energy in the glass matrix. And the fitting equation of the relaxation process is consistent with a conventional Kohlrausch stretched exponential function. The origin of the second harmonic generation (SHG) is because of the dipole reorientation caused by the photo-induced anisotropy in the glass.

Fast Bits Recording in Photoisomeric Polymers by Phase-Modulated Femtosecond Laser

High storage capacity and recording-access rate are two critical factors that the current optical storage technology pursues. Efforts on shortening the laser wavelength, increasing the numerical aperture of objective, and increasing the optical drive speed have been made to reach this end. However, these methods have run their full course because of many existing difficulties on the physical and manufacturing aspect. People have to explore innovative ways to satisfy the requirement of both mass data and high rate. Multidimensional optical storage and holographic recording are two promising technologies to increase the capacity of memory and recording rate by orders of magnitude. We report on the fast bits recording in bisazobenzene functional polymer film by femtosecond laser holographic recording. The mechanism of the photo-induced anisotropy of the material is reviewed and discussed. The demonstration of multidimensional holographic recording is presented. Some influence factors in the holographic processing are discussed.

Temporal characteristics of polarization holographic gratings formed in a photosensitive polymeric film containing N-benzylideneaniline derivative side groups

A polarization holographic grating was recorded in a transparent thin film formed from polymethacrylate with N-benzylideneaniline (NBA) derivative side groups. We measured the real time diffraction properties. The data were analyzed based on a theoretical model that accounted for the distribution of optical anisotropy caused by molecular reorientation as well as for surface relief (SR) deformation caused by molecular motion. Optical anisotropy rapidly increased and then slowly decreased, with increasing recording time. This phenomenon was described based on photoisomerization and photocleavage reactions of the NBA side groups. SR deformation was also induced in the film by polarization holographic recording, without any subsequent processes. The photoinduced optical anisotropy and SR deformation were retained after the recording was turned off.

Anisotropic gratings based on patterned photoalignment of reactive mesogen

The application of patterned photoalignment of reactive mesogen by a novel alignment of a polymer to create anisotropic diffraction gratings is demonstrated. It is shown that the weak sensitivity of the photoinduced surface anisotropy of the aligning layer to additional UV exposure greatly simplifies the procedure of grating formation. Anisotropic diffraction is investigated experimentally, and the results are confirmed by theoretical simulation.

Recording and reconstruction of vector fields in a Fe-doped LiNbO_3 crystal

We propose a flexible method to record and reconstruct vector fields with space-variant polarization distribution in c-cut Fe-doped LiNbO3, based on photorefractive two-wave mixing. To our knowledge, this is the first approach for the reconstruction of vector fields without using the photoinduced anisotropy of the recording material.

Medium for polarization-sensitive diffraction gratings

New polymers applicable for optoelectronics are developed and investigated. The ability to form photoinduced anisotropy in the media based on the films of these polymers is demonstrated. The media can be used for recording of polarization-sensitive diffraction gratings.

Study on photoinduced anisotropy of liquid crystal doped in polymer film containing coumarin group

In this article, the photoinduced anisotropy of a liquid crystal molecule doped into a polymer with photosensitive group was studied. The polymer with coumarin photosensitive group as side chain can generate a polarization axis-selective photoreaction and obtain anisotropy after being exposed at a certain wavelength of linearly polarized ultraviolet light, which occurs as [2+2] cycloaddition and form anchoring force. The thin film underwent subsequent annealing in the liquid crystal temperature range and it leads to a large optical anisotropic change in the polymer film for the self-assembly of molecule. The ultraviolet dynamic spectrum was used to evaluate the absorbance of the film, and the photoinduced optical anisotropy ΔA and order parameter S were calculated. The result shows that when the exposure energy was increased to 1908 mJ/cm2, the value of the absorbance does not change with the increase in exposure energy. When annealing temperature changed from 120 °C to 240 °C, ΔA changed from 0.07 to 0.11 and S changed from 0.02 to 0.04.

Photo-induced optical anisotropy and coherent oscillations in a quasi-two-dimensional two-orbital model for LaMnO3

Ultrafast transient optical conductivity of spin-orbital ordered LaMnO3 was investigated theoretically. A quasi-two-dimensional Hubbard Hamiltonian based on a four-site Mn cluster was established to simulate the LaMnO3 system. The time evolution of the transient optical conductivity was numerically calculated. Intensity oscillations of the optical conductivity were observed, which were interpreted as the result of the quantum interference between phonon-coupled orbiton excited states. Also, the photo-induced anisotropy was observed, which revealed the broken orbital symmetry of the optical excited state.

Comparative studies of polyimides with covalently bonded azo-dyes with their supramolecular analoges: Thermo-optical and photoinduced properties

The paper describes the synthesis and characterization of aromatic polyimides with azopyridine derivatives covalently attached as a side-chain and their supramolecular analoges based on the hydrogen bonds. Moreover, azopolymers differ in the chemical structure of the polymer backbone and the content of the chromophore. Azo-functionalized polymers were obtained by a two-step synthetic approach. This includes the preparation of a precursor with pendant hydroxyl groups followed by the covalent attachment of the chromophore, that is, 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine to the polyimide backbone. The degree of functionalization of the polymers was estimated by the UV–Vis spectroscopy. Supramolecular films were built on the basis of the hydrogen bonds between the hydroxyl groups of the polymer and azopyridine derivatives, that is, 4-(4-hydroxyphenylazo)pyridine and 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine. The polymers were characterized and evaluated by FTIR, 1H NMR, X-ray, UV–Vis, DSC and TGA methods.The synthesized precursor polymers revealed the glass transition temperature (Tg) in the range of 254–366°C, whereas azopolymers and supramolecular systems exhibited lower Tg within the ranges 129–153°C and 53–171°C, respectively. The polymers were easily soluble in common organic solvents enabling to obtain films of a good optical quality. The light-induced optical anisotropy was studied with the help of the holographic recording. The polymer films were investigated by an atomic force microscopy before and after the diffraction grating recording in order to confirm the formation of surface relief gratings. To the best of our knowledge, for the first time the photoinduced anisotropy in polyimides functionalized with azopyridine units has been compared with their supramolecular systems analoges.

Photoinduced Orientation of Photoresponsive Polymers with N-Benzylideneaniline Derivative Side Groups

Axis-selective photoreaction, photoinduced orientation, and formation of surface relief (SR) gratings of liquid crystalline polymethacrylates comprised of N-benzylideneaniline (NBA) derivative side groups are explored. Irradiating with linearly polarized (LP) 313 nm light generates molecular reorientation of the NBA side groups based on the axis-selective trans–cis–trans photoisomerization and photocleavage of the NBA groups. The in-plane order parameter (S) and birefringence (Δn) are ∼0.53 and ∼0.16, respectively. Furthermore, thermally stimulated amplification of the photoinduced optical anisotropy occurs upon exposing films to LP 313 nm light and subsequently annealing in the liquid crystalline temperature range of the material. The amplified S and Δn values are ∼0.71 and ∼0.21, respectively. Finally, polarization holography using 325 nm He–Cd laser beams with various interferometric polarization conditions demonstrates the formation of SR gratings with a molecularly oriented structure based on the per...

Photoinduced features of energy bandgap in quaternary Cu2CdGeS4 crystals

The quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using a cw 532 nm green laser with energy density about 0.4 J cm−2. The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for polarized and unpolarized photoinducing laser light to separate the contribution of the intrinsic defect states from that of the pure states of the valence and conduction bands. To understand the origin of the observed photoinduced absorption near the fundamental edge, the benchmark first-principles calculations of the structural, electronic, optical and elastic properties of Cu2CdGeS4 were performed by the general gradient approximation (GGA) and local density approximation (LDA) methods. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior (shift of the absorption maxima in different polarizations) within the 0.15–0.20 eV energy range not only near the absorption edge; optical anisotropy was also found for the deeper inter-band transition spectral range. Peculiar features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron density distribution. Possible intrinsic defects are shown to affect the optical absorption spectra considerably. Pressure effects on the structural and electronic properties were modeled by optimizing the crystal structure and calculating all relevant properties at elevated hydrostatic pressure. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu2CdGeS4 are also reported.

Synthesis and Properties of 1-(2-methyl-5-chlorine-3-thienyl)-2-(2-ethyl-benzofuran-3-Yl) perfluorocyclopentene and Application

A novel unsymmetrical photochromic diarylethene, 1-(2-methyl-5-chlor-ine-3-thienyl)-2-(2-ethyl-benzofuran-3-yl) perfluorocyclopentene (1a), was synthesized and its photochromism and photo-induced anisotropy was investigated. Specifically, photochromism in solution and in PMMA amorphous films were studied. The diarylethene showed exhibited reversible photochromism, changing from colorless to red after irradiation with UV light both in solution and in PMMA amorphous film. This new photochromic system also exhibited remarkable optical storage character.

Azo doped polymer thin films for active and passive optical power limiting applications

Two novel optical power limiters, 2-[ethyl-(4-phenylazo-phenyl)-amino]-ethanol (E4PA) and 2-[ethyl-(4-trifluoromethyl-phenylazo-phenyl)-amino]-ethanol (E4TPA) were synthesized using a diazotization reaction. The purified azo material was made into thin films in a poly(methyl methacrylate) matrix using a gravity settling technique. The electronic nonlinearities of these films were investigated using an open aperture Z-scan technique in the fs excitation regime, resulting in nonlinear absorption due to a two-photon absorption (2PA) process. The 2PA coefficient for these films is of the order 10(-12) m W(-1) and the limiting threshold values are 1.1 J cm(-2) each. A non-degenerate pump probe set-up was employed with CW lasers to study the nonlinear behaviour arising from photo-induced anisotropy and excited-state absorption. The present study shows that these azo thin films are potential candidates for active and passive optical power limiting applications.

Photoinduced In-Plane Motions of Azobenzene Mesogens Affected by the Flexibility of Underlying Amorphous Chains

Photoinduced in-plane motions are explored for a series of surface-grafted diblock copolymers comprising of an amorphous chain block and a liquid crystalline (LC) azobenzene (Az) side chain polymer block. In this architecture, amorphous polymers with different glass transition temperature (Tg) and the LC Az polymer are stepwisely grafted on substrate surfaces by the surface initiated atom transfer radical polymerization. Amorphous polymers employed are poly(hexyl methacrylate) (PHMA), poly(butyl methacrylate) (PBMA), and poly(methyl methacrylate) (PMMA) with Tg of −20, +27, and +110 °C, respectively. The extent of photoinduced in-plane anisotropy of the LC Az layer by linearly polarized light irradiation at optimal temperatures are in the order of PHMA > PBMA > PMMA which coincides with that of lowering Tg. Thus, the photoinduced motions of the LC Az blocks are coupled with the dynamic nature of the underlying amorphous chain anchored to the substrate. The structural information obtained by the grazing incidence angle X-ray diffraction measurements further shows the important role of the amorphous polymer layer in the ordering and photoalignment of LC mesogens on the top.

Role of Au nanoparticle aggregation in laser induced anisotropy of ITO transparent substrates

• Principal role of Au NP surface aggregation on the output photoinduced birefringence is shown. • The higher changes are obtained for the samples with sizes 30 nm. • The process is slowly relaxed. A principal possibility to achieve the photoinduced anisotropy in the Au NP deposited onto the ITO substrate is experimentally shown. The sizes of the Au NP forming the corresponding nanocomposites were 20 nm, 30 nm and 40 nm. As a photoinducing light source we have used two coherent beam originating from the Er:glass laser generating at 1540 nm with frequency repetition 15 Hz as well as its second harmonic doubled frequency signal at 770 nm. The effect is sensitive to the angle between the two laser beams as well as to the Au NP sizes, inter-particle distances and topology connected with their aggregation. The effect shows slow relaxation to the initial state. The optimal conditions are achieved for nanocomposites formed by 30 nm despite the expected 20 nm. This one may be caused by crucial role of the partial aggregation which even changes the effective grain sizes. The contribution of the dipole–dipole as well as quadrupole–dipole interactions to the changes of the anisotropy is discussed. The excitation is far from the resonance which allow to predict that effective role play overlap with nanotrapping levels. So principal role may belongs to surface topology and which is studied using the birefringence directly connected with the anisotropy.

Photosensitive organic/inorganic azopolymer based nanocomposite materials with enhanced photoinduced birefringence

Abstract Different methods have been employed in order to increase the value of the photoinduced anisotropy in azopolymers and to ensure more effective polarization recording. As recently established, doping photopolymers with nanoparticles allows to achieve near 100% net diffraction efficiency in case of conventional holographic recording. For this reason, we have synthesized novel organic/inorganic composite materials by incorporating 50 nm sized ZnO nanoparticles in series of side-chain azopolymers. We obtained a considerable improvement of the photoresponse in these composite materials compared to the non-doped samples – more than 40% increase of the saturated value of the birefringence (up to Δ n = 0.08) and also reduction of the recording time with about 30%. We also observed an increase of the photoinduced anisotropy after the exposure has been stopped for one of the investigated polymers. The stability of the photoinduced birefringence in all samples suggests applications of these materials as media for polarization holographic data storage and recording of diffractive optical elements with high efficiency and unique polarization properties.

Femtosecond laser-induced permanent anisotropy in bacteriorhodopsin films and applications in optical data storage

In polymeric films of bacteriorhodopsin (BR) a photoconversion product named F540-state, which is excited by 790 nm femtosecond laser pulses, is stable either for photochemical reaction or thermal pathway. The optical properties of the F540-state were studied, and Jones-matrix theory was adopted to analyze the photoinduced anisotropy of the F540-state. Based on the permanently photoinduced anisotropy, write-once-read-many (WORM) optical data storage was demonstrated by using two polarization states of femtosecond pulsed laser. Since the polarization information is also written on the storage media, it is impossible to copy it in a common way. This storage technique has a potential application in advanced optical security.

Blends of an azomethacrylic block copolymer for volume holographic storage using low energy 10 ms light pulses

Polymer blends of a poly (methylmethacrylate) (PMMA) homopolymer and an azomethacrylic diblock copolymer (BC) are investigated for the recording of holographic polarization gratings. Blends with an azobenzene content down to 2, 1 and 0.5wt.%, and good optical quality have been prepared. Their morphology associated with microphase separation as well as the achieved photo-induced anisotropy in terms of birefringence and dichroism are similar to those obtained for the original BC. Volume holographic polarization gratings have been stored in thick films (up to 500μm) of these blends using two oppositely circularly polarized 488nm light pulses of 100ms and 10ms with a constant value of the energy density of 50mJ/cm2 (each beam). Although the efficiency of the gratings decays with time, a final equilibrium efficiency, in the range of 10−4, remains in both cases. Up to 20 polarization gratings, with efficiencies higher than 3.5×10−5, have been successfully multiplexed using light pulses as short as 10ms, which is an order of magnitude shorter than that reported in previous investigations.

Diblock copolymer–azobenzene complexes through hydrogen bonding: Self‐assembly and stable photoinduced optical anisotropy

AbstractWe have demonstrated the preparation of a series of photoaddressable supramolecular block copolymers by mixing a carboxy‐terminated azobenzene derivative, 6‐[4‐(4′‐cyanophenylazo)phenyloxy]hexanoic acid (AZO), and two polystyrene‐b‐poly(4‐vinylpiridine) (PS‐b‐P4VP) block copolymers. AZO can be selectively attached to the P4VP block of PS‐b‐P4VP through hydrogen bonding interactions. The assembly of AZO with vinylpyridine group‐containing polymers was initially investigated on a model system composed of P4VP homopolymer and AZO. Homogeneous liquid crystalline materials were obtained for ratios of AZO to vinylpyridine repeating unit, x, lower or equal to 0.50. Mixtures with higher x resulted in heterogeneous materials showing clear macrophase separation. Accordingly, a series of hydrogen‐bonded complexes of PS‐b‐P4VP and AZO, PS‐b‐P4VP(AZO)x, with x = 0.25 and x = 0.50 were prepared. Lamellar and spherical morphologies were observed for the complexes based on PS24‐b‐P4VP9.5 (Mn,PS = 24,000, Mn,P4VP = 9500) and PS24‐b‐P4VP1.9 (Mn,PS = 24,000, Mn,P4VP = 1900), respectively. Photoinduced orientation of the azobenzene units was obtained in films of P4VP(AZO)x and PS‐b‐P4VP(AZO)x with x = 0.25 and 0.50 by using 488 nm linearly polarized light and characterized through birefringence and dichroism measurements. This investigation shows a versatile and less laborious approach to azobenzene‐containing polymer materials with low chromophore content, of interest in optical application. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Photoinduced anisotropy in an ensemble of CdSe/ZnS quantum rods

The photoluminescence anisotropy in initially disordered quantum rod ensemble induced by selective photochemical reaction under continuous irradiation with polarized light is demonstrated. A specific long-time relaxation of the photoinduced anisotropy in darkness, which is attributed to rotational diffusion of nanocrystals, is observed in the minutes time scale after an extended period of irradiation. The observed bi-exponential evolution of the photoluminescence intensity under continuous irradiation suggests the development of two photoinduced processes such as photopassivation of surface defects and photooxidation of the quantum rod surface.