Photoinduced Birefringence Research Articles

Synthesis and characterization of copolymers of alkyl‐ and azo‐thiophenes: Chromic properties and photoinduced birefringence

AbstractPolyazothiophene is a type of polythiophene derivative that combines the electrical and luminescent properties of polythiophenes with the photoisomerization property of azopolymers. Extensive efforts have been made to improve the properties of polyazothiophenes, such as solubility, optical, and chromic properties. We report the preparation of copolymers of an alkylthiophene (3‐octylthiophene, 3‐OT) and an azothiophene (2‐[N‐ethyl‐N‐[4‐[(4‐(4‐nitrophenyl)azo]phenyl]amino]ethyl 3‐thienylacetate, 3‐AzoT) in different ratios as an alternative route to improve these properties. The azosubstituted monomer contents in these copolymers were 6, 9, 12, and 51% (in mol), as evaluated by elemental analysis. Their chemical structures were confirmed by FTIR and 1H‐NMR. HPSEC and thermal analysis were used to characterize the polymers. The presence of thermochromic and solvatochromic properties was demonstrated by UV‐Vis spectroscopy. Optically induced birefringence was detected only in polymers with 12 and 51 mol % of azo‐units. The introduction of different ratios of the azothiophene in the copolymer alters the polymer solubility and emissive properties. The results indicate that the polyazothiophene copolymers presented are promising active layers for optical devices and sensors. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Measurement of photo-induced birefringence in germanosilicate fibers

In this paper, we report a novel method for accurately measuring the photo-induced birefringence of germanosilicate fibers by using an all fiber Mach-Zehnder interferometer. The results indicate that the photo-induced normalized birefringence of C598-302(s) germanosilicate fiber can attain 10−5 and is multi-decaying-exponentially proportional to the UV exposure.

Structural investigations on PbO–Sb2O3–B2O3:CoO glass ceramics by means of spectroscopic and dielectric studies

PbO–Sb2O3–B2O3 glasses mixed with different concentrations of CoO (ranging from 0 to 2.0 mol%)were crystallized. The samples were characterized by x-ray diffraction, scanningelectron microscopy and differential scanning calorimetric techniques. The x-raydiffraction and scanning electron microscopic studies have revealed the presence ofCoSb2O6,Co2.33Sb0.67O4,Pb5Sb2O8,Pb3(SbO4)2,PbB4O7 andCo3O4 crystalline phases in these samples. The DSC studies have indicated thespreading of the crystallization from the inside to the surface of the samplesas the concentration of the crystallizing agent is increased. The IR andRaman spectroscopic studies have pointed out the existence of conventionalBO3,BO4,SbO4 andalso CoIII–O structural units in the glass ceramic samples. These studies have further indicated thedecreasing concentration of symmetrical structural vibrational groups with increase in theconcentration of CoO. The results of various studies, namely dielectric propertiesover a range of frequency and temperature, photo-induced birefringence, opticalabsorption, fluorescence and magnetic susceptibility at room temperature of PbO–Sb2O3–B2O3:CoO glass ceramics, have also been reported. The variations observed as a function of theconcentration of crystallizing agent in all these properties have been analyzed in the light ofdifferent oxidation states and environments of cobalt ions in the glass ceramic network.

All-Optical Modulation with a Surface Plasmon Mach–Zehnder Interferometer

We use two parallel nano-slits in a silver film to form a surface plasmon Mach–Zehnder interferometer (MZI), based on the interference of two surface plasmon waves propagating along the two surfaces of the silver film. Coating the silver film with a photoinduced birefringence polymer film, we achieve optical modulation of the MZI output by changing the refractive index of the polymer film with a pump beam. An on/off ratio of 2.7 is obtained for a probe wavelength of 865 nm.

Role of titanium valence states in optical and electronic features of PbO–Sb 2O 3–B 2O 3:TiO 2 glass alloys

PbO–Sb 2O 3–B 2O 3 glasses mixed with different concentrations of TiO 2 (ranging from 0 to 1.5 mol.%) were synthesized. The samples are characterized by X-ray diffraction, scanning electron microscopy and DSC techniques. A variety of properties, i.e. optical absorption, photoluminescence, infrared, ESR spectra, magnetic susceptibility, photo-induced birefringence (PIB) and dielectric properties (constant ɛ′, loss tan δ, a.c. conductivity σ ac over a wide range of frequency and temperature) of these glass–ceramics have been explored. The analysis of these results indicated that Ti ion surrounding ligands play principal role in the observed PIB and the sample crystallized with 0.8 mol.% of TiO 2 is the most suitable for the applications in non-linear optical devices.

Influence of the Grating Parameters on the Polarization Properties of Fiber Bragg Gratings

Due to the lateral inscription process, photo-induced birefringence is present in fiber Bragg gratings (FBGs) written into photosensitive single mode fiber. The birefringence value is generally too small to be perceived in the grating spectral response but it can lead to significant polarization dependent loss (PDL) and differential group delay (DGD) evolutions. In this paper, we first theoretically analyze the evolution with wavelength of PDL and DGD as a function of the grating parameters and the birefringence value. We demonstrate that the PDL and DGD evolutions with wavelength can be strongly enhanced by a modification of the grating parameters. Simulations carried out using the coupled mode theory and the Jones formalism are then confirmed by experiments conducted on FBGs written into photosensitive single mode fiber. Our work brings a complete characterization of polarization related phenomena in FBGs and presents a great interest for the evaluation of system performances and the design of gratings for specific applications, either for telecommunications or sensing purposes. In addition, based on the comparison between experimental and simulated evolutions, we are able to verify that the birefringence value is strongly dependent on the fluence of the laser used for the grating inscription.

Birefringent azopolymer long period fiber gratings induced by 532 nm polarized laser

Based on the photoinduced birefringence in azo materials, the model of the birefringent long period fiber gratings (BLPFGs) has been proposed and the transmission of such birefringent fiber gratings was further simulated by the coupled mode theory. In order to obtain the optimum condition for making birefringent azopolymer optical fiber (APOF) gratings the photoinduced birefringence of azo dye and methyl methacrylate (MMA) copolymers was systematically studied. After photosensitivity study of azo samples, using Teflon technique, azo dye containing single mode polymer optical fiber (POF) was fabricated. Finally, BLPFGs can be fabricated in azopolymer fiber using polarized 532 nm laser. A 120 μm LPFG with a 50% duty cycle has been successfully written within the core of the fiber with 532 nm laser by an amplitude mask method. Under polarized microphotography, the grating was observed when the optical axis was set at 45° with the direction of the first polarizing lens of the polarization microscope.

Spectropolarimetric investigation of the photoinduced dichroism and birefringence in malachite green/dichromated gelatin films

We report a spectropolarimetric investigation of the photoinduced anisotropy in dichromated gelatin films containing the dye malachite green. The investigation is done with a novel Spectral Stokesmeter which measures simultaneously and in real time (20 ms) the spectra of all the Stokes parameters of light in the spectral range 500–750 nm. This made it possible to measure not only the spectrum of the dichroism but also, for the first time to our knowledge, the spectrum of the photoinduced birefringence in these films. The results show that we can measure trustworthy dichroism larger than 0.02 and birefringence larger than 2 × 10−4.

Photoresponsive and Holographic Behavior of an Azobenzene-Containing Block Copolymer and a Random Copolymer

A block copolymer containing an azobenzene (Az) moiety in the side chain was prepared by atom transfer radical polymerization (ATRP). For comparison, a random copolymer with a similar Az content was also designed. Their photoresponsive and holographic behavior was investigated in thin films. The block copolymer film showed microphase separation with cylinder microdomains after annealing, which was then irradiated at 366 nm. It was found that trans-cis photoisomerization in the block copolymer film was slower than that in the random copolymer film. This may be due to the restricted motion of the Az moieties in the Az blocks. The photoinduced birefringence (Δn) of the block copolymer film was higher than that of the random copolymer film upon irradiation with a linearly polarized beam. In holographic recording, similar results were obtained. A block copolymer with 22-mol% Az showed a liquid-crystalline (LC) phase, while a random copolymer with 24-mol% Az exhibited no LC phase. The block copolymer showed an LC phase at a lower Az content compared with the random copolymer, suggesting that the molecular alignment of Az mesogenic groups is enhanced by cooperative motion of LCs in block copolymer films.

Photoinduced anisotropy and polarization holography in a stilbene-containing fluorinated polyimide

A stilbene-containing fluorinated polyimide with high optical and thermal stability was prepared. The photoinduced anisotropy in this polyimide was investigated by measuring its dichroism and photoinduced birefringence. A large photoinduced birefringence (Deltan approximately 10(-2)) was achieved in the polymer. Stable and pure polarization phase gratings without surface-relief structure were written in the polymer film by using two orthogonally linearly or circularly polarized 532 nm beams. The polarization gratings were studied with a 650 nm probe beam by measuring the diffraction efficiency of the probe beam and surveying the conversion of the polarization state of the probe beam.

The nonlinear optical property and photoinduced anisotropy of a novel azobenzene-containing fluorinated polyimide

A novel azobenzene-containing fluorinated polyimide was synthesized. The nonlinear optical property and photoinduced birefringence of a polyimide thin film were investigated. Large third-order nonlinear refraction (n 2=−4.49×10−11 cm2/W) was observed in the polyimide thin film by carrying out Z-scan measurement. The polyimide thin film exhibited larger nonlinear refraction than that of a mono-azo dye doped PMMA thin film (n 2=−1.63×10−12 cm2/W). The photoinduced birefringence of the polyimide thin film (▵ n∼10−2) under different pump intensities was investigated; it was much larger than that of the mono-azo dye doped PMMA thin film (▵ n∼10−3). Moreover, the time constants for birefringence growth and relaxation processes were determined.

Stretched exponential kinetics for photoinduced birefringence in azo dye doped PVA films

We fabricated azo dye (methylorange) doped poly vinyl alcohol (MO/PVA) thin films and measured the photoinduced birefringence (PIB) kinetics for several pump beam intensities and for various MO concentrations by using the pump-probe technique. A novel approach to explain the transient behaviors of the photoinduced anisotropy is presented by employing an empirical stretched exponential time response in the course of the trans–cis–trans photoisomerization of azo molecules and is compared with the experimental data, showing excellent agreement. The stretched exponent is estimated to be β=0.34±0.04, revealing amorphous nature of the MO/PVA system.

Dynamics of photoinduced dichroism and birefringence in optically thick azopolymers

Dynamics of photoinduced dichroism and birefringence have been studied experimentally and theoretically (with the help of the Dumont model) by using some comb-shaped azopolymers. It is shown that the dynamics of trans-isomer concentration and their angular distribution anisotropy can be restored from the experimentally found dichroism dynamics, with the concentration and anisotropy being averaged over the thickness for optically thick samples. At the initial stage of photoinduced anisotropy when the active role of the polymer matrix can be neglected, the experimental time dependence of dichroism is shown to comply well with the Dumont model even if the orientation memory is neglected, provided that only a part of trans-isomers participates in trans-isomerisation.

The nonlinear optical properties and photoinduced anisotropy of a novel stilbene-containing fluorinated polyimide

The nonlinear optical properties and photoinduced anisotropy of a novel stilbene-containing fluorinated polyimide were investigated. Large macroscopic second-order optical nonlinearity ( d 33 = 35.4 pm/V) was obtained in a poled thin film using the Maker fringe method. A nonlinear absorption coefficient ( β) of 0.65 × 10 −5 cm/W and third-order nonlinear refractive index ( n 2) of −1.13 × 10 −10 cm 2/W were determined by Z-scan measurements. Photoinduced birefringence (Δ n) of 1.02 × 10 −2 was observed using a continuous Nd:YAG laser at 532 nm as pump light and a continuous diode laser at 650 nm as probe light.

Photoinduced changes in a Sudan I doped PMMA thin film

Photoinduced changes in a Sudan I doped PMMA thin film were investigated. The dependence of photoinduced birefringence of the thin film on light intensity was studied, with a continuous wave (CW) 532 nm laser as pump light and a CW 650 nm laser as probe light. For all the light intensities (100–1000 mW/cm 2 ), the thin film possessed large photoinduced birefringence (Δ n ~ 10 −4 ). With a CW 532 nm laser at relatively low light intensity, a reversible polarization hologram has been recorded in the thin film. Moreover, Z-scan measurements showed that the thin film possessed large nonlinear refractive index (−7.93 × 10 −5 cm 2 /W) and nonlinear absorption coefficient (−5.50 W/cm) under a CW 532 nm excitation.

Photoinduced Birefringence in Nanostructured SiO2:DR1 Sol–Gel Films

Pump-probe photoinduced birefringence measurements were carried out in amorphous and in nanostructured sol-gel films with Disperse Red 1 (DR1) azochromophores embedded in a SiO2 matrix. X-ray diffraction (XRD) patterns determine the long-range nanostructure order of the films, exhibiting a lamellar nanostructure, with two different d-spacings, due to the presence during the sol-gel process of the Sodium Dodecyl Sulfate (SDS) or of the Cetyltrimethylammonium Bromide (CTAB) ionic surfactants. The photoinduced birefringence measurements were performed on fresh and on heat treated films as a function of the pumping time. The measurements give us information about the effect of the nanostructures on the azochromophores orientation dynamics. As a result, for the same azochromophores concentration, annealed nanostructured films exhibited the largest azochromophore mobilities but by the other side, amorphous films had the largest signal intensities. Besides, we established a phenomenological model for the analysis of the azochromophores orientation in the films as a function of the pumping time.

Photoinduced birefringence in layers of benzaldehyde-containing polymers and its temperature sensitivity

The postexposure kinetics of photoinduced birefringence in benzaldehyde-containing polymer layers at temperatures exceeding the recording temperature have been considered. It is experimentally shown that the heating-induced photoanisotropy changes have a reversible character due to the rotational relaxation of benzaldehyde units.

Synthesis of azopolymers with controlled structure and photoinduced birefringence in their LB films

The molecular architecture of azopolymers may be controlled via chemical synthesis and with selection of a suitable film-forming method, which is important for improving their properties for practical uses. Here we address the main challenge of combining the photoinduced birefringence features of azopolymers with the higher thermal and mechanical stabilities of poly(methyl methacrylate) (PMMA) using Atom Transfer Radical Polymerization (ATRP) to synthesize diblock- and triblock-copolymers of an azomonomer and the monomer methyl methacrylate. Langmuir–Blodgett (LB) films made with the copolymers mixed with cadmium stearate displayed essentially the same optically induced birefringence characteristics, in terms of maximum and residual birefringence and time for writing, as the mixed LB films with the homopolymer poly[4-(N-ethyl-N-(2-methacryloxyethyl))amino-2′-chloro-4′-nitroazobenzene] (HPDR13), also synthesized via ATRP. In fact, the controlled architecture of HPDR13 chains led to Langmuir films that could be more closely packed and reach higher collapse pressures than the corresponding films obtained with HPDR13-conv synthesized via conventional radicalar polymerization. This allowed LB films to be fabricated from neat HPDR13, which was not possible with HPDR13-conv. The enhanced organization in the LB films produced with controlled azopolymer chains, however, led to a smaller free volume available for isomerization of the azochromophores, thus yielding a lower photoinduced birefringence than in the HPDR13-conv films. The combination of ATRP synthesis and LB technology is then promising to obtain optical storage in films with improved thermal and mechanical processabilities, though a further degree of control must be sought to exploit film organization while maintaining the necessary free volume in the films.

Photorefractive Effect in the Single-Domain Doped Lithium Niobate Crystal: Dependence on the Light Intensity

Intensity dependence of photo-induced birefringence changes due to photorefractive effect in Hf 4+-doped congruent single-domain lithium niobate (LN) crystals is presented. Very weak birefringence change dependence on the pump light intensity is found. Results on birefringence change measurements in complex with second harmonic generation experiments reveal that the threshold concentration of HfO2 is slightly above 2 mol%.

Hydrogen-Bonded Polymer−Azobenzene Complexes: Enhanced Photoinduced Birefringence with High Temporal Stability through Interplay of Intermolecular Interactions

We study the photoresponsive behavior of thin films of supramolecular 4-nitro-4′-hydroxyazobenzene−poly(4-vinylpyridine) complexes. Hydrogen bonding between the phenol and pyridine moieties allows attaching a chromophore to essentially each repeat unit of the polymer, thereby suppressing macroscopic phase separation and crystallization. Moreover, the noncooperative nature of hydrogen bonding leads to random complexation of the chromophores to the polymer backbone, which enables a systematic study of the effect of chromophore concentration on the photo-orientation of the complexes. Two regimes are observed: Photoinduced birefringence increases linearly with the chromophore concentration until nominally every third polymer repeat unit is occupied. Beyond that concentration, a new regime is observed with a drastically steeper slope of increase. The latter regime is connected to the interplay between the formation of a hydrogen-bonded supramolecular complex and intermolecular interactions between the mesogenic chromophores. Such a behavior significantly enhances the birefringence at high concentrations and also leads to high remnant birefringence. Hence, the supramolecular approach yields a superior optical performance compared to guest−host polymers and even surpasses the properties of many covalently functionalized polymers, while still allowing modular tunability of the materials properties.