Photoinduced Birefringence Research Articles

Analysis of Polarization Angle on Holographic Recording Based on PQ/PMMA.

The polarization state of light waves significantly affects the quality of holographic recordings. This paper quantitatively analyzes the impact of different polarization states of signal and reference beams on the quality of holographic recordings in PQ/PMMA photopolymer systems during the holography process. By deriving the light field distribution of the interference between two light waves of different polarization states and introducing the interference fringe contrast and the modulation of the refractive index of the photopolymer, we established the relationship between the diffraction efficiency of PQ/PMMA photopolymer holographic gratings and the angle between polarization directions. Based on this relationship, simulations and experiments were conducted. The experimental results demonstrated that as the angle between the polarization directions increased, the diffraction efficiency of the material decreased, with the efficiency dropping to 24.69% of its original value when the angle increased from 0° to 50°. When the angle increased to 60°, the influence of polarization characteristics became gradually significant, and at 90°, it was entirely dominated by polarization characteristics. The photoinduced birefringence properties of the PQ/PMMA prepared in the measurement experiment were studied, and the polarization characteristics of the reconstructed light under polarization direction angles of 0°, 60°, and 90° were investigated. The results indicated that at a polarization direction angle of 60 degrees, the material exhibited a significant response to the polarization information of the signal light. Finally, holographic recordings of objects at different polarization direction angles were conducted, and the reconstructed images were used to visually reflect the impact of the polarization direction angle on the quality of holographic recordings.

High photoinduced birefringence in thermally treated layers of the azopolymer PAZO with significantly changed absorbance spectrum

In this experimental work we present the effect of thermal treatment of azopolymer thin films on their absorbance spectra and how that influences the photoinduced birefringence of the azopolymer. The azopolymer we use – PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2ethanediyl, sodium salt]) is commonly investigated material for polarization holography because it is easily accessible and has well pronounced photoanisotropic properties. In a recent study it was discovered that its absorbance spectrum gradually changes upon heating to 250°C and as a result the absorbance in the range 400-600 nm is increased. That raises the question is it possible to increase the photoinduced birefringence for wavelengths that are far away from the peak of absorbance of the azopolymer PAZO, located approximately at 360 nm. For example, there are powerful lasers at 532 nm, which could be used for optical recording instead of traditionally used lasers in the blue and UV range. To answer this question, we investigated thin film samples of PAZO deposited on quartz substrate and measured continually their spectra of absorbance while heating the samples from room temperature to 250°C. We also measured the photoinduced birefringence using pump laser at 532 nm before and after the thermal procedure. Thus, we report significant increase of the birefringence at 532 nm after the thermal treatment and mention some potential applications.

Nanocomposite Photoanisotropic Materials for Applications in Polarization Holography and Photonics.

Photoanisotropic materials, in particular azodyes and azopolymers, have attracted significant research interest in the last decades. This is due to their applications in polarization holography and 4G optics, enabling polarization-selective diffractive optical elements with unique properties, including circular polarization beam-splitters, polarization-selective bifocal lenses, and many others. Numerous methods have been applied to increase the photoinduced birefringence of these materials, and as a result, to obtain polarization holographic elements with a high diffraction efficiency. Recently, a new approach has emerged that has been extensively studied by many research groups, namely doping azobenzene-containing materials with nanoparticles with various compositions, sizes, and morphologies. The resulting nanocomposites have shown significant enhancement in their photoanisotropic response, including increased photoinduced birefringence, leading to a higher diffraction efficiency and a larger surface relief modulation in the case of polarization holographic recordings. This review aims to cover the most important achievements in this new but fast-growing field of research and to present an extensive comparative analysis of the result, reported by many research groups during the last two decades. Different hypotheses to explain the mechanism of photoanisotropy enhancement in these nanocomposites are also discussed. Finally, we present our vision for the future development of this scientific field and outline its potential applications in advanced photonics technologies.

Measurement of azo dye film with large photo-induced birefringence and complex refractive index

Azo dyes are valuable functional materials due to their sensitive response to and effective modulation of polarized light. It is essential to find out not only the optical anisotropy but also the complex refractive index of azo dye materials for photonics applications. We report here such measurements of highly ordered photoaligned azo dye films. The film has an apparent dichroic ratio of about 60. We model the photoaligned azo dye film systems using the Berreman transfer matrix. By means of regression fitting of the polarization dependent transmittance and reflectance data, we were able to determine the complex refractive indices of the aligned film at the wavelength range of 400–1000 nm. This material is found to have exceptionally large optical birefringence (Δn ∼ 0.75 at 632.5 nm and Δn ∼ 0.48 at 850.6 nm). The aligned azo dye film is valuable to applications such as coatable polarizers and optical modulation devices.

Mechanism of Photoinduced Gyrotropic Phenomena in Amorphous As2S3

Little is known why illumination of linearly polarized continuous‐wave laser light can induce gyrotropy (optical rotatory power) and macroscopic circular/screwing deformations in amorphous As2S3. Gyrotropy appears through scattering and deflection of the incident light in combination with negative photoinduced birefringence. The gyrotropy could also trigger atomic motions under photoinduced fluidity, which produces macroscopic deformations.

Enhancement of the photoinduced birefringence and inverse relaxation of a liquid crystal azopolymer by doping with carbon nanostructures

• Carbon nanotubes can enhance the photoinduced birefringence of azopolymers. • The in situ doping method provides a good polymer-nanotube interaction. • The occurrence of the inverse relaxation depends on the doping method. In this work, the effect of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the photoinduced behavior of one side-chain liquid crystal azopolymer is studied. Two distinct methods of doping the azopolymer with CNTs or CNFs are followed: during (in situ method) and after (mixing method) polymerization. Thermotropic studies show that dopants do not have a significant effect on the liquid-crystalline properties of the azopolymer, which develops a smectic A-type phase whose lamellar structure is preserved after cooling to room temperature. On the other hand, the light-induced birefringence of the azopolymer doped by the in situ method (∼ 0.095) is higher than that reached with the corresponding one doped by the mixing method (∼ 0.05). In switch-on/switch-off experiments, both the typical and the inverse relaxation processes of the photoinduced orientation are observed. The inverse relaxation occurs only in the azopolymer doped by the in situ method, which seems to provide a good interaction between the azobenzene units and the nanostructures. In addition, it was found that no minimal threshold value of irradiation dose is required to make the inverse relaxation happen. A model based on non-covalent interactions is proposed to explain the inverse relaxation promoted by nanostructures introduced by the in situ method.

Photo-Induced Anisotropy of Methyl Red Azo Dye in Polymer Film

A photo-induced anisotropy (PIA) of Methyl Red (MR) azo dye embedded in PMMA polymer solid films was investigated by studying dichroic spectra and photo-induced birefringence. The reversible PIA of MR dye was measured in situ during photoactive irradiation process. Photoinduced order increases with the rise of exposure power. It is shown that PIA dark relaxation slows down at higher exposure energy. The MR rotational diffusion was evaluated at different exposure modes. These data are useful for the characterization of Methyl Red azo dye as a liquid crystal photo-aligning material.

Application of the pyrazolone derivatives as effective modulators in the opto-electronic networks

Materials engineering plays a pivotal role in the modern technologies and individual solutions for various applications. In this contribution we describe two pyrazolone-based organic chromophores that differ slightly in structure, but represent a significantly various spectroscopic response. Based on the quantum-chemical calculations, namely TD-DFT model, HOMO-LUMO maps estimation, we have implemented both derivatives in the 2nd and 3rd order nonlinear optical (NLO) experiments. Pyrazolone-based organic systems occurred to be sensitive and appealing all-optical modulators as well as SHG and THG signals generators. The presence of a small methoxy group or its lack in the dye structure directly affects the kinetics of photoinduced birefringence, and its magnification. Moreover, it allows to determine the ease modulation of higher harmonic of the light induction according to the provided experimental polarization spatial conditions. Such approach enables to design and implement individually adjusted organic components to the complex opto-electronic networks and highly effective photonic architectures.

Stress-induced and photo-induced birefringences in As2S3 glass

The birefringence in As2S3 glass that is induced by either uniaxial stress or illumination with linearly-polarized light (ħω = 2.0 and 2.3 eV) is comparatively studied using a specially-designed phase meter operating at a wavelength of 633 nm (∼2.0 eV). The photoelastic constant of As2S3 is negative (−6 × 10−12 Pa−1): the refractive index parallel to an applied uniaxial stress becomes smaller, which is contrastive to that (+3.5 × 10−12 Pa−1) in SiO2 glass. On the other hand, the photoinduced birefringence in As2S3 is also negative; i.e., the refractive index in the direction parallel to the electric field of excitation light becomes smaller than the perpendicular. These resembling features with comparable magnitudes of the stress- and photo-induced birefringences imply common structural origin. We propose that the negative birefringences can be understood by a unified model that assumes anisotropically-aligned segmental layer structures which are produced by mechanical and photo-structural atomic motions.

Effect of 2-Methylthiazole Group on Photoinduced Birefringence of Thiazole-Azo Dye Host-Guest Systems at Different Wavelengths of Irradiation.

The photoinduced birefringence behaviors of host–guest systems based on heterocyclic thiazole–azo dyes with different substituents, dispersed into PMMA matrix, were investigated under three excitation wavelengths, i.e., 405 nm, 445 nm or 532 nm. The wavelengths fell on the blue side, near the maximum or on the red side of the absorption bands of trans-azo dyes, respectively. We found that photoinduced birefringence was generated at a similar extent in all studied systems, except the system containing a 2-methyl-5-benzothiazolyl as thiazole–azo dye substituent. For this material, the achieved birefringence value was the highest among the whole series, regardless of the excitation wavelength. Moreover, we identified the optimal irradiation wavelength for efficient birefringence generation and showed that large absorption of excitation light by trans isomer does not account for achieving a significant degree of molecular alignment. The obtained results indicate that thiazole–azo dye with a 2-methyl-5-benzothiazolyl substituent shows promising photoinduced birefringence, and can be considered a dye potentially suitable for optical applications.

Analysis of light diffraction by azobenzene-based photoalignment layers.

Photoalignment materials, such as the azobenzene-based PAAD series studied here, are becoming increasingly important in liquid crystal-based optical devices and displays. Yet their properties and, in particular, their response to light, are still not fully understood. We investigate, experimentally and theoretically, the photoinduced birefringence, the order parameter and the formation of surface relief gratings, as well as the diffraction caused by them. We show that some of the azobenzene PAAD materials are suitable for the formation of surface relief gratings with high modulation depth, while others exhibit strong photoinduced birefringence. The two effects are inversely correlated: the stronger the surface relief grating is, the weaker is photoinduced birefringence. Analytical formulas based on the Raman-Nath approximation and numerical simulations of Maxwell's equations are used to quantify the diffraction caused by the induced diffraction gratings, showing excellent agreement between theory and experiment.

All-Optical Polarization Manipulation Through Orbital Polarization Holography

Spin-orbital optical phenomena are closely related with light-matter interactions and have been of great interest in the last few years. Here, the effect of optical orbital angular momentum (OAM) on polarized waves carrying spin angular momentum (SAM) has been investigated experimentally by means of orbital polarization holography and analyzed with Jones matrices theoretically. We report that all-optical OAM-to-polarization manipulation can be realized with a controllable holographic grating recorded through the interference of orthogonally polarized beams in various helical modes in a kind of photo-alignment supermolecular liquid-crystalline films. The polarization states of diffraction beams can be controlled through adjusting the spatial degree of freedom of the recording light field. The OAM-controlled polarization manipulation is discussed in terms of Jones matrices and photoinduced birefringence. Because of the realization of OAM-to-SAM conversion, this work may find applications in a variety of devices.

Photoinduced birefringence pattern based on selective induction of photoreactivity with inkjet technology

A random copolymethacrylate containing benzoic acid and benzaldehyde side groups (P1), and a diblock copolymer of P1 and a polymer with heptadecafluorodecyl side groups (P2) were synthesized by means of reversible addition-fragmentation chain transfer (RAFT) polymerization. The linearly polarized (LP) UV light exposure initiated the thermally stimulated photoinduced reorientation of P1(P2)/4-methoxyaniline (AN) films due to the axis-selective photoisomerization of in situ formed N-benzylideneaniline (NBA) groups. A photoinduced birefringent pattern was achieved by sublimation or inkjet coating with AN on the P1(P2) film, which formed NBA moieties in the desired region. Clear birefringent pattern was attained when using P2 because the fluorine-containing block improved the NBA formation at the AN-coated region.

Microscopic-characterization of photo-induced birefringence of azo-polymer thin film by focused surface plasmon

In this paper, we develop a technique to quantitatively evaluate the orientational dependence of local anisotropic properties (of azo-polymers fabricated in the form of thin films ∼ 100 nm) by means of focused surface plasmon. The magnitude of photo-induced birefringence and orientation of fast axis at microscopic sites in thin films are extracted by the developed method on evaluating the propagation constants of surface plasmon obtained from the elliptical absorption pattern in the reflected spatial frequency distribution. Low birefringence (∼0.01) has been successfully measured with ultra-low probe volume.

Influence of the size of Au nanoparticles on the photoinduced birefringence and diffraction efficiency of polarization holographic gratings in thin films of azopolymer nanocomposites

This study is aimed to determine the influence of the size of gold (Au) nanoparticles (NP) doped in azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) on the photoinduced birefringence of the nanocomposite thin films as well as on the diffraction efficiency of polarization holographic gratings recorded in them. For this reason, we used spherical Au NP with five different dimensions – 10, 20, 30, 40 and 50 nm. Concentrations of the NP in the nanocomposites varied from 0 a.u. (non-doped samples) to 16 a.u. where 1 a.u. corresponds to 0.015 wt %. Birefringence was induced with He–Cd laser (λ = 442 nm) and measured at 635 nm using DPSS laser. We observe an enhancement of photoinduced birefringence, which is most pronounced for the Au NP with size 20 nm. The polarization holographic gratings were inscribed with two orthogonal circular polarizations (left- and right-handed). AFM measurements were performed to determine the properties of the resulting surface relief gratings. Our results indicate that both the diffraction efficiencies and surface relief modulation of the gratings inscribed in the nanocomposite thin films are higher compared to those in the non-doped samples.

Photoresponsive behaviour of “T-type” azopolyimides. The unexpected high efficiency of diffraction gratings, modulations and stability of the SRG in azopoly(ether imide)

In this paper, we compare the light-induced properties of three “T-type“ polyimide containing azobenzene chromophores with the properties of their structural counterparts containing azopyridine moieties. The processes of photoinduced birefringence generation and surface relief grating (SRG) formation are efficient in azobenzene polyimides, while in the azopyridine analogues the processes are non-detectable. Density functional theory calculations performed to reveal a possible structural reason for the lack of optical response in the azopyridine materials showed a formation of intramolecular H-bonds. Azopoly(ether imide) with 2,2′-diphenylopropane structure despite relatively low chromophore content exhibited one of the highest diffraction efficiency (ca. 20%) and the largest modulation of the SRG (260 nm) as have ever been observed for azopolyimides. Two years after inscription, modulation of the SRG was still observed without a decrease in the relief depth, which is important in applications requiring long-term stability of SRGs.

Tautomeric influence on the photoinduced birefringence of 4-substituted phthalimide 2-hydroxy Schiff bases in PMMA matrix.

The photoinduced birefringence of two 4-substituted phthalimide 2-hydroxy Schiff bases, containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties has been investigated in PMMA matrix. Their optical behaviour as nanocomposite films was revealed by combined use of DFT quantum chemical calculations (in ground and excited state) and experimental optical spectroscopy (UV-Vis and fluorescence). The results have shown that solid-state reversible switching takes place by enol/keto tautomerization and Z/E isomerization. Birefringence study was performed in the PMMA nanocomposite films using pump lasers at λrec = 355nm and λrec = 442nm. Fast response time and high stability of anisotropy up to 58% for (4) and 95% for (5) after turning off the excitation laser, was observed, which makes these materials appropriate candidates for cutting-edge optical information technology materials. The possibility for multiple cycles of recording, reading and optical erasure of the photoinduced birefringence at λrec = 442nm in 5 has been demonstrated. The obtained results have shown that the maximum value of the measured birefringence is close to the anisotropic characteristics of the most frequently used azo materials.

Tunable microring resonators using light-activated functional polymer coatings.

We demonstrate tunable microring resonators (TMRs) based on light-activated functional polymer coatings deposited on glass optical fibers. TMRs were fabricated using two layers of polydimethylsiloxane-based compounds: one incorporating an azobenzene dye and one using a fluorescent ytterbium and erbium-doped sodium yttrium fluoride powder. The latter yields a photoluminescent composite producing green up-conversion emission under infrared pumping. This visible emission triggers photoinduced birefringence effects in the azobenzene layer, thereby modifying the spectral features of the TMR devices. The shift in the resonance peaks as a function of pump power is linear, yielding a tuning range of 1.3 nm. Aside from the observed photoinduced effects, we also discuss the photothermal effects involved in the tuning mechanism.

Photopatterning of Azobenzene-Containing Liquid Crystalline Triblock Copolymers: Light-Induced Anisotropy and Photostabilization.

Photochromic liquid crystalline block copolymers (PLCBCs) are currently playing a significant role as light-responsive materials because of their light controllable features over multiple length scales. Herein, a study of the photoinduced optical anisotropy derived by the combination of orientation phenomena at molecular and supramolecular levels in a novel kind of side-chain PLCBCs with mesogenic phenyl benzoate groups and pyridine units that is hydrogen bonded with azobenzene-containing phenol is reported. Based on the polymeric architectures and composition, the supramolecular configuration self-organizes in different microphases that affect the material response to the external stimuli. Simple, 1D, polarization holograms are recorded to evaluate the photoinduced birefringence. The first step, light patterning, involves the orientation of the azobenzene units and precedes a thermal treatment that amplifies the induced anisotropy through the cooperative orientation of the mesogenic units. By selective extraction, the azobenzene units can be removed, making the material transparent to the visible light. Excellent photostability of the material birefringence is obtained, whose final value is strongly affected by the block copolymer's architecture. The versatility in the molecular design, the fine control of the photoinduced features by external parameters, and, finally, the possibility to achieve photostability make these materials of great potential for developing optical and photonic devices.

Second harmonic generation and spectroscopic characteristics of TiO2 doped Li2O–Al2O3–B2O3 glass matrix

In this present study, Li2O–Al2O3–B2O3 glass matrix doped with a small percentage of TiO2 (ranging from 0 to 1 mol%) was prepared and characterized by X-Ray Diffraction (XRD), FTIR and Differential Scanning Calorimetric (DSC) techniques. After that, Optical Absorption (OA), Photo Luminescence (PL), Electron Paramagnetic Resonance (EPR) and Second Harmonic Generation (SHG) have been performed as a function of TiO2 concentration. The outcome of these studies spelled out different oxidation states of Ti ions. However, sample containing up to about 0.8 mol% of the TiO2, mostly will be in the state of Ti4+ with substitutional octahedral arrangement in the glass chain and is responsible for PL. Beyond this percentage, it is noticed that some parts of Ti ions reduced Ti4+ to Ti3+ state. The results of Photo-Induced Second Harmonic Generation (PSHG) together with other spectroscopic studies were discussed for all chosen samples and indicated that the Photo-Induced Birefringence (PIB) of sample with 0.8 mol% TiO2 is the most suitable for NLO (non-linear optics) devices.