Photoinduced Anisotropy Research Articles

Photoinduced Anisotropy in Thin Films of Azobenzene-Containing Liquid Crystalline Supramolecular Complexes of Various Polymer Architecture.

Light patternable colorless liquid crystalline (LC) polymers are promising materials for functional photonic devices with broad applications in optical communication, diffractive optics, and displays. This work reports photoinduced optical anisotropy in thin films of azobenzene-containing (Azo) LC block copolymer supramolecular complexes, which can be decolorized after light patterning providing colorless patterned birefringent polymer films. The supramolecular complexes are prepared via intermolecular pyridine-phenol hydrogen bonding between a low-molecular-weight Azo phenol and host LC AB diblock and ABA triblock copolymers consisted of LC phenylbenzoate (PhM) blocks and poly(vinylpyridine) units. The molecular architecture of the host polymers and the morphological pattern formed by the complexes can affect orientational behavior of Azo groups under irradiation with linearly polarized light. Photoorientation of hydrogen-bonded Azo groups is accompanied by the cooperative orientation of non-photochromic PhM units, which form individual microphases and stabilize the orientation of Azo groups. This effect is specific for block copolymer complexes and it is absent for random copolymer complex, which is used as a reference sample. Optical anisotropy induced in films of the block copolymer complexes can be amplified by heating above the glass transition temperature and subsequent rinsing with diethyl ether allows colorless birefringent polymer films to be prepared.

Considerations for ultrafast photomagnetism in manganese(III)-based single-molecule magnets

Manipulation of magnetic materials is a cornerstone of digital data storage technologies. Recently, it has been shown that femtosecond laser pulses are capable of switching the magnetization in a material between two stable configurations faster than ever before. One state-of-the-art method is to use laser pulses to control the magnetic anisotropy by photoexciting crystal-field transitions. The photoinduced change in anisotropy applies a torque to the magnetic moment, which reorientates it in a different direction. So far, research has focused solely on condensed matter materials. However, there is a huge variety of molecule-based magnetic materials that have been and continue to be developed. In particular, single-molecule magnets (SMMs) provide a highly tunable platform and have the added advantage of operating on nanometer length scales. This review discusses recent research in the area of ultrafast magnetism in SMMs, with a focus on manganese(III)-based transition metal complexes. Experimental data are reviewed, showing that control of the strength of the photoinduced anisotropy, the lifetime of excited states, and the dephasing times are possible and can be used to develop some design criteria for the best optically controllable SMMs.

Lone-Pair-Enabled Polymorphism and Photostructural Changes in Chalcogenide Glasses.

S- and Se-based chalcogenide glasses are intrinsically metastable and exhibit a number of photo-induced effects unique to this class of materials, reversible photostructural changes and photo-induced anisotropy being major examples. These effects are usually interpreted in terms of the formation of valence alternation pairs and 'wrong' bonds. In this work, using density functional theory simulations, we demonstrate for the case example of As2S3 that a strong decrease in the optical band gap can be achieved if a polymorphic transformation of the local structure from orpiment to that of tetradymite takes place. For the formation of the latter, the presence of lone-pair electrons in near-linear atomic configurations is crucial. Our results represent a novel approach to understanding the photo-induced structural changes in chalcogenide glasses as being due to the presence of polymorphism, and will lead to their wider use in various photonic devices.

A BODIPY‐Based Molecular Rotor in Giant Unilamellar Vesicles: A Case Study by Polarization‐Resolved Time‐Resolved Emission and Transient Absorption Spectroscopy

AbstractBODIPY and BODIPY‐derived systems are widely applied as fluorophores and as probes for viscosity detection in solvents and biological media. Their orientational and rotational dynamics in biological media are thus of vital mechanistic importance and extensively investigated. In this contribution, polarization‐resolved confocal microscopy is used to determine the orientation of an amphiphilic BODIPY‐cholesterol derivative in homogeneous giant unilamellar vesicles (GUV) made from 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine (DOPC). The BODIPY‐moiety of the molecule is placed near the polar headgroups, and the cholesterol moiety is embedded in the membrane along the acyl chain of the lipids. The rotational relaxation of fluorophore is conventionally investigated by time‐resolved emission anisotropy (TEA); and this method is also used here. However, TEA depends on the emission of the fluorophore and may not be useful to probe rotational dynamics of the non‐emissive triplet states. Thus, we employ femtosecond transient absorption anisotropy (TAA), that relies on the absorption of the molecule to complement the studies of the amphiphilic BODIPY in DCM and GUV. The photoinduced anisotropy of the BODIPY molecule in DCM decays tri‐exponentially, the decay components (sub‐5 ps, 43 ps and 440 ps) of anisotropy are associated with the non‐spherical shape of the BODIPY molecule. However, the anisotropy decay in homogenous GUVs follows a biexponential decay; which arises from the wobbling‐in‐a‐cone motion of the non‐spherical molecule in the high viscous lipid bilayer media. The observations for the BODIPY‐chol molecule in the GUV environment by TAA will extend to the investigation of non‐emissive molecules in cellular environment since GUV structure and size resembles the membrane of a biological cell.

Polarimetric Features of PEPC Polymer Doped with Photoisomerizable SY3 Azodye Chromophore

Photo-induced anisotropy measurements were carried out in thin films of azopolymers PEPC with Solvent Yellow 3, with the concentrations of 10 wt% and 30 wt% of polymers with azo groups in their side-chain. The experimental dependences of the azimuths of the probe beam at the pump beam angles for samples were studied. The photoinduced dependence of the sample birefringence of the probe beam at the incident polarization angle of the pump beam was carried out by the polarimetric method for the studied samples. The geometric phase of anisotropic carbazole-containing azopolymers, in contrast to the dynamic phase, can be measured by the proposed polarimetric method without involving interferometry methods. This is possible because the geometric phase is not controlled by the difference in optical paths but is the result of a change of polarization inside the films due to photoinduced changes in its spatial structure.

Ultrafast decay of carrier momentum anisotropy in graphite

Time- and angle-resolved photoelectron spectroscopy is employed to study the thermalization of a nonequilibrium carrier distribution in the Dirac cone of graphite generated upon absorption of linearly polarized near-infrared laser pulses. The data reveal a characteristic time constant of $(20\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$ for the decay of a photoinduced momentum anisotropy in the nascent carrier population. Additionally, the nascent carrier spectral peak shows a downshift in energy by $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{eV}$ within the first $30\phantom{\rule{0.16em}{0ex}}\mathrm{f}\mathrm{s}$ after excitation, which is associated with the emission of strongly coupled ${A}_{1}^{\ensuremath{'}}$ optical phonons at $K$. In agreement with previous findings for graphene, a pronounced momentum anisotropy is also observed for an intermediate quasithermalized state of the carrier population. A fully thermalized distribution (with respect to energy and momentum) is finally established on a characteristic timescale of $(40\ifmmode\pm\else\textpm\fi{}10)\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. The results underscore the complexity of carrier thermalization on ultrafast timescales resulting from the interplay of carrier-carrier and carrier-phonon interaction.

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.

Synthesis, characterization, spectral properties and evaluation of the photophysical behavior of novel Congo Red based polymers

New polyazomethines were synthesized by acid-catalyzed polycondensation of Congo Red azo dye with two dialdehydes to form polymers with azo- and azomethine groups in main chain. Polycondensation proceeded at a reflux temperature in DMF solution with formic acid as a catalyst. Copolymers containing azo- and azomethine groups in main chain were characterized by 1H NMR, FT-IR, and UV–Visible spectrometry, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC). The polymers exhibited photoinduced anisotropy as a result of E-Z photoisomerization. Holograms in thin polymeric films for parallel and perpendicular orientation of the object and reference beams polarization have been recorded. In the case of perpendicular polarization, diffraction efficiencies of the hologram gratings are higher comparing to the parallel. This may be due to the polymer matrix structural rearrangement peculiarities.

Impact of fullerene on the holographic properties of PQ/PMMA photopolymer

Through combining with free radical polymerization and supramolecular chemistry techniques, we herein demonstrate that C60 can dramatically enhance the intensity holographic performance (both signal and reference coherent beams are polarized perpendicularly to the plane of incidence) of phenanthraquinone doped polymethyl methacrylate (PQ/PMMA) photopolymer, and the angle-multiplexing of 321 gratings and 81 holographic patterns can be readily recorded separately. More surprisingly, although C60 can effectively enhance the intensity holography, it reduces the polarization sensitivity of PQ/PMMA photopolymer, resulting in a substantial weakening of orthogonally polarized holography (signal and reference coherent beams are polarized perpendicularly and parallelly to the plane of incidence, respectively). Further analysis reveals that the reduction of the polarization sensitivity should originate from the strong π-π stacking interaction between C60 and PQ photosensitizers, which causes C60 and PQ/PMMA construct supramolecular (C60 as the host) and limits the orientation of PQ during the photopolymerization. The constructed supramolecular also leads to a diminished photoinduced anisotropy of PQ/PMMA photopolymer. The current study thence has not only successfully synthesized photopolymer materials with potential for holographic storage applications, but also investigated the microphysical mechanism of the impact of fullerene on the holographic properties of PQ/PMMA photopolymer and clarified the structure-property relationship between the orientation of PQ photosensitizers and the polarization sensitivity of PQ/PMMA photopolymer.

Electrically modulated Josephson junction of light-dressed topological insulators

In a Josephson junction mediated by a two-dimensional irradiated topological insulator, the current-phase relation can be electrically modulated, including the 0-$\ensuremath{\pi}$ phase transition and the anomalous phase shift of the current. Qualitatively combining analyses and numerical simulations, we find that the $0\text{\ensuremath{-}}\ensuremath{\pi}$ phase transition and anomalous phase shift can be controlled by a gate voltage and a transverse electric field, respectively. These possible electrical modulations result from the photoinduced anisotropic helical edge states. Due to the anisotropy and spin-momentum locking nature in the edge channels, an effective Zeeman field is caused by an electrical potential whose orientation automatically matches the spin alignment of the edge modes. The photoinduced anisotropy provides potential applications in Flouqet engineering helical supercurrent and electrically modulated topological superconducting devices.

Thermally stable birefringent films from a liquid crystalline polymer having side groups containing N-benzylideneaniline

Polymethacrylate liquid crystalline polymer having side groups containing N-benzylideneaniline moiety was synthesized, and its photoinduced reorientation behavior was investigated using linearly polarized UV light. Films of these polymer exhibited axis-selective trans-cis-trans photoreorientation of the side groups upon irradiation. Annealing the film in the liquid crystalline temperature range of the polymer after irradiation greatly amplified their otherwise minimal photo-induced optical anisotropy. Due to the extended mesogenic structure of the side groups, the polymer showed significant thermal stability and a high degree of optical anisotropy after annealing.

Single-shot imaging of ultrafast all-optical magnetization dynamics with a spatiotemporal resolution.

We present a laboratory system for single-shot magneto-optical (MO) imaging of ultrafast magnetization dynamics with less than 8fs temporal, micrometer spatial resolutions and a MO Faraday's rotation sensitivity of 4 mdeg/μm. We create a stack of MO images repeatedly employing a single pair of pump and defocused probe pulses to induce and visualize MO changes in the sample. Both laser beams are independently wavelength-tunable, allowing for a flexible, resonant adjustable two-color pump and probe scheme. To increase the MO contrast, the probe beam is spatially filtered through a 50 μm aperture. We performed the all-optical switching experiment in Co-doped yttrium iron garnet films (YIG:Co) to demonstrate the capability of the presented method. We determine the spatiotemporal distribution of the effective field of photo-induced anisotropy, driving the all-optical switching of the magnetization in the YIG:Co film without an external magnetic field. Moreover, using this imaging method, we tracked the process of the laser-induced magnetization precession.

Controlling the anisotropy of a van der Waals antiferromagnet with light.

Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3 Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

Thermal imidization and properties of polyimide with azobenzene side-chain for photo-alignment film

Azobenzene-containing polyimides (azo-PIs) have exceptional potential for photo-alignment of liquid-crystal (LC) molecules. In this study, the synthesis of PIs with azo-side chains (azo-PIs) via thermal imidization of azo-containing poly(amic acid) (PAA) precursors was investigated. The results indicated that the increasing azo-containing diamine promoted the imidization progress, leading to a relatively lower imidization temperature for the fabricating of azo-PI films. These azo-PIs films were investigated for application as photo-induced anisotropy films in liquid-crystal devices (LCDs). It was found that the pretilt angle of the photo-aligning azo-PI film was tunable by changing the mole ratio of azo-amine monomers. These azo-PI films exhibited good thermal stability, outstanding mechanical property and excellent performance in controlling the LC molecule alignment.

Photoactivated Refractive Index Anisotropy in Fluorescent Thiophene Derivatives.

The optical control of anisotropy in materials is highly advantageous for many technological applications, including the real-time modulation of another light signal in photonic switches and sensors. Here, we introduce three thiophene derivatives with a donor–acceptor structure, which feature different positions of an electron-acceptor nitrile group, and both photoalignment and luminescence properties. Quantum chemical calculations highlight the presence of trans-forms stable at room temperature and metastable cis-isomers. Besides photoluminescence peaked at 440–460 nm and 0.4 ns lifetime, the three nonlinear optical chromophores exhibit photoinduced anisotropy of the refractive index closely depending on the specific molecular structure, with higher values of birefringence at lower driving signal being obtained for ortho substitution of the nitrile group. All-optical modulation of an external light beam at rates of hundreds of hertz is demonstrated in the fluorescent systems. This finding opens an interesting route to multispectral photonic switches embedded in the active layers of light-emitting devices.

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.

Electrooptical Effect in Polymeric Composites Containing a Heterometallic Cu/V/Mo Complex

The influence of an electric field on the transmission of linearly polarized light by polyvinylbutyral composite films doped with particles of the heterometallic complex (NH4)2{[Cu(dien)(H2O)]2[α-V2Mo6O26]}∙5H2O (dien = diethylenetriamine) was studied. The absorption became anisotropic if an electrostatic field was applied during irradiation of the composites by linearly polarized light. The observed electrooptical effect was explained by photo-induced optical anisotropy that was caused by the linearly polarized light and changed in space due to reorientation of the dipole moment of photogenerated electron–hole pairs in the composite under the action of the external electric field.

Photoinduced anisotropy of DR13/PMMA thin films upon irradiation at various wavelengths

Photoinduced anisotropy of poly(methyl methacrylate) (PMMA) thin film doped with disperse red 13 (DR13) dye isomers was investigated. This anisotropy was analyzed by measuring the temporal changes in the isotropic and anisotropic absorption coefficients for various pump wavelengths (both sides of the isobestic point). The rates for photoisomerization were determined as p515=0.41s-1 and p375=0.20s-1 under irradiation with circularly polarized pumping beam. After irradiation, the thermal relaxation was evaluated to be 0.052s-1. During irradiation with linearly polarized light, some abnormal phenomena were monitored in the early time. The mechanism of angular redistribution caused by linearly polarized light irradiation is discussed in detail.

A family of azoquinoline derivatives: Effect of the substituent at azo linkage on thermal cis-trans isomerization based on an experimental and computational approach

The subject of the research is a series of azo-dyes based on the quinoline structure, which are practically omitted as materials which use photoinduced optical anisotropy for applications. The work presents the kinetics of thermal cis-trans isomerization reaction of 5-azo-8-hydroxy-2-methylquinoline and 5,5'-((1E,1′E)-[1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(2-methylquinolin-8-ol) compounds. The impact of the structure of the substituent at the azo linkage on the cis-trans isomerization kinetics was investigated experimentally and theoretically using the UV–Vis spectroscopy and the density-functional theory (DFT). The kinetics of the dark-return of the cis-to trans-isomer of azoquinolines was investigated after UV-irradiation. It was found that attaching the nitro substituent to the azoquinolines structure retards the photoisomerization process in comparison to the other substituents (i.e. F, Cl, Br, N atoms or CH3, OH, COOH, OCH3 groups). The calculated rate constants were in qualitative agreement with the experimental values. To the best of our knowledge, the cis-trans isomerization kinetics for azoquinolines has not been studied yet.

Fundamental Limits on the Repetition Rate of Photomagnetic Recording

In the last decade it was demonstrated that the fastest way to write information employs ultrashort laser pulses. Naturally such experiments raise questions about the ultimate limit of repetition rate at which light can switch a medium between stable bit states. Here the authors demonstrate that with femtosecond pulses it is possible to write and rewrite magnetic bits in iron garnet with a frequency of up to 20 GHz, with the maximum repetition rate being defined by the frequency of ferromagnetic resonance in the field of photoinduced magnetic anisotropy. This finding reveals the principles to be employed in achieving magnetic recording at frequencies far beyond today's state of the art.