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Two-wave mixing adaptive interferometer based on a liquid crystal light valve with a semiconductor GaAs substrate is realized and studied at 1064 nm wavelength. The local response of the dynamic hologram recorded in the liquid crystal layer of the light valve allows for detection of small phase modulations of the object beam. The characteristics of the interferometer are estimated experimentally. The temporal adaptability lies in the subsecond range. The large optical nonlinearity of the cell is favorable for measurements of small displacements with high sensitivity.

Globally, grassland and farmland birds have experienced population declines due to habitat loss associated with increasing agricultural land-use intensity. These modified environments can reduce insect availability for aerial insectivorous birds and agricultural development is a leading hypothesis for declines in this guild. Our objective was to investigate the relationship between landscape composition and diet of nestling Barn Swallows (Hirundo rustica) raised in a mixed agroecosystem in southern Ontario, Canada. We used nestling feather stable-isotope (δ13C and δ15N) measurements and DNA barcoding of nestling fecal matter to elucidate nestling diets. Nestling feather isotope values were related to agricultural land use, indicating differences in diet, or diet source that varied with proportion of agricultural crops. In 1 year, we found reduced diet richness in areas with increased amounts of row crop, otherwise measures of taxonomic richness and composition of nestling diet showed no relationship with the proportion of row crop, which suggests similar diets in heavily cropped and less cropped landscapes. Additionally, amount of water in the surrounding landscape was associated with increased diet richness. Overall, isotopic measurements and fecal barcoding suggest that nestling Barn Swallows raised within our agroecosystem were provisioned insects from agricultural food webs for at least part of their diet. There was little evidence to support nestling diet composition or richness changes with increased agricultural intensity, at current (low) nesting densities.

The polar states of uniaxial ferroelectric nanoparticles interacting with a surface system of electronic and ionic charges with a broad distribution of mobilities is explored, which corresponds to the experimental case of nanoparticles in solution or ambient conditions. The nonlinear interactions between the ferroelectric dipoles and surface charges with slow relaxation dynamics in an external field lead to the emergence of a broad range of paraelectric-like, antiferroelectric-like ionic, and ferroelectric-like ferroionic states. The crossover between these states can be controlled not only by the static characteristics of the surface charges, but also by their relaxation dynamics in the applied field. Obtained results are not only promising for advanced applications of ferroelectric nanoparticles in nanoelectronics and optoelectronics, they also offer strategies for experimental verification., Comment: 35 pages, 13 figures

Using the Landau-Ginzburg-Devonshire approach, we study screening-induced phase transitions in core-shell ferroic nanoparticles for three different shapes: an oblate disk, a sphere, and a prolate needle. The nanoparticle is made of a ferroic CuInP2S6 core and covered by a "tunable" screening shell made of a phase-change material with a conductivity that varies as the material changes between semiconductor and metallic phases. We reveal a critical influence of the shell screening length on the phase transitions and spontaneous polarization of the nanoparticle core. Since the tunable screening shell allows the control of the polar state and phase diagrams of core-shell ferroic nanoparticles, the obtained results can be of particular interest for applications in nonvolatile memory cells., Comment: 22 pages, 6 figures, 1 Appendix

In the framework of the Landau-Ginzburg-Devonshire (LGD) approach, we studied the possibility of controlling the polarity and chirality of equilibrium domain structures by a homogeneous external electric field in a nanosized ferroelectric core covered with an ultra-thin shell of screening charge. Under certain screening lengths and core sizes, the minimum of the LGD energy, which consists of Landau-Devonshire energy, Ginzburg polarization gradient energy, and electrostatic terms, leads to the spontaneous appearance of stable labyrinth domain structures in the core. The labyrinths evolve from an initial polarization distribution consisting of arbitrarily small randomly oriented nanodomains. The equilibrium labyrinth structure is weakly influenced by details of the initial polarization distribution, such that one can obtain a quasi-continuum of nearly degenerate labyrinth structures, whose number is limited only by the mesh discretization density. Applying a homogeneous electric field to a nanoparticle with labyrinth domains, and subsequently removing it, allows inducing changes to the labyrinth structure, as the maze polarity is controlled by a field projection on the particle polar axis. Under specific conditions of the screening charge relaxation, the quasi-static dielectric susceptibility of the labyrinth structure can be negative, potentially leading to a negative capacitance effect. Considering the general validity of the LGD approach, we expect that an electric field control of labyrinth domains is possible in many spatially-confined nanosized ferroics, which can be potentially interesting for advanced cryptography and modern nanoelectronics., Comment: 19 pages, 5 figures

For the first time, we demonstrated successful post-synthesis incorporation of metal dopants at elevated temperature into a host structure of Sn2P2S6, known as the grandfather of dichalcogenide ferroelectrics with a formula M2P2X6 (M = metal and X = chalcogen). With the example of Cu, we show that the integration of dopant atoms into the bulk of an already grown crystal could be easily tuned up to 0.5 at% affecting the structural, optical, vibrational, electrical, and ferroelectric properties. Thermally diffused copper atoms in Sn2P2S6 bulk are in the metallic state, inducing a multiaxial expansion of the Sn2P2S6 unit cell for 2–3.4%. The energy reduction between indirect and direct optical transitions was observed, combined by small hardening for acoustic and soft optical vibrational modes originating in the partial substitution of Sn by Cu in the Sn2P2S6 crystal lattice. Similar to hydrostatic pressure, the structurally bonded copper initiates a small downward shift in the critical temperature Tc. The presence of copper has a substantial impact on the shape smearing of the ferroelectric domains as well as on the behaviour of the dielectric permittivity. The real part of the dielectric constant e′ reaches its maximal value at intermediate concentrations of Cu 0% < x < 0.5%, showing an ∼20% increase with respect to the parent structure. The incorporated metal atoms provoke a monotonous expansion of the ferroelectric P–E loops along the increased dopant content direction. An increase in the electrical conductivity at higher Cu concentration reveals a trend for inducing a metal-like behaviour.

We aim to realize a novel nanotechnology-based biosensor specifically utilized to detect harmful bacteria in potable water. The nano-inspired device makes use of a chemically functionalized gold nanorods array (for the selective selection of specific pathogens) layered with a photo-responsive nematic liquid crystal (NLC) film for real-time and high sensitivity detection. The first experimental results are presented and discussed.

Optical propagation with transverse magnetic and transverse electric polarizations is investigated in metamaterials composed of thin-film metallo-dielectric (MD) structures. The analysis is performed using a novel simplified anisotropic transfer matrix method, the Berreman matrix method with effective medium theory, and finite element methods. In the process, appropriate spatial transfer functions are defined for propagation in the effective bulk medium and used to show negative refraction and linear focusing for certain polarizations in hyperbolic metamaterials. Simulations are compared with experimental results for MD transmission filters. Possible extensions to tunable systems are discussed, using electro-optic substrates and thin-film phase change materials, along with other applications.

Polar chiral structures have recently attracted much interest within the scientific community, as they pave the way towards innovative device concepts similar to the developments achieved in nanomagnetism. Despite the growing interest, many fundamental questions related to the mechanisms controlling the appearance and stability of ferroelectric topological structures remain open. In this context, ferroelectric nanoparticles provide a flexible playground for such investigations. Here, we present a theoretical study of ferroelectric polar textures in a cylindrical core-shell nanoparticle. The calculations reveal a chiral polarization structure containing two oppositely oriented diffuse axial domains located near the cylinder ends, separated by a region with a zero-axial polarization. We name this polarization configuration "flexon" to underline the flexoelectric nature of its axial polarization. Analytical calculations and numerical simulation results show that the flexon's chirality can be switched by reversing the sign of the flexoelectric coefficient. Furthermore, the anisotropy of the flexoelectric coupling is found to critically influence the polarization texture and domain morphology. The flexon rounded shape, combined with its distinct chiral properties and the localization nature near the surface, are reminiscent of Chiral Bobber structures in magnetism. In the azimuthal plane, the flexon displays the polarization state of a vortex with an axially polarized core region, i.e., a meron. The flexoelectric effect, which couples the electric polarization and elastic strain gradients, plays a determining role in the stabilization of these chiral states. We discuss similarities between this interaction and the recently predicted ferroelectric Dyzaloshinskii-Moriya interaction leading to chiral polarization states., Comment: 53 pages, 5 figures, supplementary materials on 31 pages

Multi-layered metamaterial structures show promise in a wide variety of optical applications such as superlenses, electromagnetic cloaking, tunable filters, sensors, and spatial light modulators. Optical transmission analysis of multilayer metallo-dielectric stacks with overall thickness less than the wavelength of light can be modeled using effective medium theory and the Berreman matrix method. For multilayer anisotropic stacks of arbitrary thickness, a rigorous 4 × 4 transfer matrix embodiment is typically used. In this work, a 2 × 2 anisotropic transfer matrix method is developed to analyze optical propagation through multilayer uniaxial stacks of arbitrary thicknesses. Optical transmission of a multilayer silver–zinc oxide stack deposited on a quartz substrate is modeled with this 2 × 2 anisotropic transfer matrix method and reconciled with experimental observations. Results indicate that this numerical approach is applicable to in situ assessment of the complex refractive indices of constituent metal and dielectric layers. Additionally, the anisotropic 2 × 2 transfer matrix method enables the possibility of modeling the transmission of the same metallo-dielectric structure deposited on an electro-optic, uniaxial substrate. Simulation results predict that adjusting the bias field across the substrate results in an electrically tunable transmission filter.

Two intersecting light beams illuminate cholesteric liquid crystal with helicoidal polymer network. Space-charge field of photorefractive substrate forces polymer fibrils to move and induce director grating. Coupling of light beams within the grating leads to weak beam gain coefficient greater than in solid photorefractive crystals.

In this report we review recent progress in controlling surface plasmons by using liquid crystals and present our results on electrical tuning of Tamm Plasmons with help of cholesteric liquid crystals.

Planar cholesteric liquid crystals (CLCs) are well known for having vibrant reflective coloration that is associated with the handedness and the pitch length of the helicoidal twist of the liquid crystalline molecules. If one observes these films at oblique angles, the reflected colors blue-shift with increasing angles from normal. On the other hand, uniform lying helix (ULH) CLCs, where the helicoidal axis lies in the plane of the substrate, are well-known but are not typically associated with vibrant colors. Here, we examine the unique optical properties of CLCs at oblique incidence angles, specifically the spectral and polarization changes associated with switching between planar and ULH CLCs for various incidence angles. At small angles of incidence (0° < ψ < 45°, where ψ is the angle of incidence relative to the surface normal at the substrate-CLC interface), the electrically driven helical reorientation from planar to ULH results in a blue-shifting of the color and circularly polarized to unpolarized switching behavior. At large angles (45° < ψ < 90°), the behavior is reversed, with a red-shifting color change occurring and the polarization switching from unpolarized to circularly polarized. Modeling of the light propagation through ULH CLCs is used to confirm the change in position and polarization characteristic of the reflection band with incidence angle observed experimentally. This study provides a new perspective on ULH CLCs and reveals a unique reconfigurable angular chromaticity.

It is generally accepted that chemically synthesized nanoparticles lose their ferroelectricity (spontaneous polarization) as the particles become smaller. In contrast, ball-milled ferroelectric nanoparticles have an enhanced ferroelectric response at remarkably small sizes (≤10 nm). Although prior theory suggests that surface stress influences ferroelectricity, the source of such a stress and how it physically influences ferroelectricity in zero-dimensional nanoparticles has remained a mystery. In this paper, we demonstrate that the top-down approach of wet ball-milling not only results in fragmented materials on the nanoscale, but it also is responsible for a mechanochemical synthesis of metal carboxylates forming at the nanoparticles' surface. We prove that the presence of such a compound with a particular type of binding mode chemisorbed at the nanoparticles' surface is responsible for producing surface stress. This surface stress results in a stabilization and dramatic enhancement of the spontaneous polarization, which is 5 times greater than that of the bulk material and 650 times greater than what is measured in materials fabricated using standard chemical synthesis techniques. The results of this study have further led to the development of a new process that produces ferroelectric nanoparticles (≤10 nm) with uniform shape and size using a combination of wet chemistry and mechanochemical synthesis.

There has been considerable interest in nanomaterials for the development of anisotropic molecular alignment layers for electronic and biomedical applications. This review covers the recent progress in the design, synthesis, and characterization of automatic liquid crystal (LC) alignment layers. Amphiphilic hybrid nanomaterials consisting of organic parts and inorganic cores can interact favorably on LC media and substrates resulting in the uniaxial orientation of LC molecules in a specific direction. Among such systems, this review emphasizes giant surfactants showing automatically self-assembled 2D monolayers. Polymerizable giant surfactants allow us to build robust molecular alignment layers via a one-bottle approach. We believe that the automatic LC alignment layer constructed by doping giant surfactants can realize practical applications in smart materials.

Photovoltaic fields induced in x-cut Fe-doped lithium niobate (Fe:LiNbO3)were used to achieve optically induced defect formation and light valving in a vertically aligned nematic liquid crystal. Initially, the optical axis of the LC was vertically aligned (along the surface-normal of the planar, photovoltaic substrates) throughout the whole sample. Samples were exposed with a focused continuous wave laser beam and investigated via microscopic imaging in-between crossed polarizers. The optical axis of the planar, x-cut Fe:LiNbO3 substrates was in the substrate plane and oriented parallel to one of the polarizers, which resulted in an initially dark state. Optically induced surface fields (with high in-plane components) generated within the substrates led to director reorientations and defect formation. Accordingly, the samples were locally switched into a transmissive state. The area affected by exposure was larger (300 μm) than the FWHM of the Gaussian exposure beam (14 μm). Switching from dark to bright states (light valving) could be achieved in the investigated samples much more eficiently than in previously investigated samples with z-cut Fe:LiNbO3-substrates. Realignments of the LC director were induced at lower optical power density (140 mW/cm2) than would be required to excite the intrinsically present nonlinear optical responses in a nematic LC such as the light induced Fredericks transition.

We perform numerical simulations to compare the Berreman matrix method using effective medium results for an anisotropic material with exact calculations of a multi-layer metallo-dielectric stack using the transfer matrix method and finite element techniques. Results are given for a wide band of wavelengths and incident angles. For fixed sample thickness the number of layers is increased to study convergence of the optical characteristics (transmittance and reflectance). It is shown that the Berreman matrix method with effective medium results for an anisotropic material provides a fast and reliable estimate of the optical characteristics of the composite material. The Berreman technique readily leads to the transfer function matrix for propagation in anisotropic materials.

We develop a theory describing two-beam energy exchange in a hybrid photorefractive cholesteric cell with photosensitive molecules. A cholesteric liquid crystal (LC) layer is placed between two inorganic substrates. One of the substrates is photorefractive. Weak and strong light beams are incident on the hybrid cell. The interfering light beams induce both a periodic space-charge field in the photorefractive substrate and the photo-transformed molecules (PM) in the LC layer. The PM have the helical twisting power different from that of the initial molecules. The space-charge field penetrates into the cholesteric LC slab and modulates the director interacting with the LC flexopolarization. The PM are periodically distributed in the cell and modulate the cholesteric pitch. The periodic director modulation (director grating) arising in the cell is a sum of two in-phase gratings, the flexoelectric effect driven grating and the PM-driven grating. The director grating gives rise to the dielectric permittivity grating. Each light beam diffracts from the induced permittivity grating leading to an energy exchange between the beams. We calculate the signal beam gain coefficient and analyze its dependence on the PM parameters and concentration. We show that doping the LC with photosensitive molecules provides a control of the gain of the two-beam energy exchange in the hybrid cholesteric cell.

Recording of dynamic holograms in the near infrared spectral range is reported for a liquid crystal light valve with a GaAs semi-insulating substrate. The interaction of weak signal and strong pump waves are studied. It is demonstrated that an out-of-phase local dynamic grating is recorded. Optimal amplitude and frequency of the applied voltage are found for maximum amplification of the weak beam. Step-like external phase modulation of the signal wave with amplitude π/2 is used to change the type of response from local to nonlocal and to achieve in such a way additional transient signal beam amplification. Study of the cells with different thicknesses of the liquid crystal layer at different grating spacings allows a seventeen-fold gain of the weak beam in the device with the thickest liquid crystal layer of 16 μm at a grating period of 1000 μm (the largest possible in our set-up). The amplitude of the refractive index modulation and nonlinear coupling constant n2 are estimated from the experimental results.

We propose using a liquid crystal (LC) layer placed on top of graphene ribbons or a graphene monolayer to control surface plasmons. The plasmon frequency depends on the LC director orientation which can be controlled by an applied voltage. The surface plasmons excitation is sensitive to the light polarisation and can be manipulated using the orientation properties of LCs. We further suggest using periodically anchored LCs instead of a relief grating to excite and control the plasmons in monolayer graphene. The suggested scheme can be used as the basis for a tunable optical filter or a modulator.

Dynamics of planar-planar director reorientation under an electrostatic field is studied in nematic cell with the director easy axis gliding. Two cases of the electric field interaction with easy axis at the cell substrate is assumed, linear and quadratic in the electric field. In the case of quadratic interaction the director instability has a threshold. Dependence of the director dynamics on the anchoring energy and the electric field interaction with the easy axis are studied. We calculated the light transmittance of the cell and show that it reflects the director dynamics.

We demonstrate experimentally that Tamm plasmons can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. Not only do Tamm plasmons survive the nano-patterning of the metal film, but they also become sensitive to external perturbations, as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface we were able to red-shift a Tamm plasmon by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of this notoriously inert excitation within a 10 nm range.

In the framework of the Landau-Ginzburg-Devonshire approach coupled with electrostatic equations, we performed finite element modeling of the polarization behavior in a ferroelectric barium titanate core covered with a tunable paraelectric strontium titanate shell placed in a polymer or liquid medium. A stable two-dimensional vortex is formed in the core after a zero-field relaxation of an initial random or poly-domain distribution of the polarization, where the vortex axis is directed along one of the core crystallographic axes. Subsequently, sinusoidal pulses of a homogeneous electric field with variable period, strength, and direction are applied. The field-induced changes of the vortex structure consist in the appearance of an axial kernel in the form of a prolate nanodomain, the kernel growth, an increasing orientation of the polarization along the field, and the onset of a single-domain state. We introduced the term "kernel" to name the prolate nanodomain developed near the vortex axis and polarized perpendicular to the vortex plane. In ferromagnetism, this region is generally known as the vortex core. The vortex states with a kernel possess a manifold degeneracy, appearing from three equiprobable directions of vortex axis, clockwise and counterclockwise directions of polarization rotation along the vortex axis, and two polarization directions in the kernel. This multitude of the vortex states in a single core are promising for applications of core-shell nanoparticles and their ensembles as multi-bit memory and related logic units. The rotation of a vortex kernel over a sphere, possible for the core-shell nanoparticles in a soft matter medium with controllable viscosity, may be used to imitate qubit features., Comment: 54 pages, 12 figures, 4 appendices

We demonstrate experimentally that optical Tamm states corresponding to Tamm plasmons can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. Not only do such states survive the nano-patterning of the metal film, but they also become sensitive to external perturbations, as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface we were able to red-shift Tamm plasmons by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of these notoriously inert excitations within a 10 nm range.

We demonstrate the generation of spectrally tunable phase-dependent wavefronts, using the 2D Airy as the primary test case, via a polymer-stabilized cholesteric liquid crystal (PSCLC) element. Specifically, we use a novel spatial light modulator (SLM) based projection system to photo-align the initial helix angle landscape of the PSCLC so that it imparts the appropriate cubic phase profile to the reflected beam. This element is spectrally selective, with a reflection bandwidth of ≈ 100 nm, and electrically tunable from λ = 530 nm to 760 nm. Under both green and red laser illumination, the element is shown to conditionally form an Airy beam depending on the position of the electrically tailored reflection band. We briefly demonstrate the generality of this approach by producing PSCLC elements which form a computer-generated hologram and a higher-order Mathieu beam.

Farmland bird populations have declined with increasing agricultural intensification possibly due to putative reductions in prey insects and effects of pesticide exposure. Presence of agriculture may be especially relevant for aerial insectivorous songbirds whose primary diet is flying insects. Here, we investigated the effects of agricultural land use on nestling body condition, an important determinant of post-fledging survival, for barn swallows (Hirundo rustica), an aerial insectivore breeding within an agro-ecosystem in southern Ontario, Canada. Our scale-of-effect analysis revealed that nestling and pre-fledging body condition varied most strongly with the proportion of row crop within 100 m of the natal barn. Unexpectedly, this correlation was positive for both nestling body condition (2016 only) and for pre-fledging condition (2016 and 2017). We found a weak positive effect of row crop on number of young fledged. We speculate that the positive effects of agricultural row-cropping on condition and number of young fledged was due to higher prey availability and/or more open foraging habitat around barns surrounded by row crops. Alternatively, higher nestling condition in high agriculture environments could reflect an insurance policy to increase survival during the post-fledging period. Our results suggest that, in our southern Ontario study area, the degree of agricultural conversion does not negatively influence individual nestling condition and number of young produced for barn swallows. We recommend future research on this species to examine reproductive success in more intense agricultural landscapes and possible effects of pesticide exposure.

Energy gain of weak signal interacting with a strong pump at the director grating in photorefractive hybrid cell is studied. The cell is filled by bent-core cholesteric LC with small twist/bend elastic constants. The director grating is written by the space-charge field penetrating into LC from the photorefractive substrate. Due to a decreased twist/bend elasticity of the LC, the grating magnitude increases causing an increase of the energy gain. Influence of the elastic constants ratio, flexoelectric coefficients, and cholesteric pitch is studied. We show that the gain coefficient can reach higher values in bent-core cholesterics compared to conventional ones.

The local environment at polarized solid-liquid interfaces provides a unique medium for chemical reactions that could be exploited to control the selectivity of non-faradaic reactions. Polarized interfaces are commonly prepared by applying a voltage to an electrode in an electrolyte solution, but it is challenging to achieve high surface charge densities while suppressing faradaic reactions. Ferroelectric materials have permanent surface charge densities that arise from the dipole moments of ferroelectric domains and can be used to create polarized solid-liquid interfaces without applying a voltage. We studied the effects of ferroelectric oxides on the selectivity of a Rh porphyrin-catalyzed carbene rearrangement. The addition of ferroelectric BaTiO3 nanoparticles to the reaction solution changed the product ratio in the same direction and by a similar magnitude as performing the reaction at an electrode-electrolyte interface polarized by a voltage. The results demonstrate that colloidal suspensions of BaTiO3 nanoparticles act as a dispersible polarized interface that can influence the selectivity of non-faradaic reactions.

Harvested ferroelectric nanoparticles of BaTiO3 and LiNbO3 were dispersed in Ferroelectric Liquid Crystals (FLCs) with very high spontaneous polarization (Ps). The electrooptic and dielectric parameters were documented. The dipoles of ferroelectric nanoparticles and those of FLCs are partially cancelled in an antiparallel manner. The role of cell parameters like thickness of the Nylon 6 polymer layer and the change in preparation of the polymer layer due to different mechanical rubbing cycles has been described for one high-Ps FLC and for high-Ps FLC/BaTiO3 nanocolloids. Cell properties depend strongly on anchoring forces which might interact with dipoles both from ferroelectric nanoparticles and high-Ps FLCs.

Electron paramagnetic resonance (EPR) is used to monitor photoinduced changes in the charge states of sulfur vacancies and Cu ions in tin hypothiodiphosphate. A Sn2P2S6 crystal containing Cu+ (3d10) ions at Sn2+ sites was grown by the chemical vapor transport method. Doubly ionized sulfur vacancies ( V S 2 +) are also present in the as-grown crystal (where they serve as charge compensators for the Cu+ ions). For temperatures below 70 K, exposure to 532 or 633 nm laser light produces stable Cu2+ (3d9) ions, as electrons move from Cu+ ions to sulfur vacancies. A g matrix and a 63,65Cu hyperfine matrix are obtained from the angular dependence of the Cu2+ EPR spectrum. Paramagnetic singly ionized ( V S +) and nonparamagnetic neutral ( V S 0) charge states of the sulfur vacancies, with one and two trapped electrons, respectively, are formed during the illumination. Above 70 K, the neutral vacancies ( V S 0) are thermally unstable and convert to V S + vacancies by releasing an electron to the conduction band. These released electrons move back to Cu2+ ions and restore Cu+ ions. Analysis of isothermal decay curves acquired by monitoring the intensity of the Cu2+ EPR spectrum between 74 and 82 K, after removing the light, gives an activation energy of 194 meV for the release of an electron from a V S 0 vacancy. Warming above 120 K destroys the V S + vacancies and the remaining Cu2+ ions. The photoinduced EPR spectrum from a small concentration of unintentionally present Ni+ ions at Sn2+ sites is observed near 40 K in the Sn2P2S6 crystal.

Electron paramagnetic resonance (EPR) is used to establish the role of iodine as an electron trap in tin hypothiodiphosphate (Sn2P2S6) crystals. Iodine ions are unintentionally incorporated when the crystals are grown by the chemical-vapor-transport method with SnI4 as the transport agent. The Sn2P2S6 crystals consist of Sn2+ ions and (P2S6)4- anionic groups. During growth, an iodine ion replaces a phosphorus in a few of the anionic groups, thus forming (IPS6)4- molecular ions. Following an exposure at low temperature to 633 nm laser light, these (IPS6)4- ions trap an electron and convert to EPR-active (IPS6)5- groups with S = 1/2. A concentration near 1.1 × 1017 cm-3 is produced. The EPR spectrum from the (IPS6)5- ions has well-resolved structure resulting from large hyperfine interactions with the 127I and 31P nuclei. Analysis of the angular dependence of the spectrum gives principal values of 1.9795, 2.0123, and 2.0581 for the g matrix, 232 MHz, 263 MHz, and 663 MHz for the 127I hyperfine matrix, and 1507 MHz, 1803 MHz, and 1997 MHz for the 31P hyperfine matrix. Results from quantum-chemistry modeling (unrestricted Hartree-Fock/second-order Moller-Plesset perturbation theory) support the (IPS6)5- assignment for the EPR spectrum. The transient two-beam coupling gain can be improved in these photorefractive Sn2P2S6 crystals by better controlling the point defects that trap charge.

Two-wave mixing adaptive interferometer based on photorefractive crystal allows for compensation of temporal disturbances in ambient environment and operation with speckled beams. The crystal should exhibit large effective trap density, low dark conductivity and large photoconductivity. Deliberately doped semiconductor may meet these requirements. In the present work the photorefractive, spectroscopic and magneto-optical study of CdTe:Sn is performed aiming to estimate these characteristics and to describe the space-charge formation. The photon energies for optical ionization/neutralization of the tin ions are estimated. The crystal is characterized as a medium for two-wave mixing adaptive interferometer with excellent performance.

The possibility of controlling the domain structure in spherical nanoparticles of uniaxial and multiaxial ferroelectrics using a shell with tunable dielectric properties is studied in the framework of Landau-Ginzburg-Devonshire theory. Finite element modeling and analytical calculations are performed for Sn2P2S6 and BaTiO3 nanoparticles covered with high-k polymer, temperature dependent isotropic paraelectric strontium titanate, or anisotropic liquid crystal shells with a strongly temperature dependent dielectric permittivity tensor. It appeared that the tunable paraelectric shell with a temperature dependent high dielectric permittivity (~300 - 3000) provides much more efficient screening of the nanoparticle polarization than the polymer shell with a much smaller (~10) temperature-independent permittivity. The tunable dielectric anisotropy of the liquid crystal shell (~ 1 - 100) adds a new level of functionality for the control of ferroelectric domains morphology (including a single-domain state, domain stripes and cylinders, meandering and labyrinthine domains, and polarization flux-closure domains and vortexes) in comparison with isotropic paraelectric and polymer shells. The obtained results indicate the opportunities to control the domain structure morphology of ferroelectric nanoparticles covered with tunable shells, which can lead to the generation of new ferroelectric memory and advanced cryptographic materials., Comment: 49 pages, 11 figures, 3 appendices

The Berreman analysis for plane wave propagation through an anisotropic metamaterial is modified to derive the transfer function matrix for propagation in such a structure. The eigenvalues of the Berreman matrix, which determine the transfer function, depend on the anisotropy. Using the transfer function, propagation of TM and TE polarized beams in hyperbolic metamaterials is analyzed theoretically, and numerically based on the developed theory. For TM polarization, self-focusing of Gaussian beams in a hyperbolic metamaterial can be analytically explained using the q-parameter approach. Furthermore, the transmission coefficient for plane wave propagation through an anisotropic metamaterial slab is extended to a transmission transfer function, which is then used to analyze the propagation of beams through hyperbolic metamaterials and the ensuing spatial shifts. For TM polarized beams, negative refraction is verified. The main objective of this paper is to provide simple analytical tools for understanding self-focusing and negative refraction in anisotropic hyperbolic metamaterials as an alternative to, and check the validity of, rigorous numerical computations. Propagation of the longitudinally (z-) polarized optical field profile is derived. The transfer function approach should be useful for the analysis of arbitrary beam profiles through composite metamaterials.

The objective of this work is to investigate the electromagnetic (EM) propagation properties (transmission and reflection) of titanium dioxide (TiO 2 )-vanadium dioxide (VO 2 ) multilayer thin film structures during the phase transition cold-VO 2 to hot-VO 2 . In this study, Berreman 4 × 4 matrix is used to explore the propagation properties of thin film structures.

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive cholesteric cell with a short-pitch helix oriented parallel to the cell substrates (so-called uniformly lying helix configuration). Weak and strong light beams incident on the hybrid cell interfere and induce a periodic space-charge field in the photorefractive substrate of the cell, which penetrates into the cholesteric liquid crystal (LC). Due to the flexoelectro-optic effect an interaction of the photorefractive field with the LC flexopolarization causes the spatially periodic modulation of the helix axis in the plane parallel to the cell substrates. Coupling of a weak signal beam with a strong pump beam at the LC permittivity grating, induced by the periodically tilted helix axis, leads to the energy gain of the weak signal beam. Dependence of the signal beam gain coefficient on the parameters of the short-pitch cholesteric LC is studied.

Photorefractive sensitivity of antimony doped Sn 2 P 2 S 6 can be increased at ambient temperature by preexposure of the sample with an intense auxiliary light beam. It is shown that the largest enhancement of sensitivity occurs if the photon energy of preexposure light is close to the crystal bandgap, it decreases gradually with increasing wavelength. The preexposure gives rise also to a pronounced transient light induced absorption which vanishes approximately one order of magnitude faster than the decay of the sensitized state.

We analyze a theoretical model of the five layer nanorod-mediated surface plasmon resonance (SPR) sensor comprising a nematic LC layer. The light propagation through the layered system is studied solving the Fresnel equations under SPR conditions. We calculate the light reflectance angular spectrum and show that control of the orientational state of the LC layer enables us to manipulate position of the reflectance curve minimums, their depth and sensitivity. It allows one to choose an interval of the light incidence angles convenient for work without need in angular tuning of sensor at replacing of the analyte.

We report experimental studies on the phase transitions and physical properties of colloidal suspensions of BaTiO3 nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4ʹ-cyanobiphenyl and 4-octyl-4ʹ-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectric anisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational viscosity decrease considerably. The experimental results are discussed based on the possible contribution of BaTiO3 nanoparticles and free surfactant molecules in the suspensions.

The Berreman matrix method has been previously used to model electromagnetic plane wave propagation through a hyperbolic metamaterial, and to determine transmission and reflection coefficients as a function of wavelength and varying angles of incidence. The Berreman matrix approach is now used to derive the propagation transfer function matrix in such materials. The eigenvalues of the Berreman matrix, which determine the transfer function, depend on the anisotropy. Beam propagation in such anisotropic materials are simulated using the transfer functions of all components of the electric (and magnetic) fields. Implications of this on negative refraction and the self-lensing of beams are explored.

We demonstrate spectrally tunable Airy beams via a chiral Bragg reflector. Specifically, we photo-align a cholesteric liquid crystal such that it conditionally imparts a cubic phase dependent on voltage applied across the sample.

This paper reports the electro-optical properties of high resistivity nematic liquid crystals sandwiched between ferroelectric polymer films. Interactions between liquid crystals and the film result in a series of interesting optical and electro-optical features. For example, the visualization of ferroelectric domains by means of liquid crystals has been known for decades. However, here we demonstrate that liquid crystals can also reveal the fractal dimension of multi—domain poly(vinylidene fluoride)-based films. Unidirectionally rubbed films made of poly(vinylidene fluoride)-based (PVDF) materials align liquid crystals (LC) homogeneously, with the pretilt angle on the order of 1–2 degrees. This property was implemented in the design of hybrid cells composed of liquid crystals sandwiched between PVDF-based films. The designed PVDF|LC|PVDF cells exhibit tunable electro-optical performance originating from the presence of the PVDF-based films. More specifically, (i) the threshold voltage characterizing the transition of liquid crystals from a planar to a homeotropic state can be tuned by varying the film thickness, and (ii) total fall time (turn-off time) can be controlled by varying the frequency and amplitude of the driving voltage. This frequency dependence of the fall time is strongly pronounced at a relatively high voltage applied across the cell. In the low frequency regime, an increase in the turn-off time can be approximated as a linear function of the applied electric field. An electric-field induced polarization of the PVDF-based films is considered a major reason leading to the afore-mentioned amplitude and frequency dependence of the switching time.

Exciting experimental results on the response properties of hybridized photo responsive liquid crystal test cells are reported, where iron doped lithium niobate substrates were used to photo generate electric fields and indium tin oxide coated cover glasses were used to confine these photo generated fields in a liquid crystal layer. Samples were investigated in a modified inverted optical polarizing microscope with white probe light (crossed polarizers) and exposed with a Gaussian laser beam focused to a small spot (14 μm FWHM). Test cells filled with nematic LC showed homeotropic director alignment. Upon exposure, this alignment was maintained at the exposure spot center and the LC director was selectively realigned in a surrounding single ring. This ring had a thickness of a few microns and its diameter increased with increasing exposure intensity (112 μm at 0.7 mW, 204 μm at 1.1 mW). This characteristic director realignment was traced back to the optically generated electric field distributions by simulations. In samples filled with chiral nematic LC, uniformly standing helix alignment was found. Textural transitions were induced at the focus position, which again led to the formation of well-defined circular defects. We could show that these defects can be permanently stored within the chiral nematic LC. Polarized optical microscopy of a rotated sample revealed that a point like defect with +1 topological charge was enclosed in each of these defects. Photovoltaic fields generated in small lithium niobate particles dispersed in a LC were found to cause promising optical responses and particle movement.

Light-induced modulations of the refractive index and pattern formation are desirable to generate complex photonic structures via exposure to light. Here we show that local modulations of the effective refractive index and reconfigurable defects can be locally induced in a hybridized thin birefringent film of a nematic liquid crystal (LC) on a photoresponsive (generating photoinduced electric fields) iron doped lithium niobate surface via exposure to a focused laser beam. Samples were studied with a tailored imaging approach, which provided the ability to investigate these optically excited, field-induced responses on a microscopic level. Upon exposure with a focused laser beam, the fluent LC was expanded on the substrate's surface and localized field-induced defects were optically created. Both umbilic (central) and line defects were observed. The formation of field-induced umbilic defects was modeled in numerical simulations. In addition, line defects were experimentally studied. It was seen that line defects interconnected the centers of two central defects (field-induced defects, which were present at the upper and lower surfaces of the LC layer). In addition, line disclinations separating reverse tilt domains (caused by the inhomogeneous distribution of the photogenerated fields) were seen. These line disclinations were pinned to the central defects. By exposure with two adjacent focused laser beams two umbilic defects were created side by side and interconnected with a line defect (the line defects pinned to each umbilic defect were joined in a single defect line). An alternative technique is presented to field-induce promising photonic motives (microlenses, resonators, line defects) in a liquid crystalline, hybridized birefringent film on a microscopic scale by using a low-power laser (opposed to the high power necessary to induce optical Kerr responses in a neat LC).