Unusual Optical Behavior Research Articles

A multiyear photopolarimetric study of the semi-regular variable V CVn and identification of analog sources

The semi-regular variable star V Canum Venaticorum (V CVn) is well known for its unusual linear polarization position angle (PA). Decades of observing V CVn reveal a nearly constant PA spanning hundreds of pulsation cycles. This phenomenon has persisted through variability that has varied by two magnitudes in optical brightness and through variability in the polarization amplitude over 0.3 and 6.9%. Additionally, the polarization fraction of V CVn varies inversely with brightness. This paper presents polarization measurements obtained over three pulsation cycles. We find that the polarization maximum does not always occur precisely at the same time as the brightness minimum. Instead, we observe a small lead or lag in relation to the brightness minimum, spanning a period of a few days up to three weeks. Furthermore, the PA sometimes exhibits a non-negligible rotation, especially at lower polarization levels. To elucidate the unusual optical behavior of V CVn, we present a list of literature sources that also exhibit polarization variability with a roughly fixed PA. We find this correlation occurs in stars with high tangential space velocities, for instance, “runaway” stars, suggesting that the long-term constant PA is related to how the circumstellar gas is shaped by the star’s high-speed motion through the interstellar medium.

Insights into the Optical Properties of Triarylboranes with Strongly Electron‐Accepting Bis(fluoromesityl)boryl Groups: when Theory Meets Experiment

AbstractThe photophysical properties (absorption, fluorescence and phosphorescence) of a series of triarylboranes of the form 4‐D−C6H4−B(Ar)2 (D=tBu or NPh2; Ar=mesityl (Mes) or 2,4,6‐tris(trifluoromethylphenyl (Fmes)) were analyzed theoretically using state‐of‐the‐art DFT and TD‐DFT methods. Simulated emission spectra and computed decay rate constants are in very good agreement with the experimental data. Unrestricted electronic computations including vibronic contributions explain the unusual optical behavior of 4‐tBu−C6H4−B(Fmes)2 2, which shows both fluorescence and phosphorescence at nearly identical energies (at 77 K in a frozen glass). Analysis of the main normal modes responsible for the phosphorescence vibrational fine structure indicates that the bulky tert‐butyl group tethered to the phenyl ring is strongly involved. Interestingly, in THF solvent, the computed energies of the singlet and triplet excited states are very similar for compound 2 only, which may explain why 2 shows phosphorescence in contrast to the other members of the series.

Shape memory properties and unusual optical behaviour of an interpenetrating network of poly(ethylene oxide) and poly(2‐hydroxyethyl methacrylate)

AbstractAn interpenetrating polymer network (IPN) with shape memory properties was prepared by using poly(2‐hydroxyethyl methacrylate) (PHEMA) and poly(ethylene oxide) (PEO). PHEMA acts as a fixed phase and PEO as a switching phase. The switching action of PEO is due to the reversible process of melting and crystallization. It was observed that the shape recovery of the IPN increases with increasing crosslinker concentration up to an optimum value and decreases thereafter. In addition to the shape memory property, the IPNs show a reversible change in optical properties from translucent to opaque. The change in optical properties is quite different from that observed in a semicrystalline polymer system where the transparency increases as a result of the melting of crystals. This behaviour of the IPN is explained in terms of H‐bonding of PEO with PHEMA. Fourier transform infrared spectroscopy was used to study the H‐bonding between PEO and PHEMA. © 2019 Society of Chemical Industry

Imaging Analysis and Digital Restoration of the Holy Face of Manoppello—Part I

The Veil of Manoppello is an icon of the face of Christ (Holy Face). Its particular characteristic is being semitransparent. The face is visible on both sides (front–back) and, depending on the lighting and observation conditions, shows some differences in the anatomical details. An analysis of this icon has allowed us to clarify some aspects of the possible physical mechanism underlying its unusual optical behavior. It is a linen fiber fabric consisting of very thin threads with a thickness of about 0.1 mm, separated by distances even double the thickness of the threads, so that about 42% of the Veil is empty space. The fibers constituting the linen threads may have been cemented by an organic substance of chemical composition similar to cellulose, presumably starch, eliminating the air between them. Such a structure causes the optical behavior of the medium to be intermediate between those of a translucent medium (thin cemented linen threads) and a transparent one (empty space between the threads). The problem of digital image restoration in spatial terms has also been tackled, since the Holy Face is deformed due to distortions of the meshes of the Veil, caused by the yielding of the very fine structure of the fabric.

Dynamic‐ and Light‐Switchable Self‐Assembled Plasmonic Metafilms

AbstractLarge‐area dynamic switchable plasmonic metafilms composed of a network of gold nanoparticles (Au NPs) and the thermo‐responsive polymer poly(N‐isopropylacrylamide) (PNIPAM) are self‐assembled by casting Au@PNIPAM core–shell nanoparticles. These dense plasmonic films present an unusual optical behavior arising from the macroscale effective medium response of the film and the microscopic interactions of the gold nanoparticles. Such intensely colored plasmonic metafilms can be thermally actuated across large areas via direct heating, allowing extraction of the tuning of the effective dielectric permittivity of this metamaterial. By contrast, local plasmonic heating by illuminating the Au NPs in these metafilms enables sub‐second and highly reversible contraction of the plasmonic metafilms, capable of pumping water. This work provides a paradigm for switchable metafilms and metasurfaces that can be harnessed for different applications such as sensing, imaging optics, and image displays.

Hydrocarbon alteration in the bituminous salt of the Kłodawa Salt Dome (Central Poland)

The so called bituminous salts occurring in the Kłodawa dome, located in Central Poland, differ from the surrounding salts by their colour change from light to dark brown. This colour is associated with an extremely large amount of hydrocarbon, mainly located in the inclusions. The presence of numerous fluid inclusions has been documented in previous petrologic studies, distinguishing seven main types of fluid inclusion assemblages (FIA) in terms of size, shape as well as the ratio of filling material. However, four types of inclusions were selected in the current investigations according to their unusual optical behaviour. Raman micro-spectroscopy a modern, non-destructive method was used for investigating a single inclusion being a part of FIA. Presented in this paper Raman spectra revealed a unique pattern of bands characterizing the content of the inclusions. The hydrocarbons show a very complex character reflected in the appearance of a strong fluorescence background. A well-marked heterogeneity characterized the inclusions, by diversity in the intensity of the background and in the pattern of the bands characterizing the presence of certain components. One can distinguish the presence of carbonaceous matter showing the different degrees of order. The depth profile and the analysis of the various points of the inclusions indicate that the carbonaceous matter is not evenly distributed in the inclusions but forms a thin, disorganized film on their walls. This film was also found in sites where the inclusions are filled with brine. The certain characteristics associated with the presence of the incipient phase transformation of the organic matter, or slightly transformed organic matter and the lack of light hydrocarbons as well as a number of petrologic features of inclusions indicate that these salt rocks have been subdivided into thermal transformations, accompanied by the recrystallization of the halite and the escape of the more volatile compounds such as methane, ethane, etc.

Effects of the d-donor level of vanadium on the properties of Zn1−xVxO films

We report the effect of d-levels of vanadium atoms on the electronic band structure of ZnO. Polycrystalline layers of Zn1−xVxO with 0 ≤ x ≤ 0.08 were synthesized using magnetron sputtering technique. Electrical measurements show that electron concentration increases with vanadium up to x = 0.04 and then decreases and films become insulating for x > 0.06. Optical characterization reveals that the absorption edge shifts to higher energy, while the photoluminescence (PL) peak shows a shift to lower energy with increasing vanadium content. This unusual optical behavior can be explained by an anticrossing interaction between the vanadium d-levels and the conduction band (CB) of ZnO. The interaction results in an upward shift of unoccupied CB (E+) and the downward shift of the fully occupied E− band derived from the vanadium d-levels. The composition dependence of optical absorption edge (E+) and PL peak (E−) can be fitted using the Band Anticrossing model with the vanadium d-level located at 0.13 eV below CB of ZnO and a coupling constant of 0.65 eV.

Controlling the structure and rheology of polyimide/nanoclay composites by condensation polymerization

AbstractA novel in situ condensation polymerization method for controlling the structure and dispersion of clay in the polyimide (PI) matrix has been investigated. Initially, the viscosity of the polymerizing poly(amic acid) (PAA) solution increases rapidly with increasing shear rate and polymerization time. In the presence of clay, the PAA solution viscosity became significantly higher than that for the neat PAA solution at initial polymerization stage; however, it attained a final value, which was significantly lower than that for a neat PAA solution after a long polymerization time. The PAA/clay solution synthesized by in situ polycondensation reaction also showed an interesting optical activity between 500 and 600 nm. The PAA/clay solution prepared by a two‐step method involving polymerization followed by the addition of clay shows no unusual optical behavior above 400 nm. The imidization of PAA is enhanced in the presence of clay. The imidization temperature decreased significantly in the presence of clay, and the extent of imidization increased with increasing amount of clay and cure temperature. X‐ray diffraction and microscopy studies show the existence of exfoliated clays at low concentration of clay and at high curing temperatures. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Fluorescence enhancement for the complex PAMAM–BSA in the presence of photonic crystal heterostructures

The paper presents the optical characterization of the multilayer film composed of styrene (ST)–poly(ethylene glycol) methyl ether methacrylate (PEGMA 1100)–gold nanoparticles (Au)–poly(amidoamine) PAMAM (G4)–bovine serum albumin (BSA). The addition of the last layer composed of BSA resulted in an unusual optical behaviour, i.e. increase of the fluorescence emission intensity, respectively the intensity of the UV–vis reflection, compared with the ST–PEGMA 1100–Au–G4 film. The explanation could be attributed to the presence of photonic crystal heterostructures. The multilayer film has been characterized by optical microscopy, AFM, UV–vis, and fluorescence.

Influence of nanoparticles on elastic and optical properties of a polymeric matrix: Hypersonic studies on ethylene–vinyl alcohol copolymer–titania nanocomposites

High resolution Brillouin spectroscopy (HRBS) backscattering elastic data in nanocomposites of ethylene–vinyl alcohol copolymer (EVOH) and TiO 2 nanoparticles present anomalous dependence with concentration, while Young’s modulus and microhardness data show the expected behaviour. When performing HRBS with the 90A scattering geometry to asses the effective elastic constant, the expected behaviour for low concentration of TiO 2 nanoparticles is again obtained. This unusual disagreement can be solved assuming that the inclusion of TiO 2 nanoparticles induces anomalous refractive index behaviour at the applied laser wavelength for the different EVOH–TiO 2 nanocomposites. Comparison with experimental elastic and optical data obtained in isotactic-polypropylene–TiO 2 nanocomposites proves that EVOH–TiO 2 nanocomposites show an unusual optical behaviour at the laser wavelength, possibly due to a singular bonding between the EVOH polymer and the TiO 2 nanoparticles.

Innocent or guilty? Redox activity in and magnetic and optical behaviour of dinuclear molydenum complexes

The phenomenon of ‘non-innocence’, first articulated by Jørgensen in 1966, is briefly reviewed. Spectroelectrochemical studies of a range of dinuclear complexes of the type [Mo(NO)Tp*Cl2(bridge)] (bridge = dipyridyls) and [Mo(O)Tp*Cl2(bridge)] (bridge = diphenolates) which are redox active, show that oxidised or reduced forms of these species exhibit ‘non-innocence’. The spectral behaviour is associated with metal-to-ligand or ligand-to-metal charge transfer phenomena, probably the first time that monodentate bridging ligands have been implicated in ‘non-innocent’ behaviour. These bridging ligands also determine the nature and extent of magnetic interaction between the unpaired spins in [Mo(NO)Tp*Cl2(bridge)] and [Mo(O)Tp*Cl2(bridge)], the dominant mechanism of spin-exchange relying on the extent of {ie291-01}-delocalisation within the bridging ligands. The unusual optical behaviour of these dinuclear complexes when oxidised (oxomolybdenum diphenolates) or reduced (nitrosyl molybdenum dipyridyls) has led to the exploration of electrochromism as a means to develop variable optical attenuators operating in the near-infrared region.

Quantum-sized Au structures on Si(111) studied by spectroscopic ellipsometry

Spectroscopic ellipsometry has been used to follow the initial stages of the growth of Au on both singular and vicinal Si(111) surfaces. The imaginary part of the pseudo dielectric function shows an distinctive sharp peak at 2.7 eV, at the point where the single-domain pseudo-5 × 1 low energy electron diffraction pattern is strongest for Au growth on the stepped Si(111) surface. The singular system behaves differently. The unusual optical behaviour of the Au-induced 5 × 2 reconstruction is attributed to the recently-discovered quasi one-dimensional electronic structure of this system.

Real-time observation of atomic ordering in (001) In0.53Ga0.47As epitaxial layers.

We report the first real-time observation of atomic ordering in a semiconductor alloy during epitaxial growth. (001) ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}$As layers deposited below about 400 \ifmmode^\circ\else\textdegree\fi{}C exhibit a strong anisotropy in the broadening parameters of the ${E}_{1}{,E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions that is observable with reflectance-difference spectroscopy. This unusual optical behavior results from intraband mixing driven by triple-period ordering along both [111] and $[11\overline{1}]$ directions simultaneously. This microstructure is realizable with stoichiometry ${\mathrm{In}}_{0.556}{\mathrm{Ga}}_{0.444}\mathrm{As}$ virtually identical to that needed to lattice match InP.

The crystal structure of 1,3,5-triammo-2,4,6-trimtrobenzene: Centrosymmetric or non-centrosymmetric?

The crystal structure of the title compound (TATB) has been determined as centrosymmetric (space group P1̄, Z = 2) by X-ray diffraction, and yet the solid substance displays non-negligible second-harmonic generation. The contradiction is analyzed by means of potential energy calculations for the crystal packing of TATB, using a set of newly calibrated empirical potentials for the NO·H-N hydrogen bond. It is shown that two non-centrosymmetric (P1) crystal structures are energetically similar to the one with P1̄, Z = 2, and that libration or reorientation of the whole molecule in its plane within the crystal may be possible, especially at high temperatures. An explanation of the unusual optical behaviour of TATB is attempted in the light of these results.