Anisotropic Optical Absorption Research Articles

Anisotropic hole subband states and interband optical absorption in

Using a multiband k\ensuremath{\cdot}p theory, we have analyzed the anisotropic hole subband states and interband optical absorption in [mmn]-oriented GaAs/AlAs quantum wells (QW's). For QW's grown in the [001] and [113] direction, the spacing of hole subbands and the slope of the subband dispersion is considerably larger than that for QW's grown in the [110] direction. Anisotropy of the dispersion is the largest for [110] grown QW's. We calculate the interband optical absorption due to free electron-hole transitions. While basic features of the absorption coefficient \ensuremath{\alpha}(\ensuremath{\omega}) are independent of the growth direction, we find that the in-plane anisotropy of \ensuremath{\alpha}(\ensuremath{\omega}) differs strongly for [110] and [113]. It is opposite in sign for heavy hole and light hole transitions. Our calculations are in good agreement with available experimental data. They indicate that in [113]-oriented GaAs/AlAs structures, the corrugation of the interfaces has a small effect on the optical anisotropy observed experimentally at the fundamental absorption edge. \textcopyright{} 1996 The American Physical Society.

Optical properties in fractional-layer-superlattice quantum wires calculated by multi-band effective mass theory

We present the results of theoretical studies numerically analyzing the optical absorption and gain properties in AlGaAs fractional-layer-superlattice (FLS) quantum wires. With our calculation method we can treat any asymmetrical FLS wire with arbitrary dimensionality from 2D to 1D and to calculate not only absorption spectra but also gain properties. Our theory accurately reproduces the optical absorption anisotropy, experimentally evaluated for an AlGaAs FLS quantum wire. It is numerically shown how optical features evolve as the quantum confinement changes from 2D to 1D by varying the FLS lateral modulation. The small modulation of Al content in the AlGaAs FLS layer is found to change the optical properties markedly and improve the gain characteristics largely.

Direct measurement of the optical absorption in YBa 2Cu 4O 8

In this work we present direct measurements of the anisotropic optical absorption of YBa 2Cu 4O 8. By measuring the photon energy dependent response of a composite belometer with the sample as absorber and its comparison with the characteristics of a known bolometer we are able to deduce the absorptivity (and thus also the reflectance) of the investigated sample extremely accurately.

Large CoAPO-5 single crystals: Microwave synthesis and anisotropic optical absorption

Large CoAPO-5 crystals were synthesized by microwave heating of a fluoride-containing aluminophosphate gel. By polarization microscopy and polarization-dependent single-crystal ultraviolet—visible (UV—VIS) spectroscopy, the as-synthesized crystals show an optical anisotropy of light absorption. It is proposed that this effect originates from structure-related symmetry distortions of the tetrahedral Co2+ environment in the AFI framework. Consequently, the effect of anisotropic optical absorption can be taken as a proof for the incorporation of cobalt in the AFI framework.

The density of states and anisotropic optical absorption of a disordered polymer film

Motivated by a three-dimensional LDA band structure calculation for trans-polyacetylene (PA) we construct an effective two-dimensional model for a film of conjugated polymers. In real materials a non-perfect structural order is found. It has its origin in the finite conjugation length of single chain segments, impurities and crystalline defects. We model these effects through a fluctuating contribution to the interchain hopping amplitude. Using a supersymmetric path integral formalism we obtain the averaged one-particle Green function giving the electronic density of states (DOS) and furthermore the current correlation function determining the optical absorption coefficient and DC conductivity. As a consequence of the interchain coupling we find a broad shoulder at the band edge in the DOS and the optical absorption. The main effect of disorder is seen in a broadening of the interband transition peak and an increasing intraband absorption. The anisotropy in the absorption coefficient parallel and perpendicular to the chain direction allows us to fix the interchain coupling strength; the line shape contains information about the effective disorder strength.

Optical anisotropy and columnar structure of obliquely deposited thin films containing fine metal particles ag

Anisotropic optical properties and nanostructures of thin films prepared by oblique co-deposition of Ag and SiO 2 were studied. At low concentrations of Ag, a columnar structure grew towards the direction of the incident SiO 2 vapor and small Ag particles were embedded in that structure. An inhomogeneous distribution of Ag was observed in cases where the Ag was deposited from the opposite side of the surface normal to the direction of the incident SiO 2 vapor. This inhomogeneous distribution of Ag particles gives rise to optical anisotropy, i.e., the optical transmittance of p-polarized light varies drastically depending on the angle of incidence, while that of s-polarized light remains unchanged. A possible application of the anisotropic optical absorption properties of AgSiO 2 hybrid films is in the manufactur of an inorganic thin film polarizer.

Anisotropy in the optical absorption of Ag–SiO2 thin films with oblique columnar structures

An attempt has been made to clarify the anisotropy in the optical absorption and nanostructure of thin films prepared by oblique codeposition of Ag and SiO2. At a low concentration of Ag, the columnar structure grows toward the incident direction of vapor flux of SiO2, and Ag is dispersed in that structure as small metallic particles. An inhomogeneous distribution of Ag particles is obtained when Ag is deposited from the side of the normal to the film surface opposite to that of the incident direction of SiO2 flux. The optical transmittance is strongly dependent on the angle of incidence and the polarization of the incident light due to the effects of the small Ag particles embedded in the oblique columnar structure. The anisotropic optical absorption of Ag and SiO2 hybrid films can be used as an inorganic polarizer.

Angular-dependent photoselection and anisotropy of optical absorption for photochromic molecules in glassy poly(methyl methacrylate)

It is shown that films of poly(methyl methacrylate) which contain the thermally-stable photochrome 7,7a-dihydro-2,4,7,7,7a-pentamethylbenzo-[b]-furan-5,6-dicarboxylic anhydride II may be rendered optically dichroic using polarized laser light (λ = 514·5nm). The dichroism is produced by angular-dependent photoselection of photochromic molecules in the photoconversion of photochrome II to fulgide I. The absorption anisotropy has been measured as a function of the angle between the planes of polarization of the pump and probe beams, and it is found to be time-invariant for this photochrome/glassy polymer system at room temperature. It is suggested that the time-dependence of absorption anisotropy of thermally-stable photochromic molecules in host polymer or liquid media should provide a direct means of studying their reorientational motions, and should be applicable over the time-range 10-11 to 10+3 s.

The 〈110〉 oriented FCl− molecule–ion in KFa)

The heteronuclear diatomic molecule–ion FC1− with its molecular axis oriented along a crystal face diagonal, i.e., FC1− 〈110〉, has been produced by ionizing radiation in single crystal KF doped with PbCl2 and KC1. Evidence to support this species identification as well as its orientation relative to the lattice is obtained from anisotropic optical absorption spectral measurements as well as from electron spin resonance (ESR) absorption spectra. Optical absorption spectral data, estimates of the molecular reorientation temperature, as well as ESR absorption spectral parameters are given for the FCl− 〈110〉. A qualitative explanation of the differences of the optical and ESR parameters of FCl− 〈111〉 and FCl− 〈110〉 is discussed.

Spectroscopic standard for tetrahedrally coordinated ferric iron: ? LiAlO2:Fe3+

γ LiAlO2 doped with Fe3+ in the tetrahedral site has been examined by extended X-ray absorption fine structure (EXAFS) analysis, and Mossbauer and optical spectroscopy. The isomer shift (IS) is −0.026 mm/s (Fe-Pd); the quadrupole splitting (QS) is 0.62 mm/s. Anisotropic optical absorption is prominent at ∼391, 452, and 463 nm. The K-edge absorption spectrum shows a prominent absorption near 7,113 ev typical of tetrahedrally coordinated Fe3+.

Fundamental Optical Absorption and Valence Bands in Alkali Silver Halide Crystals

Anisotropic fundamental optical absorption bands have been found at 4.4 eV in K 2 AgI 3 and Rb 2 AgI 3 , and at 4.9 eV in Rb 2 AgBr 3 . The X-ray photoelectron spectrum of Rb 2 AgI 3 indicates that the uppermost valence band is the p-band of halogen. Optical absorption spectra are interpreted in terms of transitions of the charge transfer type from the halogen to the silver ion. The photo-excitation in the fundamental absorption region have been observed to induce luminescence and a radiation damage effect at low temperature.

Electronic properties of polymers. Anisotropic light absorption and photoelectron emission of oriented polyethylene films in the vacuum ultraviolet region

Anisotropic optical absorption of oriented ultrathin films, (down to 50 nm thickness) of low density polyethylene (LDPE) was measured in the vacuum ultraviolet (VUV) region between 6 and 12 eV. Clear anisotropy was found in the observed spectra. The lowest-energy absorption feature at 9 eV was polarized perpendicular to the chain axis, and the second at about 11.5 eV parallel to the chain axis. The edge of the lowest perpendicular absorption was determined to be 8.05±0.1 eV. The usefulness of the anisotropic absorption data in testing two theories was verified; the anisotropy predicted by the independent system model disagrees with the data, but the energy band model gives some promising results. The photoemission threshold of the oriented LDPE was determined to be 8.5±0.1 eV. The spectral dependence of the anisotropy ratio of the photoemission quantum yield was found to coincide with that of the optical absorption in the energy range above 9.5 eV. This coincidence was explained in terms of the three-step model of photoemission.

Optical vibronic absorption spectra in 14.8 MeV neutron damaged sapphire

Preliminary observations of several anisotropic optical absorption bands are reported for high-purity sapphire following exposure to a pure 14.8 MeV neutron beam. These bands are compared with and found to be similar to those reported following fission-spectrum exposure, and are associated with lattice displacement damage. Two of these bands at 3.47 and 4.09 eV show similar sharpline features at low temperature; this ≈200 cm −1 vibronic progression it tentatively associated with a local mode.

Infrared dichroism of glassy polycarbonate at small strains

AbstractThe dichroism of the 889, 1364, and 3063 cm−1 infrared absorption bands of glassy, amorphous polycarbonate has been measured as a function of the strain in the range 0 to 2% at 23°C. The data obtained for these three bands superpose rather well over this strain range. Negligible dichroism is observed up to about 0.6% strain; above this level, the dichroic ratio increases in an approximately linear manner. Independent mechanical data, obtained under comparable conditions of time‐scale and temperature, are cited which show that a transition from approximately linear to marked non‐linear viscoelastic behavior occurs with glassy polycarbonate in the range 0.7 to 1.0% strain. The coincidence on the strain axis of a relatively abrupt increase in optical absorption anisotropy with a distinct change in Young's modulus is discussed in terms of a recent molecular theory of deformation of glassy polymers. It is suggested that the data are consistent with the view that the transition from linear to nonlinear viscoelastic behavior in glassy polycarbonate is marked by the onset of significant rotation around backbone bonds.

High-sensitivity linear dichroism as a tool for equilibrium analysis in biochemistry. Stability constant of DNA--ethidiumbromide complex.

A stoichiometrical application of a sensitive method for linear dichroism (LD) detection is suggested for biochemical purposes. The complex formation between a binding site on a polynucleotide and a ligand may be studied with high precision if the following conditions are fulfilled: (1) The polymer can be given a fixed degree of orientation. (2) The site has a specific orientation with respect to the orientation axis of the polymer (e.g., intercalation). (3) The ligand has an anisotropic optical absorption property. The method was applied to studying the complex between DNA and ethidiumbromide, which was detected by LD with precision of +/- 0.5 X 10(-7) M in a 4 X 10(-4) M DNA solution, i.e., 0.1% occupation of the total site concentration can be detected. The complexation could be explained by a single type of site (n = 0.14 +/- 0.01 sites per nucleotide residue) and a stability constant K1 = (2.5 +/- 1) X 10(5) M-1 at 0.2 M ionic strength. From the specific LD an average angle 60 degrees was concluded between the helix axis and the long axis of the ethidiumbromide molecule. This value formally contradicts the Watson-Crick model or the intercalation model but may be explained by extension and deformation effects on the xhain by the flow.

Pulsed electric dichroism studies of the amylose tri-iodide complex

The anisotropic optical absorption of macromolecules can be studied, when the molecules are in dilute solution, by subjecting the system to pulsed electric fields. Measurements were made on solutions of six amylose samples of varying molecular weights ( M) in the range 1.4 × 10 5 to 1 × 10 6, after the biopolymer had been complexed with iodine. The rates of molecular orientation enabled values of τ, the rotary relaxation time, to be obtained. When analysed, these indicated molecular flexibility. The “weakly-bending rod” model appeared to satisfy the experimental data, although a single value of the persistence length ( q) did not describe the behaviour. As found in previous studies on other helical polymers, q varied continuously with M. The electrical properties and the degree of orientation in the field indicated that only induced and not permanent dipole moments were present. These induced dipole moments were analysed in terms of an anisotropy of the molecular polarisability. This was consistent with the model of a flexible helix. A value of 28 × 10 −24 F m was evaluated for the excess polarisability difference per helical pitch. Values of the molecular extinction coefficients for light of wavelength 625 nm when polarised parallel to and perpendicular to the individual helix axes, indicated that the relevant electronic transition is not confined to the long helical axis as had been previously assumed.

1031. Investigation of anisotropy of optical absorption in metallic films: V S Prokopenko and V M Gzogyan,Izv VUZ Fiz, No 9, 1971, 131– 133 ( in Russian)

1031. Investigation of anisotropy of optical absorption in metallic films: V S Prokopenko and V M Gzogyan,Izv VUZ Fiz, No 9, 1971, 131– 133 ( in Russian)

Anisotropy of optical absorption in metal films deposited on a dielectric substrate

Anisotropy of optical absorption in metal films deposited on a dielectric substrate

Investigation of anisotropy of optical absorption in metal films

Investigation of anisotropy of optical absorption in metal films

Coupling of Electromagnetic Waves in CdS

A theoretical and experimental study is made of the coupling of the ordinary and extraordinary electro-magnetic waves in CdS. The coupling, which may be induced by an external magnetic field or stress, occurs only at energies near the accidental crossing of the index-of-refraction curves ${n}_{0}(E)$ and ${n}_{e}(E)$. This effect was pointed out by J. J. Hopfield and D. G. Thomas, who showed that this condition, when combined with the anisotropic optical absorption present in the crystal, leads to a discrete absorption line. The oscillatory behavior of the coupled energy and the magnitude of these absorption effects are quantitatively explained. These effects can be used to produce a unique type of optical filter which may be either band-pass or band-reject. Finally, the Faraday rotation in CdS normal to the optic axis is measured at the isotropic point.