Two-photon absorption and energy band structure of orthorhombic Hg2Cl2 crystals

Dependence of two-photon absorption (TPA) rate on the state of polarization of a laser beam is investigated in the low-temperature orthorhombic modification of Hg₂Cl₂ crystals. Theoretical calculations of the dependence of TPA rate on the direction of polarization vector of the beam are performed for centrosymmetric points, Y, Z, T, R andS of the Brillouin zone. The domain structure of real crystals is taken into consideration and it is shown that periodicity of the polarization dependence may indicate whether TPA is due to transitions either at, Y, Z, T orR, S points. The polarization dependence of TPA cannot, however, distinguish between points inside these two groups. Comparison of theory with a low-temperature (T 8·5 K) experimental curve of polarization dependence is discussed. It is shown that the experiment can be explained in main features by a model of noninteracting oriented linear dipoles. Further, on the basis of TPA measurements, a simple energy band structure of Hg₂Cl₂ is proposed regarding Hg₂Cl₂ crystal as a linear chain of molecules.The authors express thanks to Dr. . Barta for supplying the Hg₂Cl₂ crystals, Dr. Z. Bryknar for critical comments and Dr. B. Velický for stimulating discussion. We thank also Dr. V. Kohlová for assistance during measurement.     

Elastic constants,the ΔE effect,and retarded polarization processes in a ferroelectric niobate ceramic

A resonant method was used to determine the ultrasonic velocity in circular plates of a ferroelectric niobate ceramic having the composition (Pb_0.57Ba_0.43)Nb₂O₆, over the frequency range 3–9 MHz at 19 °C. The average values of the elastic constants are calculated for unpolarized samples. A study is made of the Young's modulus (the E effect) and of the polarization vector P as functions of the polarizing field and the time for which this field is applied. The E effect and P, which increase monotonically but nonlinearly with increasing , approach limiting values as functions of the polarization time in different manners. A comparative analysis of the E and polarization curves shows that the domain structure can be thought of as slightly different from that found from a scheme proposed for its behavior during the polarization process.Translated from Izvestiya VUZ. Fizika, No. 7, pp. 61–65, July, 1970.     

Design of polarization independent InGaAs/InGaAlAs coupled quantum well structure in 1.55 μm wavelength region

A novel polarization independent InGaAs/InGaAlAs quantum well (QW) structure in the 1.55 m wavelength region is proposed. A coupled QW structure with tensile strain in the QW and/or barrier region is considered for the reduction of the optical gain difference between TE and TM modes in the wide spectral range. A triple-coupled QW structure with alternative strain (tensile/compressive/tensile) is found to be the most effective in reducing the polarization gain difference. This is because the transition strength difference of each polarization is reduced by energy states coupling. The optimized triple-coupled QW structure shows polarization independence for wide carrier density and wavelength range, which is suitable for polarization independent operation of QW based semiconductor devices, such as semiconductor optical amplifiers.     

Magnetooptical light concentration by regular domain structures

Expressions are obtained for the diffracted field in the Fresnel and Fraunhoffer diffraction domains. It is shown that the field in the diffraction pattern is the superposition of two orthogonally polarized fields. The distribution of a field with polarization orthogonal to the incident radiation depends substantially on the domain structure parameters. Thus, for zeros of the function f ()=J₀((/)) assigned as radii of the annular domains, a magnetooptical light concentration in an annular image is possible, while for f()=cos(²/2R ₁ ² ) the light is concentrated at a point, i.e., the annular domain structure plays the part of a lens. The effects of light concentration are examined for both the case of annular systems and strip domains.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 31–38, October, 1978.     

Higher-order polarizations on the Virasoro group and anomalies

In a previous paper the authors showed that the space of (first order) polarized functions on the Virasoro group is not, in general, irreducible. The full reduction was explicitly achieved by taking the orbit of the enveloping algebra through the vacuum. This additional step provided the proper quantization in the strong-coupling domain 0<c1. In this paper we introduce the concept of higher order polarization as a generalization of that of polarization. We prove that the imposing of the additional (higher-order) polarization conditions is equivalent to the taking of the above-mentioned orbit. This demonstrates that the generalized (higher-order) polarization conditions suffice to obtain the irreducible Hilbert spaces. We also discuss the need for higher order polarizations in terms of anomalies.Research partially supported by the Comision Interministerial de Ciencia y Tecnologia (CICYT)     

Fast Room Temperature Detection of State of Circular Polarization of Terahertz Radiation

We report on a room temperature detector which allows to determine and monitor the state of polarization of terahertz radiation with picosecond temporal resolution. The detector is based on the circular photogalvanic effect recently observed in GaAs/AlGaAs quantum wells. The circular photogalvanic effect yields in response to elliptically polarized radiation a current signal proportional to the degree of circular polarization. The peak current signal occurs in unbiased samples for circular polarization, vanishes at linear polarization and changes sign by switching the helicity from right-handed to left-handed. The detector consists of a (113)A MBE grown pGaAs/AlGaAs multiple quantum well structure. The response has been measured in the wavelength range between 76 m and 280 m at normal incidence of the radiation on the sample.     

Domain structure in ferroelectric PbNb2O6

Single crystals of ferroelectric PbNb₂O₆ were grown employing a modification of the technique of Goodman. The results obtained on the domain structure were analysed and compared with those reported on BaTiO₃ and KNbO₃. The domain structure observed here corresponds to the twinning on (110) plane of the unit cell reported by Francombe and Lewis or the subcell reported by Labbe and others. The (001) planes were observed, as the crystal habit is such as to produce (001) planes, and the cleavage plane is also (001). Also the analysis of the observations can be done easily under these conditions. The domains observed are 90° domains with polar axis in (001) plane. Wedge shaped domains and spikes are present as in BaTiO₃ and KNbO₃. The twinning can occur also on plane producing a domain line at 90° with that due to twinning on (110). This gives patterns of perpendicular lines similar to those in KNbO₃ and BaTiO₃. Crystal structure considerations show that the domain structures with polarization in and out of the observed (001) plane are not possible, and also were not observed. In this sense, it is a two dimensional ferroelectric. The studies showed a peculiar grain structure in the crystals, and it can be explained on the basis of the growth habit of the crystal. The polarizing microscope is particularly useful in analysing the domain structure along with the grain structure.     

Investigation of the mechanism of polarization switching in ferroelectric capacitors by three- dimensional piezoresponse force microscopy

A mechanism for the switching behavior of (111)-oriented Pb(Zr,Ti)O₃-based 1×1.5 m² capacitors has been investigated using three-dimensional piezoresponse force microscopy (3D-PFM). A combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM) has been used to map the out-of-plane and the in-plane components of the polarization. The three-dimensional polarization distribution was reconstructed by quantitative analysis of the PFM amplitude images of poled PZT capacitors while taking into account contrast variations in the PFM phase images. The switching behavior of the capacitors was determined by comparison of the static domain patterns in the same capacitors after both positive and negative poling. While 180° degree switching was observed, surprisingly, the switching process was dominated by 90° polarization vector rotation. Furthermore, central regions of the capacitors were characterized by the presence of charged domain boundaries, which could lead to imprint (preference of one polarization state over another). PACS 85.50.Gk; 77.80.Fm; 77.80.Dj; 68.37.Ps     

Polarization of atomic line radiation

In order to test the predictions of the general theory for the polarization of atomic radiation excited by electron impact, we measured the polarization of atomic lines in electron-alkali atom crossed-beam experiments. The results show a typical behaviour for the polarization of lithium and sodium resonance lines, which is contrary to the earlier measurements such as those ofSkinner andAppleyard: the polarization of the resonance lines increases monotonically from higher energies to the excitation threshold. The threshold polarization of the first resonance lines of Li⁶, Li⁷ and Na²³ is in good agreement with the calculations byFlower andSeaton. These results demonstrate that polarization is very sensitive to the natural level width, and to the fine and hyperfine structure separations of the excited states. Polarization of further lines has been investigated: an alkali line from anS to aP state is unpolarized as expected, but the polarization of a line from aD to aP state decreased in an unexpected manner near the threshold energy.     

Direct observation of magnetization reversal of epitaxial MnAs films

The magnetization reversal of MnAs epitaxial films on GaAs (0 0 1) substrates was investigated using a Kerr microscope. The direct observation of the change in domain structure under magnetic fields revealed characteristic magnetization reversal process of MnAs films with a ladder-type domain structure. The nucleus of the magnetization reversal region appears and propagates to neighboring α-MnAs lines, and then the ladder-type structures cover all over the surface. Finally the domain wall displacement occurs to expand the domain. The change in magnetic domain reflects the characteristic ridge/groove structures of MnAs films.     

Polarization switching in heterophase nanostructures: PLZT relaxor ceramics

The polarization switching is experimentally investigated in hot-pressed PLZT-x/65/35 ceramics with a lanthanum content from 5 to 12 at. %. The specific features in the temperature dependence of the polarization switching in a heterophase state are interpreted by analyzing the change in the switched charge measured over wide ranges of fields and temperatures. Particular emphasis is placed on the role of depolarization fields induced by interphase boundaries. A model of the evolution of polydomain nanostructures with a change in the temperature and in the response to an external field is considered. It is assumed that the low-temperature dielectric anomaly and the temperature hysteresis are associated with the loss of stability of a large-scale domain structure and its transformation into a nanodomain state.     

The coronograph-polarimeter: An algorithm for creation of solar corona polarization images

An algorithm for polarization imaging of the white light corona has been devised on the basis of the Stokes vector presentation and the IDL software applied to statistical data. The use of the algorithm for a 24-frame color negative polarization film of the corona of November 3, 1994, digitized in the green light allowed new data to be obtained on the distribution of the polarization degree in the white (summary of the blue and red spectral interval emissions) corona. On the average, the monotonic increase in the polarization degree, which is characteristic for the van de Hülst model, was not found over the range (1.2–2.0)R in heliolatitudes ±45°.The polarization degree was virtually constant in the region (1.2–1.4)R .The apparent plateau in the polarization degree distribution is obviously due to the deviation of the model with local spherical symmetry from the actual 3D structure of the inner corona.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 10, pp. 1298–1305, October, 1996.     

Singularities of ferroelectric properties of BaTiO3 crystals obtained with different crystallization rates

Ferroelectric properties are investigated of BaTiO₃ particles as a function of their growth rates. This latter was given as the rate of reduction of the temperature = dT/dt of the mother solution BaTiO₃-KF and varied between the limits 3.6–43 K/h. It is established that as a grows the reversible macroscopic polarization P_m diminishes according to the law , where P_m0 = 22·10^–2 C/m² and k = 6·10^–2 h/K. The coercive force E_c increases here while the domain contribution to measured in weak fields diminishes. It is shown that diminution of the growth rate results in improvement of the reverse polarization characteristics of BaTiO₃ crystals, a rise in the efficiency of their mono-domainization that would permit utilization of crystals obtained for = 3.6 K/h for optical purposes.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–78, November, 1990.     

Electronic structure,lattice dynamics,and magnetoelectric properties of double perovskite La<Subscript>2</Subscript>CuTiO<Subscript>6</Subscript>

The results of ab initio calculations of the electronic structure, vibrational properties, and the magnetoelectric effect in the La₂CuTiO₆ crystal with double perovskite structure are presented. The lattice dynamics calculation shows the presence of unstable modes in the phonon spectrum of the high-symmetry cubic phase with space group \(Fm\overline 3 m\). Condensation of two most unstable modes belonging to the center and the boundary point X of the Brillouin zone leads to the formation of a nonpolar stable phase with space group P2₁/n. The calculation taking into account spin polarization shows that the magnetic ground state is E*-type antiferromagnetic with doubled magnetic cell and with the two spin-up and two spin-down configuration of magnetic moments of copper ions along the [010] crystallographic direction. Such ordering of magnetic moments leads to polar space group and polarization formation. The polarization magnitude is estimated as 71 μC/m².     

Domain structure of “transcritical” films of permalloy

The domain structure of transcritical films of permalloy between 0·5 and 40 m thick has been studied by the powder method. A zigzag curvature of domain walls has been found ath> >3 m. The effect of external magnetic fields on powder depositions has been investigated, a model of domain structure supposing the closing of magnetic flux being proposed.Submitted for publication, not presented at the IVth ICMTF.     

On the electronic structure of graphite

Comprehending investigations of the electronic structure of polycrystalline as well as monocrystalline graphite have been performed by means of X-ray emission, self absorption and X-ray induced photoemission techniques. On the basis of these combined investigations a model for the determination of the relative cross sections for the photoemission process has been established and applied to graphite, where it yields _s / _p =32. The anisotropy and polarization of the CK-radiation of monocrystalline graphite is discussed in terms of the binding properties of the graphite lattice. The predictions are verified by measurements of the CK-emission employing a crystal monochromator which acts simultaneously as a nearly perfect analyzer for the polarization of the monochromatized radiation. By means of the self absorption technique the unoccupied part of the conduction band has been investigated.     

Relaxational polarization of dielectrics

Conclusion It follows from the foregoing that the main equations for the static and complex dielectric permittivity have now been sufficiently completely formulated, allowing the permittivity to be rigorously determined with short-range interaction taken into account. In the first case, this involves the use of the Gibbs distribution, and in the second the Kubo formalism. However, although ... the development of equilibrium statistical mechanics may be regarded as complete (from the American foreword to [14]), the reduction of the Gibbs distribution to take into account only the dipole -dipole interactions of nearest-neighbor molecules, as is done in the Kirkwood and Fröhlich theories, cannot be regarded as entirely satisfactory. For such an approach, it is necessary to know the accurate structure of the molecule and the position of the nearest-neighbor molecules, which requires special investigations. In addition, other forms of interaction are omitted, and elastic polarization is not taken rigorously into account.A much more difficult problem confronts theories of nonequilibrium relaxational polarization, in that the development of nonequilibrium statistical mechanics is much worse, since it considers considerably more difficult time-dependent problems and the fundamental question of irreversibility (from the same foreword). In the present review it has been shown that the basic problem of the theory of nonequilibrium relaxational polarization is to find the relation between the macroscopic and molecular distribution functions and to determine the explicit form of these functions. Rigorous solution of this problem for specific materials will allow the laws governing the establishment of relaxational polarization to be determined, together with the corresponding dispersion relations. To this end, in turn, it is necessary to know the relation between the molecular relaxation function and those defects which determine the relaxational process [74].Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 56–73, January, 1979.