Magnetophotonic crystals: nonreciprocity, birefringence and confinement

Photonic crystals in magnetic materials open up a number of interesting possibilities for the study of nonreciprocal effects in confined geometries, including enhanced Faraday rotation and optical unidirectionality. The development of integrated devices based on nonreciprocal magneto-optic phenomena requires understanding the effects of geometrical confinement on light propagation and magnetization in these systems. This article introduces a model for controlling the band gap by exploiting the birefringence in film-based planar magneto-optic structures. It also presents a study of magnetic remanence control through magnetization confinement in the resonant microcavity of one-dimensional planar magnetophotonic crystals.     

一种高双折射低限制性损耗光子晶体光纤设计

本文设计了一种适用于长距离光纤通信的新型光子晶体光纤。该光纤包层内椭圆形和圆形空气孔呈交错排列,纤芯两侧为两个小椭圆空气孔。利用有限元分析方法对所设计光纤的传输特性进行分析并对其结构进行了优化,确定了最佳结构。结果表明,波长为1550 nm时,此新型光子晶体光纤在最佳结构下可提供高达3.51×10^-2的高双折射和低至1.5×10^-9 dB/m的限制性损耗。与现存的引入椭圆形空气孔的光子晶体光纤相比,本文中的光子晶体光纤的双折射系数有较大提高,限制性损耗系数降低了5个数量级。另外,本文还详细研究了光子晶体光纤的色散随光子晶体光纤结构的变化以及其布里渊增益特性,并分析了其可制造性。基于其高双折射和低限制性损耗特性,此种光纤可应用于长距离光纤通信系统。     

Anisotropy of the nonreciprocal linear birefringence in crystals

Results are presented of an experimental investigation of nonreciprocal linear birefringence (a difference between the phase velocities of counterpropagating waves having the same linear polarization) in lithium iodate and potassium dihydrophosphate crystals in a magnetic field. An original double-pass optical measuring system using two linear quarter-wave phase plates (Fresnel rhombs) is used to demonstrate a new method of studying the effect which can give a fairly high sensitivity and avoid accompanying effects. Pis’ma Zh. Tekh. Fiz. 24, 13–18 (February 26, 1998)     

Analysis on the birefringence property of lyotropic liquid crystals below Krafft temperature

The effect of lyotropic liquid crystalline material, below Krafft temperature, in changing the polarization of the optical beam has been demonstrated. It is found that the lamella structures formed in the material below Krafft temperature are birefringent and this birefringence can be controlled by flowing it in rectangular channel under a suitable flow rate.     

The birefringence spectroscopic studies on ferroelectric glycine phosphite (GPI) single crystals

The ferroelectric glycine phosphite single crystals have been grown from aqueous solution by both conventional and Sankaranarayanan-Ramasamy methods. The modified channelled spectrum method has been adopted for spectral dependence of optical birefringence studies over the wavelength range of 480–620 nm, which show that both the crystals exhibit relatively high birefringence values. The photoluminescence excitation studies were carried out for the grown crystals in a wide wavelength range between 300 nm and 600 nm at 224 K. The crystals were also subjected to scanning electron microscopy analysis in order to determine the ferroelectric domain pattern configuration.     

Rayleigh scattering, absorption, and birefringence of large-size bulk single-crystal sapphire

While the thermomechanical properties of sapphire make it an excellent candidate of test mass for advanced laser interferometers, its optical quality is not well understood or well controlled. We have studied the results from high-resolution measurements of scattering, absorption, and birefringence in test-mass samples to better understand issues of quality. Samples show large-scale scattering structures clearly linked to the crystal-growth process. Samples characterized by the presence of point defects have significantly lower scattering (except at the point defects). In general on a large scale, high scattering also correlates with higher absorption and higher average birefringence inhomogeneity. However, on a smaller scale there is not a clear point-to-point correlation between scattering and absorption. Often a large-scale scattering structure is spatially displaced by tens of millimeters from a similar absorption structure, indicating that quite separate microscopic mechanisms give rise to scattering and absorption. The spatial displacements indicate that absorption centers and scattering centers are laid down during crystal growth at different distances from the solid-liquid interface. We suggest that absorption may be linked to F centers, while scattering may be linked to impurities such as iron.     

Random conjugated polybenzazole copolymers: Synthesis, characterization, and exciton confinement effects in photophysical properties

New conjugated random copolymers of poly(p-phenylene benzobisoxazole) (PBO) and poly(2,5-benzoxazole) (ABPBO) with various compositions have been synthesized and characterized by Fourier transform infrared (FTIR), wide-angle X-ray diffraction (WAXD), UV-absorption, and photoluminescence spectra. The side-to-side distance between two neighboring PBO-ABPBO chains can be modulated by the composition ratio. The absorption, excitation, and emission peaks for the copolymers in methanesulfonic acid can also be regulated by the copolymer composition from the homopolymer ABPBO solution to homopolymer PBO solution. However, the emission peaks of copolymers in solid state are quite different from those of their homopolymers. The copolymers showed blue shifted, structured emission spectra centered with higher quantum efficiency at 470 nm compared with the parent homopolymer PBO, indicating the effects of exciton confinement in the random copolymers. The relationship between the structure and properties for the random copolymers can be interpreted roughly by the sequence length distribution of the different segments estimated from the copolymerization statistics.     

FRP confinement of masonry: analytical modeling

International and National Building Codes provide requirements for design and construction of new masonry structures, but design provisions for the repair, retrofitting, and rehabilitation of masonry structures are not always available and included in the same documents. Due to the extremely large variability in masonry performances, equations of general validity cannot often be provided, namely relationships suitable for every masonry type. Despite the great research effort in the experimental field, considerable theoretical work is still needed to fully outline a definitive analytical model to predict the behavior of FRP confined masonry. Most of the available models, empirical in nature, have been calibrated against their own sets of experimental data, or they are simply derived from concrete. Even if large amount of results obtained for concrete led to consolidated design guidelines, they cannot be simply extended to masonry. In this study, a mechanically based confinement model is proposed based on mechanical parameters able to differentiate similar masonry types and to highlight that they present different confinement performance. Crucial aspects of masonry confinement will be also discussed, namely: lateral dilation; confinement effectiveness; lateral pressure also in non-circular shapes; effective strain of FRP.     

Topological birefringence and combined Rytov-Magnus effect

It is shown that the symmetry properties of the locally isotropic inhomogeneous medium of an optical fiber cause circular and linear topological birefringence. The circular birefringence δn _C in graded-index fibers is ∼(λ/ρ)² (where λ is the wavelength and ρ is the core radius), while the linear birefringence is δn _L ∼(λ/ρ)³. This topological birefringence is characterized not only by the polarization basis (as in crystals for example) but also by the magnitude and sign of the topological charge of the guided vortex. This topological birefringence forms the basis of the instability of the fiber IV vortex and is observed experimentally as the combined Rytov-Magnus effect. Pis’ma Zh. Tekh. Fiz. 25, 41–46 (March 12, 1999)     

Thermally induced birefringence in Nd:YAG slab lasers

We study thermally induced birefringence in crystalline Nd:YAG zigzag slab lasers and the associated depolarization losses. The optimum crystallographic orientation of the zigzag slab within the Nd:YAG boule and photoelastic effects in crystalline Nd:YAG slabs are briefly discussed. The depolarization is evaluated using the temperature and stress distributions, calculated using a finite element model, for realistically pumped and cooled slabs of finite dimensions. Jones matrices are then used to calculate the depolarization of the zigzag laser mode. We compare the predictions with measurements of depolarization, and suggest useful criteria for the design of the gain media for such lasers.     

Circular dichroism and birefringence in unconventional superconductors

We present a theoretical analysis of circular dichroism and birefringe in unconventional BCS superconductors with appropriate broken symmetries. We show that for the effect to exist, that in addition to broken time-reversal and two-dimensional parity symmetries, it is necessary to take into account the weak particle-hole asymmetry of the low-energy excitations of the metallic state. Circular dichroism and birefringence are shown to arise from the order parameter collective mode response of the superconductor; in the clean limit the contribution to the current response from the single-particle excitations does not give rise to circular dichroism or birefringence, even with particle-hole asymmetry. The magnitude of the circular dichroism is found to be small for the classes of superconductors which are thought to be likely candidates to exhibit the requisite broken symmetries, namely the heavy fermions and oxide superconductors. The order of magnitude of the elliptical polarization of a linearly polarized incident wave is V_f/c(/_L) (/E_f) ln(E_f/), which is roughly 10^–7–10^–8 rad at frequencies of order the gap, and decreases at least as fast as (2/)² at higher frequencies.     

Quark confinement and the renormalization group

Recent approaches to quark confinement are reviewed, with an emphasis on their connection to renormalization group (RG) methods. Basic concepts related to confinement are introduced: the string tension, Wilson loops and Polyakov lines, string breaking, string tension scaling laws, centre symmetry breaking and the deconfinement transition at non-zero temperature. Current topics discussed include confinement on R(3)×S(1), the real-space RG, the functional RG and the Schwinger-Dyson equation approach to confinement.     

Measurement of birefringence of low-loss, high-reflectance coating of m-axis sapphire

The birefringence of a low-loss, high-reflectance coating applied to an 8-cm-diameter sapphire crystal grown in the m-axis direction has been mapped. By monitoring the transmission of a high-finesse Fabry-Perot cavity as a function of the polarization of the input light, we find an upper limit for the magnitude of the birefringence of 2.5 x 10(-4) rad and an upper limit in the variation in direction of the birefringence of 10 deg. These values are sufficiently small to allow consideration of m-axis sapphire as a substrate material for the optics of the advanced detector at the Laser Interferometer Gravitational Wave Observatory.     

Two-dimensional measurement technique for birefringence vector distributions: measurement principle

The two-dimensional measurement principle for a birefringence vector distribution in transparent materials is analyzed. The system nonuniformity that results from the system components makes the two-dimensional measurement principle quite different from that of a point-measurement method, and the measurement principle requires a two-dimensional analysis. A pulsed optical phase modulation is employed to simplify the two-dimensional mathematical analysis. As a result, concepts are proposed of the system function that characterizes the system nonuniformity that results from the system components and of the intrinsic function that is related to the birefringence vector distribution in a birefringent sample. The influence of the system nonuniformity on the two-dimensional measurement is eliminated by measurement of the intrinsic function, whereas its two values allow for the mathematical separation of the birefringence vector components. The effectiveness of the two-dimensional analysis is illustrated by measurement of a birefringence vector distribution, which is induced by an internal stress distribution in a poly(methyl methacrylate) plate, owing to the photoelastic effect.     

Optical vortices in crystals: Formation,annihilation, and decay of polarization ombilics

The field of polarization states of a paraxial Gaussian beam transmitted via two crystals with crossed optical axes exhibits phase perturbations on the side of the second crystal. These perturbations are manifested by the formation, annihilation, decay, and merging of some special field regions called ombilics. Placed behind the crystals, an analyzer of the circular polarization is capable of revealing single and bound optical vortices in such a field, thus providing a means of monitoring the movement of these vortices.     

Single crystals of chevrel-type compounds: Growth,stoichiometry and electrical resistivity

A series of rhombohedral Mo chalcogenides (Chevrel compounds) based on S or Se have been melted under argon pressures up to 100 atm. We have found that single crystals of an appreciable size (1 to 3 cm³) can be grown by the Bridgman-Stockbarger technique. Density measurements on these crystals, together with x-ray and microscopic observations lead to the conclusion that the formula M_xMo₆X₈ describes correctly the great majority of these compounds. The sulfides with M = Pb, Sn as possible exceptions seem to be stabilized by a small excess of Mo relative to the stoichiometric ratio Mo:S=6:8. No evidence for the presence of S defects was found. The electrical resistivity of PbMo_6.2S₈ has been found to vary linearly as a function of temperature in the range T_c ≤ T ≤ 50 K, the lowest measured value at the normal state being $\text{? = 90 μΩ}\text{cm}$.     

Probing properties of water under confinement: infrared spectra

We have computed infrared (IR) spectra of water confined between nonpolar surfaces by using ab initio calculations. We show that electronic charge fluctuations at the interface, occurring even in the case of highly hydrophobic substrates, are responsible for specific features present in IR signals and for important differences between IR spectra and vibrational density of states. We also find that most of the frequency shifts observed under confinement originate from the modified hydrogen bonded network in close proximity to the interface.     

Growth and quantum confinement in AgI nanowires

Silver iodide nanowires were synthesized in W/O microemulsions by using cyclohexane/Triton X-100/n-pentanol system. Most likely, surfactants form rod-like aggregates that can serve as template for growth of two dimensional nanomaterials. It was found that the length of the AgI nanowires increases as a function of aging time, while the diameter decreases. Final length of the AgI nanowires is several microns, while the diameter is smaller than 3 nm. Morphological changes are accompanied by optical and structural changes. Large blue shift of excitonic peak from bulk value at 420 to 326 nm was observed as a consequence of the size quantization effect. Decrease of diameter is followed by the amorphization of AgI nanowires.