Optical nonlinearity enhancement for an anisotropic semiconductor–insulator composite

We investigate the effect of geometric anisotropy on the optical nonlinearity enhancement for a periodic composite with a rectangular array of elliptic semiconducting cylinders in an insulating host. By using a series expression of the space-dependent electric field obtained by a simple Fourier method in a periodic composite, we calculate the frequency dependence of the effective third-order nonlinear susceptibility as a function of anisotropy. The results show that the height of the nonlinearity enhancement peak may be increased by several orders of magnitude as the aspect ratio of the ellises is decreased or the lattice edge length ratio is increased. At resonance frequency, there exists a strong anomalous dispersion. We also investigate the effect of the volume fraction of the semiconductor phase for composites with a square array of circular semiconducting cylinders. Received: 23 November 2000 / Accepted: 2 August 2001 / Published online: 17 October 2001     

The effective Kerr constant of an electrooptical composite material

A general relationship is derived for the effective Kerr constant of an electrooptical composite material containing nonlinear centrosymmetric dielectric microcrystals; anisotropy of the inclusions and their nonsphericity were taken into account. It is shown that nonsphericity of the particles makes it possible to substantially enhance the Kerr constant of the composite in the case when the glass-matrix permittivity is much smaller than that of the inclusions. The size effects in ferroelectrics, on the one hand, and the required low losses for scattering, on the other, determine the optical range in which the particle shape anisotropy can be efficiently used. This range covers the IR and long-wavelength part of the visible region.     

Surface plasmon enhancement of an optical anisotropy in porous silicon/metal composite

We report on the theoretical results obtained by applying the modified effective medium theory to a composite material consisting of mesoporous Si on (110)-oriented substrate with pores filled with silver through, e.g., electroplating process. The theory developed permits a self-consistent determination of the effective dielectric permittivity tensor of such materials. It is shown that the optical anisotropy of such a composite can be greatly enhanced at some wavelength ranges. While this anisotropy is generally uniaxial as in non-metal-filled mesoporous Si etched on (110) substrate, the sign of the anisotropy (i.e., positive or negative) changes in some portions of the spectrum. The optimum material parameters for an experimental observation of the theoretically predicted effects are determined. PACS 78.20.-e; 78.20.Bh; 78.20.Ci; 78.20.Fm; 78.30.Fs; 78.55.Mb     

Dual-wavelength laser emission from an organic microcavity with terahertz beating

We report on the dynamics of laser emission from an anisotropic organic microcavity filled with the guest-host composite of tris-(8-hydroxy quinoline) aluminium (Alq₃) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). In a single microcavity, a linesplitting of 0.18 THz between two perpendicularly polarized laser modes is observed. We ascribe this effect to an optical anisotropy in the distributed Bragg reflectors surrounding the organic layer. The temporal behavior of the electromagnetic field is studied by an up-conversion technique and shows an optical beating of 0.18 THz. Two modeling approaches are used to gain insight in the temporal evolution and phase behavior of the two modes. Both point towards the presence of a phase-coupling mechanism in this system. PACS 42.55.Sa     

Polarization selection rules in scattering of cavity polaritons

We theoretically discuss scattering of polaritons in semiconductor microcavities by means of a microscopic model. Taking into account the composite character of excitons (formed by an electron and a hole), we analyze the relation between polarizations of incoming and outgoing polariton states under resonant excitation by linearly polarized laser beams with opposite in-plane momenta. In addition to these polarization selection rules, we investigate the nonlinear processes up to the sixth order and we show the origin of an induced anisotropy due to the excitation beams which is responsible for the operation of an optical gate based on polariton–polariton scattering in a microcavity.     

Optical study of an untwinned single crystal: ab-plane anisotropy

We report on the ab-plane polarized reflectance of an untwinned single crystal over the frequency range from 80 to (10 meV-4 eV) at temperatures between 10 and 300 K. We find a clear anisotropy in the ab-plane optical conductivity above and below , which is very similar to that formerly published data of (M.A. Quijada et al., Z. Phys. B 94, 255 (1994)). We employ both the one-component and two-component analyses to the optical data, which suggest that the normal-state infrared anisotropy of originates not only from the mass anisotropy, but also from the scattering rate anisotropy. Our results provide evidence that the electronic structures within the plane are anisotropic. In the superconducting state, there is a definite ab-plane anisotropy to the far-infrared absorption. This anisotropy could be due either to anisotropy of the superconducting gap or to anisotropy of the mid-infrared component to the conductivity. We also observe the superconducting condensate is anisotropic: The value of the superconducting penetration depth in the a-direction is slightly smaller than that along the b-axis. Received 16 July 1998     

Development of a “Built-in” Scanning Near Field Microscope Head for an Atomic Force Microscope System and Stress Mapping of an Al2O3/ZrO2 Eutectic Composite

We constructed a scanning near-field optical microscope (SNOM) on a commercially available atomic force microscopy (AFM) apparatus (SPM-9500J2; Shimadzu Corp.) to measure the stress distribution in ceramic composite materials. Features of our SNOM system are: (1) a compact SNOM head substituted for the original AFM head; (2) a wide scanning range (125 × 125 μm²) inherited from the original scanner; (3) use of conventional shear-force regulation; (4) an optical system for the illumination-collection (I-C) mode; (5) excitation by a 488 nm line of an Ar-ion laser, and (6) light detection by photon counting or a polychromator equipped with an electronically cooled charge coupled device (CCD). This SNOM system was used to measure the surface structure and stress distribution of an Al₂O₃/ZrO₂ eutectic composite. We simultaneously measured topographic images and fluorescence spectra of an Al₂O₃/ZrO₂ eutectic composite. We estimated its peak intensity, peak position, and peak width from the fluorescence spectrum during scanning, which respectively correspond to the abundance of Al₂O₃, stress in the grain, and the anisotropy of that stress. Mapping images showed that the stress and its anisotropy were weaker in the center of the Al₂O₃ grain than its boundary between Al₂O₃ and ZrO₂. That observation suggests that Al₂O₃ underwent intense anisotropic stress induced by volume expansion in the phase transition of ZrO₂ from the cubic phase to the monoclinic phase during preparation.     

Optical anisotropy of aligned pentacene molecules on a rubbed polymer corresponding to the electrical anisotropy

We report on the optical anisotropy of a pentacene film on a rubbed (poly)vinylalcohol (PVA) layer related to the electrical performances of the pentacene organic field effect transistors (OFETs) depending on the direction of a current flow. The optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene OFET on the rubbed PVA layer, however, the optical anisotropy is observed and the anisotropy of the electrical performances directly corresponds to the optical anisotropy of the pentacene thin-film on the rubbed PVA layer.     

Polarization transformers for optical anisotropy: II. Transformation of properties of optical anisotropy of reciprocal and nonreciprocal elements

The methods of transformation of an arbitrary phase optical anisotropy using a set of quarter-wave plates are considered. For this purpose, we use a formal analogy between the Jones matrices of these anisotropic elements and the matrices of transformation of the polarization basis states. We consider all types of reciprocal and nonreciprocal optical phase anisotropy. We show that the minimum set of anisotropic elements sufficient for such a transformation is a set of four quarter-wave plates. For nonreciprocal systems, this set should be complemented with Faraday rotators, whose number depends on the initial and final type of the nonreciprocity.     

The influence of a complexing agent ion on the magnitude of the optical anisotropy of lanthanidomesogens

Measurements of the refractive indices and optical anisotropy of a series of liquid crystalline coordination compounds based on lanthanides with an identical ligand environment have been carried out. It was found that the magnitude of the optical anisotropy of the investigated complexes was several times smaller than the anisotropy of organic liquid crystals. Analysis of the results showed that the variation of the ion complexing agent had little effect on the magnitude of the optical anisotropy in the mesogenic complexes containing the same ligands. The even-odd alternation of the optical anisotropy of lanthanidomesogens at the increasing the number of protons in the ions of the lanthanides has been observed.

     

Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials

We study the optical properties of metamaterials made from cut-wire pairs or plate pairs. We obtain a more pronounced optical response for arrays of plate pairs, a geometry that also eliminates the undesirable polarization anisotropy of the cut-wire pairs. The measured optical spectra agree with simulations, revealing negative magnetic permeability in the range of telecommunications wavelengths. Thus nanoscopic plate pairs might serve as an alternative to the established split-ring resonator design.     

Ultrafast laser-induced phase transitions in tellurium

We present experimental investigations of ultrafast phase transitions in tellurium following excitation by an intense femtosecond laser pulse. Femtosecond time-resolved polarization-sensitive microscopy is used to monitor the temporal evolution of optical anisotropy (birefringence) of the irradiated material. The decay of optical anisotropy associated with the loss of order in crystalline tellurium is fluence-dependent and occurs within 0.5–3 ps.     

All-optical switching and passive mode-locking based on non-linear polarization rotation in a semiconductor quantum well amplifier

Owing to differences in the selection rules of electron transitions between TE and TM modes in a quantum well structure, and optical amplifier with such a structure manifests linear birefringence and anisotropy of gain-absorption saturation. The anisotropy leads to the evolution of power-dependent polarization in the optical amplifier. We demonstrate experimentally efficient non-linear self-switching based on this phenomenon in an InGaAs/GaAs single quantum well amplifier. Also, by utilizing the non-linear polarization evolution, we show numerically passive mode-locking of a semiconductor laser with a ring cavity.     

Optical anisotropy of InAs/GaSb broken-gap quantum wells

We investigate in detail the optical anisotropy of absorption of linearly polarized light in InAs/GaSb quantum wells grown on GaSb along the [001] direction, which can be used as an active region of different laser structures. The energy level positions, the wave functions, the optical matrix elements, and the absorption coefficients are calculated using the eight-band k · p model and the Burt-Foreman envelope function theory. In these calculations, the Schr?dinger and Poisson equations are solved self-consistently taking the lattice-mismatched strain into account. We find that a realistic Hamiltonian, which has the C _2v symmetry, results in considerable anisotropy of optical matrix elements for different directions of light polarization and different directions of the initial-state in-plane wave vector, including low-symmetry directions. We trace how the optical matrix elements and absorption are modified when spin-orbit interaction and important symmetry breaking mechanisms are taken into account (structural inversion asymmetry, bulk inversion asymmetry, and interface Hamiltonian). These mechanisms result in an almost 100% anisotropy of the absorption coefficients as the light polarization vector rotates in the plane of the structure and in a plane normal to the interfaces.     

Surface-induced optical anisotropy of inhomogeneous media

We demonstrate that the presence of interfaces induces anisotropy in the optical properties of thin inhomogeneous layers. Several mechanisms are discussed that can control the properties of this surface-induced anisotropy. We found that the effective refractive indices for s- and p-polarized fields are different and depend on the thickness of the layer, concentration and optical properties of inclusions in the layer, and the angle of incidence.     

Two-photon absorption in hexagonal-CdS

The “composite sample technique” has been used to investigate the two-photon absorption coefficient (β) of CdS at 3.91 eV. The absolute value of β and its optical anisotropy have been studied for samples with different carrier densities. The present results may provide a new interpretation of experiments on CdS and other semiconductors reported in the literature.     

Two-step condensation of lattice bosons

We present a theoretical study of Bose-Einstein condensation in highly anisotropic harmonic traps. The bosons are considered to be moving in an optical lattice in an overall anisotropic harmonic confining potential. We find that two-step condensation occurs for lattice bosons at much reduced harmonic potential anisotropy when compared to the case of an ideal Bose gas in an anisotropic harmonic confinement. We also show that when the bosons are in an isotropic harmonic confinement but with highly anisotropic hopping in the optical lattice, two-step condensation does not occur. We interpret some of our results using single boson density of energy states corresponding to the potentials faced by the bosons.     

Optical anisotropy of liquid-crystal lanthanide complexes

Refractive and optical anisotropy indices have been measured for a number of liquid-crystal lanthanide complexes with different counter ions, ligands, and complexing agents. The optical anisotropy of the complexes is found to range from 0.002 to 0.09, i.e., to be one to two orders of magnitude smaller than that of classical calamitic liquid crystals. Analysis of the chemical structure of mesogenic complexes showed that the complexes containing counter ions with sizes much smaller than the ligand sizes and mixed-ligand complexes containing less than five ligands exhibit the largest optical anisotropy. The optical anisotropy of other mixed-ligand complexes with a large number of ligands is much smaller. The reason is the different degrees of anisometry of complexes and, as a consequence, the difference in the polarizability anisotropy and, correspondingly, the optical anisotropy of the mesophase.     

Controlling spin exchange interactions of ultracold atoms in optical lattices

We describe a general technique that allows one to induce and control strong interaction between spin states of neighboring atoms in an optical lattice. We show that the properties of spin exchange interactions, such as magnitude, sign, and anisotropy, can be designed by adjusting the optical potentials. We illustrate how this technique can be used to efficiently "engineer" quantum spin systems with desired properties, for specific examples ranging from scalable quantum computation to probing a model with complex topological order that supports exotic anyonic excitations.