Nonlinear Anisotropies Research Articles

Structural, Electronic and Nonlinear Optical Properties of Novel Derivatives of 9,12-Diiodo-1,2-dicarba-closo-dodecaborane: Density Functional Theory Approach

Abstract Using density functional theory (DFT) methods, we shed light on the structural, optical, electronic, and nonlinear optical (NLO) properties of three derivatives of 9,12-diiodo-1,2-dicarba-closo-dodecaborane(12) (C2H10B10I2). The DFT and time-dependent DFT methods are considered very precise and practical to optimize the ground and excited state geometries, respectively. A vibrant intramolecular charge transfer from highest occupied molecular orbitals (HOMOs) to the lowest unoccupied molecular orbitals (LUMOs) was observed in all compounds. The geometrical parameters of the experimental crystal structure, i.e. bond lengths/angles, have been successfully reproduced. The HOMO and LUMO energies, as well as their energy gaps (E g), were also calculated and compared with each other for all derivatives. The effect of attached groups on electronic, optical, and NLO properties along with detailed structure-property relationship was discussed. For NLO response, the CAM-B3LYP functional along with relatively larger basis set 6-31+G** (for hydrogen, carbon, boron, and oxygen atoms) and LANL2DZ (for iodine atoms) have been used to optimize the compounds at ground states. The calculation of second-order NLO polarizabilities (β tot) shows that compounds 2 and 3 possess the β tot amplitudes of 3029 and 4069 a.u., respectively, with CAM-B3LYP method that are reasonably larger than similar prototype molecules. Owing to their unique V-shapes, the nonlinear anisotropy values are found to be 0.63, 0.34, and 0.44 for compounds 1–3, respectively, which show the significant two-dimensional character of these compounds. Thus, the NLO amplitudes as well as the nonlinear anisotropies indicate that the above-entitled compounds are good contenders for optical and NLO applications.

Improving on numerical simulations of nonlinear CMB anisotropies

An Adaptative-Particle-Particle-Particle-Mesh code (HYDRA) plus a ray-tracing procedure was used in [1] to perform an exhaustive analysis of the weak lensing anisotropy. Other nonlinear Cosmic Microwave Background anisotropies, such as the Rees-Sciamaand the Sunyaev-Zel.dovicheffects are also being studied by using the same tools. Here we present some advances in our study of these nonlinear anisotropies. The primary advance is due to the use of better simulations with greater particle densities and appropriate softening, although other parameters have also been adjusted to get better estimates. Thus, we improve on a previous paper [2] where the Rees-Sciamaeffect was studied with Particle-Mesh simulations. We focus particular attention on the resolution improvement and its consequences.

A new type of organic–inorganic hybrid NLO-phore with large off-diagonal first hyperpolarizability tensors: a two-dimensional approach

We report a novel type of organic-inorganic hybrid material with rare two-dimensional nonlinear optical (NLO) properties. The density functional theory (DFT) calculations combined with the finite-field (FF) method show that the designed molecules (6,9 organo-derivatives of B10H14) could carry the characteristic NLO properties of both organic and inorganic materials. Interestingly, due to their unique V-shaped structures, they have large off-diagonal first hyperpolarizability tensors or nonlinear anisotropy, which is an advantage in their practical applications over conventional donor-π-acceptor (D-π-A) NLO-phores. The systematic substitutions of terminal donor/acceptor groups as well as the extension of π-conjugation along the V-shape in these derivatives have been evaluated to guide a purpose-oriented synthesis of NLO material. All the systems in the present study have been categorized into Set-I and Set-II with D-π-A-π-D and A-π-D-π-A configurations, respectively. These designed derivatives show large amplitudes of βz values. For example, systems 3N (6,9-[(N=C=Ph-NO2)2]-B10H12) and 4N (6,9-[((N=C=Ph)2-NO2)2]-B10H12) have βz amplitudes as large as 34.16 and 276.91 × 10(3) a.u. which are 380 and 3000 times larger than those of a typical NLO molecule of urea, respectively. Remarkably, the substitution of nitrogen atoms with 6,9 hydrogen atoms in decaborane is shown to cause a lone pair back donation to vacant p orbitals of 6,9 boron atoms of the decaborane basket. This p orbital overlapping verticalizes the V-arms of the decaborane derivatives and boosts their nonlinear anisotropies due to their larger off-diagonal tensor components. The nonlinear anisotropy values are significantly larger, ranging from 1 (minimum in system 1) to 31.90 (maximum in system 3A) due to their unique V-shape. Comparison of their efficiencies with standard molecules demonstrates that our designed organic-inorganic hybrid molecules have significant potential as excellent candidates for NLO applications.

Nonlinear CMB temperature anisotropy from gravitational perturbations

Non-linear CMB temperature anisotropies up to the third-order on large scales are calculated. On large scales and in the Sachs-Wolfe limit, we give the explicit expression for the observed temperature anisotropy in terms of the primordial curvature perturbation up to the third-order. We derived the final bispectrum and trispectrum of anisotropies and the corresponding non-linear parameters, in which the contributions to the observed non-Gaussianity from primordial perturbations and from the non-linear mapping from primordial curvature perturbation to the temperature anisotropy are transparently separated.

Behavior of nonlinear anisotropies in bouncing Bianchi I models of loop quantum cosmology

In homogeneous and isotropic loop quantum cosmology, gravity can behave repulsively at Planckian energy densities leading to the replacement of the big bang singularity with a big bounce. Yet in any bouncing scenario it is important to include nonlinear effects from anisotropies which typically grow during the collapsing phase. We investigate the dynamics of a Bianchi I anisotropic model within the framework of loop quantum cosmology. Using effective semiclassical equations of motion to study the dynamics, we show that the big bounce is still predicted with only differences in detail arising from the inclusion of anisotropies. We show that the anisotropic shear term grows during the collapsing phase, but remains finite through the bounce. Immediately following the bounce, the anisotropies decay and with the inclusion of matter with equation of state $w<+1$, the universe isotropizes in the expanding phase.

Matching molecular and optical multipoles in photoisomerizable nonlinear systems

We experimentally investigate the coupling rules between molecules and optical fields in the all-optical poling scheme. The efficiency and symmetry of the photoinduced quadratic nonlinearity is shown to be dependent on tensorial selection rules connecting the symmetries of the optical poling fields and the molecular tensors. We show that an optimal poling is obtained by matching the molecular symmetry to either a quasi-dipolar or a purely octupolar poling field, each being engineered by use of circular polarizations. Moreover, we define optimized poling conditions allowing the direct measurement of molecular nonlinear anisotropies. This research can be applied to nonlinear and photoisomerizable multipolar molecular systems.

Coherent control of the optical nonlinear and luminescence anisotropies in molecular thin films by multiphoton excitations.

Photoinduced orientational distributions are implemented with one- and two-photon absorption interference in polymer films containing chromophores that exhibit luminescent and nonlinear properties. The odd- and even-order parameters of the final distribution are probed by simultaneous measurement of second-harmonic generation (SHG) and two-photon fluorescence (TPF). We show the possibility of engineering local SHG and TPF anisotropies by controlling the polarization states and intensities of the writing optical fields. Complex multipolar orders are modeled with an irreducible spherical tensor-based formalism jointly applied to the molecular polarizabilities and field tensors.

Polarization loxodrome of a vertical-cavity semiconductor laser

We present a theoretical analysis of the polarization dynamics of a vertical-cavity surface-emitting laser with either linear anisotropies or nonlinear anisotropies. In both cases, we demonstrate that the polarization excursions follow a loxodrome spiral curve on the Poincaré sphere of the polarization.

Polarization fluctuations in vertical-cavity semiconductor lasers

We report, theoretically and experimentally, how polarization fluctuations in vertical-cavity semiconductor lasers are affected by optical anisotropies. We develop a spin-eliminated (class A) description of laser polarization and show how the various model parameters can be extracted from the experimental data. In practice, the linear anisotropies are often much stronger than the nonlinear anisotropies, so that the polarization modes defined by the linear anisotropies form a useful basis. For this case we derive a one-dimensional model for polarization noise, with simple expressions for the relative strength of the polarization fluctuations and the rate of polarization switches. For the other, more extreme, case where the nonlinear anisotropies are as strong (or even stronger) than the linear anisotropies, the spin-eliminated description remains valid. However, in this case the concept of polarization modes is shown to lose its meaning, as a strong four-wave-mixing peak appears in the optical spectrum and polarization fluctuations become highly nonuniform.

Spontaneous pulsations in gas class-A lasers with weakly anisotropic cavities

Spontaneous pulsations and polarization multistability resulting from the nonlinear interaction of two standing waves with an active medium in a single-longitudinal-mode Fabry-P\'erot gas laser with a weakly anisotropic cavity have been studied at different cavity anisotropies and different transitions between the working levels with and without an axial magnetic field applied to the active medium. As predicted theoretically, a series of polarization dynamical phenomena have been observed experimentally in the He-Ne $(\ensuremath{\lambda}=1.15\ensuremath{\mu}$m) laser in the case of orthogonal elliptically polarized eigenstates of the cavity. Periodic oscillations of two qualitatively different forms (distorted sinusoids and large spikes), the transition between two stationary orthogonal elliptically polarized waves in the vicinity of the line center tuning which occurs through the region with polarization instability, as well as switches between the orthogonally polarized components when the output is nonstationary have been found. It is shown that the diversity of nonstationary polarization phenomena in gas lasers with weakly anisotropic cavities is due to spontaneous pulsations of intensities, ellipticities, and azimuths of two emitted waves, caused by the competition of the nonlinear active medium and empty cavity anisotropies. The results of the theoretical modeling are in quantitative agreement with the experimental results.

Coupled-mode description for the polarization state of a vertical-cavity semiconductor laser

For symmetry reasons, vertical-cavity semiconductor lasers might be expected to show polarization isotropy. Practical devices, however, operate in a well-defined polarization state, which indicates the presence of residual anisotropies. We have performed a systematic experimental study of these residual anisotropies by applying additional anisotropies in a controlled and continuous way. The results are compared with a coupled-mode model that describes the effect of linear anisotropies on the polarization eigenmodes. We conclude that the polarization state of a practical device is dominantly determined by these linear anisotropies; nonlinear anisotropies are found to play at most a minor role.

Optical Third-Harmonic Generation in Poly(p-Phenylene Benzobisthiazole) Langmuir-Blodgett Film

Abstract The nonlinear optical anisotropy of ply(p-phenylene benzo-bisthiazole) (PBT) Langmuir-Blodgett (LB) films has been investigated by optical third-harmonic generation measurement. Polymer backbones of PBT in LB films were aligned along the dipping direction. The χ(3) xxxx, χ(3) yyyy and χ(3) xxyy values were determined by rotational Maker fringe method to be 11.3 × 10−12 esu., 1.5 × 10−12 esu. and 0.9 × 10−12 esu, respectively. These nonlinear optical anisotropies can be explained well by assuming a Gaussian orientational distribution of one-dimensional π-conjugated chains.