Optical band gap engineering of ZnO nanophosphors via Cu incorporation for ultraviolet–violet LED

The European Physical Journal Plus - For the Earth, water is at the core of sustainable development and at the heart of adaptation to climate change. For the Enceladus, the sixth-largest moon of...     

Anisotropy of the nonlinear optical susceptibility χ(3) in polydiacetylene single crystals

The anisotropy of ⁽³⁾ in Poly-bis(p-Toluene Sulfonate) of 2.4-hexadiyne-1.6-diol (PTS) PolyDiAcetylene (PDA) single crystals is investigated by Degenerate Four-Wave Mixing (DFWM) at a wavelength of 720 nm. The symmetries of ⁽³⁾(–;, –,) identify the non-vanishing and independent components of the tensor. For the calculation of the ⁽³⁾ elements from the measured values, the anisotropy of both the linear optical index of refraction and the absorption coefficient is included in the coupled wave approach. A magnitude of 10^–10 esu is measured for the ⁽³⁾ component if all beam polarizations are parallel to the polymer chains. The least upper bound for all other measurable components is estimated to be 10^–12 esu. The results for the intensity dependence of the DFWM signal are explained in terms of a thermooptic effect. Measurements on the orientational dependence of the DFWM signal in the samples are carried out, and the influence of the anisotropy of the linear optical parameters on the measured curves is discussed. A method for the preparation of thin layered polymer single crystals of poly-bis (4-ButoxyCarbonyle-Methylene-Urethane) of 4.6-decadiyne-1.1-diol (4-BCMU) and the first DFWM measurements on these samples are presented.     

Do solitonlike self-similar waves exist in nonlinear optical media?

We show analytically that bright and dark spatial self-similar waves can propagate in graded-index amplifiers exhibiting self-focusing or self-defocusing Kerr nonlinearities. The intensity profiles of the novel waves are identical with those of fundamental bright or dark spatial solitons supported by homogeneous passive waveguides with the same type of nonlinearity. Thus, we reveal a previously unnoticed connection between spatial solitons and self-similar waves. We also suggest that the discovered self-similar waves can be used in a promising scheme for the amplification and focusing of spatial solitons in future all-optical networks.     

Orientational order parameter studies in two symmetric dimeric liquid crystals – an optical study

The optical technique developed by [W. Kuczynski, B. Zywucki, and J. Malecki, Determination of orientational order parameter in various liquid-crystalline phases, Mol. Cryst. Liq. Cryst. 381 (2002), pp. 1–19; B.J. Zywucki and W. Kuczynski, IEEE transactions on optical phenomena – The orientational order in nematic liquid crystals from birefringence measurements, Dielectr. Electr. Insul. 8 (2001), pp. 512–515] is fabricated and used to determine the orientational order parameter in two dimeric liquid crystalline compounds nematic and SmA phases of α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane (m.OnO.m) homologous series. The compounds studied are 5.O8O.5 and 5.O10O.5 which exhibit nematic and SmA, and nematic phases, respectively. The orientational order parameter in both the phases of nematic and SmA phases of the compound one and the nematic phase of the compound two are obtained using the principle of Newton's rings which gives directly the birefringence, δn of the liquid crystal dimer. The merits of the technique used are presented over the conventional techniques for the determination of orientational order parameter. The results for the two compounds are compared with those values estimated from n _e, n _o and density using the two internal field models due to Vuks and Neugebauer applicable to nematic phase.     

Fast nonlinear optical mechanism of photoconversion in systems of lyotropic ionic liquid crystals–viologen impurities

We study a new nonlinear optical mechanism of reversible photoconversion between excited dimers and cation radicals, the reduction products of electrochromic molecules (viologens), in lyotropic ionic liquid crystals of metal alkanoates. The results of the theoretical study are compared with experimental data obtained from dynamic holographic recordings as well as nonlinear absorption measurements. The estimated value of the photoconversion probability is higher than the probability of non-radiative relaxation of excited molecules. The value of the cubic nonlinearity is χ ^(3)?～?4?·?10^?7?esu for colored cells of these materials.     

Solitary wave for a nonintegrable discrete nonlinear Schr?dinger equation in nonlinear optical waveguide arrays

We study a nonintegrable discrete nonlinear Schr?dinger (dNLS) equation with the term of nonlinear nearest-neighbor interaction occurred in nonlinear optical waveguide arrays. By using discrete Fourier transformation, we obtain numerical approximations of stationary and travelling solitary wave solutions of the nonintegrable dNLS equation. The analysis of stability of stationary solitary waves is performed. It is shown that the nonlinear nearest-neighbor interaction term has great influence on the form of solitary wave. The shape of solitary wave is important in the electric field propagating. If we neglect the nonlinear nearest-neighbor interaction term, much important information in the electric field propagating may be missed. Our numerical simulation also demonstrates the difference of chaos phenomenon between the nonintegrable dNLS equation with nonlinear nearest-neighbor interaction and another nonintegrable dNLS equation without the term.     

Energy efficient nonlinear optics in silicon: are slow-light structures more efficient than nanowires?

We compare the energy performance of four-wave mixing in nanowires and slow-light photonic crystals and outline the regimes where each platform exhibits salient advantages and limitations, including analysis of the impact of future fabrication improvement. These results suggest a route towards energy efficient silicon integrated photonics.     

A frequency analysis of the ν9 band of CD3CD3: experiment and ab initio calculations

The infrared gaseous spectrum of CD₃CD₃ has been measured in the range of 530–670cm^?1 to investigate vibration—torsion effects in the ν₉ band. Three separate spectra all taken under different experimental conditions were recorded. The lines with (ΔK = ?1) and with high values of K show torsional splittings that are substantially larger than expected from the observed barrier height. These splittings are caused primarily by Coriolis-type interactions between the torsional stack of ν₉ = 1 and the corresponding stack for the ground vibrational state. Because of a near-degeneracy that exists between the states (ν₉ = 0, ν₄ = 3) and (ν₉ = 1, ν₄ = 0), three subbands (K, σ) = (15,1), (16,2), (17,3) are resonantly perturbed. For these cases, perturbation-allowed 3ν₄ torsional transitions have been identified. Here σ= 0, 1, 2 or 3 labels the torsional sublevels. Measurements from the ν₉ and 3ν₄ bands, frequencies from the far-infrared torsional spectra in the ground vibrational state, and lower state combination differences from the ν₉ + ν₄ ? ν₄ band were fitted to within experimental uncertainty using an effective Hamiltonian which considered three torsional stacks; one for the ground vibrational state and two for ν₉ = 1. In all, 22 parameters were determined using a total of 2001 lines. Of these, three parameters were the interstack couplings, eight are from the ground vibrational state and 11 are from the excited vibrational state. Two barrier-dependent torsion—rotation parameters, which were essential for obtaining a satisfactory fit, were calculated by ab initio methods.     

Weighted nonlinear phase shift with group velocity dispersion to assess the nonlinear penalty in C L band long-haul fiber optical amplified transmission link

To assess the penalty due to nonlinear effect in C L band long-haul optical amplified transmission link,a new parameter of modified nonlinear phase shift (φD) is proposed,which is the accumulated nonlinear phase shift weighted by a normalized group velocity dispersion (GVD).Based on the numerical simulation result of broadband long-haul hybrid Raman/erbium-doped fiber amplified transmission line,it is validated thatφD is more reasonable and suitable than the previous proposed nonlinear phase shift (φNL) for broadband applications.     

Aperiodic crystals: A contradictio in terminis?

Although in the prevailing view a necessary condition for having a crystalline phase is lattice periodicity, it has become clear in the last decades that there are physical systems with many properties of the usual crystalline state but without three-dimensional lattice periodicity. Incommensurate modulated crystals have been known now for some time, and a couple of years ago much excitement was raised by the discovery of quasicrystals, systems with long-range order but with five-fold symmetry axes, which exclude lattice periodicity.A discussion is given of the various generalizations of the concept of lattice periodicity. In fact, these go from ordinary periodic crystal st structures to almost chaotic ones. One of these is the notion of quasiperiodicity. Section two deals with a special type of these quasiperiodic systems, tilings or space fillings with tiles or blocks of a small number of types. In section three the symmetry of quasiperiodic systems is discussed. Here the embedding into a higher-dimensional space is the key concept. Section four deals with N-dimensional crystallographic groups that occur as symmetry groups of quasiperiodic systems, so called superspace groups. In section five the diffraction from quasiperiodic systems is treated, and in section six it is shown that in some cases quasiperiodic structures may be approximated by periodic ones, and that periodic systems sometimes are more conveniently described by quasiperiodic ones. The emphasis in the symmetry discussion is on quasicrystals.This is even more so in the remaining sections. Section seven gives a brief account of the many experimental data, section eight describes what is known about the microscopic structure. Imperfections are even more important for quasiperiodic systems than for periodic ones. They are discussed in section nine.Not only microscopically do quasiperiodic systems have similarities with ordinary crystals, but also macroscopically. The morphological laws may be generalized to quasiperiodic systems, as shown in section ten. The consequences of quasiperiodicity on the physical properties is still to a large extent unclear. Mathematically they differ much from periodic systems. A discussion of a number of results is given in section eleven.     

High-temperature superconductor Ba2YCu3O7-¢L: plasmon and ultraviolet optical transition studies

We present the first electron energy loss data on a high-temperature superconductor. A detailed comparison with photoemission energy distribution curves and partial-yield spectra identifies the nature of the corresponding features. This analysis shows that the bulk plasmon energy is 24.6 eV, and that there are two peaks in the density of empty states, 2.3–2.5 eV and 4.3–4.6 eV above the Fermi level.     

Polarization field gradient effects in inhomogeneous metal–ferroelectric bilayers: Optical response and band gap tunability

Optical constants, reflectivity response and direct band gap energy (E_g^d) were calculated and simulated by developing an electrodynamic-based model for a three medium system, namely vacuum/ferroelectric film/metallic substrate. Depolarization effects due to the contact between the metallic substrate and the FE film, as well as the spatially dependent profile of the dielectric susceptibility ε(z)$\epsilon (z)$ enter into the formalism by adapting the phenomenological Landau–Ginzburg–Devonshire theory (LGD). Absorption coefficient is obtained from the Lambert–Beer–Bouguer (LBB) approximation and the direct band gap energy as a function of the characteristic length is calculated by using the general Tauc power law. Numerical simulations lead to range of values for tunable E_g^d from 2.6 to 2.8 eV for characteristic lengths up to 30% the thickness of the film, in concordance with recent reports.     

Energy gap revealed by low-temperature scanning-tunnelling spectroscopy of the Si(111)-7×7 surface in illuminated slightly doped crystals

Physical properties of the Si(111)-7×7 surface of low-doped n- and p-type Si samples are studied in the liquid helium temperature region by scanning-tunnelling microscopy and spectroscopy. Conduction required for the study is provided by illumination of the surface. Application of illumination completely removes the band bending near the surface and restores the initial population of the surface states. Our results indicate the existence of the energy gap 2Δ?=?40?±?10?meV in the intrinsically populated Si(111)-7×7 surface.     

B~3Σ_u~- X~3Σ_g~- transition in selenium dimer:ab initio multireference configuration interaction calculations

Theoretical investigation of low-lying electronic states and B 3Σu-X3Σg- transition properties of selenium dimer using size-extensivity singly and doubly excitation multireference configuration interaction theory with nonrelativistic all-electron basis set and relativistic effective core potential plus its split valence basis set is presented in this paper. The spectroscopic constants of ten low-lying Λ-S bound states have been obtained and compared with experiments. Spin-orbit calculations for coupling between B3Σu- sates and repulsive 1Πu,5Πu states have been made to interpret the predissociation mechanisms of the B3Σu- state. The lifetimes of B3Σu-(ν=0～6) have been calculated with scalar relativistic effects included or excluded,respectively,and reasonably agree with experimental values.     

Variations of the band gap of Hg1 ? x Cd x Te solid solutions in response to a magnetic field

The optical transmission spectra of Hg_{1 ? x} Cd _x Te epitaxial layers with a magnetization gradient have been investigated experimentally. The magnetization gradient has been artificially created by the opposite arrangement of the poles of the magnets. The possibility of transforming the shape of the spectral dependences of the optical transmission of the HgCdTe films after their exposure to a magnetic field has been demonstrated. Assumptions about the mechanism underlying this phenomenon have been made.