Magnetic anisotropy of two-dimensional nanostructures: Transition-metal triangular stripes

The magnetic anisotropy energy (MAE) of one-dimensional stripes having infinite length and triangular lateral structure are investigated in the framework of a self-consistent tight-binding method. One observes discontinuous changes in the easy magnetization direction along the crossover from one to two dimensions. The MAE oscillates as a function of stripe width and depends strongly on the considered transition metal (TM). The MAE of the two-leg ladder is strongly reduced as compared to that of the monoatomic chain and the convergence to the two-dimensional limit is rather slow.     

Optical size resonances in nanostructures

The existence of optical size resonances in atomic nanostructures is proved. The properties of optical size resonances strongly depend on the interatomic distances and on the polarization of an external radiation field. The properties of linear and nonlinear size resonances are considered in the case of two-dimensional nanostructures. The linear optical size resonances are described based on a closed system of equations for dipole oscillators and nonlocal field equations taking into account the dipole-dipole interactions of atoms in the radiation field. Using a stationary solution to these equations, it is demonstrated that two isotropic atoms with definite intrinsic frequencies form an anisotropic system in the radiation field, possessing two or four size resonances depending on whether the component atoms are identical or different. The nanostructure composed of two different atoms possesses two size resonances with positive dispersion and two other resonances with negative dispersion. The frequencies of the size resonances significantly differ from the intrinsic frequencies of isolated atoms entering into the nanostructure. By changing the angle of incidence of the external wave, it is possible to excite various size resonances. The properties of nonlinear optical size resonances excited by an intense radiation field were theoretically and numerically studied using the modified Bloch equations and nonlocal field equations. Dispersion relationships for the nonlinear resonances were derived and the inversion properties of atoms in the nanostructure were studied for various polarizations of the external optical wave.     

Optical properties of several ternary nanostructures

To investigate the optical properties of the ternary nanostructures, the nanodisk, core–shell, and three-sphere structures are constructed. The extinction coefficients and electric near-field distributions of these structures are calculated by the discrete dipole approximation(DDA) method. The result shows that the nanodisk structure has the best extinction efficiency in the three structures. Furthermore, several three-material combinations of the nanodisk structures are investigated. The ternary nanodisk structure composed of TiO₂ and two noble metals(Au, Ag or Pt) has higher extinction coefficient and near-field intensity than the nanodisk consisting of Au, TiO₂ and a semiconductor(Pb Se, Ge, MoS2, CdSe, CdS or TiO₂).Especially, TiO₂/Ag/Pt has the best extinction efficiency and the max electric near-field intensity. And the extinction spectra of TiO₂/Ag/Pt and TiO₂/Ag/Au structures are complementary in the visible range. This work conduces to the further research into ternary nanostructure and provides essential information about its performance in visible range.     

Continuation of acoustic near-fields

This paper deals with the analytic continuation of a coherent pressure field specified on a finite sheet located close to and conformal to the surface of a vibrator. This analytic continuation is an extension or extrapolation of the given (measured) field into a region outside and tangential to the original finite sheet, and is based on the Green's function (the transfer function) relating acoustic quantities on the two conformal surfaces. The continuation of the measured pressure field is an inverse problem that requires the use or regularization theory, especially when noise is present in the data. An iteration algorithm is presented that is successful in continuing the pressure field into the tangential sheet. The results are accurate close to the original boundary and taper (decay) toward zero with distance away from it. The algorithm is tested on numerical and experimental data from a point-driven rectangular plate. Results show the successful extrapolation (continuation) of this data into an area nearly double that of the original pressure field. This algorithm is not limited to planar surfaces and can be applied to arbitrarily shaped surfaces.     

Optical absorption spectroscopy of one-dimensional silicon nanostructures

During the last decade there has been a great development in nanoscience and nanotechnology. The technology of nanostructures synthesis and characterization has grown rapidly and optical spectroscopy has become a very useful characterization technique, since it provides information on the structural, electronic, optical and dynamical properties of materials. Nanostructures have unique physical properties that are different from bulk materials. A wealth of interesting and new phenomena are associated with nanometer-sized structures, such as size-dependent emission or excitation, metallic and semiconductor behavior, etc. Here we present an overview of the linear optical response of one-dimensional silicon nanostructures. In particular, we make a theoretical study of the effects of the size and shape of one-dimensional silicon structures on the absorption spectrum, focusing on the calculation of the linear optical response of clean and hydrogen-adsorbed armchair (6,6) silicon nanotubes. We discuss the changes of the absorption spectrum of silicon nanowires with different diameters and analyze the behavior of the band gap as we go from bulk silicon to one-dimensional silicon nanostructures with nanometer-size diameters.     

Optical properties of one-dimensional subwave plasmonic nanostructures

Nanogratings formed by parallel gold nanowires on a quartz substrate have been fabricated. Their transmission and reflection spectra have been studied experimentally. Numerical simulation of the transmission (reflection) spectra has been performed. The simulated spectra agree well with the experimental ones. A package has been proposed, which combines the experimental techniques of the structure fabrication and theoretical methods for calculating the parameters of optical spectra. This package is a promising tool for designing and fabricating optoelectronic devices such as filters, polarizers, and switches.     

Optical bistability of planar metal/dielectric nonlinear nanostructures

We study theoretically a nonlinear response of the planar metal/dielectric nanostructures constituted from periodical array of ultra thin silver layers and the layers of Kerr-like nonlinear dielectric. We predict hysteresis-type dependences of the components of the tensor of effective dielectric permittivity on the field intensity allowing the change in material transmission properties from transparent to opaque and back at extremely low intensities of the light. It makes possible to control the light by light in all-optical nanoscale devices and circuits.     

内嵌圆饼空心方形银纳米结构的光学性质

采用离散偶极子近似方法计算了内嵌圆饼空心方形银纳米结构的消光光谱以及其近场的电场强度分布,并进一步与空心方形纳米结构的消光光谱和表面电场做比较.结果表明,在耦合作用下内嵌圆饼空心方形银纳米结构不仅产生了新的共振模式,而且新的共振模式在传统表面增强拉曼散射的激发波长范围内,进而可以弥补由于实验上运用纳米切片法所制备的空心方形纳米结构尺寸较大导致其共振吸收峰在远红外波长范围的不足.此外,可以通过改变内嵌圆饼空心方形银纳米结构的形貌参数调节其表面等离子体共振峰的共振波长,以满足在表面增强拉曼散射、生物分子或化学分子探测上的应用.     

Optical properties of nanostructures in layered metal tri-iodide crystals

The optical response of excitons confined in characteristic nanostructures in layered metal tri-iodide crystals introduced by some irregular stackings from the bulk structures is reviewed. In BiI₃ a specific stacking fault takes place during crystal growth constructing macroscopic planar defects. In this space conspicuous localized exciton transitions occur below the intrinsic absorption edge. Another stacking disorder introduced by applying external stress in this crystal brings about a new nanostructure domain of symmetry D_3d different from that of bulk symmetry C²_3i. The optical transitions due to new structures appear in the lower energy region as an absorption and luminescence line series. The similar nanostructures are induced in SbI₃ crystals under the hydrostatic pressure. In these nanostructures, the electronic structure is analyzed by a model based on the confined excitons in a nanoscale disk-like shape space. The magnetic field effect confirms the structure in the wave function-size scale. The nanoscale disk-like structure of BiI₃ in CdI₂ matrices is also obtained by a hot wall technique and mixed crystal annealing, which is realized by observing the size distribution with an electron microscope. In a BiI₃ disk in CdI₂ Stokes shifted photoluminescence bands appear. The Stokes shifts of the luminescence bands are understood by considering the size-dependent exciton-phonon interaction. In these nanostructures large optical nonlinearity under the intense laser field was obtained.     

Optical limiting in pulsed laser deposited VO2 nanostructures

Being a Mott type oxide, at a temperature of ~ 68 °C and ambient pressure, stoichiometric VO₂ undergoes a first order metal-insulator transition, which is accompanied by a reversible abrupt change in the band gap opening. From an optical point of view, this metal-insulator transition manifests itself by a significant and reversible variation of the refractive index under either a thermal stimuli or by photo-induction. This contribution reports on the ultrafast optical limiting in the IR regime of pulse laser deposited VO₂ nanostructures.     

Optical limitation in two-dimensional nonlinear photonic crystal with triangular lattice

Optical limitation in a 2D nonlinear photonic crystal (NPC) has been studied in this Letter. Since the optical limitation is due to Bragg scattering induced by the variation of nonlinear refractive index, it is sure that the optical limitation can be realized in nonlinear photonic crystal. The light propagation characteristics in a two-dimensional nonlinear photonic crystal with triangular lattice has been calculated by using the finite-difference time-domain algorithm, which is constructed by placing certain number of nonlinear dielectric rods in a linear photonic crystal. The optical limiting effects at 1.300 and 0.504 μm have been obviously obtained for TE polarization and TM polarization, respectively.     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.     

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.     

Optical emission from excess Si defect centers in Si nanostructures

Four groups of Si nanostructures with and without beta-SiC nanocrystals were fabricated for clarifying the origin of a blue emission with a double-peak structure at 417 and 436 nm. Spectral analyses and microstructural observations show that the blue emission is related to the existence of excess Si atoms in these Si nanostructures. The energy levels of electrons in Si nanocrystals with vacancy defects formed from the excess Si atoms are calculated and the characteristics of the obtained density of states coincide with the observed double-peak emission. The present work provides a possible mechanism of the blue emission in various Si nanostructures.     

Optical investigations of ZnxCd1-xS nanostructures

Zn_xCd_1-xS nanostructures with (x = 0, 0.25, 0.5, 0.75, 1) have been grown on glass substrates using spray pyrolysis technique. X-ray diffraction results have showed that Zn_xCd_1-xS nanostructures were formed with hexagonal and cubic structures. The structural parameters have been evaluated as a function of concentration (x). Also, the optical properties that depend on the mole fraction (x) are investigated for Zn_xCd_1-xS nanostructures.     

Fundamentals and properties of zinc oxide nanostructures: Optical and sensing applications

In this paper we will give an overview of the status of catalytic growth and of low-temperature chemical growth of ZnO nanostructures performed in our laboratory. Particularly results employing different substrates will be discussed. The second part deals with structural and optical properties of ZnO nanorods. The results from high resolution transmission electron microscope (HRTEM), scanning electron microscope (SEM), photoluminescence (PL), Cathodoluminescence (CL), and Electroluminescence (EL), on single nanowires will be shown. Our results on surface morphology, bulk and the position of the catalyst as well as the optical properties including UV emission, lasing and white emission will all be presented and discussed. In the third part experimental results from electroluminescence of ZnO nanorods on different substrates in the UV in addition to excellent white light emission obtained from samples grown at low temperature are to be given and discussed. Finally the sensing of molecules in water by ZnO nanorods will be discussed from a theoretical point of view. Also fundamental properties of polaritons and excitons in ZnO nanostructures are to be highlighted.     

Optical properties of Cantor nanostructures made from porous silicon: A sensing application

The fabrication and characterization of pSi based triadic Cantor multilayer structures is reported. Transfer matrix method was used to calculate the corresponding reflectivity spectrum and was compared with the experimental data. A good agreement between the theoretical and experimental reflectivity spectra is reported for the wavelengths corresponding to the non-absorbing region for silicon. The reduction in the quality of the transmission modes has been attributed to the dispersion and backscattering from the interfaces of the complete structure. As the optical response of pSi based sensor depends on the characteristics of the multilayered photonic structure, a comparison of sensitivity of 3rd order Cantor structure with a regular microcavity structure of similar optical thickness resulted in an optical response dependent on the choice of the refractive indices. An optimal choice of the refractive indices for the Cantor structures resulted in an increased red-shift (due to the enhancement of the refractive indices of the corresponding layers), after the surface modification of the photonic device. Such structures offer the possibility to improve the sensing response of the pSi-based photonic structures.