硅薄膜的短波红外光学特性和1.30μm带通滤光片

在短波红外区域(1～3μm),硅薄膜材料因其具有折射率高、透明性好、膜层应力易匹配等诸多优点而得到广泛应用。基于改进后的Sellmeier模型拟合出了制备的硅薄膜的短波红外光学特性,以此为基础,选用硅和二氧化硅两种材料,设计并制备出中心波长在1.30μm,相对带宽2.46%的带通滤光片。利用了硅薄膜在波长小于1.0μm波段的吸收特性较好地扩展了带外截止范围。测量结果表明,具有2个谐振腔的带通滤光片峰值透射率达到85.8%,半功率带宽控制在约32nm,带外截止范围覆盖了波长小于1.75μm的光谱区域。     

铁磁-绝缘层-d波超导结中的Andreev反射特性

考虑到铁磁层中的磁交换作用和界面散射效应,利用推广的Blonder-Tinkham-Klapwijk散射理论,计算铁磁-绝缘层-d波超导结中的准粒子输运系数与隧道谱.研究表明：铁磁层中的交换场能抑制Andreev反射,使得依赖于准粒子入射角的Andreev反射表现出瞬息波的行为,呈现一些新的隧道谱特征. 关键词： Andreev反射 d波超导 隧道谱     

Excitation Spectrum of Spin-1 Bosonic Atoms in Mott Insulating Phase

The effective action for spin-1 bosonic atom in an optical lattice is derived. The quasiparticle and quasihole dispersions are calculated for different cases by using a functional integral formalism. For all cases, the excitation spectra are analyzed. All the quasiparticle and quasihole excitations start with a gap.     

Andreev levels in a graphene–superconductor surface

In order to consider the Dirac-like spectrum of graphene we employ the Bogoliubov de Gennes–Dirac formalism to determine the quasiparticle Andreev levels in an NS surface (normal–superconductor). The normal region is characterized by a width L while the superconducting region is semi-infinite and both regions are made of doped graphene. The quasiparticle energy spectrum is originated by the Andreev reflections that occur in the NS interface. It is shown that this spectrum depends on the width of the normal region and the Fermi energy in each region. When the Fermi energy in the normal metal is lower than the gap of the superconductor region, the spectrum is affected by specular Andreev reflections. The equation that is obtained to find the spectrum is very general and we solve it for some particular cases. We find that the energy spectrum oscillates when the Fermi energy in graphene is changed. Finally we obtain under some approximations an equation for the energy spectrum which is similar in structure as those obtained for an INS conventional junction.     

基于第一性原理计算的掺杂柔性黑硅吸收光谱特性分析

针对宽波段微型光谱仪缺乏宽波段柔性探测器这一难题,提出了一种在紫外-可见-近红外波段内具有高吸收效率的掺杂柔性黑硅作为探测器吸光材料。首先,基于第一性原理计算了S和F元素掺杂后柔性黑硅的电子结构、能带结构和紫外-可见-近红外波段的光学吸收特性,得到了不同元素及浓度掺杂时,柔性黑硅的光学吸收系数。其次,将第一性原理计算结果与时域有限差分算法相结合,建立了柔性黑硅的吸收光谱模型。结果表明,掺入S和F元素后柔性黑硅的能带带隙均减小,吸收截止波长发生红移,且掺杂浓度越高,光学吸收系数越大。在1 500 nm波长处,50%浓度的S元素掺杂黑硅的吸光系数是1.5%浓度的S元素掺杂黑硅的吸光系数的8.3倍,50%浓度的F元素掺杂黑硅的吸光系数是1.5%浓度的F元素掺杂黑硅的吸光系数的3倍。在相同掺杂条件下,表面具有小尺寸微结构的柔性黑硅在近红外波段具有最高的吸收效率。最后,测试了制作的柔性黑硅样品,其吸光效率在紫外-可见波段高于95%,在近红外波段为70%~80%。     

Linear and nonlinear optical absorption coefficients in an asymmetric graded ridge quantum wire

In this article, the optical absorption coefficients in an asymmetric ridge quantum wire within the framework of the density matrix formalism are studied. The energy spectrum and wave functions of a quantum wire with graded confinement potential using the effective mass approximation are analytically calculated. The results show that parameters such as the asymmetry and width of the potential well change the position and magnitude of the absorption peak and saturation intensity. The incident optical intensity also has a great effect on the total absorption.     

Determination of doping effects on Si and GaAs bulk samples properties by photothermal investigations

The knowledge of doping effects on optical and thermal properties of semiconductors is crucial for the development of opto-electronic compounds. The purpose of this work is to investigate these effects by mirage effect technique and spectroscopic ellipsometry SE.

The near gap optical spectra are obtained from photothermal signal for differently doped Si and GaAs bulk samples. However, the above bandgap absorption is determined from SE. These spectra show that absorption in the near IR increases with dopant density and also the bandgap shifts toward low energies. This behavior is due to free carrier absorption which could be obtained by subtracting phonon-assisted absorption from the measured spectrum. This carrier absorption is related to the dopant density through a semi-empirical model.

We have also used the photothermal signal phase to measure the influence of doping on thermal diffusivity.     

Introduction of quantum finite-size effects in the Mie's theory for a multilayered metal sphere in the dipolar approximation: Application to free and matrix-embedded noble metal clusters

A mixed classical/quantum model for calculating the optical response of free and matrix-embedded multilayered metal spheres in the dipolar approximation is presented. The conduction electrons are quantum-mechanically treated in the framework of the time-dependent local-density-approximation formalism (TDLDA), whereas the surrounding matrix, the ionic metal backgrounds and the non-metallic materials are classically described through homogeneous charge distributions or/and dielectric media. Except for the TDLDA calculations, the present formalism is completely analytical and can be applied to coated spheres with any number of metal or dielectric layers. Contrary to the previous TDLDA-based models involving an inner or/and an outer dielectric medium (one or two interfaces), all the dielectric effects (screening and absorption) are self-consistently calculated. In particular, the interband transitions and the mutual interplay between the conduction and core electrons are self-consistently treated. The deficiencies of the previous models are analyzed, and the results are compared with the classical Mie's theory, over the entire spectral range. The building-up of the classical absorption spectrum, consisting of the surface plasmon resonance and the interband transitions, is clearly observed as the cluster size increases. Received 15 March 1999 and Received in final form 11 October 1999     

Statistical properties of a paired nucleus with fixed quasiparticle number

The pairing Hamiltonian, diagonal in the quasiparticle space, has been used to determine the statistical and thermodynamical properties of a system with fixed number of quasiparticles. The formalism can describe the phase transition occuring at fixed energy as the system relaxes from the initial to the equilibrium quasiparticle number.     

Optical-absorption spectrum of silicon containing internal elastic stresses

The spectrum of the coefficient of optical absorption in silicon in the presence of internal (elastic, mechanical) stresses is calculated. Band splitting, including the weight factor of the split subbands, is allowed for. An ordinary equation for the optical-absorption coefficient is obtained by allowing for a correction that reflects the real relation between the energy of quantum pulses and the actual forbidden gap. The results obtained can be used in practice for assessing the internal stresses of silicon crystals and structures as well as for diagnostics of recombination parameters of imperfect silicon. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 576–580, July–August, 1998.     

Zero-phonon linewidth and phonon satellites in the optical absorption of nanowire-based quantum dots

The optical properties of quantum dots embedded in a catalytically grown semiconductor nanowire are studied theoretically. In comparison to dots in a bulk environment, the excitonic absorption is strongly modified by the one-dimensional character of the nanowire phonon spectrum. In addition to pronounced satellite peaks due to phonon-assisted absorption, we find a finite width of the zero-phonon line already in the lowest-order calculation.     

Description of odd transitional nuclei in terms of core-quasiparticle models

A general core-quasiparticle coupling (CQPC) formalism is presented that properly accounts for the particle-hole structure in odd-A transitional nuclei. The method is a consequent generalization of the Coriolis-coupling approach. The formalism is applied to the coupling of a quasiparticle to the γ = 30° triaxial rotor and to a γ-unstable core. A comparison with the experimental ¹⁹¹Pt spectrum shows that the odd particle is not a sufficiently sensitive probe to distinguish between the two core models.     

Two-photon spectroscopy in AgCl

Two-photon absorption constant β⁽²⁾ has been measured in the indirect gap of AgCl between 4 and 4.4 eV. The absorption constant has been derived from the excitation spectrum of the recombination luminescence. The absolute value of β⁽²⁾ is in fair agreement with that calculated for a two-photon phonon-assisted transition.     

From Si nanowires to porous silicon: the role of excitonic effects

We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modeling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.     

Exchange and correlation effects in electronic excitations of Cu(2)O

State-of-the-art theoretical methods fail in describing the optical absorption spectrum, band gap, and optical onset of Cu(2)O. We have extended a recently proposed self-consistent quasiparticle approach, based on the GW approximation, to the calculation of optical spectra, including excitonic effects. The band structure compares favorably with our present angle-resolved photoemission measurements. The excitonic effects based on these realistic band structure and screening provide a reliable optical absorption spectrum, which allows for a revised interpretation of its main structures.     

Femtosecond pulse propagation in silicon waveguides: Variational approach and its advantages

We investigate the propagation characteristics of ultrafast pulses inside silicon waveguides considering frequency chirp associated with each input pulse. Effects of linear losses, two-photon absorption, and free-carrier dynamics are included analytically within the framework of a modified variational formalism and the results are validated by comparing them with full numerical simulations. It is found that an initial chirp helps in maintaining the pulse shape and spectrum in the anomalous-dispersion regime, thereby resulting in soliton-like propagation of ultrashort optical pulses.     

Linear and nonlinear intersubband optical absorptions in an asymmetric rectangular quantum well

The linear and nonlinear intersubband optical absorptions in Al_xlGa_1-xlAs/GaAs/Al_xr Ga_1-xrAs asymmetric rectangular quantum well are studied within the framework of the density matrix formalism. We have calculated the electron energy levels and the envelope wave functions using the effective mass approach. In addition, we have obtained an expression for saturation intensity. It is shown that the parameters such as asymmetry and width of potential well not only shift the peak positions in absorption spectrum but also considerably modify their height. These results suggest that the absorption process can be easily controlled by the structure parameters of an asymmetric rectangular quantum well. Also, the incident optical intensity has a great effect on the total absorption spectrum. We have seen that the absorption peak is reduced by half when the optical intensity is approximately 0.8 MW/cm² for well width L=90 ? and β=0.5. Moreover, it is seen that the saturation intensity is quite sensitive to the structure parameters of an asymmetric rectangular quantum well. Thus, the results presented here can be useful for electro-optical modulators and photodetectors in the infrared region.     

Ab initio absorption spectra and optical gaps in nanocrystalline silicon

The optical absorption spectra of Si(n)H(m) nanoclusters up to approximately 250 atoms are computed using a linear response theory within the time-dependent local density approximation (TDLDA). The TDLDA formalism allows the electronic screening and correlation effects, which determine exciton binding energies, to be naturally incorporated within an ab initio framework. We find the calculated excitation energies and optical absorption gaps to be in good agreement with experiment in the limit of both small and large clusters. The TDLDA absorption spectra exhibit substantial blueshifts with respect to the spectra obtained within the time-independent local density approximation.     

Defect absorption and optical transitions in hydrogenated amorphous silicon

Using an empirical model for the density of states functions associated with hydrogenated amorphous silicon, with defect states taken into account, we examine how the distributions of such states shape the optical response of this material. The contributions to this response attributable to the various types of optical transitions are also determined. Finally, we demonstrate that we are able to capture the spectral dependence of the optical absorption coefficient associated with a defect absorption influenced sample of hydrogenated amorphous silicon using our empirical formalism for the density of states functions associated with this material.     

Dynamics of Electronic States and Magnetoabsorption in 3D Topological Insulators in a Quantizing Magnetic Field

Quantum states have been calculated analytically; the dynamics of a wave packet in a magnetic field has been investigated, and the optical absorption coefficient has been calculated for surface states in 3D topological insulators of the Bi₂Te₃ family. We have detected a qualitative effect of the hexagonal warping of the spectrum on the structure of wavefunctions at the Landau levels, its manifestation in the features of the wave packet dynamics in a quantizing magnetic field, as well as in the frequency dependence of the optical absorption coefficient, in which new peaks that are absent in the isotropic model of the spectrum appear depending on the polarization of the incident wave. The effects considered here can be manifested in the optical and transport experiments with topological insulators, which makes it possible to determine the parameters of their band structure.