Quantized longitudinal exciton-polaritons in periodic metal–semiconductor nanostructures

We theoretically investigate the optical properties of one-dimensional photonic crystals composed of two alternating layers, namely a semiconductor film and a metallic one. The nonlocal optical response of the semiconductor is here described by using a resonant excitonic dielectric function, whereas the local response function of the metal film is modeled with Drude formula. We calculate optical spectra of the metal–semiconductor 1D photonic crystal for both s- and p-polarization geometries. In both cases the spectra exhibit a rich resonance structure due to the coupling of size-quantized excitons inside the semiconductor film with light. We show the difference between s- and p-polarization reflectivity as the angle of incidence is increased. In the p-polarization geometry, besides transverse exciton-polariton modes, longitudinal polarization waves are excited producing additional spectral resonances. The spectra become radically different when the frequency corresponding to the minimum of the first photonic pass-band is close to the exciton resonance, since such a frequency is distinct for s- and p-polarized modes. We also show how reflectivity spectra for both polarizations are modified with varying the metal filling fraction which controls the width of the gap below the lowest frequency band.     

Specific features of the reflection of infrared radiation by crystalline dielectrics in a magnetic field

Magnetic-field-induced variations in the reflection spectra R(λ) of the crystalline dielectrics Al₂O₃, LiF, and MgO in the infrared band (λ = 2.5–25 μm) are investigated. It is found that the reflection spectra exhibit specific features in the neighborhood of wavelengths corresponding to the excitation of optical phonon modes in the above-mentioned crystals and that a magnetic field causes an appreciable variation in the reflectivity at these wavelengths. To qualitatively describe the effect of a magnetic field on the reflection of light, the magnetoreflection spectra ΔR/R are investigated. The spectra ΔR/R exhibit sharp peaks in the neighborhood of wavelengths at which the materials under investigation are characterized by minimal reflectivity. The values of ΔR/R for p-polarized infrared radiation in a magnetic field of about 12 kOe amount to about 0.5% for Al₂O₃ at λ ≈ 9.6 μm, 7% for LiF at λ ≈ 11.1 μm, and 0.07% for MgO at λ ≈ 11.7 μm.     

Magnetic-field oscillations of the critical current and the pinning force in PbTe/PbS semiconductor superlattices and YBa2Cu3O7–δ films

The magnetic-field dependences of the critical current I _c and the pinning force F _p in single-crystal semiconductor PbTe/PbS superlattices on KCl and YBa₂Cu₃O_7–δ films in magnetic fields oriented perpendicular to the plane of the samples or parallel to the current were investigated. Oscillations of I _c and F _p were observed for superlattices with a parallel orientation of the magnetic field and with two directions of the field for the YBa₂Cu₃O_7–δ film. A model was proposed for the vortex structures which correspond to extrema of the pinning force in superlattices. It was shown that the single points (extrema and points of inflection) of the field dependences of F _p for superlattices and a YBa₂Cu₃O_7–δ film appear for critical values of the magnetic field equal to H _cr = (p/q)H ₀, where p and q are integers, H ₀ = 838.37 Oe, and H ₀ = ch/π eR ₀ ² is determined by the length R ₀ = 88.607 nm, somewhat less than the reciprocal of the Rydberg constant R _∞ = me ⁴/π?³ c (R _∞ ^?1 = 91.127 nm). It was inferred on the basis of published data that the temperature and magnetic-field dependences of the properties of superlattices and HTSC materials follow general laws.     

Higher multipole surface plasmons,plasmon bands in periodic metallic heterostructures,and plasma modes of semiconductor superlattices

The electronic collective excitations are investigated for a number of different systems whose equilibrium electron density n₀(z) is spatially varying. The relation between higher multipole surface modes, waveguide like plasma modes of a low electron density layer embedded in a high density host, and plasmon bands of periodic metallic heterostructures and semiconductor superlattices is pointed out.     

Quasiperiodic magnonic superlattices with mirror symmetry

We address the spin wave modes propagating in Fibonacci, Thue–Morse, and double period quasiperiodic magnonic superlattices. These structures are made of layers of a metamagnetic material alternating with layers of a nonmagnetic material, presenting mirror symmetry. Our calculations are carried out in the magnetostatic regime for the antiferromagnetic phase. Our model takes into account the presence of an external applied magnetic field, which is perpendicular to the interfaces of the superlattice, as well as the crystalline anisotropic contribution to the inner magnetic field. The magnetostatic bulk and surface modes are obtained by using the transfer matrix technique. The metamagnetic material considered here is FeBr₂, however, our results can be extended to other materials. Our numerical results show the behavior of these modes, for small frequencies of the energy spectra. The results reported here can be experimentally observed by light scattering techniques.     

Modulation polarimetry of the topological effect in gold-organic nanocomposite films

The phenomenon of surface plasmon resonance in composite films consisting of gold nanoclusters in matrices of organic molecular materials calix[4]arene and poly(N-vinylcarbazole) has been investigated. The internal reflection coefficients R _s ² and R _p ² of s- and p-polarized light and their physical difference ρ = R _s ² ? R _p ² have been measured according to the Kretschmann scheme as a function of the angle of light incidence θ at different wavelengths λ in the range 400–1000 nm. The angular characteristics reflect the cluster structure of the films, which is confirmed by electron microscopy. A topological size effect has been revealed. This effect is associated with the dependence of the excitation energy efficiency of surface plasmons on the azimuth of the linearly polarized light, the shape, and the distribution of nanoclusters in the coordinate space. The dependences ρ(λ) demonstrate that the local plasmon resonance is excited by both s- and p-polarized light, whereas the polariton resonance is excited by s-polarized light. The sign of the curvature of the dependence ρ(θ) determines the predominance of the excitation energy efficiency of electromagnetic modes with one of the two states of polarization of the excitation radiation.     

Enhanced photosensitivity of InGaZnO-TFT with a CuPc light absorption layer

A copper phthalocyanine (CuPc) organic semiconductor is capped onto an amorphous indium–gallium–zinc-oxide (InGaZnO) thin film transistor (TFT) to enhance the photosensitivity of InGaZnO-TFT. The CuPc organic semiconductor is served as a light absorption layer and forms a p–n junction with the InGaZnO film. After 60 s white light illumination, light responsivity (R) of InGaZnO-TFT with a CuPc light absorption layer reaches a value of 148.5 A/W at a gate-source voltage (V_GS) of 20 V, which is much larger than that (31.2 A/W) of the conventional InGaZnO-TFT. The results are attributed to the following mechanism. First, a CuPc layer is employed as the light absorption layer. Second, CuPc/InGaZnO p–n junction enables the injection of electron into InGaZnO film. Our results indicate that using CuPc as light absorption layer is an effective approach to improve the photosensitivity of InGaZnO-TFT.     

Thermodynamic instability of Ag/Au and Cu/Pd metal superlattices

We show how the formation energies of A_pB_q superlattices with arbitrary periods p and q and layer orientation Ĝ can be predicted via a 'cluster expansion' technique, given the formation energies of short period structures from first-principles calculations. We predict both bulk and epitaxial energies as well as the energies of the fully intermixed (alloyed) superlattices. Applications to Ag/Au and Cu/Pd superlattices illustrate our method, as well as a global classification scheme for superlattice stability.     

Optical measurements of silicon wafer temperature

An optical technique for precise, non-contact, and real time measurement of silicon wafer temperature that uses the polarized reflectivity ratio R_p/R_s is described. The proposed method is based on temperature dependence of the optical functions of silicon. Expected strong temperature sensitivity is obtained near band gap. Simultaneous monitoring of temperature and oxide layer thickness is discussed using measurements at four wavelength 365 nm, 405 nm, 546 nm, and 820 nm.     

Suppression of the singularly localized states in dimer quasiperiodic Fibonacci superlattices

Using the transfer-matrix technique, we have numerically investigated the effect of introducing the dimer on the nature of the states across Dimer Fibonacci semiconductor superlattices on the miniband structure of the GaAs/Al_xGa_1?xAs superlattices. By the introduction of the dimer model, the transmission spectra reveal the appearance of a miniband structure with a concomitant disappearance of the singularly localized states. This behavior is due to the interaction between the states of the dimer wells inside the potential and, therefore, the system is seen by the particle as two overlapped ordered structures.     

The resonance interaction of vibrational modes in one-dimensional nonlinear lattices

Resonances of vibrational modes were for the first time revealed for the example of the one-dimensional random Morse lattice. The observation of resonances was possible because of lattice deformation, when, at certain relative deformation values, vibrational modes satisfied the conditions of double (m _i ω _i + m _j ω _j = 0) or triple (m _i ω _i + m _j ω _j + m _k ω _k = 0) resonances. Of all the resonances observed, the resonance with the frequency ratio ω₂: ω₁ = 2: 1 was studied in detail. The dependences of mode lifetimes and the degree of energy exchange between them on such parameters as resonance frequency detuning, excitation energy level, etc. were determined. A model of two nonlinearly coupled harmonic oscillators was considered in detail on the assumption of a one-to-one correspondence between oscillators and vibrational modes. A consideration of the model problem of oscillators revealed analytic dependences of the dynamic behavior of vibrational modes on control parameters. Excellent agreement between the numerical results for the Morse lattice and analytic conclusions was obtained. It was shown that, for the Fermi-Pasta-Ulam lattice, the resonance interaction of vibrational modes was controlled by the same rules as with the Morse lattice.     

窄带隙超晶格中载流子俄歇寿命和碰撞电离率的第一性原理研究

通过第一性原理的完整形式,基于全势能线性化增广平面波方法确定的精确能带结构和波函数,推算了技术上极为重要的窄带隙半导体超晶格中载流子俄歇复合时间.少数载流子的俄歇寿命由两种相关的方法来确定:1)由Fermi-金规则直接估算,2)联系俄歇复合和其相反过程碰撞电离,建立细致平衡公式,在一个统一的结构中进行间接估算.在n掺杂HgTe/CdTe和InAs/In_xGa_1－xSb超晶格中,由直接和间接的方法确定的寿命与一些实验结果相当一致.这说明该计算模式可以作为一种精确的手段用于窄带隙超晶格材料的性能优化. 关键词： 第一性原理 俄歇复合 碰撞电离 半导体超晶格     

I.R. Lattice-vibration spectra of MnF2

The room temperature reflectivity of MnF₂ has been measured in the far i.r. between 50 and 800 cm^?1. The reflectivity spectra show four i.r.-active modes which can be identified as E_u and A_2u modes. The spectra also show structure which might be related to multiphonon effects. The dielectric functions were determined by fitting the reflectivity data with classical oscillator parameters.     

Light scattering in a layer of correlatively arranged optically soft particles

The light scattering by an ensemble of monodisperse spatially correlated optically soft spherical particles is studied in the interference approximation. A model of the interaction of particles is proposed in which the spatial correlation between particles is determined by a radius R _c exceeding the particle radius R _p. The radial distribution function is calculated in the Percus-Yevick approximation for hard spheres of the radius R _c. To simulate the radiation scattering from an individual particle of the radius R _p, the Mie equations are used. It is shown that, in a medium of correlated small nonabsorbing particles of the radius R _c > R _p, an abnormal wavelength dependence of the refractive index is possible at a low volume concentration of particles. The results obtained explain some experimentally observed features of the scattering in sodium borosilicate glasses with a small concentration of scattering centers.     

High-pressure induced magnetoresistance in <Emphasis Type="Italic">p</Emphasis>-InAs:Mn and <Emphasis Type="Italic">p</Emphasis>-CdGeAs<Subscript>2</Subscript>:Mn

The baric (P ≤ 5GPa) and magnetic-field (H ≤ 5 kOe) dependences of the transverse magnetore-sistance Δρ _xx /ρ₀ have been measured for p-InAs (R _H = 22.5 cm³/C, ρ = 0.15 Ω cm) and the new ferromag-netic semiconductor p-CdGeAs₂ (R _H = 5 cm³/C, ρ = 0.62 Ω cm), doped with a magnetic impurity (Mn), near the temperature T = 297 K. The dependences Δρ _xx /ρ₀ (P, H) for p-InAs:Mn and p-CdGeAs₂:Mn exhibit a magnetoresistive effect with an increase in pressure, and a pressure-induced magnetoresistance hysteresis is observed in p-CdGeAs₂:Mn with a pressure drop.     

Infrared reflectivity of Ag6Ge10P12

Infrared reflectivity spectra of p-type Ag₆Ge₁₀P₁₂ single crystals were measured at room temperature in the wavenumber range from 180 to 4000 cm^-1. Fits to a dielectric function including contributions from lattice oscillators and free carriers gave the parameters of five lattice vibrational modes and an effective hole mass of m_p/m_o = 2.3.     

Phonon-plasmon interaction in tunneling GaAs/AlAs superlattices

The phonon-plasmon interaction in tunneling GaAs _n /AlAs _m superlattices (m=5and 6≥n≥0.6 monolayers) was studied by Raman scattering spectroscopy. The interaction of optical phonons localized in GaAs and AlAs layers with quasi-three-dimensional plasmons strengthens as the thickness of GaAs quantum wells decreases and the electronic states in the superlattices become delocalized due to tunneling. It is assumed that the plasmons also interact with the TO-like phonon modes localized in quantum islands or in thin ruffled layers.     

Optical properties of antiferromagnetic/ion-crystal superlattices

Transmission, refraction and absorption properties of an antiferromagnetic/ion-crystal superlattice are investigated. The transmission spectra based on FeF₂/TlBr superlattices reveal that there exist two intriguing guided modes in a wide stop band. Additionally, FeF₂/TlBr superlattices possess either the negative refraction or the quasi left-handedness, or even simultaneously hold them at certain frequencies of two guided modes, which require both negative magnetic permeability of antiferromagnetic layers and negative permittivity of ion-crystal layers. Frequency regimes of the guided modes will be dependent on the magnitude of the external magnetic field. Therefore, handedness and refraction properties of the system can be manipulated by modifying the external magnetic field. Absorption spectra exhibit that absorption corresponding to guided modes is noticeable.     

Effect of the Optical Pumping and Magnetic Field on the States of Phase Separation Domains in Eu0.8Ce0.2Mn2O5

The effect of optical pumping and applied magnetic field on the characteristics of ferromagnetic layers in one-dimensional superlattices is studied. At low enough temperatures, these layers correspond to phase separation domains in RMn₂O₅ and R_0.8Ce_0.2Mn₂O₅ multiferroics. The formation of such domains occurs owing to the charge ordering of Mn³⁺ and Mn⁴⁺ ions and to the finite probability for e_g electrons to tunnel between these pairs of ions. The volume occupied by such superlattices is rather small, and they can be treated as isolated ferromagnetic semiconductor heterostructures, spontaneously formed in the host crystal. The sequences of ferromagnetic resonances related to the superlattice layers in Eu_0.8Ce_0.2Mn₂O₅ are studied. The characteristics of these resonances give information on the properties of such layers. For the first time, it is demonstrated that the optical pumping gives rise to a new metastable state of superlattices, which can be recovered by the magnetic field cycling to the state existing before the optical pumping. It is found that the superlattices recovered by the magnetic field exist up to temperatures higher than those in as-grown crystals.

     

FP-LMTO study of structural,electronic and optical properties of wurtzite (CdS)n/(CdSe)n superlattices

In this paper, we have conducted a first-principles study of the structural, electronic and optical properties of (CdS)_n/(CdSe)_n superlattices (where n is numbers of monolayers) in the wurtzite phase (B4), using the Full-Potential Linear Muffin-Tin Orbital (FP-LMTO) method within the Local Density Approximation (LDA) technique, in order to describe the exchange correlation energy. The calculated electronic properties indicate that all (CdS)_n/(CdSe)_n superlattices configurations, possess a semiconductor behavior with same energy gaps. We have seen more carefully and accurately that the different superlattices configurations have no effect on the electronic properties; in particular, we did not observe any dependence between the band gap behavior and the used layers.