Characterisation of GaN–In x Ga1−x N quantum-well lasers

An efficient analytical model to calculate the optical gain of the quantum-well laser of the GaN–In _x Ga_1?x N material system is used in this paper. Based on the anisotropic effective mass theories, empirical formulas delineating the relations between optical gain, emission wavelength, well width and material compositions are obtained for such a quantum-well lasers.     

Exciton and intracenter radiative recombination in ZnMnTe and CdMnTe quantum wells with optically active manganese ions

The emission spectra of Zn_1?x Mn _x Te/Zn_0.6Mg_0.4Te and Cd_1?x Mn _x Te/Cd_0.5Mg_0.5Te quantum-well structures with different manganese concentrations and quantum-well widths are studied at excitation power densities ranging from 10⁵ to 10⁷ W cm^?2. Under strong optical pumping, intracenter luminescence of Mn²⁺ ions degrades as a result of the interaction of excited managanese ions with high-density excitons. This process is accompanied by a strong broadening of the emission band of quantum-well excitons due to the exciton-exciton interaction and saturation of the exciton ground state. Under pumping at a power density of 10⁵ W cm^?2, stimulated emission of quantum-well excitons arises in CdTe/Cd_0.5Mg_0.5Te. The luminescence kinetics of the quantum-well and barrier excitons is investigated with a high temporal resolution. The effect of the quantum-well width and the managanese concentration on the kinetics and band shape of the Mn²⁺ intracenter luminescence characterized by the contribution of the manganese interface ions is determined.     

The phonon dispersion relations in a disordered Ni1-xPtx system

The phonon dispersion relations of disordered alloys Ni_1-xPt_x were measured for five specimens covering the full range of atomic concentration by means of neutron inelastic scattering. At the resonant frequencies well separated double peak structures were observed in the phonon spectra. For the specimens with $\text{x ≧ 0.30}$, the energy splitting of the phonon dispersion curve for the L-branch takes place at a higher frequency than that for the T-branch. The maximum phonon energy decreases with increasing Pt concentration but deviates from the theoretical prediction based on the mass defect CPA model. For x = 0.95 the localized mode cannot be observed as a well defined peak and does not seem to be split off from the main phonon band.     

Determination of the indirect band edge of GaAs by quantum-well bandfilling (Lz∼100Å)

Photoexcitation-bandfilling of a quantum-well Al_xGa_1?xAs-GaAs-Al_xGa_1?xAs (x～0.6, L_z～100Å) heterostructure is used to generate visible-red recombination radiation that cuts-off at high energy (1.864 eV, 4.2°K) due to electron transfer to indirect minima. This leads to an estimate of ΔE = E_L?E_Γ～310 meV.     

Generation of phosphor nanoparticles for temperature sensing by laser ablation in liquid

Nano-size phosphor particles of Y_2.97Ce_0.03(Al_1?x Gd _x )₅O₁₂ were fabricated by ablating commercial micron-size powders in deionized water. We show that these colloidal phosphor nanoparticles suspended in deionized water can be used as a liquid sensor for all-optical, non-contact measurements of temperature with nanosecond time resolution. The nanophosphors can be used as temperature-sensing reporters in many applications where real-time measurements of temperature are necessary to understand physical processes, such as the mechanisms of temperature–time profiles in laser ablation.     

Lattice dynamics of a disordered Ni1−xPtx system in the pseudo unit cell model

The phonon dispersion relations for a disordered Ni_1?xPt_x system have been derived using the pseudo unit cell model proposed by Varshney et al. The results are presented for a few values of x along the [00ζ] direction and compared with recent measurements of Kunitomi et al. Reasonable agreement is found between the calculated and measured dispersion curves at all concentrations. In particular the localised impurity mode for low Ni concentrations does not split off from the main phonon band, in agreement with experiment and in contrast with the mass defect CPA prediction.     

Cavity size and temperature dependence of stimulated emission in single Zn1 − xCdxSe/ZnSe quantum-well lasers

We observed stimulated emission originating from light-hole transitions in single Zn_1 − xCd_xSe quantum-well structures, using high excitation photoluminescence spectroscopy as a function of temperature and cavity length. We found that the lasing wavelength depends on the cavity length, and can be switched by varying the sample temperature. The occurrence of lasing from light-hole recombinations in short cavities is also accompanied by an increase in the threshold of stimulated emission, with respect to larger cavities.     

The influence of the skin effect on FMR in MnFe ferrites

FMR experiments on the system Mn _x Fe_3?x O₄ have been performed at 7·8 and 15·2 (or 16) GHz in the temperature range 80–300 K. The temperature dependences of the linewidth, resonance field and lineshape have been studied on spherical samples with diameter 0·25–1 mm. It has been shown that the observed dependences from greater part reflect the decrease of the skin depth with increasing temperature and are connected with the influence of the skin effect. This mechanism is discussed in more detail. The maxima in theΔH vs.T plot are compared to similar ones predicted for an infinite slab. The behaviour of the resonance field and lineshape is shown to be essentially dependent on the position of the uniform mode frequency to the upper limit of the spin-wave manifold. An approximative method has been given for treating resonance in the surface layer of the spherical sample and the corresponding surface mode. This mode and a further mode similar to the Walker magnetostatical (3,1,1) mode have been observed at higher temperatures. It has been experimentally verified that they can coexist with the nearly uniform mode.     

Optical properties of MgxZn1−xO nanowire photonic crystals

We investigate the optical properties of two-dimensional periodic arrays of well-aligned Mg_xZn_1−xO nanowires, i.e., Mg_xZn_1−xO nanowire photonic crystals. The nanowire photonic crystal can exhibit a photonic band gap in the visible range. As the mole fraction of Mg, x, increases, the edge frequencies of the band gap increase and the band gap size decreases. The characteristics of relative band gap and vacant point defect mode are also studied with varying x. From the finite-difference time-domain simulations, we show that the light extraction from nanowires can be controlled by varying the distance between optically excited nanowires and a waveguide, and the mole fraction of Mg. Controlling the light extraction from nanostructures can be useful in the implementation of nanoscale light emitting devices.     

Electromagnetically induced transparency in double quantum dot under intense laser and magnetic fields: from Λ to Ξ configuration

We theoretically investigated the effects of non-resonant intense laser and magnetic fields on the optical properties of asymmetric GaAs/AlGaAs double quantum dot related to the occurrence of electromagnetically induced transparency, using compact density-matrix formalism and effective mass approximation. The chosen structure has the advantage to present x-lambda(Λ)-configuration or y-ladder(Ξ)-configuration for EIT occurrence, depending on lasers polarization, at low values of the non-resonant laser, and to change the configuration from Λ to Ξ at the increase of the x-polarized non-resonant laser intensity. We discussed in detail the influences of the control laser field intensity, non-resonant laser strength and polarization, and magnetic field intensity on the absorption coefficient, refraction index and group index. It is found that: (i) the control laser or the non-resonant laser at the same control laser intensity influences more the system being in x-Ξ-configuration than in x-Λ-configuration and have intermediate effects on y-Ξ-configuration; (ii) the magnetic field has the greatest influence on the system being in x-Λ-configuration and the lowest for y-Ξ-configuration; (iii) the increment of the non-resonant intense laser or magnetic fields induces a red-shift of the transparency windows and sub (super) luminal frequency intervals for the Λ-configuration but a blue-shift for both Ξ-configurations.     

Visible light transmission properties of double metal thin gold films with sub-wavelength hole arrays

The transmission spectrum of linearly polarized visible light through double metal thin films perforated with nano-hole arrays is investigated and simulated by using the three dimensional finite-difference time-domain method. The results show that the transmission spectra can be controlled by changing the longitudinal interval G between films and, their lateral displacements L_x and L_y, which are parallel and perpendicular to the polarization direction of the incident light, respectively. We have two important peaks (due to guided mode and SP mode) in these spectrums. The variation in longitudinal distance results a wavelength shift in guided mode peak of transmission spectrum while the wavelength of SP mode peak remains fixed. The lateral displacement L_x leads to the higher transmission of the guided mode peak, while the lateral displacement L_y suppresses the transmission of this peak. Here we try to discuss the physical explanations of these spectral behaviours by surface plasmon waves on the metal films and by using the concepts of surface plasma (SP) and guided modes in our double metal structure.     

The effect of Fe doping on structural, magnetic and electrical transport properties of CaMn1−xFexO3 (x=0-0.35)

A comprehensive study of the effect of Fe doping on CaMnO₃ is carried out by means of experiments on the structural, transport conduction, and magnetic properties of CaMn_1−xFe_xO₃ (0≤x≤0.35). With a sol-gel process for sample preparation, Fe is substituted for Mn up to x=0.35. This substitution substantially brings out the lattice expansion and gradually suppresses the antiferromagnetism. For x=0.08 and 0.10 in particular, the magnetization curves with a field-cooled mode under the field of 1 kOe behave as those of a ferrimagnetic-like system and present low-temperature negative magnetization. For x≥0.15, the negative magnetization phenomenon disappears, and a ferromagnetic component coexists with an antiferromagnetic one, but the antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show insulating behavior for all compositions. Fe doping, even at a level as low as x=0.02, can cause a marked resistivity increase in the temperature range studied. Further increasing the Fe content causes the resistivity to gradually decrease due to the increasing carrier presence.     

Modified Elliptic Gamma Functions and 6d Superconformal Indices

We construct a modified double elliptic gamma function which is well defined when one of the base parameters lies on the unit circle. A model consisting of 6d hypermultiplets coupled to a gauge field theory living on a 4d defect is proposed whose superconformal index uses the double elliptic gamma function and obeys W(E ₇)-group symmetry.     

Mössbauer-effect investigation of the magnetic phases in the Mn1−xCrxSb system

The magnetic phases of the Mn_1?xCr_xSb system, at 4.2°K, have been investigated through the Mössbauer effect in Sb¹²¹. The ferromagnetic component of the magnetization induces an effective magnetic field at the Sb nuclei which is observed to decrease with increase in Cr concentration. Bulk magnetization and neutron diffraction data are given for comparison and the ‘double exchange’ interaction model is considered in the discussion of the results.     

光子晶体微腔中高偏振单偶极模的研究

三角晶格排列的光子晶体微腔中的偶极模式是简并的,通过改变其晶格的对称性可以消除模式简并.晶格的整体形变破坏了晶格对称性从而影响光场的分布,同时还改变了电磁场的偏振分布.晶格整体形变使得简并的偶极模式分离成x极和y极偶极模式.通过计算分析发现分离后的模式具有良好的偏振特性,从而为实现单偏振光子晶体激光器提供了一种很好的途径.文中针对光子晶体薄板结构的微腔,主要计算了偶极模中x极偶极模式在不同拉伸时以及不同填充因子情况下的Q值,并分别计算了x关键词： 光子晶体 偶极模 品质因子 偏振度     

Index of refraction and d. c. electrical conductivity in Ge40−xSbxS60 glasses

The spectral dependence of the index of refraction (n) in Ge_40?xSb_xS₆₀ glasses has been described by a simple one-oscillator approximation using the Wemple-DiDomenico treatment (W-D). The possibility of application of the Moss formula or W-D in some amorphous materials for estimation of the optical gap (E _g) fromn or vice versa has been examined. From the results of optical and d.c. electrical conductivity measurements and bond statistics suggestions it can be concluded that many types of defect states are created in Ge_40?xSb_xS₆₀ glasses, which pin the Fermi level in the vicinity ofE _g/2.     

Temperature and composition dependence of vibrational modes in Sm1−xLaxAlO3

At room temperature and for x₀ ～ 0.43, the system Sm_1?xLa_xAlO₃ undergoes a first order phase transition in which an orthorhombic structure transforms into a rhomboedral structure as x increases. This phase transition was investigated by Raman scattering experiments on polycrystalline compounds in the range 0 ? x ? 1. The dependence of low frequency modes on x is reported and compared with their well known temperature dependence. Soft modes have been observed in the orthorhombic (D_2h¹⁶) and rhomboedral (D_3d⁶) phases. It was found that the first order phase transition is probably driven by a double degenerate mode whose two components in the low symmetry phase display a linear composition dependence of their squared frequencies. The source of anomalous behaviour of Raman bands near the phase transition is discussed.     

AC susceptibilities and size effect in Ln0.7(Sr,Ca)0.3MnO3 CMR manganites

The low field susceptibility versus temperature has been studied for the series of colossal magnetoresistive (CMR) manganites Ln_0.7Ca_0.3−xSr_xMnO₃ (LnPr, x = 0.10, 0.05, 0; LnNd, x = 0). The effect of the average size of the interpolated cation upon T_c is confirmed for the larger sizes (LnPr, x = 0.10, 0.05) showing a classical ferromagnetic behavior at low temperature in agreement with the neutron diffraction studies. For a smaller size of the A-site cation (LnPr or Nd, x = 0) an original behavior is observed: the _χ′(T) curves show a spin-glass-like behavior with a cusp at T_cusp, whereas the neutron diffraction data in zero magneteic field evidence a ferromagnetic component starting around T_cusp.     

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.     

Tunable double-beam optical bistability in an asymmetric double quantum-well system

We have proposed a scheme for double-beam optical bistability in a tunnel-coupled asymmetric double quantum-well driven by two optical fields circulating inside two independently unidirectional ring cavities. In contrast to the single-cavity case where single-photon saturated absorption and self Kerr-nonlinearity are dominant, the two-photon absorption and cross phase modulation can be enhanced via tunneling induced interference and have an important influence to the formation of bistability. The bistable behavior can be controlled effectively via the system parameters such as the input and detuning of control field, the detuning of the probe field, Fano interference strength and cooperation parameter. Furthermore, the proposed scheme has the ability to manipulate the outputs of two optical cavities simultaneously. Due to the flexible design of semiconductor quantum well, our scheme is more practical than atomic system, therefore it can be utilized to achieve dual all-optical switching which has application in optical communication and computing.