Enhanced intersubband absorption in stepped double barrier quantumwells

Intersubband absorption is measured in the conduction band of GaAs and stepped GaAs/In_xGa_1-xAs multiple-quantum-wells confined by narrow AlAs barriers. Enhanced absorption from n=1 to n=2 is observed in the stepped wells. This is attributed to relaxation of the intersubband polarisation selection rule     

Intersubband optical second-harmonic generation in multiple quantumwells: radiative coupling effects

An analysis of intersubband optical second-harmonic generation (SHG) in multiple quantum wells (QW's) is presented, in which special attention has been paid to radiative coupling effects. Starting from the Maxwell-Lorentz equations, the field at the second-harmonic frequency in the QW system is determined. Following the same framework, the field at the fundamental frequency is also obtained. In a reflection geometry, the SH conversion efficiency is derived for a p-polarized incident field. Detailed numerical calculations for different numbers of QW's, angles of incidence, as well as barrier thicknesses show that the SHG conversion efficiency can be significantly modified due to radiative coupling among wells     

On the intersubband optical bistability in semiconductor quantumwells: the influence of the permanent dipole moment

Some interesting features of the intrinsic optical bistability, based on intersubband transitions in asymmetric semiconductor quantum wells, are analyzed within the two-level model. In particular, the excitation-induced levels shifting, on which the bistability relies, is found to be significantly influenced by the exchange-correlation potential, thus requiring self-consistent modeling-the simple charge separation dipole model is not accurate enough. The optical influence of the permanent dipole moment, characterizing bound states in asymmetric quantum wells, is also analyzed. This is found to have some beneficial and some adverse consequences on bistability, both of which may be quite important in realistic structures     

Microscopic theory of gain, absorption, and refractive index insemiconductor laser materials-influence of conduction-bandnonparabolicity and Coulomb-induced intersubband coupling

The influence of the conduction-band nonparabolicity and Coulomb coupling between different electronic subbands and different hole subbands on gain, absorption, and refractive index in semiconductor heterostructures is investigated. We implement these features into a fully microscopic approach. At low carrier densities, the nonparabolicity leads to a steeper increase of the absorption for increasing transition energy. In this regime, the Coulomb subband coupling allows for a shift of oscillator strength to energetically lower transitions. In the gain regime, the conduction-band nonparabolicity is shown to reduce the gain width for a given carrier density and to strongly modify the corresponding refractive index. The Coulomb coupling is especially important to determine the correct energetic position and density dependence of the gain maximum. In addition, it leads to a steeper transition from the gain to the absorptive region     

The influence of dephasing in the coupling of light with intersubband transitions

This paper studies the dispersions of the quasiparticle resulting from the coupling of light with intersubband transitions. As in the interband case, increased losses/dephasing reduce the oscillator strength of the material excitation until a critical value is reached after which only a photon branch characterized by a renormalized group velocity remains.     

The effect of resonant sublevel coupling on intersubband transitions in coupled double quantum wells

The effect of resonant sublevel coupling on intersubband transitions in double quantum wells is investigated using far-infrared spectroscopy. We study widely tuneable parabolic double quantum wells in which potential spikes of different energetic height and thickness provide tunnel barriers for the electron systems on either side of the barrier. The use of gate electrodes enables us to tune both the carrier densities as well as the respective sublevel spacings and allow for a manifold degeneracy scheme like in an artificial molecule where the atomic number of both partners can be intentionally changed. Depending on the actual experimental condition, we observe pronounced level anticrossings into a symmetric and antisymmetric state. This single-particle sublevel coupling manifests itself in a rich spectrum of the observed collective intersubband transitions which occur at the depolarization shifted intersubband energy.     

Doping effects on intersubband and interband optical transitions inGaSb-InAs superlattices

Normal incidence intersubband and interband absorptions of a novel type II GaSb-InAs superlattices can be obtained by utilizing the various doped-type cap and buffer layers. Moreover, the types and intensities of the absorptions could also be modulated by changing doping concentration. The intersubband transition can occur due to the strong mixing of the heavy-hole band and the light-hole band for InAs n-type cap and buffer layers. But the interband transition is a result of coupling between the wave-functions of the first conduction subband and the first heavy-hole subband for GaSb p-type cap and buffer layers. Both the intensities of intersubband can be modulated by changing doping concentration, and the corresponding wavelengths are in the ranges of 3-5 μm and 8-14 μm, respectively. Hence, it shows the potential application as an infrared photodetector     

Nonlinear absorption properties of AlGaAs/GaAs multiple quantumwells grown by metalorganic chemical vapor deposition

The nonlinear absorption properties of the excitonic resonances associated with multiple quantum wells (MQWs) in AlGaAs/GaAs grown by metalorganic chemical vapor deposition are reported. The dependence of the saturation properties on growth parameters, especially growth temperature, and the well width are described. The minimum measured saturation intensity for these materials is 250 W/cm², the lowest reported value to date. The low saturation intensities are the result of excellent minority carrier properties. A systematic study of minority carrier lifetimes in quantum wells are reported. Lifetimes range from 50-350 ns depending on growth temperature and well width     

Comparison of hole and electron intersubband absorption strengthsfor quantum well infrared photodetectors

It is well known that the hole intersubband absorption of normally incident (TE polarized) radiation is nonzero for p-doped quantum well infrared photodetectors (p-QWIP's) which have been fabricated without an optical grating. This present paper shows from k&oarr;·p&oarr; theory that, for typical p-QWIP designs, this hole intersubband absorption of TE polarized radiation (without the help of an optical grating) is significantly smaller than the electron intersubband absorption of TE polarized radiation in those n-doped QWIP's (n-QWIP's) fabricated with an optical grating. A second result of this present work is that, even when there is significant mixing of the light and heavy hole states, the p-QWIP absorption of TE polarized radiation (without the help of an optical grating) is still much smaller than the n-QWIP absorption of TE polarized radiation (with the help of an optical grating). The reason is that the mixing of light and heavy hole states never increases the amount of |S〉-symmetry in the mixed hole wave function beyond the amount of |S〉-symmetry which was present in the unmixed, purely light hole state. Finally, this present paper shows from k&oarr;·p&oarr; theory that strained layer growth on an (001) substrate does not significantly affect the strength of the hole intersubband absorption. The reason is that the Hamiltonian describing uniaxially strained quantum wells has precisely the same (tetragonal) symmetry as the Hamiltonian describing carrier confinement in unstrained quantum wells. All of these results are important in choosing a QWIP device design     

Modeling of intersubband and free-carrier absorption coefficientsin heavily doped conduction-band quantum-well structures

Theoretical modelings of the transition energy for intersubband absorptions, and the intersubband and free-carrier absorption coefficients in heavily doped conduction-band anisotropic semiconductor quantum-well (QW) structures are presented. The transition matrix elements for photon absorption and emission, which are not identical due to the different many-body effects involved in the photon absorption and emission processes, are rigorously derived. We also show that the linewidth broadening effect caused by various scattering processes gives a considerable increase in resonance energy, which explains the relatively large parallel-mode transition energy which cannot be inferred from previous modeling studies. In addition, theoretical modeling of free-carrier absorption in anisotropic semiconductor QW structures is presented for the first time. The calculated results are compared with the experimental values for δ-doped Si QW's     

Optimal design of GaN-AlGaN Bragg-confined structures for intersubband absorption in the near-infrared spectral range

A method is proposed for the design and optimization of structural parameters of GaN-AlGaN Bragg-confined structures with respect to peak intersubband absorption from the ground to the first excited state,1 /spl rarr/ 2 electronic transition, in the near infrared spectral range. An above-the-barrier bound state was used to extend the range of transition energies above the values available in conventional quantum wells. Intrinsic polarization fields and nonparabolicity effects were taken into account. The selection of optimal parameters, maximizing the absorption at wavelengths of 1.55 and 1.3 /spl mu/m, was performed by using a simulated annealing algorithm, and optimal structures with infinite superlattices as confinement regions were thus designed. These optimal parameters were then used to set realistic, finite structures with a small number of layers, the performance of which was re-evaluated by solving the Schrodinger-Poisson equation self-consistently for a few different levels and profiles of doping.     

Dependence of cross-phase modulation on channel number in fiber WDMsystems

The phase term appearing in the expression for cross-phase modulation due to the optical Kerr effect depends on the sum of the powers carried by each wavelength channel. For this reason, one might expect that the amount of cross-phase modulation would increase with increasing channel number, causing increased interference among channels and hence limiting the total number of channels that a WDM system can support. However, computer simulations of multichannel systems have shown no change in signal distortion as the number of wavelength channels is increased from four to eight. In a simulated three-channel system, the signal distortion of the central channel approaches that of a single-channel system as the wavelength separation is increased to approximately 2 nm. Thus, even a moderate amount of dispersion tends to cancel out the influence of cross-phase modulation, so that beyond a certain wavelength spacing, additional channels do not interfere with the channel under consideration. From these observations, we conclude that cross-phase modulation does not limit the number of wavelength channels that a single optical fiber can support. However, self- and cross-phase modulation are not the only nonlinear effects influencing fiber lightwave systems. Stimulated Raman scattering tends to transfer optical power from short-wavelength channels to channels operating at longer wavelength, degrading their signal-to-noise ratio. The efficiency of this process increases with increasing wavelength spacing. Clearly, a compromise needs to be reached between the conflicting requirements imposed by the optical Kerr effect and by stimulated Raman scattering     

Large Stark shift of the interband transition in two-step quantumwells

Large and near-linear Stark shifts of the electron-heavy-hole ground state excitonic transition were observed in photoluminescence (PL) measurements for a two-step quantum-well (TSQW) structure. The Stark shift was 40 meV while a corresponding square well shifted only 20 meV at a field of 70 kV/cm. The observed Stark shifts agreed well with calculations. The large Stark shift of the TSQW was achieved on a global-to-local state transition realized via tailor-made quantum well (QW) parameters. This structure is an ideal candidate for optoelectronic devices based on the quantum confined Stark effect (QCSE)     

Lutetium effects on the UV absorption spectra of Nd:YAG

Spectroscopic observations were made on Lu:Nd:YAG and Nd:YAG laser materials. Variations in the UV absorption strengths are described and related to the presence of Lu³⁺. These results indicate that an optimum Lu/Nd ion ratio exists that will yield minimum absorption by the host over the wavelength range0.4-0.24 mum. The reduction in absorption will minimize nonuniform heating effects that should lead to better laser performance under certain conditions.     

Hole mixing and strain effects in the conduction intersubband transitions of undoped quantum wells

We study the effects of hole dispersion and mixing in the conduction intersubband transitions and infrared dressing of undoped quantum wells (QWs). This is done considering transitions of photo-excited electrons from one conduction subband (e1) to another (e2) in the presence of Coulomb interaction with the photo-excited holes. We show that, when the dispersion of the hole subband hh1 (lh1) is mainly parabolic, these transitions occur mainly between the s-states of e1-hh1 (e1-lh1) and e2-hh1 (e2-lh1) excitons with the same principal quantum numbers (allowed transitions). When the hole subbands have nonparabolic dispersions, however, such transitions are suppressed while another type of intersubband transitions in which the initial and final exciton states have different principal quantum numbers (nonallowed transitions) are enhanced. We show the enhancement and suppression processes reach their maxima when hh1 and lh1 are about to cross over, allowing multilevel mixing of excitons to occur when the undoped QW interacts with a single intense infrared field polarized along its growth. We associate these results with spinor mixing of hh1 and lh1 and illustrate how via changing the spinor contributions in these subbands one can employ strain to manipulate the dipole moments of the intersubband transitions.     

Curvature effects on the mutual coupling of cylindrical-rectangularmicrostrip antennas

The mutual coupling between two cylindrical-rectangular microstrip antennas is studied from the application of the generalised transmission line model (GTLM). Theoretical solutions and measured results of both E-plane and H-plane coupling coefficients are presented. The curvature effects on the mutual coupling are discussed     

Dependence of the linewidth enhancement factor on the number of compressively strained quantum well in lasers

We have systematically studied the well number dependence of the linewidth enhancement factor in strained quantum-well (QW) lasers and have demonstrated experimentally that the linewidth enhancement factor can be reduced from /spl sim/9.4 to /spl sim/2.0 by increasing the number of compressively strained QW's from 2 to 8. This behavior is primarily due to an increase in the differential gain with the number of QW's.     

外加电场影响SnO2纳米晶簇光吸收

实验测量外加电场对 Ｓｎ Ｏ２ 纳米晶簇室温近紫外光吸收的影响程度,得到光吸收变化谱线和光吸收大小变化随外加电场变化曲线。它们均为一种非线性变化规律,在高电场区,光吸收变化值趋于饱和     

Electromagnetic coupling effects on the cavity measurement of antenna efficiency

Electromagnetic coupling effects on the antenna in a conducting cavity are studied theoretically and experimentally. It is observed in experiments that at the resonant frequencies of the cavity, the input resistance of the antenna attains values two or three orders of magnitude higher than that at frequencies away from resonance. It is shown via theoretical analysis that the input resistance of the antenna measured at the resonant frequencies of the cavity is not merely the loss resistance desired in computing the antenna efficiency, but is actually the sum of the loss resistance of the antenna and the coupling resistance between the antenna and cavity. This coupling effect is demonstrated quantitatively by numerical computations for dipole and monopole antennas. The computational results for the input resistance are in agreement with the measured data. A method is proposed to avoid the cavity-antenna antiresonance in the measurement.