GaAs/GaAlAs短周期超晶格中的Wannier-Stark效应

Submitted by zhangdi (zhangdi@red.semi.ac.cn) on 2009-04-13T11:45:31Z

WANNIER-STARK LOCALIZATION IN INGAAS/GAAS SUPERLATTICES AND ITS APPLICATION TO ELECTROOPTICAL DEVICES

We have observed Wannier-Stark localization in strained In0.2Ga0.8As/GaAs superlattices by low- and room-temperature photocurrent spectra measurements. The experimental results are well in agreement with the theoretical predictions. A large field-induced modulation response of the absorption edge of the superlattices at room temperature suggests the possibilities of the application to the design of various kinds of electro-optical devices operating at a wavelength of 0.98 mum, based on Wannier-Stark localization effects.

InGaAs/GaAs应变层短周期超晶格的Wannier-Stark效应

于2010-11-23批量导入

GaAs/GaAlAs超晶格的Wannier Stark效应

国家自然科学基金

WANNIER LOCALIZATION IN GAAS/GAALAS SUPERLATTICES UNDER ELECTRIC-FIELD

We have studied the Wannier-Stark effect in GaAs/GaAlAs short-period superlattices under applied electric field perpendicular to the layers by room- and low-temperature photocurrent measurements. The changes in the transition intensities with biasing are well fitted to a theoretical calculation based on the finite Kronig-Penney model on which the potential of an applied electric field is superposed. With increasing electric field, the 0h peak grows to a maximum while the -1h and +1h peaks monotonousely decrease. By a comparison of the spectra measured at different temperatures, the two peaks in the room temperature photocurrent spectra at relatively low electric field (1.0 X 10(4) V/cm) are identified to be caused by the Wannier localization effect instead of saddle-point excitons.

WANNIER QUANTIZATION OF A SUPERLATTICE SUBBAND UNDER AN ELECTRIC-FIELD

Wavefunctions of electronic Wannier-Stark states in a superlattice are calculated with a finite Kronig-Penney model. Overlap integrals between electron and heavy-hole wavefunctions centred in the same well layer, and in first- and second-neighbour wells are calculated as functions of the applied field. The results show good agreement with experimental results on photoluminescence. The problem is also treated by a one-band approximation method, which gives a closed expression for the wavefunction of the Wannier-Stark states; this is compared with the results of accurate calculations with the Kronig-Penney model.

Wannier-function approach to electron states in superlattices under an electric field

Electron quasi-stationary states in a periodic semiconductor superlattice are calculated, as linear combinations of Wannier-Kohn functions, for different values of an electric field applied along the heterostructure. A comparison with an alternative approach, which is based on the localization of quasi-stationary states, is performed. (C) 2004 Elsevier Ltd. All rights reserved.

Degenerate four-wave-mixing signals from dc- and ac-driven semiconductor superlattice

Within the one-dimensional tight-binding model;rnd chi-3 approximation, we have calculated four-wave-mixing (FWM) signals for a semiconductor superlattice in the presence of both static and high-frequency electric fields. When the exciton effect is negligible, the time-periodic field dynamically delocalizes the otherwise localized Wannier-Stark states, and accordingly quasienergy band structures are formed, and manifest in the FWM spectra as a series of equally separated continua. The width of each continuum is proportional to the joint width of the valence and conduction minibands and is independent of the Wannier-Stark index. The realistic homogeneous broadening blurs the continua into broad peaks, whose line shapes, far from the Lorentzian, vary with the delay time in the FWM spectra. The swinging range of the peaks is just the quasienergy bandwidth. The dynamical delocalization (DDL) also induces significant FWM signals well beyond the excitation energy window. When the Coulomb interaction is taken into account, the unequal spacing between the excitonic Wannier-Stark levels weakens the DDL effect, and the FWM spectrum is transformed into groups of discrete lines. Strikingly, the groups are evenly spaced by the ac field frequency, reflecting the characteristic of the quasienergy states. The homogeneous broadening again smears out the line structures, leading to the excitonic FWM spectra quite similar to those without the exciton effect. However, all these features predicted by the dynamical theory do not appear in a recent experiment [Phys. Rev. Lett. 79, 301 (1997)], in which, by using the static approximation the observed Wannier-Stark ladder with delay-time-dependent spacing in the FWM spectra is attributed to a temporally periodic dipole field, produced by the Bloch oscillation of electrons in real space. The contradiction between the dynamical theory and the experiments is discussed. In addition, our calculation indicates that the dynamical localization coherently enhances the time-integrated FWM signals. The feasibility of using such a technique to study the dynamical localization phenomena is shown. [S0163-1829(99)10607-6].

Theory of terahertz-photocurrent resonances in miniband superlattices

The currents of de and ac components and their phase-angle cosines for a superlattice under a direct bias and alternating field are calculated with the balance equations. It is found that the de currents as functions of the direct field show resonance peaks at the fields corresponding to the Bloch frequency equal to n omega. With increasing alternating field intensity the resonance peaks of higher harmonic increase, and simultaneously the first peak caused by the de field decreases. The results are in good agreement with the experimental results, indicating that this resonance can be understood in terms of electron acceleration within the miniband, i.e., it is a bulk superlattice effect, rather than caused by the electric-field localization mechanism (Wannier Stark ladder). The phase-angle cosine for the first harmonic cos phi(1) becomes negative when the Bloch frequency increases to be larger than the frequency of the ac field omega, and it also shows resonance peaks at the resonance frequencies n omega. The negative cos phi(1) may cause the energy transferred to the alternating field, i.e., oscillation of the system.

ELECTRIC-FIELD-INDUCED EXCITON-LINEWIDTH BROADENING IN SHORT-PERIOD GAAS/GAXAL1-XAS SUPERLATTICES

We have investigated the Wannier-Stark effect in GaAs/GaAl1-xAs superlattices under electric fields by photocurrent spectroscopy measurements in the range of temperatures 10-300 K. The linewidth of the Oh Stark-ladder exciton was found to increase significantly along with an increase in peak intensity when the electric field increases. We present a mechanism based on an enhanced interface roughness scattering of electronic states due to Wannier-Stark localization in order to explain this increased broadening with electric field. This electric-field-related scattering mechanism will weaken the negative differential conductance effects in superlattices predicted by Esaki and Tsu.

超晶格微带电子态随电场演化过程的电解液电反射谱研究

利用电解液电反射谱研究了In_(0.2)Ga_(0.5)As/GaAs短周期超晶格中微带电子态随电场的演化过程,不仅清楚地观察到弱场Franz-Keldysh效应和强场Wannier-Stark局域化效应,面且观察到了从弱场过度到强场过程中Franz-Keldysh效应和Wannier-Stark局域化效应互相竞争的现象.

Abnormal phenomenon of ac Stark splitting in magneto-optical traps

We experimentally study the ac Stark splitting in D2 line of cold Rb-87 atoms. The frequency span between the Autler-Townes doublets is obviously larger than that derived from theoretical calculation. Two physical effects, which increase the effective Rabi frequency, contribute to the splitting broadening. First, atoms tend to distribute in strong lield places of a inhomogeneous red-detuned light field. Second, atoms reabsorb scattered light when they are huge in number and high in density.