耦合量子阱中激子凝聚研究新进展

玻色-爱因斯坦凝聚成为探索量子世界的一种新方法,而且在半导体纳米结构中激子的凝聚研究取得了很大进展.实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离,显著提高了激子的冷却速度和寿命,成功地把激子冷却到1 K以下,观察到了激子的准凝聚状态.着重介绍冷激子系统凝聚现象、发光图案和宏观有序的激子态.理解这些简并激子系统的形成机理,为其在半导体纳米结构中最终实现玻色-爱斯坦凝聚提供新的机会.     

Hot exciton effects and relaxation in coupled quantum wells

By studying photoluminescence and photoluminescence excitation in coupled quantum wells, we have studied the influence of the tunneling time on the relaxation of the photoexcited carriers. The ability to vary the coupling strength by the barrier thickness, allowed us to evidence hot exciton relaxation effects as well as resonant Raman scattering.     

Surface relaxation frequency of ground-state exciton in quantum wells

We derive and analyze the relaxation frequency ν of exciton in a quantum well using the self-consistent Green's function method. The exciton relaxation is caused by the individual electron and hole scattering from the randomly-rough well interface. We reveal two types of ground-state exciton resonance and obtain the criteria for the transition from the asymmetric (sharp) resonance to the symmetric (broad) one. The dependence of the excitron–surface relaxation on the microscopic parameters of the interface defects, the average well width d and on the exciton characteristics is analyzed analytically. Specifically, in the case of sharp resonance the frequency ν∝d⁻⁶, whereas for broad resonance ν∝d⁻³. Moreover, ν is proportional to the ratio of the total exciton mass M over the squared reduced mass μ² (ν∝M/μ²).     

Heavy-hole and light-hole quantum beats in the polarization stateof coherent emission from quantum wells

Dual-channel spectral interferometry is used to measure the vectorial dynamics of the the four-wave-mixing signal from a GaAs-AlGaAs multiple quantum well when both heavy-and light-hole excitons are excited. The ellipticity, the orientation, and the sense of rotation of the polarization ellipse associated with this emission are observed to oscillate dramatically at the heavy-hole-light-hole quantum beat frequency. The qualitative nature of the beating can be described by a simple model based on the density matrix equations for two independent three-level systems without the inclusion of many body effects; however, these beats are superposed on a polarization that is dominated by many-body effects     

Enhanced exciton absorption in quantum-confined Stark effect of [110]-oriented InGaAsP quantum wells

A new phenomenon of the quantum-confined Stark effect has been found in InGaAsP quantum wells grown on [110]InP substrates by MOVPE. Enhanced exciton absorption concurrent with the applied voltage is observed for the first time.<>     

Enhanced spontaneous emission using quantum dots and an aperturedmicrocavity

Spontaneous emission characteristics from apertured microcavities are studied using quantum-dot light emitters. Spatial averaging over the emitter positions within the apertures significantly impacts the measured lifetime changes, but lifetime changes due to the aperture modes are still readily observed. A maximum increase of a factor of ~2.5 over the cavity-free emission rate is measured. We argue that, for a narrow bandwidth emitter, the microcavity enhancement can lead to nearly-single-mode and high-speed operation even in the spontaneous regime     

Investigation of HgCdTe waveguide structures with quantum wells for long-wavelength stimulated emission

The photoluminescence and stimulated emission during interband transitions in quantum wells based on HgCdTe placed in an insulator waveguide based on a wide-gap CdHgTe alloy are studied. Heterostructures with quantum wells based on HgCdTe are of interest for the development of long-wavelength lasers in the range of 25–60 μm, which is currently unattainable for quantum-cascade lasers. Optimal designs of quantum wells for attainment of long-wavelength stimulated emission under optical pumping are discussed. It is shown that narrow quantum wells from pure HgTe appear to be more promising for long-wavelength lasers in comparison with wide (potential) wells from the alloy due to the suppression of Auger recombination. It is demonstrated that molecular-beam epitaxy makes it possible to obtain structures for the localization of radiation with a wavelength of up to 25 μm at a high growth rate. Stimulated emission is obtained for wavelengths of 14–6 μm with a threshold pump intensity in the range of 100–500 W/cm² at 20 K.     

发光量子阱对波长上转换红外探测器效率的影响

波长上转换红外探测器在实现大面阵、低暗电流红外探测方面具有很大的发展潜力.短波长光子的发光效率是影响上转换器件效率的重要因素.设计、制作了具有不同发光阱个数的波长上转换红外探测器件,结合器件的红外响应和仿真计算,分析了发光阱个数对波长上转换效率的影响规律.研究结果表明,选择单个发光阱,有利于提高器件的发光效率,从而提高波长上转换效率.     

Single-peak excitonic emission of CdSe ultra-thin quantum wells finished with fractional monolayers

Ultra-thin quantum wells (UTQWs) of CdSe grown by atomic layer epitaxy (ALE) present very interesting features, such as intense excitonic luminescence and relatively narrow width. Grown under adequate conditions only a single excitonic peak is exhibited in the photoluminescence (PL) spectrum, indicating the absence of thickness fluctuations. The PL peak of an UTQW grown with the same nominal thickness presents a blue shift if grown at a higher substrate temperature. This energy shift is attributed to changes in composition: instead of a pure CdSe UQTW we obtain a Zn_1?xCd_xSe UTQW with Cd content depending on growth temperature. The fact that even changing the growth temperature only a single excitonic peak is observed is interpreted in terms of a homogeneous mixing of the Cd and Zn atoms at the upper interface. The lack of homogeneity would produce islands and terraces, and then thickness fluctuations that would be clearly evident in the PL spectrum. In order to verify this assertion we have grown UTQWs finished with fractional monolayers, around 0.5 ML of CdSe. We produced UTQWs with 2.5, 3 and 3.5 ML and the result is that all the samples presented a single peak, red shifted with increasing CdSe coverage and monotonic increase in the full-width at half-maximum (FWHM) of the emission. This confirms the homogenization of the CdSe–ZnSe interface.     

Fermi-edge singularities in the optical emission of doped direct and indirect quantum wells

The luminescence spectrum of doped direct and indirect quantum wells is studied. The Bethe-Salpeter equation is solved in a three band model of parabolic wells. By shifting the electron and the hole confining parabolic potentials with respect to each other, we found a strong enhancement at the Fermi edge. We take an infinite mass for the hole and we find an increasing signal of the spectrum at the Fermi edge when two conditions are fulfilled: (i) the separation between the electrons and hole gravity centers is of the order of the well width and (ii) the Fermi edge is close to the bottom of the second conduction subband. The effects of temperature are analyzed.     

Long-wavelength stimulated emission and carrier lifetimes in HgCdTe-based waveguide structures with quantum wells

The interband photoconductivity and photoluminescence in narrow-gap HgCdTe-based waveguide structures with quantum wells (QWs) (designed for long-wavelength stimulated emission under optical pumping) are investigated. The photoconductivity relaxation times in n-type structures reach several microseconds, due to which stimulated emission at a wavelength of 10.2 μm occurs at a low threshold pump intensity (~100 W/cm²) at 20 K. In the p-type structures obtained by annealing (to increase the mercury vacancy concentration), even spontaneous emission from the QWs is not detected because of a dramatic decrease in the carrier lifetime with respect to Shockley–Read–Hall nonradiative recombination.     

Terahertz emission from CdHgTe/HgTe quantum wells with an inverted band structure

The terahertz electroluminescence from Cd_0.7Hg_0.3Te/HgTe quantum wells with an inverted band structure in lateral electric fields is experimentally detected and studied. The emission-spectrum maximum for wells 6.5 and 7 nm wide is near 6 meV which corresponds to interband optical transitions. The emission is explained by state depletion in the valence band and conduction band filling due to Zener tunneling, which is confirmed by power-law current–voltage characteristics.     

Accurate methods for study of light emission from quantum wells confined in a microcavity

Photomodulated reflectance (PR) and conventional reflectance studies have been performed on an InGaP/AlGaInP/Al/sub x1/Ga/sub y1/As/Al/sub x2/Ga/sub y2/As resonant-cavity light emitting diode structure in the red spectral region. The PR spectra show prominent signals from the Fabry-Perot cavity mode and the quantum-well (QW) ground state excitonic transition. This high-precision technique, and its variations as functions of incidence angle and temperature, as reported in this article, allow one to investigate light emission from the QW confined in a microcavity with relation to the Fabry-Perot mode, and is the only known nonconductive, nondestructive method of doing so.     

Room-temperature laser emission of GaInNAs-GaAs quantum wells grown on GaAs (111)B

In this work, we present room-temperature laser emission at 1.206 /spl mu/m from GaInNAs-GaAs quantum-well (QW) laser diodes (LDs) grown on misoriented GaAs (111)B substrates for the first time. Details of the structure and the molecular beam epitaxial growth of the lasers are discussed. We found that the postgrowth rapid thermal annealing increased the optimum emission, while the in situ self annealing effect in these QWs is almost negligible. The optimum annealing cycle (30 s at 850/spl deg/C) is comparable to that found for the cladding-free GaInNAs single QW samples grown on GaAs (111)B. Finally, the optical and electrical characterization of these LD devices is presented. The LDs show a room-temperature threshold current density of 2.15 kA/cm/sup 2/, with a differential quantum efficiency of 37%, under pulsed conditions.     

非对称高斯势量子点中LO声子自发辐射率的研究

本文在偶极近似下,基于费米黄金规则,采用LL P-Pekar幺正变换变分法研究了双参 量非对称高斯势量子点(QD)中体纵光学(LO)声子的自发辐射率,并讨论了它的单参量抛 物势近似,数值结果表明:声子自发辐射率w随电声耦合强度α、色散系数η 的增加而减小,随高 斯势阱深V₀的增加而增大;声子自发辐射率w随高斯势阱宽L的变化呈现非对称“高斯分布”并受到电声 耦合强度α、色散系数η和高斯势阱深V₀的显著影响;选用双参量非对称高斯势VG描写QD中电子的受 限效应能够反映声子自发辐射的更多量子化特性,而其单参量抛物势VP近似给出的结果较为简单和粗糙。     

Two electron transitions of the exciton bound at the Si Donor confined in GaAs/AlxGa1-xAs quantum wells

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ₆), to the first excited state, 2s(Γ₆), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ₆)-2s(Γ₆) transition energy has been determined.     

On spontaneous emission

The problem of the spontaneous emission from a single excited two-level atom in the presence of N unexcited similar atoms is discussed. A formalism is presented for evaluation of the time-development operator (and thus the density matrix) which has certain desirable features for time-dependent problems in the area, e.g. of atom-field interactions, when the off-diagonal elements are large compared to the diagonal elements of the Hamiltonian. By use of this formalism, it is shown that the spontaneous emission from a single atom is often drastically altered in the presence of unexcited atoms with similar resonances. When the number of atoms present becomes very large relative to the number of modes of the electromagnetic field, the excited atom becomes trapped in its excited state     

Electrically driven uniaxial stress device for tuning in situ semiconductor quantum dot symmetry and exciton emission in cryostat

Uniaxial stress is a powerful tool for tuning exciton emitting wavelength, polarization, fine-structure splitting (FSS), and the symmetry of quantum dots (QDs). Here, we present a technique for applying uniaxial stress, which enables us in situ to tune exciton optical properties at low temperature down to 15 K with high tuning precision. The design and operation of the device are described in detail. This technique provides a simple and convenient approach to tune QD structural symmetry, exciton energy and biexciton binding energy. It can be utilized for generating entangled and indistinguishable photons. Moreover, this device can be employed for tuning optical properties of thin film materials at low temperature.     

Spectroscopy of exciton states of InAs quantum molecules

Tunnel-coupled pairs of InAs quantum dots (quantum molecules) were formed by molecular beam epitaxy in a GaAs matrix. Optical and structural properties of the obtained quantum molecules were studied. Four molecular exciton states forming a photoluminescence spectrum were revealed. The photoluminescence decay times indicate the possibility of interlevel radiative recombination from the second excited state, which is of particular importance for designing mid-infrared devices.     

Investigation on the emission wavelength of GaInNAs/GaAs strained compressive quantum wells on GaAs substrates

In this paper, we study the effect of the incorporation of nitrogen in strained GaInAs quantum well structures. We evaluate the influence of nitrogen on the conduction band energy by using the band anticrossing model. The incorporation of nitrogen appears to decrease the bandgap energy and increase the emission wavelength. The reduction of energy is due to the interaction of the energy of the conduction band with the level of nitrogen and more the concentration of nitrogen increases, more the energy of the band gap decreases. On the other hand, the emission wavelength increases, the advantage of the incorporation of nitrogen in such structures is to vary the wavelength between 0.980 and 1.3 μm while exploiting of course the composition of gallium, composition of nitrogen and the thickness of the quantum well. Less temperature insensitive devices are so intended to be fabricated.