| Quantum-confined impurity atoms as "artificial atom" quantum dots has provided one kind of new channel to prepare quantum dots. For the semiconductor quantum well system, transition energy can be tuned in a controlled manner, which can be achieved by changing the quantum-well width, the position of impurity atoms, the doping consistence and so on. This could have an impact on the potentical applicationd in photoelectron field such as far-infrared detectors;solid terahertz lasers; ultrafast electronic devices. For any quantum emitters/detectors or lasers based on impurity-states transitions, exited state lifetime(ESL) is an important parameter, which has an influence on the research and development of the far-infrared detectors and Teraherz solid-state laser technology. In this letter, we demonstrate the effect of quantum-well confinement on the excited state lifetime of Be acceptors confined in GaAs/AlAs multiple quantum wells (MQWs),using far-infrared time-resolved pump-probe experiment, by the means of constructing physical models.Firstly, the investigated GaAs/AlAs MQW were grown by molecular-beam epitaxy(MBE), and every GaAs well layer wasδ-doped at the well center with Be acceptor atoms. Different quantum well width demonstrate different quantum confinement effect on Be acceptor. Using Fourier-transform infrared absorbtion experiment, we have investigated the far-infrared absorption spectrum of Be acceptor confined in quantum-well on the different quantum well widths. Low-temperature far-infrared absorbtion measurements clearly show three principal absorption lines due to transitions of Be acceptor state from the ground state 1 s3/2 (Γ6+Γ7) to the first three odd-parity excited states 2 p3/2 (Γ6+Γ7),2p5/2 (Γ6+Γ7), 2p5/2Γ7 respectively. A variational'principle is presented to obtain the 2p→1s transition energies of quantum-confined beryllium acceptors as a function of the well width.It is found that the theoretical calculation of 2p→1s transition energies are in good agreement with the experiment results of D-like line absorption.Secondly, the direct pump-probe experiments were performed using the Free Electron Laser for Infrared experiments. The relationship between ESL and quantum-well width, temperature, laser wavelength is investigated by far-infrared time-resolved spectroscopy.The lifetime of the excited state of acceptors was measured as a function of the width of the quantum well and the temperature. It is found that the lifetime decreases monotonically with decreasing well width and is independent of temperature. The relaxation process of the 2p→1s is non-radiative. We suggest that the effect of quantum-well confinement on acoustic-phonon modes, in particular, zone-folded acoustic-phonon modes which arise from the periodic nature of the multiple quantum well sample increase the intra-acceptor scattering rate of acoustic-phonon-assisted ralaxion. At 4K temperature, the pump-probe experiments for MQW sample with well width lOnm were repeated at different FEL wavelength, respectively. It is shown that the 2p→1s relaxation time is strongly dependent on the laser wavelength. The pump-probe signals atλ=43.44.and 47μm are observed to be weaker than those atλ=46μm,the D-line position. The most possible explanation for the laser wavelength dependence of the excited state life is the diffusion of the Be atomsδ-layer in QWs.Lastly, the effect of quantum confidence on acceptor excite-state dynamics is explaned reasonably in theory by constructing physical model of Be acceptor in quantum wells.For the analysis of Schrodinger equation of the effective mass approximation, we have a clear physical ideas for the physical microscopic mechanism of scattering interaction for holes bound to the acceptors.
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