Intersubband relaxation dynamics in InGaAs/ AlAsSb multiquantum wells ͑QWs͒ is studied by single-color femtosecond pump-probe measurements. At early delay times, all samples show an exponential decay of the transient transmission occurring with time constants of the order of a picosecond. The relaxation dynamics at later delay times strongly depend on both QW thickness and doping location. A non-single-exponential decay behavior indicates extra competing relaxation channels, as further confirmed by solving three-level rate equations. It is shown that slowly decaying components are due to electron transfer to states related to indirect valleys in the wells or in the barriers. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2360242͔Intersubband transitions in semiconductor quantum wells ͑QWs͒ have been employed for midinfrared optoelectronic devices, in particular, lasers and detectors.1 Also ultrafast all-optical switching 2,3 is an interesting application due to the short relaxation times of intersubband transitions, 4 which are several orders of magnitude faster than for interband transitions. Recently there have been attempts to extend the range of intersubband devices towards the near infrared, i.e., to wavelengths shorter than 3 m ͑Ref. 5͒ and even towards the 1.55 m telecommunication range. 2,3,6 This requires material systems with a large conduction band discontinuity, such as strained InGaAs/ AlAs on GaAs ͑Ref. 7͒ or InP substrates, 8 In 0.53 Ga 0.47 As/ AlAs 0.56 Sb 0.44 lattice matched to InP, 9,10 InAs/ AlSb on GaSb, 5 or nitrides such as InGaN / AlGaN ͑Refs. 11 and 12͒ and II-VI compounds such as ZnSe/ BeTe. 13 While the latter two material systems exhibit ultrafast, subpicosecond intersubband relaxation due to their strong electron-phonon coupling, they may be less suited for quantum cascade emitters, which appear more promising in the InP ͑Refs. 14 and 15͒ or GaSb ͑Ref. 5͒ based systems.A common feature of some of the above material combinations is the indirect character of the barrier material, i.e., an X or L valley represents the lowest-energy states. In narrow QWs ͑Ͻ3 nm͒, required to achieve short-wavelength intersubband absorption, the first excited subband state may be raised above such indirect state in the barrier or even in the QW, which would strongly influence the relaxation behavior and also the performance of quantum cascade lasers. 16 Indications for such an effect have been observed in some intersubband relaxation experiments for the InGaAs/ AlAs ͑Ref. 7͒ and ZnSe/ BeTe systems.
13In the present letter we investigate the intersubband relaxation dynamics in narrow InGaAs/ AlAsSb QWs, which are the building block for short-wavelength quantum cascade lasers. 15 We employ femtosecond pump-probe spectroscopy and focus on the possible role of intervalley electron transfer in the relaxation dynamics. Though time resolved experiments on this material system have been reported before, 10 the role of indirect states has remained open. The shortest lasing wavelength in this material syste...
