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Towards Quantum Dots THz lasers


Sept. 16 2009

Realizing efficient THz emitters is a major issue of contemporary opto-electronics. An european team including researchers from the LPA-ENS has recently proven that an efficient population inversion in semiconductor quantum dots is achievable in the THz frequency range, paving the way to the use of these nano - objects in the build up of efficient THz lasers.
Quantum Cascade Lasers (QCL) are unipolar emitters that operate in a broad range of frequency, from mid infrared to THz. In present days QCL’s, based on semiconductor quantum well structures, the population inversion takes place between two electronic subbands. However, the presence of two-dimensional electronic continuums favors deleterious non-radiative desexcitation (like static defects - assisted or phonon - assisted inter subband scatterings). The use of quantum dots as active medium of QCL has a major advantage that is the discreteness of the electronic spectrum. Therefore, if the non-radiative lifetimes of these levels are long, the population inversion should be made easier than in quantum well structures and the magnitude of the threshold current might become significantly lower than those of currently available QCL lasers.
Minimizing the non radiative recombination is a key parameter of efficient lasing action. The existence of polarons (i.e. a composite excitation built from the strong coupling between electrons and LO phonons) and the part they play in the energy relaxation was established by LPA-ENS researchers in “small” (10 nm) InAs – GaAs self organized quantum dots where the energy \Delta to be relaxed is comparable to the energy of an optical phonon \hbar \omega_{LO} ( = 36 meV). Very little was known of the \Delta \ll \hbar \omega_{LO} regime that characterizes the THz range of the electromagnetic spectrum. Joint efforts of researchers from the Sheffield University (UK), Forschungzentrum Dresden (Germany) and LPA-ENS have enabled a quantitative understanding of relaxation mechanisms in large quantum dots aimed for applications to THz.
The Sheffield and Dresden teams have used free electron laser radiations to perform pump - probe measurements in InAs quantum dots. The results evidence a spectacular increase (from 50 ps to a few ns) of the lifetime of the excited electronic state when ∆ decreases from 50 meV to 10 meV (\sim 2.4 THz). The LPA-ENS team has made the theory of the energy relaxation in the quantum dots. It acknowledges the polaronic nature of the quantum dots eigenstates and accounts for anharmonicity of the vibrations to compute the lifetime of the excited states. An excited polaron state decays into an electron in the ground state of the dot and two acoustical phonons. It is shown that the relaxation frequency from the excited polaron state decreases markedly when \Delta decreases towards the THz range.
The agreement between theory and experiment is very good implying that one gets a firm grip on the relaxation mechanism in quantum dots of any size. These studies nicely end an old debate about the energy relaxation in semiconductor quantum dots. They pave the way to the use of these nano – objects in the design of novel opto – electronic devices that work in a much demanded frequency range.

Référence : Long lifetimes of quantum-dot intersublevel transitions in the terahertz range ; E. A. Zibik , T. Grange, B. A. Carpenter, N. E. Porter, R. Ferreira, G. Bastard, D. Stehr, S. Winner, M. Helm, H. Y. Liu, M. S. Skolnick & L. R. Wilson ; Nature Materials 16 Aug 2009

Contact : robson ferreira

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Population lifetime
Decay time of the excited state population of a quantum dot as a function of its transition energy. Symbols: measurements. Lines: calculations. LA and TA correspond to emitted longitudinal and transverse acoustical phonons.