Mid-Infrared Emission in InAs/GaAs Self-Assembled Quantum Dots
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ABSTRACT We have investigated the emission properties of InAs/GaAs self-assembled quantum dots in the mid-infrared. The emission relies on the intraband transitions in the valence band of the quantum dots. We first show that third-harmonic generation can be observed. The frequency tripling efficiency is enhanced by the resonances between the pump excitation and the quantum 2 dot intraband transitions. A giant third-order nonlinear susceptibility X.3" = 1.5 x 10-14 (mnV) is measured for one dot plane. The narrow spectral dependence of the nonlinear susceptibility is well explained by simulations which account for the three-dimensional confinement potential. We secondly show that the mid-infrared spontaneous emission between hole confined states can be observed under an interband optical pumping. The spontaneous emission involves transitions between either the ground and excited states or between excited states. These measurements demonstrate the potentiality of self-assembled quantum dots for mid-infrared emission. INTRODUCTION The mid-infrared properties of self-assembled quantum dots are drawing considerable interest for the development of new optoelectronic devices. These infrared properties rely on the intraband (or intersublevel) transitions between the confined states in the conduction and in the valence band. A strong analogy can be found between quantum dot intraband transitions and quantum well intersubband transitions. Quantum dot infrared photodetectors, very similar to quantum well infrared photodetectors have been recently developed [1, 2]. Apart from these devices based on an absorption mechanism, one may wonder if the quantum dots do exhibit similar emission properties in the mid-infrared as reported for intersubband transitions. It is well known that a strong resonant enhancement of the nonlinear susceptibility associated with intersubband transitions has been observed in quantum wells [3]. The mid-infrared emission between subbands has also led to the development of a new class of unipolar laser, the so-called quantum cascade laser [4]. It is therefore legitimate to investigate the emission properties of self-assembled quantum dots. One may expect that the quantum dots could exhibit properties similar to those observed in quantum wells, with even some specificities associated with the three-dimensional confinement potential.
279 Mat. Res. Soc. Symp. Proc. Vol. 571 ©2000 Materials Research Society
RESULTS Quantum dot samples and experimental setup The investigated self-assembled quantum dots have been grown by molecular beam epitaxy [5]. The samples consist of 40 InAs layers separated by 35 nm GaAs barriers. The quantum dots were embedded in a mid-infrared waveguide [6]. One sample was modulation doped with a beryllium 8-doping (p-doping) to provide a carrier density of 6 x 1010 cm-2 . The quantum dots of the second sample were non-intentionally doped. The third-harmonic generation experiments were performed at room temperature with the free-electron laser CLIO. The mid-infrared beam was injected
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