Abstract

Coherences and electronic couplings at the quantum level in semiconductor nanostruc tures are currently investigated intensely because of fundamentally new properties that may become important for new optoelectronic devices. Examples include observations of THz radiation from double quantum wells,¹ coherent control of single quantum dot states,² Bloch oscillations in superlattices,³ and molecular-like spectra from coupled quantum dots.⁴ In this work we focus on the ability to control the electronic coupling in coupled quantum wells with external E-fields leading to a strong modification of the coherent light emission, in particular at a bias where a superlattice-like miniband is formed. More specifically, we investigate a MBE-grown GaAs sample with a sequence of 15 single quantum wells having a successive increase of 1 monolayer in width ranging from 62 Å to 102 Å and with AlGaAs barriers of 17 Å. The electron and hole states without bias can therefore be sketched as in Figure 1(a) and indeed almost spectrally equidistant photoluminescence lines are observed as seen in Figure 1(c). Because of excitation well above the bandgap, creating free carriers effectively screening an externally applied field, we did not observe any spectral change as the bias field was changed.⁵ In resonant four-wave mixing experiments, however, the spectra in Figure 1(c) clearly show consistent shifts of the individual lines as the bias field is increased. Focussing on the spectrally lowest line, we can observe very fast oscillations in the coherent emission around a bias voltage of 1.6 V, see Figure 2(a). The inset shows the FWM spectrum indicating that the broad fs laser excites at least 8 lines significantly. The beat period of 380 fs can not be explained by interference between any two individual lines as the period corresponds to a bandwidth of 10.9 meV much more than the line splitting. Rather, this shows that around this voltage the hole levels have been aligned as sketched in Figure 1(b) to induce strong electronic coupling between the quantum wells resulting in the fast quantum beat. The measured beat bandwidth corresponds to having aligned the first six levels so they coherently contribute to the beats observed in Figure 2(a). In contrast to this we find much longer beat periods of 2.3 ps, corresponding to the level splitting of 1.8 meV, without a bias field as shown in Figure 2(b), where the laser has been detuned slightly to excite only the lowest levels. Moreover, the phase change for small detunings around the lowest level'' shows that these beats are mere polarization beats in the external detector and not due to any electronic coupling between the four excited levels. Thus we have shown that indeed we can externally control the degree of electronic coupling in superlattice-like quantum wells. Further studies are in progress to clarify' in detail how externally applied E-fields can modify the electronic states in these types of nanostructures relevant for applications in optoelectronic devices and in particular when minibands are formed.

© 1999 Optical Society of America