Ultrafast Dynamics of Coherences in a Quantum Hall System

Abstract

Using three-pulse four-wave-mixing optical spectroscopy, we study the ultrafast dynamics of the quantum Hall system. We observe striking differences as compared to an undoped system, where the 2D electron gas is absent. In particular, we observe a large off-resonant signal with strong oscillations. Using a microscopic theory, we show that these are due to many-particle coherences created by interactions between photoexcited carriers and collective excitations of the 2D electron gas. We extract quantitative information about the dephasing and interference of these coherences.

Correlation effects in the ultrafast dynamics of the Quantum Hall system close to v = 1

“…We spectrally resolve the signal so as to separate out the contribution from each LL, and then measure the timeintegrated FWM signal from each LL as a function of time delays. Figure 1(a) shows the experimental FWM signal vs 12 t D for 13 0 t D = [10]. Although we photoexcite mostly LL1, we observe a large offresonant signal from LL0 with strong oscillations versus 12 t D that is absent in the undoped system.…”

“…To test the predictions of the many-body theory developed in the previous section, we compare with the results of three-pulse FWM experiments [10] that probe the intra-and inter-band dynamics of the QH system. These results were obtained from a modulation doped QW, whose active region consisted of 10 periods of a 12 nm GaAs well and a 42 nm Al 0.3 Ga 0.7 As barrier, the central 12 nm doped with Si.…”

“…(11) are due to the scattering of the polarization L n P off incoherent populations or intraband coherences. By comparing the experimental results for the doped and undoped system, and noting their huge difference in the LL0 signal [6,8,10], we conclude that the latter contributions, induced by the 2DEG collective excitations (MPs), dominate the ultrafast nonlinear optical response of the 2DEG [10]. In the next section we present a model calculation based on the above microscopic theory that captures the main experimental features.…”

“…Experiments [6,8,10] have shown that the differences in the physics of the doped and undoped QW manifest themselves strongly in the ultrafast nonlinear optical response. This paper discusses a theoretical approach that extends the DCTS to the case of a strongly correlated ground state and is suitable for studying the QH regime.…”

“…Such MPs have been observed in inelastic light scattering [2][3][4] and other experiments. However, their role during the early timescales of quantum coherent dynamics in the QH system is just beginning to be explored [6][7][8][9][10]. In general, coherences play a central role in several quantum mechanical systems [11][12][13][14][15].…”