Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws. For example, the long-range Coulomb interaction between electrical charges has a simple form, yet is responsible for a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures. Semiconductors form an ideal laboratory for studying many-body interactions of electronic quasiparticles among themselves and with lattice vibrations and light1,2,3,4. Oppositely charged electron and hole quasiparticles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons5,6. The pathways between such states, however, remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing centre-of-mass momenta. In contrast, transitions between internal exciton levels, which occur in the far-infrared at terahertz (1012 s-1) frequencies7,8,9, are independent of this restriction, suggesting10 their use as a probe of electron–hole pair dynamics. Here we employ an ultrafast terahertz probe to investigate directly the dynamical interplay of optically-generated excitons and unbound electron–hole pairs in GaAs quantum wells. Our observations reveal an unexpected quasi-instantaneous excitonic enhancement, the formation of insulating excitons on a 100-ps timescale, and the conditions under which excitonic populations prevail.
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We thank J. Reno for growth of the quantum well samples, and S. L. Chuang for band-structure calculations. We also thank M. Kira, S. W. Koch, M. Woerner, T. Timusk and J. Orenstein for discussions. This work was supported by the Office of Basic Energy Sciences of the US Department of Energy, the Deutsche Forschungsgemeinschaft and the Alexander von Humboldt Foundation.
The authors declare that they have no competing financial interests.
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Kaindl, R., Carnahan, M., Hägele, D. et al. Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas. Nature 423, 734–738 (2003). https://doi.org/10.1038/nature01676
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