Ultrafast terahertz probes of transient conducting and insulating phases in an electron-hole gas
Ultrafast terahertz probes of transient conducting and insulating phases in an electron-hole gas
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DOI：
10.1038/nature01676
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发表时间：
2003-06-12
期刊：
影响因子：
64.8
通讯作者：
Chemla, DS
中科院分区：
文献类型：
--
作者：
Kaindl, RA;Carnahan, MA;Chemla, DS
自然界中的多体系统表现出复杂性和自组织性，这些复杂性和自组织性源于看似简单的定律。例如，电荷之间的长程库仑相互作用具有简单的形式，但却导致了物质中大量的束缚态，从氢原子到复杂的生物化学结构。半导体形成了一个理想的实验室，用于研究电子准粒子之间以及与晶格振动和光的多体相互作用（1-4）。不带电的电子和空穴准粒子可以共存于电离但相关的等离子体中，或者形成被称为激子的束缚氢对（5，6）。然而，在近可见光光学实验中，这些状态之间的路径仍然难以捉摸，这些实验检测到了具有消失的质心动量的激子子集。相比之下，内部激子能级之间的跃迁，发生在太赫兹（10（12）s（-1））频率的远红外线（7-9），与这种限制无关，这表明（10）它们可以用作电子-空穴对动力学的探针。在这里，我们采用超快太赫兹探针直接研究GaAs量子威尔斯中光生激子和非束缚电子空穴对的动力学相互作用。我们的观察揭示了一个意想不到的准瞬时激子增强，绝缘激子的形成在100 ps的时间尺度上，激子人口占上风的条件下。
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 light(1-4). Oppositely charged electron and hole quasiparticles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons(5,6). 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 (10(12) s(-1)) frequencies(7-9), are independent of this restriction, suggesting(10) 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.