Gaal P, Kuehn W, Reimann K, Woerner M, Elsaesser T, Hey R
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
Nature. 2007 Dec 20;450(7173):1210-3. doi: 10.1038/nature06399.
A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fröhlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties--relevant for transport on nanometre length and ultrashort timescales--are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.
例如冰中的质子、DNA分子中的离子、界面处的电子或有机或无机晶体中的电子。反过来，介质也会作用于粒子。在极性或离子性固体中，自由电子会使晶格发生畸变，使原子偏离其平衡位置。当把电子与其周围的晶格畸变一起考虑时，它就是一个单一的准粒子，称为弗罗利希极化子。极化子的基本性质及其在弱电场中的漂移运动是众所周知的。然而，它们在纳米长度和超短时间尺度上与输运相关的非线性高场性质却尚不为人所知。在此，我们表明太赫兹范围内的高电场会将砷化镓晶体中的极化子驱动到高度非线性状态，在这种状态下，除了漂移运动外，电子还会被脉冲式地从周围晶格畸变的中心移开。通过这种方式，会诱导出持续数百飞秒的相干晶格振动（声子）以及伴随的漂移速度振荡。它们会调制红外频率下介于吸收和受激发射之间的光学响应。这种量子相干过程直接影响纳米结构中的高频输运，并且可应用于新型太赫兹驱动的光学调制器和开关中。
