Internal motions of a quasiparticle governing its ultrafast nonlinear response

Abstract

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.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…The question of how the internal motion of a polaron within its potential well is affected at high electric fields was recently discussed in an experiment on GaAs by Gaal et al (2007). To illustrate the physical mechanisms, the selfinduced potential for a single polaron was calculated and is shown in Fig.…”

Intense ultrashort terahertz pulses: generation and applications

“…[23]. A high electric field in the terahertz range drives the Fröhlich polaron in a GaAs crystal into a highly nonlinear regime where an electron is impulsively driven from the centre of the surrounding lattice distortion.…”

“…Promising results of nonlinear THz spectroscopy using single-cycle pulses exceeding field strengths of 100 kV/cm have recently been reported. Examples include acceleration of carriers such as free electrons in semiconductors [22,23], and alignment of the induced dipole moment of nonpolar molecules [24]. The temporal profile of the electric field of the THz pulse can also be tailored to allow resonant pumping of collective excitations like phonons [25] or excitons [26].…”

Transition from ballistic to drift motion in high-field transport in GaAs

“…To explain our observations, we have developed a model for high-field carrier transport in semiconductors that correctly describes both the time dependence of transport on ultrashort time scales and steady-state transport [9,1,8]. This is achieved by describing electrons and phonons together as new quasiparticles, polarons, in the form of wavepackets.…”