Experimental observation of electron–hole recollisions

Abstract

An intense laser field can remove an electron from an atom or molecule and pull the electron into a large-amplitude oscillation in which it repeatedly collides with the charged core it left behind. Such recollisions result in the emission of very energetic photons by means of high-order-harmonic generation, which has been observed in atomic and molecular gases as well as in a bulk crystal. An exciton is an atom-like excitation of a solid in which an electron that is excited from the valence band is bound by the Coulomb interaction to the hole it left behind. It has been predicted that recollisions between electrons and holes in excitons will result in a new phenomenon: high-order-sideband generation. In this process, excitons are created by a weak near-infrared laser of frequency f(NIR). An intense laser field at a much lower frequency, f(THz), then removes the electron from the exciton and causes it to recollide with the resulting hole. New emission is predicted to occur as sidebands of frequency f(NIR) + 2nf(THz), where n is an integer that can be much greater than one. Here we report the observation of high-order-sideband generation in semiconductor quantum wells. Sidebands are observed up to eighteenth order (+18f(THz), or n = 9). The intensity of the high-order sidebands decays only weakly with increasing sideband order, confirming the non-perturbative nature of the effect. Sidebands are strongest for linearly polarized terahertz radiation and vanish when the terahertz radiation is circularly polarized. Beyond their fundamental scientific significance, our results suggest a new mechanism for the ultrafast modulation of light, which has potential applications in terabit-rate optical communications.

Interband Bloch oscillation mechanism for high-harmonic generation in semiconductor crystals

“…Recently, however, interest has grown towards studying HHG in condensed matter systems. Experiments generating high-order harmonics in bulk semiconductor crystals for wavelengths in the mid-infrared (mid-ir) [3,4] and THz [5,6] regimes have been performed, establishing a foundation on which attosecond electron dynamics in solids can be studied.…”

Electric field ionization of gallium acceptors in germanium induced by single-cycle terahertz pulses

“…[10][11][12] It is claimed that the field ionization process may play a key role in these nonlinear phenomena. However, the ionization process under instantaneous high electric fields is not well understood.…”

Ultrafast Dynamics of High-Harmonic Generation in Terms of Complex Floquet Spectral Analysis

“…Some have used two-color light beams, such as near infrared and far infrared, to clearly distinguish the different electronic excitations, which are sometimes called high-order sideband generation (HSG) [26][27][28][29]. Even though coherent electronic motion in solids is different from that of atoms and molecules, characteristic features of the HHG spectrum, such as the plateau and cutoff, are common to those from atoms and molecules.…”