High-order sideband generation in semiconductors: colliding quasiparticles and probing Berry curvature.

The direct measurement of Berry phases is still a great challenge in condensed matter systems. The bottleneck has been the ability to adiabatically drive a quasiparticle coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz (THz)-frequency electric fields[1-3]. I will discuss recent experimental and theoretical studies of HSG from GaAs/AlGaAs quantum wells[4]. The observed HSG spectra contain sidebands up to the 90th order. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone. A surprising dynamical birefringence is observed: high-order sidebands are usually stronger when the exciting near-infrared (NIR) and the THz electric fields are polarized perpendicular than parallel; and the sidebands exhibit significant ellipticity that increases with increasing sideband order, despite nearly linearly-polarized excitation and driving fields. We explain dynamical birefringence by generalizing the three-step model for high order harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasi-momentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways.

[1] R.-B. Liu and F.-B. Zhu, AIP Conf. Proc. 893, 1455 (2007).
[2] B. Zaks, R.-B. Liu and M. S. Sherwin, Nature 483, 7391 (2012).
[3] F. Langer, et. al., Nature 533, 7602 (2016).
[4] H. B. Banks, et. al., ArXiv.1706.08449.