# Dynamical birefringence: Electron-hole recollisions as probes of Berry curvature

## TL;DR

This paper demonstrates that high-order sideband generation in semiconductors reveals Berry phases through dynamical birefringence, enabling probing of Berry curvature via electron-hole recollisions driven by strong terahertz fields.

## Contribution

It introduces a novel method to measure Berry phases in solids using high-order sideband generation and explains dynamical birefringence through quantum interference of electron-hole recollision pathways.

## Key findings

- Observation of up to 90th order sidebands in GaAs/AlGaAs quantum wells.
- High-order sidebands are stronger when NIR and THz fields are perpendicular.
- Sideband ellipticity increases with sideband order, indicating Berry phase effects.

## Abstract

The direct measurement of Berry phases is still a great challenge in condensed matter systems. The bottleneck has been the ability to adiabatically drive an electron 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. We carried out experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum wells. The observed HSG spectra contain sidebands up to the 90th order, to our knowledge the highest-order optical nonlinearity observed in solids. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone around the \Gamma point. The principal experimental claim is a dynamical birefringence: the sidebands, when the order is high enough (> 20), are usually stronger when the exciting near-infrared (NIR) and the THz electric fields are polarized perpendicular than parallel; the sideband intensities depend on the angles between the THz field and the crystal axes in samples with sufficiently weak quenched disorder; and the sidebands exhibit significant ellipticity that increases with increasing sideband order, despite nearly linear 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.