Terahertz electro-optic effect in Bi$_2$Se$_3$ crystals

TL;DR

This paper reports the observation of a terahertz-induced electro-optic effect in Bi$_2$Se$_3$ crystals, linked to surface Dirac states, with potential implications for ultrafast optoelectronic applications.

Contribution

It demonstrates a novel terahertz electro-optic effect in Bi$_2$Se$_3$ linked to surface states, revealing both linear and quadratic responses and their symmetry properties.

Findings

Electro-optic effect follows terahertz field with sub-100 fs delay.

Linear component shows three-fold rotational symmetry.

Effect disappears after phase transition or femtosecond pre-pulse.

Abstract

We report the observation of the electro-optic effect in Bi$_2$Se$_3$ crystals induced by an intense single-cycle terahertz pulse. The effect reveals itself as a transient change of the polarization state of a femtosecond laser pulse reflected from the crystal that is exposed to the terahertz electric field. The corresponding experimental signal follows the field with a sub-100-fs delay and can be represented as a linear combination of the terahertz electric field and its square. The linear and quadratic components are of comparable magnitude. The latter is almost independent of crystal orientation, while the former demonstrates three-fold rotational symmetry in agreement with the trigonal symmetry of the crystal surface. The electro-optic effect vanishes upon phase transition to the non-topological metal state induced by indium doping and also can be quenched by a femtosecond pre-pulse. We associate this effect with surface Dirac electronic states of Bi$_2$Se$_3$ and discuss its possible mechanisms.