  The second-order nonlinear optical (NLO) processes, such as the photogalvanic
effect and second-order harmonic generation (SHG), play crucial roles in
probing and controlling light-matter interactions for energy and device
applications. To date, most studies of second-order NLO processes focus on
materials with broken spatial inversion symmetry, such as proper ferroelectrics
and noncentrosymmetric Weyl semimetals. Nevertheless, inversion symmetry of
Shubnikov groups can be broken via spin-ordering in centrosymmetric crystals.
Unfortunately, these materials are less common, and their NLO responses are
usually weak. Combining quantum perturbation theory and first-principles
simulations, we predict a giant injection-current photogalvanic effect and SHG
in a family of emerging axion insulators, the even septuple layers of MnBi2Te4
(MBT) materials that exhibit the zero-plateau quantum anomalous Hall (QAH)
effect. Their amplitudes of injection current and SHG are about two orders of
magnitude larger than those of widely used ferroelectrics, such as BiFeO3 and
LiNbO3. Moreover, unlike the usual injection current observed under
circularly-polarized light, the injection photocurrent of MBTs only emerges
under linearly polarized light, making it convenient for device applications.
These unique characters are from a combination effect of parity-time symmetry,
three-fold rotation symmetry, and significant spin-orbit coupling. These
enhanced NLO effects are valuable for characterizing subtle topological orders
in QAH systems and also shed light on novel infrared photo-detector and
photovoltaic applications based on magnetic topological materials.
