LAUSR

The second-order nonlinear optical (NLO) processes, such as photogalvanic effect and second-order harmonic generation (SHG), play crucial roles in probing and controlling light-matter interactions for energy and device applications. 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 $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ (MBT) materials. Their amplitudes of photocurrent and SHG are about 1--2 orders of magnitude larger than those of proper ferroelectrics and antiferromagnetic $\mathrm{Cr}{\mathrm{I}}_{3}$. Moreover, unlike the widely studied injection current observed under circularly polarized light, the injection photocurrent of MBT only emerges under linearly polarized light, making it suitable for device applications. These unique characters are from a combination effect of parity-time symmetry and significant spin-orbit coupling. Our predicted enhanced NLOs are valuable for characterizing subtle magnetic orders and shed light on infrared photodetecting and photovoltaic applications based on magnetic topological materials.