  The phenomena of crystal size changes and structural phase transitions
induced by light irradiation have garnered significant interest due to their
potential for tuning and controlling a wide range of material properties
through highly cooperative interactions. However, a microscopic theory that can
comprehensively explain these phenomena in correlation with photon frequency
and polarization has remained highly desirable. In this work, we propose that
nonlinear photocurrent may correspond to driving these effects, which arise
from a competition between light-injected energy and structural variations. By
conducting first-principles simulations and comparing them with two established
experiments, we show that shift current, a second-order photocurrent, can
induce photostriction and nonreciprocal structure phase transitions. The
quantitative comparisons across key parameters such as light frequency,
irradiation time, polarization, and intensity provide further support for the
nonlinear photocurrent mechanism. Beyond shift current, this microscopic
understanding proposes to utilize more types of nonlinear photocurrent to
enhance light-structure interactions and control material properties.
