Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe$_2$

Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe$_2$ has bridged 2D materials and topological physics. While the realization of QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe$_2$, the geometrical properties of the wavefunction, such as the Berry curvatures, remain entirely unstudied. On the other hand, it has been increasingly recognized that the Berry curvature plays an important role in multiple areas of condensed matter physics including nonreciprocal electron transport, enantioselective optical responses, chiral polaritons and even unconventional superconductivity. Here we utilize mid-infrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe$_2$. By optically exciting electrons across the inverted QSH gap, we observe an in-plane circular photogalvanic current even under normal incidence. The sign of the photocurrent can be further switched by applying a perpendicular displacement field. Our observations suggest an electrically switchable Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge, which opens the door for observing a wide range of quantum geometrical phenomena in both single-particle and collective modes.