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Recent experimental evidence for the quantum spin Hall (QSH) state inmonolayer WTe$_2$ has bridged two of the most active fields of condensed matterphysics, 2D materials and topological physics. This 2D topological crystal alsodisplays unconventional spin-torque and gate-tunable superconductivity. Whilethe realization of QSH has demonstrated the nontrivial topology of the electronwavefunctions of monolayer WTe$_2$, the geometrical properties of thewavefunction, such as the Berry curvature, remain unstudied. On the other hand,it has been increasingly recognized that the Berry curvature plays an importantrole in multiple areas of condensed matter physics including nonreciprocalelectron transport, enantioselective optical responses, chiral polaritons andeven unconventional superconductivity. Here we utilize mid-infraredoptoelectronic microscopy to investigate the Berry curvature in monolayerWTe$_2$. By optically exciting electrons across the inverted QSH gap, weobserve an in-plane circular photogalvanic current even under normal incidence.The application of an out-of-plane displacement field further systematicallycontrols the direction and magnitude of the photocurrent. Our observedphotocurrent reveals a novel Berry curvature dipole that arises from thenontrivial wavefunctions near the inverted gap edge. These previouslyunrealized Berry curvature dipole and strong electric field effect are uniquelyenabled by the inverted band structure and tilted crystal lattice of monolayerWTe$_2$. Such an electrically switchable Berry curvature dipole opens the doorto the observation of a wide range of quantum geometrical phenomena, such asquantum nonlinear Hall, orbital-Edelstein and chiral polaritonic effects.

Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2