  Electrical tuning of second-order nonlinearity in optical materials is
attractive to strengthen and expand the functionalities of nonlinear optical
technologies, though its implementation remains elusive. Here, we report the
electrically tunable second-order nonlinearity in atomically thin ReS2 flakes
benefiting from their distorted 1T crystal structure and interlayer charge
transfer. Enabled by the efficient electrostatic control of the
few-atomic-layer ReS2, we show that second harmonic generation (SHG) can be
induced in odd-number-layered ReS2 flakes which are centrosymmetric and thus
without intrinsic SHG. Moreover, the SHG can be precisely modulated by the
electric field, reversibly switching from almost zero to an amplitude more than
one order of magnitude stronger than that of the monolayer MoS2. For the
even-number-layered ReS2 flakes with the intrinsic SHG, the external electric
field could be leveraged to enhance the SHG. We further perform the
first-principles calculations which suggest that the modification of in-plane
second-order hyperpolarizability by the redistributed interlayer-transferring
charges in the distorted 1T crystal structure underlies the electrically
tunable SHG in ReS2. With its active SHG tunability while using the facile
electrostatic control, our work may further expand the nonlinear optoelectronic
functions of two-dimensional materials for developing electrically controllable
nonlinear optoelectronic devices.
