  Structural symmetry of crystals plays important roles in physical properties
of two-dimensional (2D) materials, particularly in the nonlinear optics regime.
It has been a long-term exploration on the physical properties in 2D materials
with various stacking structures, which correspond to different structural
symmetries. Usually, the manipulation of rotational alignment between layers in
2D heterostructures has been realized at the synthetic stage through artificial
stacking like assembling Lego bricks. However, the reconfigurable control of
translational symmetry of crystalline structure is still challenging. High
pressure, as a powerful external control knob, provides a very promising route
to circumvent this constraint. Here, we experimentally demonstrate a
pressure-controlled symmetry transition in layered InSe. The continuous and
reversible evolution of structural symmetries can be in-situ monitored by using
the polarization-resolved second harmonic generation (SHG) spectroscopy. As
pressure changes, the reconfigurable symmetry transition of the SHG pattern
from three-fold rotational symmetry to mirror symmetry was experimentally
observed in a layered InSe samples and was successfully explained by the
proposed interlayer-translation model. This opens new routes towards potential
applications of manipulating crystal symmetry of 2D materials.
