  Two-dimensional (2D) layered semiconductors with both ultrawide bandgap and
strong second harmonic generation (SHG) are essential for expanding the
nonlinear optical (NLO) applications to deep-ultraviolet (DUV) region in
nanoscale. Unfortunately, these materials are rare in nature and have not been
discovered until now. In this Letter, we predict the B2S2O9 (BSO), an existing
layered oxide, can exhibit both DUV bandgap and strong SHG effects, comparable
to the best known DUV NLO bulks. The strong SHG intensities in BSO, originated
from the ordered arrangement of polar SO4 and BO4 tetrahedra forming planar
structure, are linearly tunable by the layer thickness. Surprisingly, the
spontaneous rotations of rigid tetrahedra under strains can induce the (nearly)
zero Poisson's ratios in BSO, which simultaneously result in the robust SHG
effects against large strains, fundamentally differing from other known 2D NLO
semiconductors. The discovery of BSO may provide an unprecedented opportunity
to explore DUV NLO physics and applications in 2D limit.
