Unlocking second harmonic generation in graphene through strain-induced sublattice polarization

Graphene is renowned for its exceptional optical and electronic properties. However, its inherent centrosymmetric nature has constrained its capacity to exhibit second harmonic generation (SHG). Various prior efforts have explored the possibility of disrupting graphene's spatial inversion symmetry using external stimuli, but these methods have fallen short of fundamentally altering the lattice structure of graphene. To address this limitation, we introduce a novel approach by applying non-uniform strain to graphene. This technique effectively creates sublattice polarization, thereby breaking the lattice inversion symmetry and enabling the generation of SHG. Remarkably, this strain-induced SHG is significantly enhanced, reaching approximately 50-fold higher values at low temperatures due to resonant transitions between strain-induced pseudo-Landau levels. Notably, the second-order nonlinear susceptibility of strained graphene surpasses that of hexagonal boron nitride, a material with intrinsic broken inversion symmetry. Our results demonstrate new possibilities in engineering graphene's symmetry properties, providing a practical and scalable means to achieve second-order nonlinear optical effects.