Nonlinear optical processes in 2D Cairo pentagonal palladium phosphide sulfide

Anisotropic nonlinear optical two-dimensional (2D) materials hold great potential for advancing photonics and optoelectronics applications due to their coexistence of strong nonlinear optical response and high nonlinear anisotropy ratio, such as crystal orientation identification, bio-microscopy, optical switching. This work focused on the unique layer-dependent symmetry breaking and strong second harmonic generation (SHG) anisotropy based on palladium phosphide sulfide (PdPS), a pentagonal 2D semiconductor with Cairo tiling. PdPS exhibits even-layer inversion symmetry breaking and highly tunable SHG anisotropy, contrast to odd-layer inversion symmetry. Notably, the SHG anisotropy ratio reaches up to 32.7 for six layers (< 5 nm) PdPS. This ratio is the highest in such thin 2D materials to the best of our knowledge. Furthermore, its optical properties can be tuned easily through layer number and microdevice configuration. As layer number of PdPS decreases from six layers to two layers, the anisotropic ratio drops drastically from 32.7 to 0.89. In addition, the overall SHG signal can be increased by 10 times when integrated with a photonic crystal device. These findings suggest that PdPS holds great promise for use in polarization-sensitive and layer-engineered nonlinear photonic applications like photodetectors, polarized lasers, polarized light emitting diodes and reflective polarizer.