High-Order Harmonic Generation from Strained Metal Thio/SelenoPhosphates

The search for and design of two dimensional (2D) van der Waals materials with extraordinary nonlinear optical properties has important implications for next-generation optoelectronic devices. CuInP₂S₆ (CIPS) is an emerging layered ferroelectric material, which, when synthesized with Cu-deficiencies (Cu_1-xIn_1+x/3P₂S₆), segregates into a self-assembled heterostructure consisting of CIPS and In_4/3P₂S₆ (IPS). The consequence of this segregation is significant in- and out-of-plane strain between the CIPS and IPS phases. Here, we investigate high-order harmonic generation (HHG) from sub-micron exfoliated crystals of CIPS, Cu-deficient CIPS, and IPS using femtosecond mid-IR driving laser pulses. We find that Cu-deficient CIPS produces orders-of-magnitude higher HHG yield with lower input driving laser intensity in comparison to pure CIPS and IPS. Through ab initio calculations, we identify that the efficiency enhancements in Cu-deficient CIPS are likely due to a combination of strong electron-electron interactions in the Cu d-orbitals of CIPS and strain in the CIPS/IPS heterostructure.