Localized surface plasmon resonance induced spatial self-phase modulation of Cu₂-xS in the near-infrared region

Generating controllable spatial self-phase modulation (SSPM), such as self-diffraction, is a challenge in the near-infrared (NIR) band. Cu2-xS nanoplates (NPs) and their heterojunction nanoplates (HNPs) were confirmed to enable enhancement SSPM between 800 nm and 1550 nm, as p-type semiconductors with localized surface plasmon resonance (LSPR). Cu1.94S NPs with particle sizes of 36 nm and 53 nm both exhibit this enhancement, dominated by LSPR-induced hole interband transitions, proofed by the results of open aperture (OA) Z-scan and transient absorption (TA) spectroscopy. Furthermore, the sign inversion of the third-order nonlinear optical coefficients between 1300 nm and 1550 nm was achieved by modulating the copper-sulfur ratio from 1.94 to 1.81 in Cu2-xS. It was attributed to the introduction of additional hole carriers by increasing the amount of copper deficiency, proofed by the blue shift of the LSPR absorption peak, the increase of the carrier relaxation time, and a decrease in the saturable intensity Is. Meanwhile, the unchanged or decreased hole carrier concentration in CdS-Cu1.81S and Ag-Cu1.81S HNPs resulted in unchanged or opposite outcomes. It is presented that utilizing the LSPR of hole semiconductor materials could realize the modulation of SSPM in the NIR band.