Enhanced second harmonic generation from vertically stacked multilayer gallium selenide separated by low-index PMMA spacer layer

We report Second Harmonic Generation (SHG) enhancement in a custom-designed vertically stacked multilayer Gallium Selenide with a low-index PMMA spacer layer. The structure obtained using an evolutionary optimization algorithm consists of two 40 nm GaSe layers separated by a 195 nm PMMA layer, with a 130 nm Silicon-dioxide layer on a Silicon substrate. This results in 11x and 303x enhancement in SHG signal when compared to a single 40nm GaSe on a 130nm and 300nm SiO₂/Si substrate, respectively. This work underscores the importance of micro-cavity engineering in choosing appropriate 2D material and interspacer thickness to enhance SHG emission from popular 2D materials.