Enhanced second harmonic emission with simultaneous polarization state tuning by aluminum metal-insulator-metal cross nanostructures.

Aluminum (Al) plasmonic nanostructures have recently demonstrated remarkable optical nonlinear phenomena, such as enhanced second harmonic (SH) generation. However, the relatively weak field enhancement resulted from large optical losses associated with aluminum nanostructures in combination with the difficulties in controlling the emission polarization pose as a challenge for SH enhancement and tuning. In this paper, we show that the SH emission of aluminum nanostructures can be efficiently enhanced with the polarization properties simultaneously tunable by using metal-insulator-metal (MIM) nanostructures, constituting of Al cross nanoantenna arrays on top of Al mirrors with a SiO spacing layer. Specifically, femtosecond laser beam with a linear polarization parallel to one arm illuminates on the structure while the orthogonal arms were physically modified by the laser-induced photothermal reshaping technique to control the SH radiation by the plasmonic resonances. Under the resonance at the SH wavelength, we observed one order of magnitude larger emission enhancement compared to that at the off-resonant condition. Interestingly, the polarization states can be well manipulated simultaneously by controlling the resonances of the orthogonal arms. The enhanced SH conversion and tunable polarization states pave the way for the development of nonlinear optical sources and advanced functional metasurfaces.