Large-scale flexible membrane with resonant silicon nanowires for infrared visualization via efficient third harmonic generation

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale. Indeed, various nanophotonic designs have been developed to demonstrate high efficiencies for third harmonic generation (THG). However, fabrication approaches for these designs usually involve high-cost lithography-based technological steps. This work reports on the efficient THG in a free-standing Si nanowire array encapsulated into a flexible polymer membrane. High nonlinearity of Si material $χ1111(3)≈2.62×10−19m2/V2$ and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared (⁠$1200–2000$ nm) to visible (⁠$400–666$ nm) upconversion. The fabricated membranes demonstrate high flexibility and transparency, which make them convenient to use as infrared light visualizers.