Strong Third-Harmonic Enhancement from Sb₂Te₃ Nanopores with the Generation of Fabry-Pérot and Surface Plasmon Resonances

Topological insulators are a new kind of quantum materials with the insulating bulk and topologically-protected conducting surface/edge states. Due to the ultrahigh third-order nonlinear optical coefficient and capacity for supporting surface plasmons, topological insulators will find promising applications in optical nonlinearities. In this study, an ultrathin Sb₂Te₃-based cavity is prepared to generate the Fabry-Pérot resonance, which can effectively improve the light absorption at near-infrared wavelengths. The Fabry-Pérot resonance wavelength is controllable by changing the thickness of the Sb₂Te₃ layer. The numerical simulations agree well with the measurements. The Fabry-Pérot resonance enables a 5.5-fold enhancement of the third-harmonic signal from the Sb₂Te₃ film, which is measured by a self-built nonlinear optical micro-spectroscopy system. Furthermore, a nanopore array is fabricated on the Sb₂Te₃-based cavity employing a focused ion beam lithography. The Sb₂Te₃ nanopore array enables the generation of localized surface plasmon resonance at visible wavelengths. By combining the Fabry-Pérot and plasmonic resonances, the optical nonlinearity of Sb₂Te₃ can be further improved. The measurements show that the third-harmonic signal of the Sb₂Te₃ nanostructure can be enhanced by 19.3-fold compared to that of the Sb₂Te₃ film. This work will pave a new avenue for enhancing optical nonlinearities of topological insulators and promoting their applications in high-performance nonlinear optical devices.