Plasmonic resonance modulation of graphene by nanoscale substrate curvatures

The nanopatterning of graphene with complex geometries and configurations has been widely investigated to excite plasmonic resonances in the graphene layer with high spatial confinement and gate tunability for nanophotonic applications. However, the existing approaches suffer from time-consuming and complicated fabrication processes, edge effects, and a narrow spectral tunability range of plasmonic resonances in graphene, limiting their utility in optoelectronic applications. Here, we study the effect of substrate corrugation and curvature on the plasmonic resonances of graphene through theoretical studies of a model system - graphene on closely packed SiO₂ nanospheres with different diameters. Highly confined plasmonic waves in graphene are efficiently excited due to nanoscale deformations in graphene using a nanocorrugated substrate on silicon. The SiO₂ nanospheres, with their close-packed curvature pattern, couple the incident optical wave to the graphene plasmonic resonances, thereby creating a sharp notch (resonance peak) on the normal-incidence transmission spectra (absorption spectra) with a high-quality factor of ∼109 and a near-field intensity enhancement of ∼50². The resonant wavelength of the plasmonic modes can be tuned over a wide wavelength range in the mid-infrared by adjusting the size of the nanospheres. Finally, we demonstrate that the biaxially deformed graphene structure can detect slight variations in the refractive index of the surrounding analyte. The results demonstrate a high sensitivity of 3250 nm RIU^-1 and a large figure-of-merit of 63.2 RIU^-1 for deformed graphene on closely packed SiO₂ nanospheres. Our findings indicate that biaxially nanocorrugated graphene on closely packed SiO₂ nanospheres, with its facile fabrication process, strong plasmonic resonances, and superior sensing performance, provides a platform for ultrasensitive biosensing.