Cavity Effect of Gold Nanoparticles on Mid-Infrared Light.

Nanophotonics has attracted wide attention in photonic devices and biotechnology. Interaction of visible and near-infrared lights with metal nanoparticles (NPs) is already well explored, leading to a mount of applications, especially in high-sensitivity biodetection. However, the effects of metal NPs on mid-infrared (MIR) light are still lacking because the light cannot resonantly excite the surface electron oscillation of the NPs. Recently, gold NP (AuNP)-assisted experiments indicate that AuNPs can be used in the detection of MIR biophotons, but the underlying mechanism remains unclear. Here, constructing a cavity by two AuNPs and performing finite difference time domain simulations based on Maxwell equations, we demonstrate that even if the AuNP dimension is significantly smaller than the MIR wavelength, the AuNP-formed cavity (AuNP-cavity) still can confine the light. The confinement effect increases with an increase in the wavelength or the cavity length when the cavity length and wavelength are fixed, respectively, while it vanishes only when the AuNP dimension is less than 1000th of the light wavelength. These results can be attributed to the resonance of MIR light with the two AuNPs, and in this view, it can be said that this nanocavity overcomes the diffraction limitation of the optical system. Our findings provide an understanding of the biophoton detection mentioned above, potentially promoting the applications of metal NPs in biotechnology and even in MIR-related imaging and wave-guiding circuits.