Optical hyper-focusing in Au nanoparticle on Si/Au multilayer systems

The ability to focus light beyond the diffraction limit has been a long-standing challenge in optical science. Although many innovative techniques have been developed, ranging from near-field scanning optical microscopy to super-resolution and plasmonic approaches, these methods often rely on complex configurations and expensive setups. In this study, we propose a way for optical hyper-focusing by simply placing a gold nanoparticle on a silicon/gold multilayer metamaterial. In our configuration, the gold nanoparticle generates a localized plasmon resonance hotspot, exciting high-wave vector electric fields that propagate through the Si/Au multilayer. These fields progressively focus at the bottom interface, forming focal spots tens of nanometers in diameter across a high-loss propagation zone (∼100 nm). An optimal spot size of 44 nm and an intensity 4.5 times that of the incident light are observed at a wavelength of 780 nm using a 160 nm gold nanoparticle, surpassing both diffraction limits and the nanoparticle geometric size. This work provides insights into how multilayer metamaterials can be utilized to achieve sub-wavelength light confinement and optical hyper-focusing, opening possibilities for advanced optical applications in super-resolution imaging, nano-optics, and integrated photonic devices.