Efficient Magnetic Resonance Amplification and Near-Field Enhancement from Gain-Assisted Silicon Nanospheres and Nanoshells

Strong artificial magnetic responses play a pivotal role in the unique optical properties of metamaterials, such as cloaking, Fano resonance, and negative refraction. We investigate the magnetic resonances of gain-assisted Si nanospheres and SiO2–Si nanoshells. It is found that the super-resonances occur successively at the octupole, quadrupole, and dipole magnetic modes of the active Si nanosphere or SiO2–Si nanoshell by changing the gain coefficient. At the super-resonances, the magnetic scatterings and the local electric fields are all extremely enhanced. The corresponding surface-enhanced Raman spectroscopy (SERS) enhancement factors at the magnetic super-resonances could reach about 1010–1015, which are enough for single-molecule detection. In particular, the electric scatterings of the active Si nanosphere and SiO2–Si nanoshell are all very weak at the super-resonances. Therefore, the active Si nanosphere and SiO2–Si nanoshell should be excellent SERS substrates with weak background radiation. The e...