文摘
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 extremely strong magnetic resonances also could be used for designing new metamaterials and devices.