Study On Magnetic Field Enhancement At Optical Frequencies In Metamaterials

In light-matter interactions at optical frequencies,the magnetic component of light generally plays a negligible role,since as light interacts with matter the force exerted by the magnetic field on a charge is far smaller than the force applied by the electric field.However,under illumination of light,some specially shaped artificial magnetic atoms,such as metallic U-shaped rings or paired metallic rods,can generate localized magnetic field enhancement in their interior,the contribution of the magnetic field to the coupling of light with matter is not comparable to the contribution of the electric field component of light.It has been shown that the magnetic field enhancement at optical frequencies is likely to improve the spontaneous emission of the magnetic dipole transition,and in the theory,the Purcell factor can be expcted to be close to 2000.Therefore,how to further improve the magnetic field enhancement at optical frequencies is of new significance for the future research of nanospectroscopy.In this thesis,we propose and demonstrate two novel mechanisms for optical magnetic field enhancement in metamaterials.The thesis is mainly composed of two sections that are arranged as following:1.We have proposed a powerful approach to enhance magnetic field at optical frequencies by introducing surface plasmon polaratons(SPPs)modes into metamaterials.It has been shown that when the array period approaches resonant wavelength of magnetic surface plasmons(MSPs),the SPPs and the MSPs could couple together,and resulting into a narrow-band hybridized mode.Associated with the excitation of this hybridized mode,a 20 times enhancement of magnetic fields within in the dielectric spacer is achieved as compared with the pure MSPs resonance.In particular,the maximum magnetic fields are enhanced to be about 2143 times of the incident field.Moreover,we also found that the coupling between the MSPs and the SPPs can be tailored by the period of the metal nanowire array to affect the magnetic response of the plasmonic structure.Above the resonant wavelength of the MSPs,coupling between two kinds of resonance modes can lead to the huge magnetic field enhancement,whereas below it,coupling cannot lead to a magnetic field enhancement.2.We have presented a novel method to realize optical magnetic field enhancement in metamaterials by introducing a Fabry-Pérot(FP)cavity that supports optical modes.It has been shown that when the position of the FP cavity modes is tuned to be close to the resonant wavelength of MSPs resonance,the strong coupling between MSPs and optical cavity modes is observed,manifested by the large anticrossing of the resonant positions in the reflection spectra.It results in two branches of hybridized MP modes with a Rabi-type splitting as large as 78 meV.The maximal magnetic field intensity achieved in the metamaterials with an FP cavity is enhanced greatly and an enhancement factor >3 is observed compared with that achieved by the bare metamaterials.In particular,the maximum magnetic fields are enhanced to be about 459 times of the incident field.