Designs And Applications Of Some Optical Nano-Antennas

The wide applications of optical nano-antennas enable the manipulation of light in the subwavelength region in the near field. In this thesis, metallic and dielectric optical nano antennas are studied using different methods according to there different characteristics and their applications in surface enhanced Raman scattering (SERS) and Fano resonance are explored.Firstly, we put forward a perodic gold bowtie/gold mirror dual-resonance nano-antenna and it is used to provide local field enhancement to long wavenumber or hyper Raman scattering. Two localized surface plasmonic resonances (LSPRs) are supported in this structure and they are the electric dipole bonding resonance (EDBR) and the magnetic dipole resonance (MDR). Compared with other dual-resonance antennas, the resonant frequencies of the two LSPRs can be independently tuned and both modes can provide strong local field in the gap region. Dual-resonance optical nano-antennas can be used to simultaneously enhance the Raman excitation and scattering fields and thus has special priority in Raman enhancement at long wavenumbers. In the experiment, we test the SERS effect on C-H vibration at 2960cm-1. When Raman excitation and scattering fields are simultaneously enhanced, the total Raman intensity is much larger than those soly enhanced at the excitation or scattering frequency.Secondly, we put forward a triple-resonance gold bowtie-strip/gold mirror nano antenna. By utilizing the strong interaction between non-localized SPR and the LSPRs, we can obtain totally three near field enhancement peaks, which can be usted to simultaneously enhance the Raman excitation, Stokes and anti-Stokes signals, meaningful to the study of laser heating and the molecule vibrational pumping effect. Moreover, by introducing the gold strip, the local field is further enhanced for all the resonances since more light can be harvested and confined to the near field. For the magnetic dipole resonance, the gold strip can improve the mode excitation efficiency and thus the local field is greatly enhanced. For the non-localized SPR, gold strip reflects the propagating surface wave. Thus the directly excited and reflected surface wave interferences with each other. When the width of the gold strip is optimized, the Raman enhancement for Stokes and anti-Stokes is increased by～22.8 times and～3.6 times, respectively.Thirdly, the Fano resonance in one dimensional (1-D) silicon nanowire array is explored by coupled leaky mode theory (CLMT). We derived the reflection and transmission equations in 1-D nanowire array, relating the optical responses with the eigenvalues and the intrinsic phases. The intrinsic phases of each leaky mode are theoretically calculated and the physical understandings are given. It is found that the size ratio of the nanowire can engineer the intrinsic phase from 0 to 2π with ease. By changing the phase, arbitrary Fano lineshape can be designed. By changing the filling factor, the Fano bandwidth can be tuned.Finally, we conclude the thesis and give perspectives about further works.