Design Of Optical Nanoantennas And Their Optical Absorption Enhancement Properties

Nanoantenna is a kind of antenna with size in nanometer level. It is different fromtraditional antenna in terms of working frequency and working principle.The mainreason is that its working principle basically depends on the surface plasmon resonance(SPR) We can control the surface plasmon resonance frequency and the local fieldstrength. These features can be used in many fields, such as sub-diffraction limitimaging, new type of near field optical probe, high conversion efficiency of solar cell,subwavelength waveguides transmission, etc. Optical nanoantenna is getting more andmore attentions.This thesis is based on the finite integral method (FIT) and the characteristics ofthe S parameter calculation. This thesis delivers a method on the input impedance ofnanoantenna and the gold-plate absorption rate.We analysis the difference of traditional antenna and optical annoantenna. Thenanoparticle equivalent circuit is analyzed, and we focuse on the input impedance ofdifferent structure of nanoantenna. We study how to manipulate the input impedance bychanging the geometry and size of the nanoantenna. It is shown that with the increase ofthe radius of the antenna, the resonance frequency increases, while the input resistancedecreased; When the arm length increases, the resonance frequency is reduced and theinput resistance also increased; With the increase of gap, the resonant frequency andinput resistance both increased; As the dielectric constant of the load medium inside thegap increases, the resonance peak decreases, and input resistance decreases. All theseobservations are qualitatively in consistent with the equivalent circuit analysis.We study the absorptivity of metal plate coupled with an optical nanoantenna array.We analyzes the200nanometers thick gold-plate absorbance intensively, we alsoanalyzes the absorbance of silver nanoantenna array placed above it. Results show thatwhen the nanoantenna array is in presence,at the resonant frequency of the coupledgold-plate system the reflectivity decrease obviously, yet the correspondingtransmission is infinitely close to zero. so we can conclude that the gold-plateabsorbance has improved greatly.Moreover, Using inhomogeous nanoantenna array canbroad the absorption band dramatically, with respect to the homogenous nanoantennaarray. The method and results in this thesis can be of reference value in increasing solarnanoflat cell efficiency.