Study Of Optical Nanoantenna On Controlling Near-field Enhancement And Unidirectional Scattering

Optical antennas are emerging as powerful building blocks for exploring the potential applications in biosensing,photodetection and near-field microscopy.Optical antennas act as transducers of the visible spectrum electromagnetic fields: they convert electromagnetic power coming from the far-field,to localized electromagnetic power,or vice versa.They are thus able to convert freely-propagating light beams into sub-wavelength,high intensity hot spots,thereby increasing the excitation rate of fluorescent molecules.Reciprocally,for a molecule in an excited state,optical antennas can enhance both its radiative and non-radiative decay rates,since antenna elements act as secondary sources.Furthermore,the scattered and emitted fields can interfere in the far-field,thereby modifying the radiation pattern of the emission.Nevertheless,there are many differences between optical antenna and traditional(e.g.,microwave or RF)antennas.To fully explore the unique characteristics of the optical antenna(such as strong near-field enhancement,spectrum and directivity etc.),this thesis aims to explore the near and farfield properties of metal dimer antenna,all dielectric antenna and metal-dielectric patch antenna,respectivvely.This thesis carries out the following studies:Firstly,the interactions between different order of LSPR antenna modes in metal dimer antenna are discussed.Particularly,the optical-mehanical effects induced by Fano resonance are exmanined.It is found that Fano resonance leads to unidirectional scattering of the antenna.The corresponding near field feartures are analyized in detail.It is indicated that the optical binding force reversal and unidirectional scattering are both caused by the phase variation at the Fano resonance frequency.Furthermore,the optical emission response of a single dipole emitter coupled with a metal dimer that sustains plasmonic Fano and multipole Fano resonances is investigated.The bright mode and dark mode excitation of each particle are decided by the the position of the dipole emitter and this determines whether the Fano interactions are visibly activated.Finally,the EIT effect caused by the symmetry broken of the particles is examined.Secondly,the properties of near field and far field for all dielectric nanoantenna are explored.The underlying mechanism of high magnetic field concentrations for different geometrical parameters of an all-dielectric hollow nanodisk is discussed.The Purcell factor can be more than 300 and it is shown that MD emission can be significantly enhanced,while ED emission is suppressed when the point-like emitters are located in the hollow of the nanodisk.Furthermore,Dual-band unidirectional forward scattering with high efficiency can be realized in the antenna.Thirdly,the properties of magnetic cavity modes in the dielectric gap of multilayer metal-dielectric nanoantenna are explored.It is found that the cavity modes could be selectively excited using a dipole source,and the excitation of cavity modes is shown to be highly sensitive to the dipole position which determines the symmetry matching and the degree of field overlap between the dipole source radiation field and the cavity mode pattern.Furthermore,we show that the resonance frequencies of the antenna with different geometries can be approximately predicted theoretically.Furthermore,the coherent coupling between antenna mode and cavity mode leads to unidirectional emissions in opposite directions at different wavelengths.Additionally,with a proper spacer thickness and filling medium,it is possible to control the spectral positions of the forward and backward unidirectional emissions and to exchange the wavelengths for two these unidirectional emissions.In summary,the thesis demonstrates the great potential of optical antennas as elements to control light on the nanoscale.Within this powerful paradigm,the interaction of light with nanoscale matter can be tailored with complete flexibility.This research shall also pave a new route of optical antenna on spectroscopy,sensing,display technologies,and photovoltaics devices.