| In this thesis,several ring nanoantennas will be investigated,which can achieve spontaneous emission enhancement and far-field emission enhancement with a tunable bandwidth at visible wavelengths.Based on the azimuthally propagating surface plasmon polaritons(SPP),the SPP semi-analytical model is established to clarify the underlying physical mechanism of antenna spontaneous emission enhancement.Due to the antenna excellent resonant characteristics,the optical nanoantennas can drastically enhance the spontaneous emission rate of surrounding emitters such as molecules or quantum dots.Since the fluorescence emission wavelength of molecules or quantum dots usually covers a range from tens to hundreds of nanometers,a broadband enhancement of the spontaneous emission rate is highly required for the antenna design.Therefore,we propose a split-ring optical nanoantenna which can achieve enhancement factors of total and radiative emission rates up to high values of7411 and 1941,respectively,with a tunable bandwidth of enhanced emission(Purcell factor over 1000)up to 221 nm at visible wavelengths.To clarify the underlying physical mechanism of radiation enhancement and guide antenna design,we build up a semi-analytical model by considering the dynamical launching and intuitive multiple-scattering process of SPPs that propagate azimuthally along the curved antenna arms.The SPP model provides an intuitive picture that under two phase matching conditions(corresponding to symmetric and antisymmetric source excitation,respectively),the SPPs form paired and tunable whispering-gallery-like resonances and are then coupled to the emitter in the nano-gap or scattered into free space,which results in an enhancement of the total or radiative emission rate,respectively.To clarify the underlying physical mechanism of broadband tunability of spontaneous emission enhancement,based on the SPP model,we further build up a semi-analytical model for the quasi-normal modes(QNM)supported by the antenna,which provides an analytical expression of the frequency dependence of the electromagnetic field and indicates that the broadband enhancement of the spontaneous emission rate results from an excitation of paired symmetric and anti-symmetric QNMs with different eigenfrequencies.The proposed models clarify the impact of azimuthally propagating SPPs on the antenna radiation and may be extended to other optical antennas with curved arms.Compared with metal nanoparticle optical nanoantenna(such as the split-ring optical nanoantenna described above),the nanoaperture antenna can shield the background fluorescence noise outside the nanoaperture,which makes the fluorescence in the aperture have a high signal-to-noise ratio,and can narrow the fluorescence excitation region and beyond the diffraction limit,so as to realize the detection of a single fluorescent molecule at a physiological concentration of up to micromole.These superior properties have enabled many important applications such as fluorescence analysis,single molecule biology.Therefore,combined with the characteristics of the broadband spontaneous emission enhancement of the split-ring optical nanoantenna and the advantages of nanoaperture antenna,we propose an azimuthally-periodic split-ring nanoaperture antenna(ASNA),which can achieve enhancement factors of total and radiative emission rates up to high values of 4171 and 1235,respectively,with a tunable bandwidth of enhanced emission(Purcell factor over 700)up to 285 nm at visible wavelengths.We build up a semi-analytical model by considering the dynamical launching and intuitive multiple-scattering process of SPPs that propagate azimuthally along the curved nanoaperture.The SPP model provides an intuitive picture that under emitter excitation,the azimuthally propagating SPPs in the nanoaperture form tunable whispering-gallery-like resonances,which results in an enhancement of the total spontaneous emission rate.The resonant SPPs are further scattered into free space,resulting in the enhancement of radiative emission rate.To clarify the underlying physical mechanism of tunable bandwidth radiation enhancement,based on the SPP model,we further build up a semi-analytical model for the QNMs supported by the nanoaperture antenna,which indicates that the broadband enhancement of the spontaneous emission rate results from an excitation of QNMs with different eigenfrequencies.In addition,since the metallic nanoparticle on mirror(NPo M)antenna can accurately form nano-sized metal nano-gap,which greatly enhances the spontaneous emission rate and fluorescence intensity,this thesis investigates the azimuthally-periodic ring NPo M antenna with azimuthally-periodic or azimuthally-periodic defects,gives the generalization of the above spp model,and clarifies the impact of azimuthally propagating SPPs on the antenna spontaneous emission. |
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