Study On Directional Modulation To The Far-Field Emission Of Fluorescence By Optical Antennas

Efficient detection of fluorescence from quantum emitters(QEs),such as semiconductor quantum dots(QDs),fluorescent dyes,or J-aggregates,is vital for the fields of optical sensing,microscopic imaging,integrated optics,quantum information,molecular biology and analytical chemistry,et al.The radiation shape of the fluorescence and the energy flowing into the far field is a critical factor for efficient photon collection.Therefore,to improve these two characteristics of fluorescence has become an important part of the study of fluorescence modulation and detection.In recent years,more and more optical antennas are designed to achieve efficient modulation and detection of fluorescence.The optical antennas can realize the mutual transformation of local and free space light field.Although they are similar to the traditional radio frequency(RF)antennas in the structure shape,its scale is usually nano scale,and their transmission and reception bands lie in the visible region.These features of optical antennas make it possible to modulate fluorescence.At present,the optical antennas to achieve the fluorescence modulation which can be observed in the experiments include Yagi antennas,bull’s eye grating structures,nanorod,and V or U-shaped rod,etc.These structures are usually made of metal supportting the surface plasmons(SPs).Through the geometric effect of the structure,or the coupling between the fluorescence emitter and the surface plasmon resonant modes,the far field emission patterns of the fluorescence can be modulated,and realizing the efficient transformation among the fluorescence,local light field and free space light.In this paper,we mainly foucs on the influence of the SPs traveling wave optical antennas on the far-field radiation direction of semiconductor quantum dots.At first,we have prepared the composite structures integrated by traveling wave antenna based on silver nanowires(NWs)and quantum dots(QDs).Through the fluorescence Fourier imaging technology,we studied the modulation of the far field fluorescence radiation of the composite structures.The main research results and innovations of this paper are as follows:1.We improved the traditional wet chemical synthesis method,successfully fabricate the composite structure with various particle sizes and shapes based on silver(NW)-silica(SiO2)-QDs,and achieved the best coupling effect of fluorescence by accurately controlling the thickness of silica.The preparation process is carried out in neutral environment,which overcomes the corrosion of silver NWs.By the special method,the composite structure of V-shaped silver NWs and QDs is also obtained.2.The studies on the traveling wave optical antenna to control the fluorescence direction of quantum dots.By using the traveling wave waveguide based on silver NWs coupled with quantum dot fluorescence,the fluorescence is transformed into the propagating SPs,and the far field radiation of fluorescence is modulated by the antenna effect at the end of nano wires.It is found that by breaking the symmetry of spatial refractive index,the leakage mode on silver nanowires can be induced.By using both the leakage mode and the guided mode bound to the surface of nanowires,the far-field radiation of quantum dots can be modulated.We observed the transition of fluorescence direction from bidirectional to unidirectional radiation.By adjusting the diameter of the silver nanowires,the radiation dispersion angle can be changed from 21.5°to 45°.3.The radiation model of the composite structure is constructed by finite difference time-domain(FDTD),and dyadic Green’s function in stratified medium is derived to analysis this structure.The far-field radiation results corresponding to azimuth and polar angle are calculated by Green’s function method,and the far-field radiation Fourier surface detected in the experiment is reconstructed.The calculated results are in good agreement with the experimental results.4.The effect of V-shaped traveling wave optical antenna on the fluorescence radiation of quantum dots is studied.It is found that the V-shaped optical traveling wave antennas can realize the highly unidirectional radiation of quantum dot fluorescence.The forward and backward ratio of the radiation intensity in the Fourier image is as high as 11.8dB.5.We studied experimentally and use the antennas to achieve the the far-field fluorescence modulation from 0°to 40°for the azimuth angle(φ)in the spherical coordinates,and the change of polar angle in the spherical coordinates is from-43° to 0°(θ).The dispersion angle(△φ)for the fluorescence radiation in the far-field can be adjusted simultaneously.