Enhanced Directioal Fluorescence Emissions Of Quantum Dots In Far-field Based On Nano Metallic Cavity Structures

Over past few years,fluorescence technology is widely used in many areas,especially in biological research.Although fluorescence is a highly sensitive and advanced technique,there is still a drive for reduced detection.For example,single-molecule detection is limited by the brightness and photostability of the fluorophores.To satisfy the requirement for analyzing low concentration analyst,it is crucial to enhance the far-field directional emission of fluorophores in order to improve the sensitivity of bio-detection.But radiative decay rates of fluorophores can be significantly enhanced by near field interactions with noble metallic nanoparticles or thin films.And emission intensity of fluorophores can also be enhanced.As quantum dots(QDs)have high luminescence efficiency,tunable emission wavelength and narrow emission peaks,they process a number of advantages over traditional fluorescent materials.Therefore,in this paper we propose a metallic micro/nano structure to enhance directional fluorescence emission of QDs in far field.First of all,we propose a novel system that contains QDs binding silver nanoparticles(AgNPs)in arrayed silver cavities,which can drastically enhance the excitation and far-field directional emission of QDs.Three-dimensional(3D)finite difference time domain(FDTD)method is used to investigate the excitation and emission of QD in four systems.Comparing with other systems,the far-field directional emission of QD in our system is the largest and more than 230 times larger than that of the system ‘Si O2 cavity without AgNP'.And both AgNP and silver cavity have positive effects on the enhancement of far-field directional emission power of QD,and the silver cavity can lead to further far-field emission enhancement besides the plasmonic enhancement of AgNP.Comparing with the systems using metallic nanoparticles for plasmonic enhancement of the QD emission,our system using both metallic nanoparticles and silver cavity arrays can further enhance the far-field emission either the QD-AgNP dimer is in vertical direction or a tilted QD-AgNP dimer is in asymmetrical positions of cavity.Moreover,the silver cavity arrays in our system can prevent particle aggregation,which can provide a stable environment for detection.In real bio-detection,QD-AgNP dimers suspend randomly in a sample,and most of them are tilted and not in the symmetrical positions of cavities.According to the results,the far-field directional emission enhancement of our system is robust wherever the QD-AgNP dimer is in the silver cavity.Therefore,the system will have a great application potential in real bio-detection.Secondly,we propose a novel structure of periodic silver nanodomes structure which are placed on a silver reflector and SiO2 films.And the diameter of each nanodome structure is 162.5nm,QD is located in the gap between two adjacent nanodomes structure.And two-dimensional FDTD method is used to investigate the effect of different positions of QD and different quantities of nanodome structures on the emission intensity and the electric field distribution of QD.The results show that when there are four nanodomes,the QD is located in the gap of two adjacent nanodomes,the enhanced emission of QD in the far field is more than 4 times compared to those without nanodome structure.