Enhanced Directional Fluoresccence Emission Of Colloidal Quantum Dots Based On Micro/Nano Light Scattering Structures

Fluorescence detection is a widely used technique and can be applied in many areas,such as sensing,DNA sequencing,imaging,medical diagnostics,and devices.Although present day fluorescence technology is advanced,there are some natural limitations of traditional fluorescence.Fluorophores in solutions emit in all directions.As a result,fluorescence detection systems collect less emission,which reduced the detection sensitivity and the accuracy of detecting results.In order to improve the sensitivity of biological and medical detection,focus on the research of improving the optical signal acquisition of microscope/spectrometer has become one of the objects,and enhance the directional fluorescence which perpendicular to the substrate is one of the methods.Quantum dots?QDs?have unique advantages than other fluorescent materials,so in this paper we propose a micro/nano structure to improve directional fluorescence emission?DFE?of colloidal QDs.The finite difference time domain?FDTD?method is used to study the enhanced DFE of colloidal QDs.the enhanced DFE of quantum dot by structures is analyzed by calculating the field intensity distribution and the fluorescence power above the structures.Firstly,the principle of enhanced DFE of colloidal QDs by spherical silver nanoparticles?AgNPs?is systematically studied.By changing the size and the number of AgNPs,as well as relative position between QD and AgNPs,we found that when QD is in the middle of two Ag NPs?the diameters of two AgNPs are 90 nm and 70 nm,respectively?and the surface spacing between AgNPs is 20 nm,the DFE is enhanced by 70 times compared to the QD without AgNPs at the wavelength of 595⁶25 nm.The results show that the Mie scattering and localized surface plasmon resonance?LSPR?from AgNPs are contribute to enhanced DFE of QDs,the single-layer and non-uniform AgNPs play a major role in the enhancement of DFE.Secondly,for the above Ag NPs LSP,the enhanced fluorescence of colloidal QDs is confined to nearby of AgNPs.A slides-silver film-SiO₂ substrate-vertical silver nanorods dimer structure is proposed,by using of coupling emission of silver film and silver nanorods dimer with QD fluorescence,fluorescence propagation of silver nanorods dimer and the interference in top of nanorods to enhance the DFE of colloidal QDs.By changing the thickness of Si O₂,the length of silver nanorods dimer and the surface spacing of nanorod dimer,it is found that for a silver nanorods dimer with a diameter of 70 nm,when the thickness of SiO₂ is 20 nm,the length of the dimer is 400 nm,and the surface spacing between silver and QD is 2 nm,the enhanced DFE of QDs with emission wavelength of 595⁶25 nm is the greatest,and enhancement factor is 1022 compared with the DFE of QDs by slides-silver film-SiO₂ substrate.Finally,the absorption for the fluorescence of colloidal QDs by the above-mentioned silver nanorods dimer damp down the enhanced DFE.A silver thin film-SiO₂ hollow cylindrical composite structure is proposed to enhance the DFE of colloidal QDs.By changing the thickness,inner diameter and internal refractive index of SiO₂ hollow cylinder,we investigate the enhanced far-field DFE and the enhanced excitation of QDs.It is concluded that when the thickness of Si O₂ hollow cylinder is 2 mm,the inner radius of Si O₂ hollow cylinder is 3.2 mm and the lower refractive index of the internal solution,the enhanced super-length far-field DFE which reached more than 100 mm can be realized.