Far-field directional emission of fluorescence enhanced by dielectric microsphere and metallic planar nanolayers

Abstract

Controlling the emission characteristics of fluorescent substances and increasing the intensity of fluorescence emission are crucial for fluorescence detecting technology in single-molecule detection, biomedicine, and sensing applications. Since fluorescence emission is isotropic in nature, the collected fluorescence is only accounted for a small fraction of the total emitted fluorescence. In this paper, a composite structure composed of dielectric microsphere and metallic planar nanolayers is proposed to enhance the fluorescence far-field directional emission intensity and improve the fluorescence collection efficiency. The excitation process and the emission process of quantum dots (QDs) located between the dielectric microspheres and the gold layer are investigated by the finite difference time domain (FDTD) method. In the emission process, the emission of QDs in a homogeneous medium is isotropic. Therefore, we usually select several special polarizations in theoretical analysis state for research. In this paper, we first study the effect of the structure on the fluorescence emission enhancement of QDs when the QDs are in the <i>x-</i>, <i>y-</i>, and<i> z-</i>polarization state. Some results can be obtained as shown below. When the radiation direction of the QDs is perpendicular to the microsphere plane layered structure, the structure is coupled with the emitted fluorescence, thereby realizing the directional enhancement of the emitted fluorescence of the QDs, and the obvious fluorescence enhancement is obtained in the <i>x-</i> and <i>y-</i>polarization state. Therefore, in the research, we choose and investigate the dipole light source of <i>x</i>-polarization state. We mainly study the influence of microsphere radius, refractive index, and QDs position on the fluorescence directional enhancement. The QDs as a fluorescent material are coated in polymethyl methacrylate (PMMA) to control the distance from the gold layer to tune the fluorescence enhancement. The structure is based on the synergistic effect among plasmon coupling, whispering gallery mode and photonic nanojet, which enhances the far-field fluorescence of QDs by a factor of 230, and the fluorescence collection efficiency is as high as 70%. Comparing with the enhanced fluorescence of the dielectric microspheres and the gold sphere dimer composite structure, the distance between the gold sphere dimers is not easy to control, and the QDs should be placed at specific positions between the gold spheres. The structure we propose is more convenient to implement. In this paper, not only the emission enhancement process of QDs is studied in detail, but also the excitation process of QDs is investigated. Our proposed dielectric microsphere metal planar nanolayered structure can enhance the excitation of QDs in most areas, proving that our designed structure can effectively realize the excitation enhancement of QDs. The above results have very important applications in the fluorescence biological detection, imaging, and light-emitting devices.