Surface plasmon coupling for enhancing light emission and color conversion

Abstract

The efficiencies of light emission and absorption are two key factors for the effective operations of many optoelectronic devices. Those efficiencies can be improved through the efforts of upgrading material quality and optimizing device design. When such an improvement reaches a limit in considering the technological difficulty and/or fabrication cost, other means based on nano-photonics techniques deserve consideration. In particular, due to the development of the nano-fabrication technology and the trend of shrinking device dimension, those techniques based on near-field interactions are attractive. Among them, surface plasmon (SP) coupling is a powerful method for enhancing the emission and absorption efficiencies. Also, when color conversion is needed, the Förster resonance energy transfer (FRET) is an effective approach for transferring energy from a donor into an acceptor within a short range. In this paper, the basic principles, the fundamental behaviors, and the applications to the enhancements of light emission and color conversion of SP coupling are reviewed. The SP coupling here is referred to as that not strong enough to produce the phenomenon of Rabi splitting. For effective color conversion, the combined effects of FRET and SP coupling are also discussed. Meanwhile, the nanoscale-cavity effect is introduced to combine with FRET and SP coupling for further enhancing the emission and color conversion efficiencies. The review starts with the behaviors of the SP resonances of metal nanostructures, particularly those of metal nanoparticles (NPs), including deposited surface metal NP and chemically synthesized metal NP, due to their easy fabrication, low cost, and strong localized SP resonance. Among the metals with the negative real parts of dielectric constants for inducing SP resonances in the ultraviolet through near-infrared spectral range, Ag is the major concern in this review because of its high SP resonance strength and low dissipation. SP coupling can be understood as a process of the energy transfer from a light emitter into an SP resonance mode for creating an alternative emission channel, i.e., the coherent SP radiation. A model and a derivative simulation algorithm, which take the Purcell effect into account, are reviewed for interpreting experimental observations. SP coupling can be used for improving the performances of a light-emitting diode (LED), including the enhancements of internal quantum efficiency and electroluminescence intensity, the reduction of the efficiency droop effect, the increase of modulation bandwidth, and the generation of partially polarized light in an LED. SP coupling can also be used for increasing the efficiency of a color conversion process. In such a process, the energy donor, acceptor, and metal nanostructure can be coupled together through an SP resonance mode around the donor emission or acceptor absorption wavelength for forming a three-body coupling system. Such a coupling process can lead to an effective transfer of energy from the donor into acceptor, resulting in a high color conversion efficiency. When the distance between the donor and acceptor is shorter than a few tens nm, an FRET process can occur to further increase the energy transfer efficiency. The combination of SP coupling and FRET can produce a high color conversion efficiency. Due to the near-field Purcell effect, a nanoscale structure can change the emission behavior of a light emitter and hence its far-field radiation performance. Such a nanoscale-cavity effect can lead to the enhancements of emission efficiency, FRET, and SP coupling. In other words, through the fabrication of a nanoscale-cavity structure, FRET and SP coupling can combine with the nanoscale-cavity effect to significantly enhance the color conversion efficiency.