Wen-Hung Chuang

Surface plasmon coupling with radiating dipole for enhancing the emission efficiency of a light-emitting diode.

The experimental demonstrations of light-emitting diode (LED) fabrication with surface plasmon (SP) coupling with the radiating dipoles in its quantum wells are first reviewed. The SP coupling with a radiating dipole can create an alternative emission channel through SP radiation for enhancing the effective internal quantum efficiency when the intrinsic non-radiative recombination rate is high, reducing the external quantum efficiency droop effect at high current injection levels, and producing partially polarized LED output by inducing polarization-sensitive SP for coupling. Then, we report the theoretical and numerical study results of SP-dipole coupling based on a simple coupling model between a radiating dipole and the SP induced on a nearby Ag nanoparticle (NP). To include the dipole strength variation effect caused by the field distribution built in the coupling system (the feedback effect), the radiating dipole is represented by a saturable two-level system. The spectral and dipole-NP distance dependencies of dipole strength variation and total radiated power enhancement of the coupling system are demonstrated and interpreted. The results show that the dipole-SP coupling can enhance the total radiated power. The enhancement is particularly effective when the feedback effect is included and hence the dipole strength is increased.

Differentiating the contributions between localized surface plasmon and surface plasmon polariton on a one-dimensional metal grating in coupling with a light emitter

The authors demonstrate the differentiation between the contributions of localized surface plasmon (LSP) and surface plasmon polariton (SPP) couplings with an emitting dipole to emission enhancement in a metallic grating structure. Because of the relatively higher loss and in-plane radiation of the SPP modes, the LSP modes dominate the enhancement effect. However, because the LSP resonance energy is sensitive to the metal/dielectric interface geometry, it may be difficult to precisely implement a particular geometry and achieve the emission enhancement of a desired emission wavelength based on the LSP coupling. On the other hand, because the SPP feature can be controlled by the period of a grating structure, the implementation of the SPP coupling for emission enhancement in a practical device can be more feasible.

Numerical Study on Quantum Efficiency Enhancement of a Light-Emitting Diode Based on Surface Plasmon Coupling With a Quantum Well

We demonstrate the numerical study results of the enhancements of internal quantum efficiency (IQE) and external quantum efficiency (EQE) of a semiconductor quantum well when it is coupled with surface plasmons (SPs) induced on a grating interface between Ag and semiconductor. The IQE and EQE enhancements depend on the emission dipole position and the assigned intrinsic IQE. The SP dissipation in metal and the grazing-angle SP radiation lead to a significant difference between IQE and EQE. The enhancement of EQE is less significant when the intrinsic IQE becomes larger. In applying the SP coupling phenomenon to an InGaN-GaN quantum-well light-emitting diode, the efficiency enhancement is more significant in the green-red range, in which the intrinsic IQE is normally quite low.

Enhancement of emission from CdSe quantum dots induced by propagating surface plasmon polaritons

The influence on the emission property of CdSe quantum dots arising from propagating surface plasmon polaritons based on interconnected periodic gold nanoarrays with a large area of 1 cm2 is reported. Variation in the structural parameters allows us to tune the surface plasmon resonance to the emission band of quantum dots, which can result in an enhancement up to 54 times in the external quantum efficiency. Our strategy for the enhancement of luminescence efficiency from semiconductor quantum dots should be useful for the creation of high efficiency solid state emitters.

Transient behaviors of surface plasmon coupling with a light emitter

The transient behaviors of dipole couplings with surface plasmons (SPs) on a metal/dielectric grating interface, including surface plasmon polariton (SPP) and localized surface plasmon (LSP), are numerically demonstrated. Such a dipole-SP coupling process can lead to either enhanced dipole emission or effective pumping of a cavity-confining SP mode. Based on the time-resolved responses of a source pulse, it is found that the dipole-SP coupling features can be excited in several femtoseconds with the decay times ranging from 5 to 20 fs. From the significantly different decay times between the LSP and grating-assisted SPP features, one can classify those SP-coupling features into different application categories of efficient emission and SP energy storage.

Study on the decay mechanisms of surface plasmon coupling features with a light emitter through time-resolved simulations

The transient behaviors of the dipole coupling with surface plasmon (SP) features in an Ag/dielectric-interface grating structure in order to understand the characteristics of those dipole-coupling features are demonstrated. In particular, the major decay mechanisms of those coupling features can be identified. For comparison, the time-resolved behaviors of the resonant surface plasmon polariton (SPP) coupling feature on a flat interface are also illustrated. Among the three major grating-induced SP-dipole coupling features, two of them are identified to be localized surface plasmons (LSPs). The third one is a grating-assisted SPP, which shows two decay components, corresponding to the first stage of SPP in-plane propagation and the second stage of coupling system decay. In all the dipole coupling features, metal dissipation can dominate the energy relaxation process, depending on the assumption of damping factor. All the dissipation rates are proportional to the assumed damping factor in the Drude model of the metal. The dissipation rates of the LSP and resonant SPP features are about the same as the damping rate, implying their local electron oscillation natures. The dissipation rate of the grating-assisted SSP feature is consistent with theoretical calculation. In the LSP features under study, dielectric-side emission is prominent. The coupled energy in the grating-assisted SPP feature can be efficiently stored in the coupling system due to its low emission efficiency and effective energy confinement through grating diffraction.

Characteristics of light emitter coupling with surface plasmons in air/metal/dielectric grating structures

The dipole radiation power and far-field emission power in the process of dipole surface plasmon (SP) coupling over the spectral range of 400-800 nm in air/Ag/dielectric grating nanostructures with two dipole positions are evaluated based on the boundary integral equation method. Three kinds of SP coupling features are differentiated, including the plane-wave-excited surface plasmon polariton (SPP), dipole-excited SPP, and localized surface plasmon (LSP). The dipole radiation and coupling system emission powers depend on the metal nanostructure, dipole position, and spectral location. Generally, when the Ag layer becomes thinner, the major SPP and LSP coupling features redshift. The emission of a coupling system is mainly on the dipole side of the air/Ag/dielectric nanostructure, even though the metal layer is thin.

Effective energy coupling and preservation in a surface plasmon–light emitter coupling system on a metal nanostructure

The simulation results of the coupling of a radiation dipole with a surface plasmon (SP), which is induced on a metal/dielectric interface of a single groove (SG) plus a grating structure, are demonstrated. With the SG structure, the dipole can effectively couple energy into an SP feature, which has a mixed nature of localized surface plasmon (LSP) and surface plasmon polariton (SPP). The SPP energy is confined by a grating structure with a well designed grating period and position. With such a cavity configuration, the SPP energy can be well preserved. Both the dipole-SP coupling behaviors in the frequency and time domains are numerically illustrated. The results are useful for designing a metal/dielectric interface nanostructure for implementing a SPASER (surface plasmon amplification by stimulated emission of radiation) system.

Time-resolved behaviors of surface plasmon coupling features with a light emitter

The transient behaviors of the dipole coupling with surface plasmons in an Ag/dielectric grating structure for understanding the characteristics of those dipole-coupling features are demonstrated. The major decay mechanisms of those coupling features are identified.

Surface plasmon coupling for light emission/absorption/conversion enhancement

Surface plasmon polariton and localized surface plasmon couplings with InGaN/GaN quantum wells in bluegreen light-emitting diodes are used for enhancing their emission efficiencies by 25-200 % depending on the used quantum well crystal quality.