Gu et al. Nanoscale Research Letters 2011, 6:199 http://www.nanoscalereslett.com/content/6/1/199 The effect of varying the distance between the active emitting region and metallic NPs on the overall device efficiency enhancement also reveals that the larger local field plays an important role. Coupling and enhancing vanish as the distance is increased above a certain threshold, and the distance cannot be too small or too large. If it is too small, the non-radiative quenching pro- cess dominates and most of the energy is dissipated accordingly. On the contrary, if the distance is too large, the coupling effect vanishes since only the electron/hole pairs near the metallic NPs can effectively couple to increase the IQE. Fujiki et al. [34] have introduced a copper phthalocyanine hole transport layer in their LED structure as a spacer to avoid non-radiative quenching. In order to retain the effect, a 20-nm-thick film is used and smaller enhancement is observed if the thickness is larger. The experiment provides direct evidence of the role played by the higher local electric field in the IQE enhancement effect. A model proposed by Khurgin et al. [35] further con- firms the importance of the higher local field and demonstrates that the IQE can indeed be enhanced by LSPR via the electric field/emitter interaction. However, the use of NPs to enhance the IQE of LEDs is effective only when the original IQE is very low (<1%) and the NPs are highly disordered, as shown in Figure 3. The results are corroborated by experiments. For example, Yeh et al. [36] have studied two InGaN/GaN QW-based LEDs emitting different colors and discovered that the green one, which has a lower original IQE correspond- ing to a lower crystal quality, exhibits a more effective enhancement than the blue one. Besides, due to indirect band-gap emission, Si has a relatively low original IQE and can possibly gain the most from LSP, which will be discussed in “Applications” section. Enhancement of LEE In some situations, LSPR is expected to enhance the LEE rather than IQE. In this case, the energy of the gen- erated photons is first transferred to the metallic NPs to induce LSP followed by emission of light. While IQE enhancement has been studied, only a few cases in which the LEE is enhanced by LSP have been reported. The GaN-based LED developed by Sung et al. [37] and described in Figure 4 shows an electroluminescence [EL] increase of 1.8 times at an injection current of 20 mA. As the distance between the gold NPs and the GaN multiple QW [MQW] is very large, the argument about enhancement by emitter/field interaction no longer holds, and so the enhancement can only stem from an out-coupling of the generated photons by the gold NPs. From the viewpoint of energy transfer, nanoscaled antennae can be incorporated to enhance the absorption of generated photons and subsequent reemission. As the Page 4 of 12 Figure 3 Use of NPs to enhance the IQE of LEDs. (a) Two- dimensional ordered array of metal NPs placed in the vicinity of the QW active region of a LED. (b) Enhancement due to isolated Ag spheres on InGaN/GaN QW emitters with a separation of 10 nm as a function of the sphere radius a for different original radiative efficiencies. (c) Enhancement due to two-dimensional array of Ag spheres on InGaN/GaN QW emitters with a separation of 10 nm as a function of the sphere radius a for different original radiative efficiencies. Also shown is the optimized sphere spacing Ropt for hrad = 0.001. Reproduced from [35]. Copyright American Institute of Physics, 2008. boundary separating the fields of electronics and photo- nics is becoming more blurred, the concept of antennae developed for radio frequency and microwave communi- cation has been extended to optical frequencies, and one potential application of these nanoscaled antennae is to