Plasmonics has been studied and used for various optoelectronic applications include efficient light-emitting diodes (LEDs) . Next important challenge is to develop device applications and to extend into wide wavelength regions . Here, I present the new nanostructures and methods to tune the plasmonic resonances for wide wavelength range including deep UV.
Recently, we observed unusual localized surface plasmon (LSP) resonance spectra that have a narrow bandwidth and high intensity by fabricating multilayered Ag nanoparticle sheet structures . The peaks of the extinction spectra were clearly split into two peaks on metal substrates, while this phenomenon was not observed on a transparent substrate. This optical phenomenon should be due to the mode splitting effect by the strong coupling. The strong dipole oscillator located near the metal interface can interact with the mirror image of the dipole oscillator, which has the opposite phase. This presents a powerful and useful technique to tune the strong mode coupling effect without any lithographic structures.
Quite recently, we also found the similar peak splitting and sharpened of the LSP spectra for random metal nano-hemispheres, fabricated by thermal annealing of metal thin layers, on metal substrates through thin SiO2 spacer layer. We call such structure nano-hemispheres on mirror (NHoM). The LSP spectra of Ag NHoM became much larger and sharper, and also tunable in UV to visible wavelength region by the spacer thickness of the structure. In order to extend this technique into deep UV region, we fabricated NHoM structures by using aluminum which has the LSP resonance in ultra-deep UV regions. We obtained very strong and sharp resonance peak due to the mode splitting effect by the strong coupling at 156 nm by the by the electromagnetic simulations. As far as we know, this is the LSP spectrum which has the shortest peak wavelength in ultra-deep-UV region. The similar LSP spectra in deep UV region were obtained by experiments and found to be well tunable by the thickness of the SiO2 spacers.
I believe that our approaches using tunable plasmonics including Deep-UV region will bring high efficient plasmonic LEDs with practical use level and will develop future optic and photonic technologies for smart societies.
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Surface plasmon (SP) resonance with light waves and/or excitons can be used to enhance the emission efficiencies of light-emitting materials and devices. This approach was experimentally demonstrated by our group for enhancing the visible emission from InGaN/GaN quantum wells (QWs). Exciton–SP coupling increases the spontaneous emission rates of the excited states, causes a relative reduction in nonradiative relaxation, and ultimately increases the internal quantum efficiencies (IQEs) of such devices. This method has the potential to enable the development of high-efficiency lightemitting diodes (LEDs), eventually leading to the replacement of fluorescent lights with solid-state light sources. Next important challenge is to extend this method into UV and IR wavelength regions. We found that Al is very useful for wider tuning of the plasmonic resonance from the deep UV to the visible wavelength region. The plasmonic tuning at the IR regions were also discussed by using tantalum nanoparticle. These tunable plasmonics over the UV-IR range should bring new possibilities of applications to plasmonics and lead to new class of several smart photonic and optoelectronic applications.
The surface plasmon (SP) resonance was used to increase the emission efficiencies toward high efficiency light-emitting diodes (LEDs). We obtained the enhancements of the electroluminescence from the fabricated plasmonic LED device structure by employing the very thin p+-GaN layer. The further enhancements should be achievable by optimization of the metal and device structures. Next important challenge is to extend this method from the visible to the deep UV region. By using Aluminum, we obtained the enhancements of emissions at ~260 nm from AlGaN/AlN quantum wells. We succeeded to control the SP resonance by using the various metal nanostructures. These localized SP resonance spectra in the deep-UV regions presented here would be useful to enhance deep UV emissions of super wide bandgap materials such as AlGaN/AlN. We believe that our approaches based on ultra-deep UV plasmonics would bring high efficiency ultra-deep UV light sources.