Volume 22 Issue 9 - November 30, 2012 PDF
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Enhanced spontaneous light emission by multiple surface plasmon coupling
Chuan-Pu Liu*, Wen-Huei Chu, Yuan-Jen Chuang, Po-I Lee and Steve Lien-Chung Hsu
Department of Materials Science and Engineering, College of Engineering, National Cheng Kung University
 
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Spontaneous emission rate in semiconductors and polymers can be enhanced by surface plasmon resonance (SPR) coupling, a collective response of electrons on the surface of a conductor. However, application is restricted due to the strongly decaying field of localized SP. In this study, photoluminescence of polyfluoren (PF) copolymers can be enhanced selectively by coupling with either localized or propagating modes of SPR.
The plasmonic structure contains periodic Ag sub-micron cylinders with or without a 50 nm thick Ag film at bottom as shown in Fig.1. PF is a white-light material with emissions at 390, 438, 465, 500, and 533 nm. When without a Ag film, all the plasmonic structures can enhance light emissions, which matches with the SPR absorption energies, but to different degree at different wavelength. The enhanced light emissions at 500 and 533nm are attributed to the low frequency branch of localized SPR. Furthermore, the silver thin film provides propagating surface plasmons and enhances the blue emission band of the PF copolymer at 438 nm. Moreover, by effectively coupling the localized and propagating SPR, we can experimentally demonstrate that the photoluminescence of the PF copolymers is enhanced by 4 to 5.4 times at different wavelengths compared to enhancement by either single mode as shown in Fig.2.
Fig.1. (a) Schematic of experimental geometry for enhanced emission of multiple plasmons; (b) a SEM image of Ag cylinder array with the height of 75nm and diameter of 200nm on silicon.
Fig. 2. PL enhancement factors by normalizing PL intensity with that of the pristine PF copolymer alone, where D, P and C denote diameter, periodicity and continuous Ag film.
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