Nanophotonics/Plasmonics

The fundamental properties of luminescence were studied, and surface plasmon polaritons (SPPs) can be used to improve the light emission efficiency (see Fig.1).  SPPs are electron density waves excited at the interfaces between metals and dielectric materials.   Owing to their highly localized electromagnetic fields, they may be used for the transport and manipulation of photons on subwavelength scales. In particular, plasmonic resonant cavities represent an application that could exploit this field compression to create ultrasmall-mode-volume devices.   The localized surface plasmon resonance (LSPR) in planar structure and nanostructure has drawn great attention in many researches and applications, such as subwavelength waveguides, molecular sensors, organic solar cells, electrochromic devices, and light emitting diodes. 

The photoluminescence (PL) enhancement factor for a dipole emitter in an emitter/metal/dielectric structure depends on the thickness of the metal film.  A novel and simple samarium (Sm3+) doped polymer/Ag/SiO2 trilayer structure has beem fabricated to investigate the impact of SPPs on the emission quantum efficiency of the dipole emitter Sm3+.  As expected, SPP enhanced PL intensity has been observed at room temperature and 10K, and the maximum increase is ~10 times for 30nm thick Ag film.  When the Ag film was replaced by Ag/Au multilayer film, the Sm3+ fluorescence peak aspect ratio can be tuned.

Fig 1 Schematic showing how SPPs enhance the luminescence efficiency in rare earth doped polymer.  The Sm3+ ions are considered as dipole emitters with multiple spatial wavevectors. The SPPs propagates along the x direction, and the dark line represents the resonance wave vector of a plane wave with resonance angle £c.

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Enhanced fluorescence of Eu3+ due to LSPR using annealed silver film (nano-particle), nanocubes and nanoprisms have also been investigated.  Studies show that the enhancement depends on the scattering and absorption of the nano-materials.  Theoretical studies based on Finite Difference Time Domain (FDTD) method have been carried out to help understand the mechanisms of the excitation and the emission.

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Fig 2 SEM image of synthesized Ag nanocubes using polyol method 

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A theoretical study of the influence of a single spherical metal nanoparticle (MNP) on the fluorescence intensity of nearby emitters with two-level and multi-level energy systems has also been developed. The calculated results for the two-level systems agree well with reported experimental data.  As to the upconversion fluorescent systems, our numerical results provide the first theoretical prediction showing that the MNP may selectively enhance a certain fluorescence process among various ones.

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