Volume 24 Issue 1 - May 24, 2013 PDF
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Plasmonic resonances of gold nanoparticle pairs
Hsing-Ying Lin1,2, Chen-Han Huang1, Yun-Chiang Lan1, Chih-Han Chang2, and Hsiang-Chen Chui1,*
1Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
2Institute of Biomedical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Optics Express, Vol. 18, No. 1, 165-172 (2010)
(selected into Virtual Journal for Biomedical Optics, Vol. 5, Issue 2)
 
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Here we study the near-field optical behavior of plasmonic resonance in gold nanoparticle pairs. We directly observed the localized electromagnetic field distribution between two nanospheres is sensitively depended on the incident polarization and interparticle distance using performing series measurements through a fiber-collection mode near-field scanning optical microscope (NSOM). The qualitative near-field observation and quantitative analysis facilitate more understanding of localized hot spots in surface-enhanced Raman scattering (SERS).

It is of fundamental issue to further understand the optical coupling between resonant nanoparticles for the localized optical enhancement effect in SERS process. It has been shown that a metal nanoparticle possesses the ability to support the localized surface plasmon resonance (LSPR), which can be understood as a coherent electron oscillation of the nanoparticle in response to an incident optical field [1]. When two or more plasmon-resonant nanoparticles are closely spaced, localized surface plasmons of individual particles interact, leading to additional coupled oscillation modes in gap regions between particles [2-3]. This near-field coupling manifests in a strong optical field enhancement, known as “hot spot,” where the field intensity is found to be many orders of magnitude stronger than the incident optical field. We report a series of NSOM experiments to directly observe and analyze the distance- and polarization-dependent optical enhancements of two coupled metal nanoparticles.

Polarization-dependent optical enhancements in metal nanoparticle pairs

To understand the polarization-dependent optical enhancements of metal nanoparticle pairs in more detail, we held the fiber probe at the central position between two nanoparticles and recorded the near-field optical signal once every 15 degrees of polarization rotation angle (θ). R is the radius of a nanoparticle, and d is the distance of a nanoparticle pair. Figure 1(a) outlines the measured optical signal at various θ in 2R- and 0.8R-spaced cases. Insets exhibit topographies of these two pairs.

Changes in optical signals are well-fitted by a cos2 function. As seen from the 0.8R curve, the maximal optical intensity is achieved when θ is at 60° and 240°; the minimum intensity is achieved when θ is at 150° and 330°. This cyclic variation in optical intensity relying on changes in polarization angle directly corroborates the generated optical field is polarization-dependent, reaching a peak and bottom value respectively for the incident polarization along and orthogonal to the pair axis. The 2R curve presents a similar mode of the polarization-dependent optical enhancement with 0.8R curve. However, due to the gap increase of 1.2R inducing the weakened coupling interaction, the amplitude variation in field intensity of 2R case is much less than that of 0.8R case. The fitted data are plotted in a polar diagram, as shown in Fig. 1(b).

Figure 1 Polarization-related optical responses of nanoparticle pairs. (a) Normalized near-field optical signal for d=0.8R and d=2R as a function of polarization angle (θ). Inset: topographies of Au nanoparticle pairs. Fitting curves are plotted in the polar diagram (b).

Distance-dependent optical enhancements in metal nanoparticle pairs

Figure 2 exhibits the near-field optical intensity at the midpoint of various interparticle distances with parallel- and orthogonal-polarized excitations. The ordinate is the relative optical signal and the abscissa is gap distances of nanoparticle pair. The strongest enhancement is found if the incident polarization is approximately parallel to interparticle axis and these enhancements are even more manifest when two nanoparticles are more proximal. When the separation goes above 75 nm, the interstitial field intensity is virtually faded away. Since the separation is large enough, the pair system actually transforms into two independent single-particle systems, which without interparticle optical enhancement effect and exhibit no polarization dependence.

Figure 2 Normalized optical field intensity in the central gap region of Au nanoparticle pairs are fitted for the parallel-polarized (blue curve) and orthogonal-polarized (red-dotted curve) excitations.

Under the parallel-polarized condition, the coupling field intensity is drastically strengthened with the decreased separation distance. Hence, the localized coupling interaction and resultant EM field distribution of a nanoparticle pair are systematically affected by both the interparticle distance and incident polarization. The polarization- and distance-dependent coupling properties suggest a possible approach of controlling the spatial distribution of surface plasmonic modes and enhanced intensity of generated hot spots on a specific fabricated nanostructure that may be applied in nano-photonic devices or ultrasensitive sensors. Furthermore, this observation assists us in further understanding the field distribution of localized hot spots in SERS process.

References:
  1. C. H. Huang, H. Y. Lin, H. C. Chui, Y. C. Lan, and S. W. Chu, “The phase-response effect of size-dependent optical enhancement in a single nanoparticle,” Opt. Express 16, 9580 (2008).
  2. P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. I. Stockman, “Plasmon hybridization in nanoparticle dimers,” Nano Lett. 4, 899-903 (2004).
  3. H. Xu, J. Aizpurua, M. Käll, and P. Apell, “Electromagnetic contributions to single-molecule sensitivity in surface-enhanced Raman scattering,” Phys. Rev. E 62, 4318-4324 (2000).
  4. Z. B. Wang, B. S. Luk’yanchuk, W. Guo, S. P. Edwardson, D. J. Whitehead, L. Li, Z. Liu, and K. G. Watkins, “The influences of particle number on hot spots in strongly coupled metal nanoparticles chain,” J. Chem. Phys. 128, 094705 (2008).
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