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In this paper we investigate the near-field optical behavior of plasmon coupling in gold nanoparticle pairs. In particular, by performing series measurements through a fiber-collection mode near-field scanning optical microscope (NSOM), we directly observed the localized electromagnetic (EM) field distribution between two nanospheres is sensitively depended on the incident polarization and interparticle distance. The qualitative near-field observation and quantitative analysis facilitate more understanding of localized hot spots in surface-enhanced Raman scattering (SERS), and nano-applications in selectively controlling the spatial distribution of localized surface plasmon (SP) modes on a fabricated nanostructure by adjusting the polarization direction.
©2010 Optical Society of America
1. Introduction
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 [1–4]. 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 [5–8]. 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 [9]. Recent studies have revealed that the optical enhancement feature in metal particle dimers can give the expansion of Raman scattering cross section which may enable highly sensitive biochemical detections down to a single-molecular level in surface-enhanced spectroscopies [10–13].
It is of fundamental issue to further understand the optical coupling between resonant nanoparticles for the localized optical enhancement effect in SERS process. Previous studies have theoretically and experimentally investigated that the plasmonic property of metal nanoparticle pairs is dramatically influenced by the particle’s size, shape, composition, arrangement, surrounding medium, and characteristics of the incident excitation [14–21]. Most of these studies focused on the effect of interparticle interaction are established by means of spectroscopic and/or fluorescence-assisted analyses [22–26]. However, the direct observation of near-field interaction of plasmon resonant metal hosts associated with varying interparticle distances and relative polarization of the excitation light has not been fully demonstrated and elaborated so far.
It is very difficult to observe such an EM-field distribution of assembled nanoparticle pairs due to the insufficient spatial resolution of conventional far-field optical measurement scheme and the rigours of simultaneously keeping a record of corresponding topography and field signals. To achieve that, a fiber-collection mode NSOM equipped with a lock-in technique, used to enhance the signal to noise ratio of optical signals, is employed to directly image the field distribution. As corroborated by the orientation of interparticle axis in the topography, the polarization angle regarding to the axis of particle pair can be clearly defined. Moreover, to avoid the acquisition error induced by the residual difference of individual NSOM probes and to avoid the risk of damaging tips from repetitively approaching, gold nanoparticles of various gap distances are arranged on a quartz surface through chemical immobilization [27].
In this paper, we report a series of NSOM experiments to directly observe and analyze the distance- and polarization-dependent optical enhancements of two coupled metal nanoparticles. With a far-field excitation and near-field collection scheme, coupling behaviors within the confined region between two nanoparticles can be explored. Our observation provides a more detailed understanding of localized optical properties of nearly neighboring nanoparticles, which indicates the potential to control the spatial distribution of plasmonic modes on a designed nanostructure surface via polarization manipulation [28].
2. Experimental setup
2.1 Near-field scanning optical microscopy (NSOM)
Figure 1(a) exhibits the scheme of experimental setup. One solid-state laser (λ=532 nm) was chosen depending on the absorption peak of immobilized nanoparticles to function as the exciting light source. The incident polarization was maintained by a half-wave plate (HWP) and polarizer (PL). The continuous wave laser modulated by an optical chopper (SR450) at 2 kHz was conducted through a reflective mirror, and then focused on nanoparticles with a microscope objective (Olympus 50X, NA = 0.8). The scattering radiations generating from photon-plasmon interactions, existing on the surface of metal nanoparticles, was collected by a tapered Al-coated fiber probe with a 50-nm aperture. The fiber-based probe mounted on a tuning fork, equipped for the NSOM scanner (Veeco Aurora-3), was perpendicular to the sample surface where the distance between the tip and surface was controlled by a shear force feedback system [29]. In our measurements, the set point was maintained at half of the free oscillation amplitude in a constant-gap scanning mode. The constant scanning rate was set at 5 μm/s. The collected signal transmitted through the fiber and fiber coupler was detected by a photomultiplier tube (PMT) and then was demodulated by a lock-in amplifier (SR830). Both the topography and near-field scanning optical images were simultaneously recorded for subsequent analyses.
Fig. 1 (a) The setup of fiber-collection mode NSOM. (b) The absorption spectrum of Au nanoparticles. The spectral position of the NSOM excitation is shown with G line. Inset: SEM image of Au nanoparticles.
2.2 Samples
Sphere Au nanoparticles with various interparticle distances on quartz substrate was fabricated by a chemical immobilization technique. Gold nanoparticles were synthesized by a reduction of hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4·4H2O) by trisodium citrate dehydrate (C6H5O7Na3·2H2O) and tannic acid (C14H10O9) [27]. The mean diameter of Au colloids was 50 nm, observed by a scanning electron microscope (SEM, JEOL JSM-7001). The quartz substrate was ultrasonically cleaned using 15% acetone, 15% isopropanol, and de-ionized water for 30 minutes and then immersed in H2O2 mixed in a 1:3 ratio with H2SO4 for 20 minutes. The substrate was subsequently immersed in a 20% aqueous solution of 3-aminopropyltriethoxysilane (APTES) for 30 minutes. Amino-functional silane was used as a coupling agent between Au nanoparticles and quartz substrate. After the surplus silane on the surface was rinsed off, the silanized substrate was dried by nitrogen gas and immersed in the prepared Au colloids for 12 hours. Then, the substrate was dipped in de-ionized water for 1 minute to remove non-immobilized nanoparticles. The inset in Fig. 1(b) shows the SEM picture of immobilized Au nanospheres. Figure 1(b) gives the far-field absorption spectrum of Au nanoparticle film measured by a UV/VIS/NIR spectrometer (Hitachi U-3010). The majoradvantage of this fabrication method is to obtain diversely-spaced nanoparticles with a size on the same surface. As a consequence, there is no need to frequently change or re-approach the scanning probes to study the distance- and polarization-dependent coupling behaviors, avoiding the risk of damaging probes.
3. Results and discussion
3.1 Electromagnetic (EM) field distributions of metal nanoparticle pairs
The near-field distribution of optical field intensity is very different for the incident polarization along and orthogonal to the interparticle axis of the same nanoparticle pair. Spatial EM-field distributions of two spherical Au nanoparticles of 50 nm in diameter for the polarization-dependent optical behavior directly observed via NSOM are presented in Fig. 2 and 3 . For qualitative and subsequent quantitative analyses of these near-field patterns, the relative optical signal is defined as the measured intensity normalized relative to the plane intensity without nanoparticles. In Fig. 2, the interparticle distance (d) was estimated about 53 nm (~2R, R is the radius of a nanoparticle). When the incident polarization is tuned approximately perpendicular to the pair axis, the interstitial field intensity is not as strong as it is for a parallel-polarized excitation. Two bright spots are observed with an orthogonal-polarized excitation, as shown in Fig. 2(a), which can be attributed to the plasmon resonance of individual particles in response to the 532-nm excitation. In contrast, prominent optical field intensity at the interstitial site is observed with a parallel-polarized excitation, as displayed in Fig. 2(b). This localized optical field enhancement in the gap region is generated from the polarized interaction of opposite charges separately distributed at the faced surfaces of two excited nanospheres. The redistributed electrons cause the plasmon coupling interaction and inducing the local EM field enhancement at the interstitial junction, where is so called a hot spot in SERS. Corresponding profiles along the interparticle axis of two polarization-influenced field distributions are explicitly compared in Fig. 2(c). A massive bulge is observed along the pair axis with the parallel-polarized excitation while two weaker bumps are observed with the orthogonal-polarized excitation.
Fig. 2 Measured NSOM images of an adjacent nanoparticle pair (d=53 nm) at the excitation laser’s wavelength (532 nm) for (a) the incident polarization is perpendicular to the interparticle axis and (b) the incident polarization is parallel to the interparticle axis. Their cross-sectional views are combined in (c), showing the difference in between.
Fig. 3 Measured NSOM images of an adjacent nanoparticle pair (d=20 nm) at the excitation laser’s wavelength (532 nm) for (a) the incident polarization is perpendicular to the interparticle axis and (b) the incident polarization is parallel to the interparticle axis. Their cross-sectional views are combined in (c), showing the dramatic difference in between.
The polarization-dependent optical enhancement effect is even more influential when the gap distance is down to around 20 nm (~0.8R), as shown in Fig. 3. Similarly, two highlight spots, caused by localized plasmon resonances, are detected with the orthogonal-polarized irradiation, whereas a concentrated hilltop, resulting from the highly regional plasmon coupling, is detected with the parallel-polarized irradiation. However, the distribution of prominently enhanced EM field in Fig. 3(b) is supposed to be not only based on the effect of parallel polarization, but owing to the great impact of shortened interparticle distance as well.
Note that with the same parallel-polarized excitation, the 0.8R-spaced particle pair shows a much stronger enhancement than the 2R-spaced particle pair at the interstitial site. This non-proportional enhancement phenomenon may be explained as follows. The plasmon coupling between two spheres is anisotropic, and thus the optical field enhancement is significantly dependent on the angle of incident polarization with regard to the pair axis and the length of interstitial gap. The remarkable characteristic of polarization- and distance-dependent optical field enhancement between two nanospheres will be qualitatively and quantitatively analyzed below.
3.2 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). Figure 4(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, which indicates the difference between maximum and minimum signals corresponding to a 90° polarization rotation. 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. 4(b), where both curves demonstrate an identical current of polarization-dependent optical enhancements, but distinct signal contrasts owing to different interparticle distances.
Fig. 4 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).
3.3 Distance-dependent optical enhancements in metal nanoparticle pairs
Figure 5 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. It is worth noting that 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. In contrast, such an intensive field enhancement is almost negligible for an orthogonal-polarized excitation. The slowly increased field intensity within the tiny gap region, as shown by the red-dotted curve, is caused by the superposition from lateral optical fields of individual particles, not a coupling result. 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.
Fig. 5 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.
4. Conclusion
In conclusion, our result manifests that a coupling of the localized surface plasmon modes confined within the small region of two illuminated metal nanoparticles is highly dependent on the incident polarization angle with regard to the pair axis. 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. By directly visualizing the spatial EM field distribution in the near-field with the aid of a fiber collection-mode NSOM, the near-field coupling behaviors of assembled nanoparticles can be qualitatively and quantitatively analyzed. The polarization- and distance-dependent coupling properties suggest a possible approach of controlling the spatial distribution of SP 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.
Acknowledgments
This work was supported by National Science Council of Taiwan under the contract NSC95-2112-M-006-030 and the Landmark Project of National Cheng Kung University, Taiwan. We acknowledge the Center for Micro/Nano Science and Technology at National Cheng Kung University for providing access to experimental facilities and Cheng-Wen Huang for help with experimental works.
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