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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 21 — Oct. 12, 2009
  • pp: 18760–18767
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Laser-induced self-assembly of silver nanoparticles via plasmonic interactions

Yoshito Tanaka, Hiroyuki Yoshikawa, Tamitake Itoh, and Mitsuru Ishikawa  »View Author Affiliations


Optics Express, Vol. 17, Issue 21, pp. 18760-18767 (2009)
http://dx.doi.org/10.1364/OE.17.018760


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Abstract

We reportlaser induced self-assembly of silver nanoparticles via plasmonic interactions. By focusing a near-infrared laser in silver nanoparticle suspension, nanoparticle assembly is formed as a result of optical trapping. The shape of Rayleigh scattering spectra of the nanoassembly strongly depends on the polarization of the laser beam. Particularly, a linearly polarized laser induces the formation of arrayed structure along the laser polarization, that shows a sharp plasmon resonance band and harnesses excellent plasmonic properties applicable for nonlinear surface enhanced spectroscopy.

© 2009 OSA

Noble-metal nanoparticles exhibit a strong interaction with light due to collective oscillations of conduction electrons in the nanoparticles, known as surface plasmon resonance (SPR) [1

1. U. Kreibig, and M. Vollmer, Optical Properties of Metal Clusters (Springer, Berlin, 1995).

], which have fascinated scientists for a long time. SPR not only gives the nanoparticle a specific color but it also produces the intense local electric field with the same frequency as the excitation light source near the nanoparticle surface. This phenomenon enables spectroscopies of surface-enhanced fluorescence [2

2. M. Kawasaki and S. Mine, “Enhanced molecular fluorescence near thick Ag island film of large pseudotabular nanoparticles,” J. Phys. Chem. B 109(36), 17254–17261 (2005). [CrossRef]

], surface-enhanced Raman scattering [3

3. M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57(3), 783–826 (1985). [CrossRef]

], and surface-enhanced hyper-Raman scattering [4

4. H. Kneipp and K. Kneipp, “Surface-enhanced hyper Raman scattering in silver colloidal solutions,” J. Raman Spectrosc. 36(6-7), 551–554 (2005). [CrossRef]

]. Particularly junctions of nanoparticles formed in aggregates shows a giant local field enhancement due to particle plasmon coupling, that strongly depends on the structure and orientation of the aggregates [5

5. H. Xu and M. Käll, “Surface-plasmon-enhanced optical forces in silver nanoaggregates,” Phys. Rev. Lett. 89(24), 246802 (2002). [CrossRef] [PubMed]

8

8. H. Xu, “Calculation of the near field of aggregates of arbitrary spheres,” J. Opt. Soc. Am. A 21(5), 804–809 (2004). [CrossRef]

]. Consequently, recent studies have been focused on the fabrication and structural control of the aggregate of noble-metal nanoparticles promising the giant local-field enhancements.

In this paper, we report the laser-induced self-assembly of silver nanoparticles via plasmonic interactions investigated by Rayleigh scattering spectroscopy. The shape of Rayleigh scattering spectra of the silver nanoassembly produced in the laser focus strongly depends on the polarization of the laser beam. Particularly, a linearly polarized laser beam induces the formation of arrayed structure along the laser polarization, that shows a sharp SPR band and harnesses excellent plasmonic properties applicable for nonlinear surface enhanced spectroscopy.

The colloidal silver was prepared by a slight modification of a well-known citrate reduction method [25

25. H. Yoshikawa, T. Adachi, G. Sazaki, T. Matsui, K. Nakajima, and H. Masuhara, “Surface enhanced hyper-Raman spectroscopy using optical trapping of silver nanoparticles for molecular detection in solution,” J. Opt. A, Pure Appl. Opt. 9(8), S164–S171 (2007). [CrossRef]

]. Electron micrograph in Fig. 1 (a)
Fig. 1 (a) SEM image of and silver nanoparticles and (b) absorption spectrum of these colloidal samples. (c) Schematic of experimental setup.
showed that the colloidal solution consists of nearly spherical nanoparticles, whose average diameter is 27 nm. Absorption spectra of these colloidal samples (Fig. 1 (b)) were measured by using a commercial uv-visible absorption spectrometer. They showed a single peak attributed to the SPR, and a broadening or an additional band indicating aggregation or polydispersion was not observed. This spectral feature proves a monodispersion of colloidal particle. The size distribution of the sample confirmed by means of dynamic light scattering had 7% standard deviation. The final silver particle concentration was about 2 × 10−9 M. As depicted in Fig. 1 (c), a near-infrared laser beam (wavelength: 1064 nm) from a continuous-wave (CW) Nd3+:YAG laser was introduced into an inverted microscope and focused into the colloidal solution via an oil immersion microscope objective lens (Olympus UPlanApo 100 × , N.A. = 0.5-1.3). Since this objective lens corrects for both chromatic and spherical aberration at high level, the aberration was not a significant issue for this study. The polarization of the linearly polarized laser beam was rotated by a λ/2 plate. Circular polarization was produced using a λ/4 plate. Since the height of the laser focus in solution was set to be 20 μm apart from the top of a coverslip, the surface effects were negligibly small. For Rayleigh scattering measurements, a collimated unpolarized beam of white light from a tungsten halogen lamp was illuminated onto the silver colloidal solution through an oil immersion dark-field condenser lens (Olympus U-DCW NA = 1.4). Rayleigh scattering light from the colloidal silver was collected with a microscope objective and introduced to a CCD spectrometer. An absorptive sheet polarizer was placed in front of the detector to analyze the polarization of the Rayleigh scattered light.

A thin layer of the silver colloidal solution was prepared by sandwiching it between two glass plates. A 300-μm pinhole was set at the conjugate spot of the laser focus to limit the detection area of the Rayleigh scattering light to a spot 1 μm in diameter centered on the focal point. Rayleigh scattering spectra of silver nanoparticles optically trapped in the laser focus were obtained by calculating I t(λ) − I b(λ), where I t(λ) and I b(λ) are Rayleigh scattering spectra from the sample without and with laser irradiation, respectively.

As linearly polarized laser beam was focused into the silver colloidal solution, Rayleigh scattering light from trapped silver nanoparticles gradually increased. A surface plasmon resonance (SPR) band could be measured after several sec and then its shape changed during the laser irradiation. In every measurement, single plasmon band shifted to low energy, as shown in Fig. 2 (a)
Fig. 2 Consecutive Rayleigh scattering spectra (from bottom to top) of silver nanoparticles optically trapped in the laser focus with (a) linear and (b) circular polarization. These spectra were recorded every 5 sec from the start of laser irradiation.
. The splitting of this band and the sudden appearance of another band were not observed in this case. Figure 3 (a)
Fig. 3 Rayleigh scattering spectra of assemblies of the silver nanoparticles formed in the laser focus with (a) linear and (b) circular polarization. These spectra were recorded at 20 sec after the start of laser irradiation
shows four typical Rayleigh scattering spectra measured at 20 sec after the start of laser irradiation. Since the photon energy range of the tungsten halogen lamp was from 1.57 to 3.1 eV, SPR band of isolated silver nanoparticles at 3.2 eV could not be clearly detected. On the other hand, since assemblies of silver nanoparticles generally show a SPR band in energetically lower region than monomer nanoparticles [1

1. U. Kreibig, and M. Vollmer, Optical Properties of Metal Clusters (Springer, Berlin, 1995).

], all of SPR bands shown in Fig. 3 (a) are assigned to those of assemblies. Moreover, Rayleigh scattering light of the trapped silver nanoparticles showed polarization anisotropy, as shown in Fig. 4
Fig. 4 Normalized intensity of the plasmon band maxima vs. rotation angles of an analyzer. The dashed line indicates a fitted cos2 θ curve. The arrows show the laser polarization direction and analyzer direction. The analyzer was placed in front of the detector as shown in Fig. 1 (c).
. When the analyzer is parallel (θ = 0, 180°) and perpendicular (θ = 90°) to the polarization of the laser beam, the intensity of the plasmon resonance was maximized and minimized, respectively. Furthermore, this angular dependence fits to a cos2 θ curve, thereby showing that the bands shown in Fig. 3 (a) are due to a dipole mode oscillation of plasmon resonance. This polarization dependence of the Rayleigh scattering spectra clearly suggests the formation of the anisotropic assembly of silver nanoparticles in the laser focus, which is supported by Ref [17

17. M. Pelton, M. Liu, H. Y. Kim, G. Smith, P. Guyot-Sionnest, and N. F. Scherer, “Optical trapping and alignment of single gold nanorods by using plasmon resonances,” Opt. Lett. 31(13), 2075–2077 (2006). [CrossRef] [PubMed]

,18

18. C. Selhuber-Unkel, I. Zins, O. Schubert, C. Sönnichsen, and L. B. Oddershede, “Quantitative optical trapping of single gold nanorods,” Nano Lett. 8(9), 2998–3003 (2008). [CrossRef] [PubMed]

]. that the long axis of an anisotropic nanoparticle aligns with the trapping laser polarization. Thus we estimated the general aspects of Rayleigh scattering spectra of 27 nm-sized silver nanoparticles according to the extended Mie theory [26

26. D. W. Mackowski and M. I. Mishchenko, “Calculation of the T matrix and the scattering matrix for ensembles of spheres,” J. Opt. Soc. Am. A 13(11), 2266–2278 (1996). [CrossRef]

,27

27. D. W. Mackowski, “Discrete dipole moment method for calculation of the T matrix for nonspherical par- ticles,” J. Opt. Soc. Am. A 19(5), 881–893 (2002). [CrossRef]

]. It was confirmed that their linearly arrayed structures have longitudinal plasmon bands in the wavelength range between1.57 and 3.1 eV and the peak positions shift to lower energy with increasing array length, while the transverse plasmon band around 3.2 eV hardly changes with array length. That is, scattering spectra shown in Fig. 3 (a) can be ascribed to the linearly arrayed structures of silver nanoparticles whose long axis aligns along the laser polarization direction and Fig. 2 (a) show their growing process in the laser focus.

Actually, the same measurement by using the laser beam with circular polarization gives a contrastive result. Figure 2 (b) and 3 (b) show Rayleigh scattering spectra obtained by circular polarization. Distributions of the peak energy E res and bandwidth Γ of the SPR (Fig. 5
Fig. 5 Histograms of (a, c) peak wavelength and (b, d) bandwidth of the plasmon resonance bands in the spectral measurements repeated under the same conditions as for Fig. 1. (a, b) linear and (c, d) circular polarization of the laser beam.
)demonstrates that the plasmon band in the case of circular polarization shows a broader Γ and a higher E res than that in the case of linear polarization (average E res and Γ: 1.85 and 0.128 eV for linear, 2.38 and 0.534 eV for circular polarization). This result suggests that the aggregate structure produced in the laser focus with circular polarization is relatively isotropic as compared to that in the case of linear polarization, because less anisotropy of aggregates with the same number of nanoparticles increases a peak energy and broadens a bandwidth of longitudinal plasmon modes.

Attractive interaction between laser induced dipoles can be explained to be originated from a gradient force by an intense optical field produced between a gap of two nanoparticles. Especially, silver and gold nanoparticles produces an extremely enhanced optical field in their nanogap, as well-known as hot spot, because the interactive dipole is coupled with SPR [29

29. A. N. Grigorenko, N. W. Roberts, M. R. Dickinson, and Y. Zhang, “Nanometric optical tweezers based on nanostructured substrates,” Nat. Photonics 2(6), 365–370 (2008). [CrossRef]

]. That is, if the alignment of nanoparticles is attributed to the laser induced interaction, hot spots must be produced in the laser induced self-assembly. This is confirmed as follows. Turning off the white light used for Rayleigh scattering measurement, the emission was observed from the laser focus, as shown in Fig. 7
Fig. 7 Emission spectra of assemblies of the silver nanoparticles formed in the laser focus. (a) linear and (b) circular polarization of the laser beam. The inset in (a) displays an enlarged spectrum plotted as a function of Raman shift from double frequency of the irradiated laser beam.
. A sharp band at 2.34 eV (532 nm) is attributed to the hyper-Rayleigh scattering (HRS) of the laser beam at 1.17 eV (1064 nm). Particularly in the case of a linearly polarized laser, the other two sharp bands were observed at 2.17 and 2.14 eV, and they are not shown in the case of circular polarization. These spectral peaks correspond to 1394 and 1583 cm−1 hyper-Raman bands of symmetric and asymmetric stretching vibrations (inset of Fig. 7 (a)) that could be originated from the carboxylate group of the citrate ion adsorbed on the colloidal silver during citrate reduction synthetic process [30

30. A. Sivanesan, P. Kannan, and S. A. John, “Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode,” Electrochim. Acta 52(28), 8118–8124 (2007). [CrossRef]

,31

31. M. Kerker, O. Siiman, L. A. Bumm, and D. S. Wang, “Surface enhanced Raman scattering (SERS) of citrate ion adsorbed on colloidal silver,” Appl. Opt. 19(19), 3253–3255 (1980). [CrossRef] [PubMed]

]. Occurrence of surface enhanced hyper-Raman scattering (SEHRS) due to SPR of silver nanoparticles has been confirmed by using a pulse laser excitation. We previously demonstrated SEHRS by means of CW laser excitation, but the target molecule was rhodamine 6G dye, which is excited via resonance Raman process [25

25. H. Yoshikawa, T. Adachi, G. Sazaki, T. Matsui, K. Nakajima, and H. Masuhara, “Surface enhanced hyper-Raman spectroscopy using optical trapping of silver nanoparticles for molecular detection in solution,” J. Opt. A, Pure Appl. Opt. 9(8), S164–S171 (2007). [CrossRef]

]. Even in the case of resonance SEHRS of rhodamine 6G, considering the cross section of hyper Raman scattering (the order of typically 10−65 cm4·s) and obtained signal intensity, it is estimated that the enhancement factor reaches 1020. The present SEHRS from citrate ions is not caused by resonance Raman process, meaning that a giant enhancement over 1020 occurs accompanied by the laser induced self-assembly of silver nanoparticles. In addition, this remarkable plasmonic property of the present array structure is proved by the especially sharp SPR band shown in Fig. 2 (a).

In conclusion, we have demonstrated that laser induced self-assembly of silver nanoparticles via plasmonic interactions results in the formation of linear array structures possessing exciting plasmonic properties. Our experimental results will constitute the novel strategy to utilize SPR in a small volume of solution, that is applicable to biomolecular sensing in a micro chamber, a microfluid, or single cell directly.

Acknowledgments

Y T and T I are supported by “WAKATE B (No. 16760042 and No. 21710090)” respectively, and Priority Area “Strong Photon-Molecule Coupling Fields (No. 470)” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. HY is gratefully acknowledges the financial support Foundation ATI of the current study.

References and links

1.

U. Kreibig, and M. Vollmer, Optical Properties of Metal Clusters (Springer, Berlin, 1995).

2.

M. Kawasaki and S. Mine, “Enhanced molecular fluorescence near thick Ag island film of large pseudotabular nanoparticles,” J. Phys. Chem. B 109(36), 17254–17261 (2005). [CrossRef]

3.

M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57(3), 783–826 (1985). [CrossRef]

4.

H. Kneipp and K. Kneipp, “Surface-enhanced hyper Raman scattering in silver colloidal solutions,” J. Raman Spectrosc. 36(6-7), 551–554 (2005). [CrossRef]

5.

H. Xu and M. Käll, “Surface-plasmon-enhanced optical forces in silver nanoaggregates,” Phys. Rev. Lett. 89(24), 246802 (2002). [CrossRef] [PubMed]

6.

L. A. Sweatlock, S. A. Maier, H. A. Atwater, J. J. Penninkhof, and A. Polman, “Highly confined electromagnetic fields in arrays of strongly coupled Ag nanoparticles,” Phys. Rev. B 71(23), 235408 (2005). [CrossRef]

7.

M. Inoue and K. Ohtaka, “Surface Enhanced Raman Scattering by Metal Spheres. I. Cluster Effect,” J. Phys. Soc. Jpn. 52(11), 3853–3864 (1983). [CrossRef]

8.

H. Xu, “Calculation of the near field of aggregates of arbitrary spheres,” J. Opt. Soc. Am. A 21(5), 804–809 (2004). [CrossRef]

9.

C. Hosokawa, H. Yoshikawa, and H. Masuhara, “Optical assembling dynamics of individual polymer nanospheres investigated by single-particle fluorescence detection,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 70(6), 061410 (2004). [CrossRef]

10.

C. Hosokawa, H. Yoshikawa, and H. Masuhara, “Cluster formation of nanoparticles in an optical trap studied by fluorescence correlation spectroscopy,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72(2), 021408 (2005). [CrossRef] [PubMed]

11.

Y. Tanaka, H. Yoshikawa, T. Asahi, and H. Masuhara, “Laser microfixation of highly ordered J-aggregates on a glass substrate,” Appl. Phys. Lett. 91(4), 041102 (2007). [CrossRef]

12.

Y. Tanaka, H. Yoshikawa, and H. Masuhara, “Laser induced self-assembly of pseudoisocyanine J-aggregates,” J. Phys. Chem. C 111(50), 18457–18460 (2007). [CrossRef]

13.

P. Jordan, J. Cooper, G. McNay, F. T. Docherty, D. Graham, W. E. Smith, G. Sinclair, and M. J. Padgett, “Surface-enhanced resonance Raman scattering in optical tweezers using co-axial second harmonic generation,” Opt. Express 13(11), 4148–4153 (2005). [CrossRef] [PubMed]

14.

P. M. Hansen, V. K. Bhatia, N. Harrit, and L. Oddershede, “Expanding the optical trapping range of gold nanoparticles,” Nano Lett. 5(10), 1937–1942 (2005). [CrossRef] [PubMed]

15.

K. Svobada and S. M. Block, “Optical trapping of metallic Rayleigh particles,” Opt. Lett. 19(13), 930–932 (1994). [CrossRef]

16.

S. Ito, H. Yoshikawa, and H. Masuhara, “Laser Manipulation and Fixation of Single Gold Nanoparticles in Solution at Room Temperature,” Appl. Phys. Lett. 80(3), 482–484 (2002). [CrossRef]

17.

M. Pelton, M. Liu, H. Y. Kim, G. Smith, P. Guyot-Sionnest, and N. F. Scherer, “Optical trapping and alignment of single gold nanorods by using plasmon resonances,” Opt. Lett. 31(13), 2075–2077 (2006). [CrossRef] [PubMed]

18.

C. Selhuber-Unkel, I. Zins, O. Schubert, C. Sönnichsen, and L. B. Oddershede, “Quantitative optical trapping of single gold nanorods,” Nano Lett. 8(9), 2998–3003 (2008). [CrossRef] [PubMed]

19.

L. Bosanac, T. Aabo, P. M. Bendix, and L. B. Oddershede, “Efficient optical trapping and visualization of silver nanoparticles,” Nano Lett. 8(5), 1486–1491 (2008). [CrossRef] [PubMed]

20.

F. Svedberg, Z. Li, H. Xu, and M. Käll, “Creating hot nanoparticle pairs for surface-enhanced Raman spectroscopy through optical manipulation,” Nano Lett. 6(12), 2639–2641 (2006). [CrossRef] [PubMed]

21.

A. S. Zelenina, R. Quidant, and M. Nieto-Vesperinas, “Enhanced optical forces between coupled resonant metal nanoparticles,” Opt. Lett. 32(9), 1156–1158 (2007). [CrossRef] [PubMed]

22.

Z. Li, H. Xu, and M. Kall, “Optical forces on interacting plasmonic nanoparticles in a focused Gaussian beam,” Phys. Rev. B 77(8), 085412 (2008). [CrossRef]

23.

P. C. Chaumet and M. Nieto-Vesperinas, “Optical binding of particles with or without the presence of a flat dielectric surface,” Phys. Rev. B 64(3), 035422 (2001). [CrossRef]

24.

A. J. Hallock, P. L. Redmond, and L. E. Brus, “Optical forces between metallic particles,” Proc. Natl. Acad. Sci. U.S.A. 102(5), 1280–1284 (2005). [CrossRef] [PubMed]

25.

H. Yoshikawa, T. Adachi, G. Sazaki, T. Matsui, K. Nakajima, and H. Masuhara, “Surface enhanced hyper-Raman spectroscopy using optical trapping of silver nanoparticles for molecular detection in solution,” J. Opt. A, Pure Appl. Opt. 9(8), S164–S171 (2007). [CrossRef]

26.

D. W. Mackowski and M. I. Mishchenko, “Calculation of the T matrix and the scattering matrix for ensembles of spheres,” J. Opt. Soc. Am. A 13(11), 2266–2278 (1996). [CrossRef]

27.

D. W. Mackowski, “Discrete dipole moment method for calculation of the T matrix for nonspherical par- ticles,” J. Opt. Soc. Am. A 19(5), 881–893 (2002). [CrossRef]

28.

J. N. Israelachvili, Intermolecular and Surface Forces (Academic, New York, 1992).

29.

A. N. Grigorenko, N. W. Roberts, M. R. Dickinson, and Y. Zhang, “Nanometric optical tweezers based on nanostructured substrates,” Nat. Photonics 2(6), 365–370 (2008). [CrossRef]

30.

A. Sivanesan, P. Kannan, and S. A. John, “Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode,” Electrochim. Acta 52(28), 8118–8124 (2007). [CrossRef]

31.

M. Kerker, O. Siiman, L. A. Bumm, and D. S. Wang, “Surface enhanced Raman scattering (SERS) of citrate ion adsorbed on colloidal silver,” Appl. Opt. 19(19), 3253–3255 (1980). [CrossRef] [PubMed]

OCIS Codes
(290.5870) Scattering : Scattering, Rayleigh
(220.4241) Optical design and fabrication : Nanostructure fabrication
(350.4855) Other areas of optics : Optical tweezers or optical manipulation
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Optical Trapping and Manipulation

History
Original Manuscript: June 17, 2009
Revised Manuscript: July 31, 2009
Manuscript Accepted: August 21, 2009
Published: October 2, 2009

Virtual Issues
Vol. 4, Iss. 12 Virtual Journal for Biomedical Optics

Citation
Yoshito Tanaka, Hiroyuki Yoshikawa, Tamitake Itoh, and Mitsuru Ishikawa, "Laser-induced self-assembly of silver nanoparticles via plasmonic interactions," Opt. Express 17, 18760-18767 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-21-18760


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References

  1. U. Kreibig, and M. Vollmer, Optical Properties of Metal Clusters (Springer, Berlin, 1995).
  2. M. Kawasaki and S. Mine, “Enhanced molecular fluorescence near thick Ag island film of large pseudotabular nanoparticles,” J. Phys. Chem. B 109(36), 17254–17261 (2005). [CrossRef]
  3. M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57(3), 783–826 (1985). [CrossRef]
  4. H. Kneipp and K. Kneipp, “Surface-enhanced hyper Raman scattering in silver colloidal solutions,” J. Raman Spectrosc. 36(6-7), 551–554 (2005). [CrossRef]
  5. H. Xu and M. Käll, “Surface-plasmon-enhanced optical forces in silver nanoaggregates,” Phys. Rev. Lett. 89(24), 246802 (2002). [CrossRef] [PubMed]
  6. L. A. Sweatlock, S. A. Maier, H. A. Atwater, J. J. Penninkhof, and A. Polman, “Highly confined electromagnetic fields in arrays of strongly coupled Ag nanoparticles,” Phys. Rev. B 71(23), 235408 (2005). [CrossRef]
  7. M. Inoue and K. Ohtaka, “Surface Enhanced Raman Scattering by Metal Spheres. I. Cluster Effect,” J. Phys. Soc. Jpn. 52(11), 3853–3864 (1983). [CrossRef]
  8. H. Xu, “Calculation of the near field of aggregates of arbitrary spheres,” J. Opt. Soc. Am. A 21(5), 804–809 (2004). [CrossRef]
  9. C. Hosokawa, H. Yoshikawa, and H. Masuhara, “Optical assembling dynamics of individual polymer nanospheres investigated by single-particle fluorescence detection,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 70(6), 061410 (2004). [CrossRef]
  10. C. Hosokawa, H. Yoshikawa, and H. Masuhara, “Cluster formation of nanoparticles in an optical trap studied by fluorescence correlation spectroscopy,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72(2), 021408 (2005). [CrossRef] [PubMed]
  11. Y. Tanaka, H. Yoshikawa, T. Asahi, and H. Masuhara, “Laser microfixation of highly ordered J-aggregates on a glass substrate,” Appl. Phys. Lett. 91(4), 041102 (2007). [CrossRef]
  12. Y. Tanaka, H. Yoshikawa, and H. Masuhara, “Laser induced self-assembly of pseudoisocyanine J-aggregates,” J. Phys. Chem. C 111(50), 18457–18460 (2007). [CrossRef]
  13. P. Jordan, J. Cooper, G. McNay, F. T. Docherty, D. Graham, W. E. Smith, G. Sinclair, and M. J. Padgett, “Surface-enhanced resonance Raman scattering in optical tweezers using co-axial second harmonic generation,” Opt. Express 13(11), 4148–4153 (2005). [CrossRef] [PubMed]
  14. P. M. Hansen, V. K. Bhatia, N. Harrit, and L. Oddershede, “Expanding the optical trapping range of gold nanoparticles,” Nano Lett. 5(10), 1937–1942 (2005). [CrossRef] [PubMed]
  15. K. Svobada and S. M. Block, “Optical trapping of metallic Rayleigh particles,” Opt. Lett. 19(13), 930–932 (1994). [CrossRef]
  16. S. Ito, H. Yoshikawa, and H. Masuhara, “Laser Manipulation and Fixation of Single Gold Nanoparticles in Solution at Room Temperature,” Appl. Phys. Lett. 80(3), 482–484 (2002). [CrossRef]
  17. M. Pelton, M. Liu, H. Y. Kim, G. Smith, P. Guyot-Sionnest, and N. F. Scherer, “Optical trapping and alignment of single gold nanorods by using plasmon resonances,” Opt. Lett. 31(13), 2075–2077 (2006). [CrossRef] [PubMed]
  18. C. Selhuber-Unkel, I. Zins, O. Schubert, C. Sönnichsen, and L. B. Oddershede, “Quantitative optical trapping of single gold nanorods,” Nano Lett. 8(9), 2998–3003 (2008). [CrossRef] [PubMed]
  19. L. Bosanac, T. Aabo, P. M. Bendix, and L. B. Oddershede, “Efficient optical trapping and visualization of silver nanoparticles,” Nano Lett. 8(5), 1486–1491 (2008). [CrossRef] [PubMed]
  20. F. Svedberg, Z. Li, H. Xu, and M. Käll, “Creating hot nanoparticle pairs for surface-enhanced Raman spectroscopy through optical manipulation,” Nano Lett. 6(12), 2639–2641 (2006). [CrossRef] [PubMed]
  21. A. S. Zelenina, R. Quidant, and M. Nieto-Vesperinas, “Enhanced optical forces between coupled resonant metal nanoparticles,” Opt. Lett. 32(9), 1156–1158 (2007). [CrossRef] [PubMed]
  22. Z. Li, H. Xu, and M. Kall, “Optical forces on interacting plasmonic nanoparticles in a focused Gaussian beam,” Phys. Rev. B 77(8), 085412 (2008). [CrossRef]
  23. P. C. Chaumet and M. Nieto-Vesperinas, “Optical binding of particles with or without the presence of a flat dielectric surface,” Phys. Rev. B 64(3), 035422 (2001). [CrossRef]
  24. A. J. Hallock, P. L. Redmond, and L. E. Brus, “Optical forces between metallic particles,” Proc. Natl. Acad. Sci. U.S.A. 102(5), 1280–1284 (2005). [CrossRef] [PubMed]
  25. H. Yoshikawa, T. Adachi, G. Sazaki, T. Matsui, K. Nakajima, and H. Masuhara, “Surface enhanced hyper-Raman spectroscopy using optical trapping of silver nanoparticles for molecular detection in solution,” J. Opt. A, Pure Appl. Opt. 9(8), S164–S171 (2007). [CrossRef]
  26. D. W. Mackowski and M. I. Mishchenko, “Calculation of the T matrix and the scattering matrix for ensembles of spheres,” J. Opt. Soc. Am. A 13(11), 2266–2278 (1996). [CrossRef]
  27. D. W. Mackowski, “Discrete dipole moment method for calculation of the T matrix for nonspherical par- ticles,” J. Opt. Soc. Am. A 19(5), 881–893 (2002). [CrossRef]
  28. J. N. Israelachvili, Intermolecular and Surface Forces (Academic, New York, 1992).
  29. A. N. Grigorenko, N. W. Roberts, M. R. Dickinson, and Y. Zhang, “Nanometric optical tweezers based on nanostructured substrates,” Nat. Photonics 2(6), 365–370 (2008). [CrossRef]
  30. A. Sivanesan, P. Kannan, and S. A. John, “Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode,” Electrochim. Acta 52(28), 8118–8124 (2007). [CrossRef]
  31. M. Kerker, O. Siiman, L. A. Bumm, and D. S. Wang, “Surface enhanced Raman scattering (SERS) of citrate ion adsorbed on colloidal silver,” Appl. Opt. 19(19), 3253–3255 (1980). [CrossRef] [PubMed]

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