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4Department of Electrical Engineering, National Taiwan University, No.1 Section ... OCIS codes: (250.5230) Photoluminescence; (240.6680) Surface plasmons.
Surface plasmon effects on two photon luminescence of gold nanorods Da-Shin Wang,1,2 Fu-Yin Hsu,3 and Chii-Wann Lin,1,2,4* 1

2

Institute of Biomedical Engineering, National Taiwan University, No.1 Section 4,Roosevelt Road, Taipei,Taiwan 10617 Center of Nano Science and Technology, National Taiwan University, No.1 Section 4,Roosevelt Road, Taipei,Taiwan 10617 3 Institute of Bioscience and Biotechnology, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, Taiwan, 20224 4 Department of Electrical Engineering, National Taiwan University, No.1 Section 4,Roosevelt Road, Taipei,Taiwan 10617 *[email protected]

Abstract: Gold nanorods emit strong photoluminescence under two photon excitation; the efficient two photon lumininescence (TPL) arises from the local field enhancement assisted by surface plasmons. The surface plasmon effects on the TPL efficiency and spectrum are investigated by measuring the TPL of gold nanorods with various aspect ratios. A large TPL efficiency is found when incident light wavelength coincides with the longitudinal surface plasmon mode of the gold nanorods. However, the emission spectra of nanorods with various aspect ratios look similar and exhibit modest surface plasmon features, which implies a major non-radiative decay of excited surface plasmons. ©2009 Optical Society of America OCIS codes: (250.5230) Photoluminescence; (240.6680) Surface plasmons.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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(C) 2009 OSA

Received 18 May 2009; revised 17 Jun 2009; accepted 18 Jun 2009; published 22 Jun 2009

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1. Introduction The photoluminescence of gold metal was first reported in 1969 by Mooradian [1]. The photoemission is thought to arise from the direct transitions between electrons in the conduction band below the Fermi level and holes in the d bands. Although suggested by Mooradian as a band-structure probe, the photoluminescence of noble metals remained relatively unexplored for nearly two decades until 1986, when Boyd [2] reported multiphotoninduced luminescence on roughened noble metal surfaces. Since the efficiency of single photon luminescence is very small, ~10−10 [1], even the luminescence can be excited by multiphoton absorption, it is not likely to be detected without an enhancement mechanism. There are two important effects contributing to the local field enhancement on roughened metal surfaces. First, fields tend to concentrate at the protrusions on metal surfaces, known as the lightning-rod effect [3]. Second, the collective oscillation of the electrons on metal surfaces can be induced by optical fields; the resonance greatly amplifies the local field, which is referred to as the local-plasmon effect [3]. In recent years, there has been a renewed interest in photoluminescence from metal surfaces, primarily in the photoluminescence from noble metal nanoparticles. Much of the interest is generated by the potential biomedical applications of nanoparticles. Two photon luminescence (TPL) is particularly appealing for biomedical use because of its high resolution, low photodamage and 3D-imaging capability. Besides, the near-infrared excitation light is where the absorption of water and biological samples are minimized.The two photon luminescence of various gold nanoparticles, including nanospheres, nanorods and nanoshells, has been tested and reported to be highly efficient [4–8]. Similar to the mechanism on a roughened surface, the photoluminescence is enhanced through the mediation of surface plasmons (SPs). The efficient, non-bleaching and non-blinking photoluminescence makes gold nanoparticles an ideal probe in imaging, as it is used as a contrast agent in most reports [7–9]. However, the photoluminescence of gold nanoparticles has not yet reached its full potential for applications. Since surface plasmon modes are sensitive to the local dielectric properties, the local ionic movement or fluctuation of charge density could modify the surface plasmon mode by affecting the local dielectric constant and thus causes a change in the photoluminescence. Thus, by monitoring the photoluminescence of the targeted nanoparticles, it is possible to sense local biological events that involve charge density changes, such as the action potential firing of neurons. Yet, prior to the promising use of photoluminescence as a sensor, the local plasmon effects on the photoluminescence spectra need to be well studied. This article is intended to study the local plasmon effects on the two photon luminescence of gold nanorods. It is well known that the surface plasmon modes can be modulated by varying the aspect ratio of gold nanorods or by modifying the ambient refractive index. Hence, we measured the two photon luminescence of gold nanorods with various aspect ratios and in media with different refractive index. The experiment was carried out for gold nanorods in aqueous solution, very similar to the previous work on single photon luminescence done by Mohamed et al. [10], but simply on a two-photon basis. Unlike the single photon luminescence of gold nanorods, which has a spectrum peak strongly dependent on the aspect ratios of the nanorods [10], the difference in the TPL spectrum for nanorods with various aspect ratios is quite modest. We have observed quite similar TPL spectra for gold nanorods with aspect ratios from 1.3 to 5.3 and in media of varied refractive index. #111292 - $15.00 USD

(C) 2009 OSA

Received 18 May 2009; revised 17 Jun 2009; accepted 18 Jun 2009; published 22 Jun 2009

6 July 2009 / Vol. 17, No. 14 / OPTICS EXPRESS 11351

However, a great variation in the emission efficiency was observed among the nanorods with various aspect ratios. The variation in the TPL efficiency is also clearly seen when the ambient refractive index is varied. This indicates that the surface plasmon plays an important role in TPL efficiency, but the emission spectrum is determined by the intrinsic electronic properties of the rod particles. 2. Methods 2.1 Experiment Gold nanorods were synthesized by using the seed-mediated methods as described elsewhere [11]. In short, Au seeds were prepared by mixing solutions of 0.1M NaBH4 and 2.5 × 10−4M HAuCl4, followed by vigorous stirring. For the rod formation, a sufficient amount of surfactant, cetyltrimetyl ammonium bromide (CTAB), was added into the solution. By varying the amount of surfactant, gold nanorods with different aspect ratios can be obtained. The extinction spectra of gold nanorods with various aspect ratios were measured by a UV-Visible spectrophotometer (Cary 50Conc, Varian) and the longitudinal surface plasmon (LSP) peaks are found to be at 540, 590, 680, 740, 790, 820, and 930nm, which is equivalent to aspect ratios from 1.3 to 5.3 according to the linear dependence of LSP peaks on the aspect ratios of gold nanorods [12] (see Fig. 1). The axial length of the nanorods is between 10 and 15nm as indicated by the TEM (transmission electron microscopy) images. The concentration of each nanorod solution was adjusted so the optical density (OD) of LSP band is about 1; the nanorod solutions are then used for the TPL measurement.

Fig. 1. The extinction spectra for gold nanorods with various aspect ratios. The numbers at the top and the right of each trace indicate the position of the longitudinal surface plasmon band and the aspect ratio of that nanorod, respectively.

The setup for TPL measurements is schematically shown in Fig. 2. The light source was a Ti:sapphire laser (Tsunami, Spectra Physics) at a 80 MHz repetition rate with a pulse width