Plasmon Resonance of Finite One-Dimensional Au Nanoparticle Chains Q.-H. Wei, ² K.-H. Su, S. Durant, and X. Zhang* Department of Mechanical and Aerospace Engineering, UniVersity of California at Los Angeles, Los Angeles, California 90095 Received March 11, 2004; Revised Manuscript Received April 20, 2004 ABSTRACT We report experimental and theoretical studies on the plasmon resonances of finite one-dimensional chains of Au nanoparticles excited by evanescent light waves with polarization parallel to the chains. The experimental results show that the plasmon resonance peak wavelengths of these finite 1D chains are significantly red-shifted in comparison to that of single Au nanoparticle. Contrary to previous findings, the peak wavelengths are observed to be a nonmonotonic function of particle numbers in the chain. This phenomenon is reproduced in the theoretical results obtained by using the transfer-matrix method and is shown to occur only for larger particles where phase retardation effects are important in plasmon coupling. Considerable recent interest has been paid to nanometal particles due to the numerous potential applications of their extraordinary optical properties. 1-11 The strong interactions of nanometal particles with visible light originate from the excitation of collective oscillations of conduction electrons within these particles, an elementary excitation termed “surface plasmons”. 13 The surface plasmons can be detected as resonance peaks in the light scattering spectra of these nanoparticles. The plasmon resonance energy of a particular nanometal particle depends on its size, shape, composition, and its surrounding medium. 14 Coupling between surface plasmons of neighboring particles leads to energy shifts and energy confinements between particles, an effect playing an important role in surface enhanced Raman scattering experi- ments (SERS). 10-12,15-19 Near-field plasmon coupling in periodic arrays has been extensively studied. Of particular interest, recent theoretical and experimental work suggests that one-dimensional nano- particle arrays can be utilized to transport energy. 10-12,20-21 The minimum size of the guided modes in nanoparticle arrays is not limited by diffraction, which may enable nanoscale optical devices if the propagation loss can be minimized. Furthermore, near-field scanning optical microscopic studies of 1D chains of nanoparticles showed that enhanced local electromagnetic fields are primarily confined between par- ticles, which may be utilized for producing efficient SERS- active substrates for molecular sensing applications. 22 Since the energy transportation in a chain is sensitive to interparticle plasmon coupling, thorough understanding of the resonance wavelength dependence on the chain length and particle spacing could provide useful guidance for designing na- nooptical devices. Far-field spectroscopic studies and finite- difference time-domain (FDTD) simulations of chains of 50 nm diameter Au spheres show that the peak wavelength of plasmon resonances increases (decreases) with the chain lengths for the longitudinal (transverse) mode and saturates when the chain is longer than about seven particles. 20 In this letter, we report experimental and theoretical studies on the plasmon resonances of finite 1D chains of Au nanoparticles excited by optical waves with a polarization parallel to the chain (longitudinal mode). The experimental results show that the plasmon resonance peak wavelength of a finite 1D chain of Au nanoparticles is significantly red- shifted in comparison to that of single Au nanoparticles. In contrast to previous findings for smaller particles, 20-21 the peak wavelength is found to be nonmonotonic and oscillating with the variation of the chain length. Theoretical calculations based on the transfer matrix method show satisfactory agreement with this experimental observation and demon- strate further that this resonance peak oscillation occurs only for large particles. This finding may help to provide a better understanding of the plasmon resonances in coupled nano- particle chains. The gold nanoparticles were prepared on quartz substrates by electron beam lithography (EBL) and a standard lift-off process. 10 nm-thick indium tin oxide (ITO) films were sputtered on the quartz substrates to reduce charging effects during the EBL process, and 100 nm-thick poly(methyl methacrylate) (PMMA) films were used as a positive * Corresponding author. E-mail: xiang@seas.ucla.edu. ² Present address: Applied NanoBioscience Center, Arizona State University, Tempe, AZ 85287-4004; e-mail: Qihuo.Wei@asu.edu. NANO LETTERS 2004 Vol. 4, No. 6 1067-1071 10.1021/nl049604h CCC: $27.50 © 2004 American Chemical Society Published on Web 05/08/2004 Downloaded via UNIV OF CALIFORNIA BERKELEY on May 4, 2020 at 19:12:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.