Delivered by Ingenta to: Rice University, Fondren Library IP : 95.25.180.202 Sat, 04 Aug 2012 04:39:25 Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Computational and Theoretical Nanoscience Vol. 8, 1424–1427, 2011 Optical Characterization of Hexagram Metallic Nanoholes Shaoli Zhu ∗ and Wei Zhou School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore Optical characterization of a two-dimensional silver hexagram nanohole arrays was carried out. Finite-difference and time-domain method was used to calculate the transmission and localized electric field distribution of the nanohole arrays. According to the theoretical design result, focused ion beam (FIB) nanofabrication technology was employed to fabricate the hexagram nanoholes in the silver film sputter-coated on a glass substrate. The FIB fabricated nanoholes were shown to be distributed on the glass substrate regularly. Near-field scanning optical microscope (NSOM) was used to measure the electric field distribution near the surface of the nanoholes. All results show that the two-dimensional silver hexagram nanohole arrays can be fabricated by FIB successfully and that the optical intensity distribution near the nanoholes is uniform. The hexagram metallic nanoholes may be utilized for imaging, focusing and nanobiosensing. Keywords: Optical Characterization, Finite-Difference and Time-Domain, Hexagram Metallic Nanoholes, Focused Ion Beam. 1. INTRODUCTION The optical characterizations of the metal nanoholes have been under investigation since the first predictions by Bethe 1 in 1944. In the past decade, many scien- tists focus on studying the transmission processes. Many theoretical 2–7 and experimental 8–11 studies had been carried out to explore the fundamental properties, fabrication, and utilization of metal nanostructures. The materials, shapes, periodicity and the arrangement of the nanoholes play a significant role for the excitation of surface plasmons. 12 By changing these parameters, it is possible to change the transmission properties to desired wavelength regions and manipulate the light propagation through the film. 13 In this paper, we report the two-dimensional metal- lic hexagram nanohole array to enhance the transmis- sion phenomenon. Finite-difference time-domain (FDTD) method was used to design and calculate the transmis- sion and the localized electric field distribution. Our designed nanohole arrays were fabricated using focused ion-beam (FIB) milling method. The optical properties of the nanohole arrays were explored by the near-field scanning optical microscope (NSOM) system. Both the calculated and experiments detection results show that the hexagram nanohole array can significantly enhance the localized electric field and the transmission. It has potential ∗ Author to whom correspondence should be addressed. applications in nanobiosensor , imaging and nano-device in the future. 2. DESIGN HEXAGRAM NANOHOLES USING FDTD In order to derive the optical properties of the hex- agram nanohole array, we used professional software- three-dimensional finite-difference time-domain (FDTD) 14 method to model the transmission and optical field distri- butions. Figure 1 shows our designed geometrical model of the Ag hexagram nanohole array. The hexagram nanohole array is arranged in the symmetry two-dimensional infi- nite arrays. This hexagram nanohole array lies in x-y plane and the incident light polarized of x-axis propa- gates along z axis. The out-of plane heights of the Ag nanoholes is 40 nm. The in plane widths of each nanoholes is 1500 nm. The period of the nanohole array is 1500 nm and the refractive index of the medium surrounding the Ag nanohole is 1.0 (in air). For the Ag material, we used Drude model to calculate the electronic distribution on the surface of the hexagram nanohole array. For the nanoholes, a glass substrate n = 152 was used, the thickness is 1000 nm. According to the design model shown in Figure 1, we calculated the transmission in the z direction and the local- ized electric field distribution. The simulation parameters of FDTD algorithm are set as follows: the incident light 1424 J. Comput. Theor. Nanosci. 2011, Vol. 8, No. 8 1546-1955/2011/8/1424/004 doi:10.1166/jctn.2011.1831