Sub-wavelength Imaging through Metallic Nanorod Array
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0919-J05-06
Sub-wavelength Imaging through Metallic Nanorod Array Atsushi Ono1,2, Jun-ichi Kato2, and Satoshi Kawata1,2 1 Osaka University, Suita, Osaka, 565-0871, Japan 2 RIKEN, Wako, Saitama, 351-0198, Japan
ABSTRACT We proposed a subwavelength imaging system at optical frequency region with metallic nanorod array. We calculated the field distribution at the different planes of imaging process using the finite-difference time-domain (FDTD) algorithm and found that the spatial resolution was 40 nm, which was much beyond the diffraction-limit and was limited by the array pitch. The typical configuration is a hexagonal arrangement with 40 nm periodicity of silver rods of 50 nm height and 20 nm diameter. The image formation highly depends on the coherence and the polarization of the dipole sources, the array pitch, and the source-array distance. The principle of our near-field imaging is based on the longitudinal resonance of the localized surface plasmon along a metallic nanorod. The spectral responses of the device are also investigated.
INTRODUCTION Negative index material is expected to exhibit interesting optical properties. Especially, superlens effect, which is predicted by John B. Pendry in 2000, is very attractive to overcome the diffraction limit in optical imaging [1]. Although there is no negative index material in nature, Pendry numerically suggested that several metals, only dielectric constant is negative at optical frequencies, behave like a superlens under the electrostatic limit and for the p-polarized fields. We traced the Pendry’s model by the FDTD simulation and the result was shown in figure 1. Figures 1(a) and 1(b) show the calculation model and the intensity field distribution of the vertical cross section for the silver thin film. The thickness of silver thin film was 40 nm. The object and the image plane were 20 nm away from the entrance and the exit surface of the film. Strong field enhancement was observed at the both interfaces. The operation wavelength was 318 nm. In this wavelength, the dielectric constant of silver was set to –1 that was impedance matching condition. Figure 1(c) shows the intensity distribution at the image plane. Nanospot, which indicates sub-diffaraction limited resolution, was observed in the image plane. The full width at half maximum (FWHM) was 60 nm. X. Zhang experimentally demonstrated this superlens effect by constructing nanolithography system with silver thin film in 2005 [2]. Silver thin film has an ability of super lens by the enhancement effect of the lateral propagation mode of surface plasmon polariton. The optical superlens promises exciting avenues to nanoscale optical imaging and ultrasmall optoelectronic devices.
Figure 1. Pendry’s superlens and the FDTD simulation results. (a) Superlens is constructed by silver thin film with 40 nm thickness. (b), (c) Intensity field distributions at the cross sectional plane of the silver thin film and the image plane.
THEORY We proposed a sub-wavelength imaging system at optical frequency regime in an array o
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