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We fabricate periodic arrays of simple-square and composite-square air hole nanostructures on a 120 nm thick Au film deposited on glass substrate by focused ion beam technology and study their optical properties by examining the optical near-field distribution via scanning near-field optical microscopy in the near-infrared region. The simple-square nanostructure only contains one square air hole in each unit cell, while the composite-square one contains the same size square air hole in the center and eight smaller square air holes in the periphery. The measured optical near-field patterns for the two nanostructures show very different distribution features. High intensity light spots locate within the central square air hole in the simple-square structure, while they sit at the peripheral smaller square air holes in the composite-square structure. Numerical simulations based on the plane-wave transfer-matrix method have been carried out to analyze the optical near-field patterns for the two metallic nanostructures and agree well with the experimental data. The results indicate that light interaction with metallic nanostructures is very sensitive to even a small change in the subtle geometrical feature. Meanwhile, by comparing near-field patterns with dielectric nanostructures theoretically, we also find that optical confinement is better for gold nanostructures than for dielectric nanostructures. [ABSTRACT FROM AUTHOR]

Abstract: The near field intensity distribution and the near-field transmission spectrums of metallic nanostructures simulated by the finite-difference time-domain method are present in the text. Better field intensity localization and larger field intensity enhancement exist in the near field intensity distribution images for nanoparticle array in the near-infrared wavelength range. The minimum transmission intensity collected at the metal film surface can exceed 1.0 and the electric field intensity can limit in a number of nanometer. Meanwhile, these nanoparticle array structures can extensively apply to biochemical sensor devices design and so on. [Copyright &y& Elsevier]