Fabrication of Large-Area Patterned Nanostructures for Optical Applications by Nanoskiving Qiaobing Xu, † Jiming Bao, ‡ Robert M. Rioux, † Raquel Perez-Castillejos, † Federico Capasso, ‡ and George M. Whitesides* ,† Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts, 02138, and HarVard School of Engineering and Applied Sciences, HarVard UniVersity, 29 Oxford Street, Cambridge, Massachusetts, 02138 Received June 11, 2007 ABSTRACT Cost-effective and convenient methods for fabrication of patterned metallic nanostructures over the large (mm 2 ) areas required for applications in photonics are much needed. In this paper, we demonstrate the fabrication of arrays of closed and open, loop-shaped nanostructures by a technique (nanoskiving) that combines thin-film deposition by metal evaporation with thin-film sectioning. These arrays of metallic structures serve as frequency-selective surfaces at mid-infrared wavelengths. Experiments with structures prepared using this technique demonstrate that a closed-looped structure has a single dominant resonance regardless of the polarization of the incident light, while open structures have resonances that are anisotropic with respect to the polarization of the electric field. Finite-difference time-domain (FDTD) simulations reproduce the scattering spectra of these frequency-selective surfaces, provide an explanation of the wavelength of the experimentally observed resonances, and rationalize their polarization dependence based on the patterns of current induced in the nanostructures. This paper describes the fabrication by nanoskiving 1-3 of large-area (∼9 mm 2 ), thin (∼100 nm), free-standing epoxy slabs incorporating regular arrays of metallic nanostructures. We measured the transmission spectra of these nanostructures and compared them with simulated scattering spectra using finite-difference time-domain (FDTD) calculations. Experi- mentally, closed-loop structures have a single dominant resonance regardless of the polarization of the incident light. Open structures s e.g., L- or U-shaped structures s have resonances that are anisotropic with respect to the polariza- tion of the electric field. The FDTD calculations adequately reproduce the spectra and provide a simple explanation for the wavelengths of resonances, and of their polarization dependence, based on the patterns of current induced in the nanostructures by the incident light. We believe that the ability to fabricate and manipulate free-standing metallic nanostructures will find applications in the fabrication of materials having negative index of refraction and of three- dimensional metamaterials. 4-10 Patterned arrays of metallic nanostructures have wide applications in photonics, in (for example) negative index materials (NIMs), 4-10 frequency-selective surfaces (FSS), 11-20 optical polarizers, optical filters, and nanostructures for surface-enhanced Raman spectroscopy (SERS). 21,22 FSS are two-dimensional periodic arrays of metallic structures that transmit or reflect radiation at specific frequencies; FSS are useful in beam splitters, filters, and polarizers. 13-15 In FSS, the reflection or transmission is strongest when the frequency of the incident electromagnetic field matches the plasmon resonance of the metallic structures comprising the FSS. This resonant frequency is mainly determined by the size and shape of the unit metallic nanostructures comprising the FSS. The bandwidth of the resonance s as well as the total reflectivity or transmittance s depends on the density and periodicity of unit elements. 15 For a FSS consisting of identical, straight, metallic wires (an antenna array) of length l, the longest wavelength of resonance is approximated by eq 1, where λ r is the resonance wavelength and n eff is the effective refractive index of the medium. In eq 2, n 1 and n 2 are refractive indices of the media above (air, n ) 1) and below (supporting substrate) the metallic nanostructure. 15,23 At the resonant wavelength λ r , the wire behaves approximately as a simple electric dipole. For simple * Corresponding author. Telephone: (617) 495-9430. Fax: (617) 495- 9857. E-mail: gwhitesides@gmwgroup.harvard.edu. † Department of Chemistry and Chemical Biology. ‡ Harvard School of Engineering and Applied Sciences. λ r /2 ∼ n eff l (1) n eff ) ( n 1 2 + n 2 2 2 ) 1/2 (2) NANO LETTERS 2007 Vol. 7, No. 9 2800-2805 10.1021/nl0713979 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007