    Plasmon resonances in near-field coupled 2D Au Nanoparticle Arrays P. K. Sahoo 1 , Y. Ekinci 2 , A. Christ 3 , O. J. F. Martin 3 , and H. H. Solak 1 1 Paul Scherrer Institut, Laboratory for Micro- and Nanotechnology CH – 5232 Villigen-PSI, Switzerland, Fax: +41 56 310 2646, email: harun.solak@psi.ch 2 Swiss Federal Institute of Technology Zurich (ETH), Laboratory of Metal Physics and Technology, CH-8093 Zürich, Switzerland 3 Swiss Federal Institute of Technology Lausanne (EPFL), Nanophotonics & Metrology Labora- tory, CH-1015 Lausanne, Switzerland Abstract When Au nanoparticles are close proximity to each other in a regular two-dimensional array, coupling of plasmon resonances of the individual particles leads to a collective response. Such systems are of interest espe- cially because of their potential application in analytical techniques such as Surface Enhanced Raman Spectros- copy (SERS). We used extreme ultraviolet interference lithography (EUV-IL) and a shadow evaporation tech- nique to fabricate two-dimensional arrays of Au dots with a periodicity of 100 nm. The gap between the particles that controls the extent of coupling was varied in a range from 50 nm to below 10 nm. Optical measurements show two resonances at 520 nm and 620 nm, with the latter gaining strength as the gap is reduced. Extensive experimental theoretical investigations using a FDTD algorithm demonstrate that the low-energy resonance can be assigned as a collective surface plasmon resonance arising from the strong near-field coupling between the nanoparticles. 1. Introduction Effects of near-field coupling on plasmon resonance of metal nanostructures such as binary particles [1-5], chains of particles [6], and cylindrical wires [7] have been subjects of intense research recently. Plasmon resonance in near-field-coupled 2D particle arrays is particularly interesting due to the ability to obtain high field enhancement over large areas [8-10]. Such systems are potentially use- ful in SERS or in exploitation of nonlinear phenomena such as second-order or super-continuum white light generation. Obtaining large enhancement factors over large areas in a uniform and reproducible way is of critical importance for such applications . The central challenge in this respect is the fabrica- tion of arrays with extremely narrow gaps for optimum enhancement of the field. Previously, plas- monic responses of nanoparticle arrays formed by self-assembly of colloidal particles in a hexagonal close-packed structure were reported [8, 9]. However, square-arrays where the field enhancement is expected to be stronger [10] cannot be obtained by self-assembly methods. Here, we address this prob- lem by fabricating coupled periodic Au nanoparticle arrays with sub-10 nm gap using a top-down fab- rication method, namely four beam EUV-IL and a simple shadow deposition method. The transmis- sion spectra and the effect of the dot spacing are explained by comparison to FDTD simulations. 2. Fabrication of 2D-dot arrays and characterization The dot arrays were fabricated on quartz substrates starting by spin-coating of an 80 nm-thick photoresist (PMMA) film. The samples were exposed with EUV-IL [11] at the XIL beam line of the Swiss Light Source (SLS) and developed. Depending on the exposure dose, arrays of PMMA dots with varying dot-size were obtained with a fixed period of 100 nm. Subsequently, Au was shadow- evaporated onto the samples at an oblique angle of 15 degrees, while the substrate was rotated around the normal direction. As a result of self-shadowing during the film growth, arrays of Au dots isolated from each other were produced (Fig. 1). The shallow deposition angle and the small gap between the particles ensure that a very small amount of Au - on the order of 1-2 nm - is deposited onto the under- lying quartz surface. The arrays typically cover an area of about 0.5 mm 2 . Transmission spectra were 571