Geometrical and fluidic tuning of periodically modulated thin metal films Giovanni Gilardi a,b,c, * , Sanshui Xiao a, ** , Romeo Beccherelli c , Antonio d’Alessandro b,c , N. Asger Mortensen a a DTU Fotonik, Department of Photonic Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark b Dipartimento di Ingegneria dell’informazione, elettronica e telecomunicazioni, Sapienza Universita ` di Roma, Via Eudossiana 18, 00184 Rome, Italy c Consiglio Nazionale delle Ricerche – Istituto per la Microelettronica e Microsistemi (CNR-IMM), Via del fosso del cavaliere 100, 00133 Rome, Italy Received 20 July 2011; received in revised form 7 December 2011; accepted 16 December 2011 Available online 27 December 2011 Abstract We numerically demonstrate near-zero transmission of light through two-dimensional arrays of isolated gold rings. The analysis of the device as an optofluidic sensor is presented to demonstrate the tuning of the device in relation to variations of volume and refractive index of an isotropic fluid positioned over the structure. We also evaluate the performance of the device with respect to geometrical parameters of the rings. # 2011 Elsevier B.V. All rights reserved. Keywords: Diffraction and gratings; Surface plasmons; Metal optics 1. Introduction Optofluidic sensors have emerged as one of the crucial elements in the current trend of miniaturized optofluidic compartments for lab-on-a-chip systems. Integration of these systems offers significant advantages including minimized consumption of reagents, portability, increased automation and reduced costs. Identification and quantification of the analyte can be accomplished by measuring the refractive index. This can be measured by several techniques such as evanescent waves in liquid waveguides [1], long period grating [2], interference at the liquid–solid interface [3], Fabry–Perot interferometer [4], Mach–Zehnder interferometry [5], ring resonators [6] or surface plasmon resonance [7]. As first demon- strated by Ebbsen et al. [8], making a periodic arrangement of subwavelength holes in an otherwise optically opaque metal film may induce an extraordinary amount of light transmittance for certain frequencies, the so-called extraordinary optical transmission. The enhanced transmittance is attributed to the excitation of surface plasmon polaritons (SPPs) [9], i.e., surface waves that arise from the interaction between light beams and the collective plasma oscillations of the conduction electrons in the metal. One may intuitively expect that an ultrathin and semi-transparent film would turn even more transparent if it were to be perforated by sub-wavelength www.elsevier.com/locate/photonics Available online at www.sciencedirect.com Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 177–182 * Corresponding author at: DTU Fotonik, Department of Photonic Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. ** Corresponding author. E-mail addresses: gilardi@die.uniroma1.it (G. Gilardi), saxi@fotonik.dtu.dk (S. Xiao). 1569-4410/$ – see front matter # 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.photonics.2011.12.003