PHYSICAL REVIEW B 84, 245424 (2011) Plasmonic interaction of visible light with gold nanoscale checkerboards S. Anantha Ramakrishna, P. Mandal, K. Jeyadheepan, and N. Shukla Department of Physics, Indian Institute of Technology, Kanpur 208016, India S. Chakrabarti, M. Kadic, S. Enoch, and S. Guenneau Institut Fresnel, UMR CNRS 6133, Aix-Marseille Universit´ e, Campus universitaire de Saint-J´ erˆ ome, FR-13397 Marseille, France (Received 6 July 2011; published 13 December 2011) Intersecting corners and checkerboards of negative refractive index materials (NRIM) represent highly singular electromagnetic systems that involve very highly enhanced local fields and the local density of modes. It is well known that plasmonic metallic systems can mimic the behavior of NRIM in the near-field limit at optical frequencies. Opaque gold films have been structured by focused ion-beam technologies at submicrometer scales in a checkerboard fashion and their optical properties measured. Subwavelength square holes in thick gold films placed in checkerboard fashion show a broadband extraordinary transmission of light at visible wavelengths. We find that the smaller the square holes, the larger is the transmission over a band of wavelengths from 650 to 950 nm suggesting that such structured surfaces have very unusual effective medium properties, which is confirmed by the band-structure diagrams computed with finite elements. Theoretical results also confirm the experimental transmission measured to be well over 80% from 750 to 950 nm for a checkerboard with 150 nm × 150 nm square holes. This unusual broadband nature of checkerboard structured films is confirmed by the dark-field reflection spectra. Microscopic studies reveal that these structures have enhanced interaction of light at the edges and corners. These checkerboards are also found to give rise to an enhancement of fluorescence by imbedded dye molecules. There is a strong correspondence between the theoretical predictions and the experimental measurements. DOI: 10.1103/PhysRevB.84.245424 PACS number(s): 42.70.Qs, 73.20.Mf, 81.05.Xj, 81.07.−b I. INTRODUCTION In 1967, Veselago proposed a thought experiment in which materials with simultaneously negative permittivity (ε) and magnetic permeability (μ) were shown to have a negative refractive index. 1 A ray analysis allowed him to conclude that a slab of such a negative refractive index material (NRIM) can act as a flat lens that imaged a source on one side to a point on the other. But this result remained an academic curiosity for almost thirty years, until Pendry and co-workers 2,3 proposed designs of structured materials that would have negative effective ε and μ. These so-called metamaterials are indeed structured at subwavelength length scales (typically λ/10 to λ/6), making it possible to regard them as almost homogeneous. The first experimental realizations were chiefly achieved at GHz frequencies, 4,5 but metamaterials in the near infrared and optical frequencies 6,7 have been proposed and demonstrated. Further, Pendry showed that the flat lens proposed by Veselago was very unusual in that the image resolution produced by this lens in principle, did not have any limitation as the evanescent near-field components of radiation that contain subwavelength information were also involved in the image formation. 8 Pendry also suggested that using plasmonic metals at optical frequencies for constructing lenses with superdiffraction resolution would be possible as they mimic the behavior of NRIM when all the length scales in the system are small compared to the wavelength (near-field limit). 8 Such a superlensing effect was demonstrated at optical frequencies through a silver slab film in Ref. 9 with the image resolution of about λ/5 limited only by the levels of absorption in the silver material. It was subsequently shown by Pendry and Ramakrishna 10 that the superlensing effect with a slab of negative refractive index medium can be generalized to mate- rials that are anisotropic and spatially inhomogeneous. Using a geometric transformation, it was shown, 10 as a consequence of this theorem, that two rectangular (semi-infinite) intersecting wedges of NRIM act as an imaging system whereby a source gets imaged onto itself. This system, originally studied by Notomi 11 using a ray picture, was thus shown to involve the evanescent modes also, and was a unique resonator. Guenneau et al. 12 subsequently generalized this imaging effect to a rectangular checkerboard lattice where alternating cells have positive (ε = μ =+1) and negative (ε = μ =−1) refractive index. It was shown that a source placed in one cell would reproduce itself in every other cell of the infinite lattice. Unusual transmission properties through checkerboard lenses were further investigated by Chakrabarti et al. 13 The properties of corners and checkerboards in the presence of dissipation have also been studied using geometric transforms. 14 In other developments on understanding the behavior of corners on NRIM, Monzon et al. 15 derived an analytical solution for a finite-sized NRIM wedge in the presence of a source. He et al. 16 studied some modes of a resonator with NRIM wedges and constructed an open cavity using triangular wedges of a photonic crystal that shows the negative refraction effect (also see Ref. 17). In a parallel development, Ebbesen et al. demonstrated in 1998 that resonant excitation of surface plasmons enhance transmission of light through arrays of subwavelength-sized holes in metallic films. 18 This triggered off intense debates about the plasmonic mechanism responsible for the phe- nomenon as well as immense development in the area of 245424-1 1098-0121/2011/84(24)/245424(11) ©2011 American Physical Society