Templates ‘08 p.316 PLASMON MEDIATED RESONANT TRANSMISSION THROUGH AN UNDULATED METAL FILM USED AS INFORMATION CARRIER Y. Jourlin (1) , A.V. Tishchenko (1) , C. Pedri (1) , S. Reynaud (1) , S. Tonchev (2) , O. Parriaux (1) , A. Last (3) , J. Mohr (3) , Y. Lacroute (4) (1) Laboratoire H. Curien UMR CNRS 5516, F-42000 Saint-Etienne, jourlin@univ-st-etienne.fr (2) Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria (3) Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, (4) LPUB UMR CNRS 5027, Université de Bourgogne, F-21078 Dijon, France Abstract: A small amplitude sinusoidal undulation of a continuous thin metal film embedded in a homogeneous medium gives rise to an effect of resonant transmission through the excitation of the plasmon modes propagating along the film. This simple topographic feature can be used to encode high density information at the surface of an easily replicable element. © 2008 Microoptics Group (OSJ/JSAP) 1. Introduction It has been known since the early age of waveguide optics that the association of a periodic corrugation with a planar guided wave confers to the grating highly wavelength, angular and polarization selectivity in the neighbourhood of waveguide mode resonance [1]. The best known effect is that of abnormal (or resonant) reflection [2] which has been used in laser mirrors [3]. The surface wave needs not be a true guided mode, it can also be a leaky mode of a dielectric layer in which case anomalously high diffraction efficiency in reflection can be obtained with polarization and adjustable wavelength and angular selectivity [4]. Leaky mode enhanced diffraction is used for instance in CPA compression gratings [5]. 2. The plasmon resonant transmission The question naturally arises whether the surface wave in the grating field neighbourhood can be a TM surface plasmon propagating along an undulated metal surface. It was shown that grating coupling of a surface plasmon can suppress metal reflection if the grating absorption and the radiation losses are suitably balanced [6]. If however, the radiation strength of the grating coupled plasmon can be made larger, high and narrow band resonant reflection does take place under the condition of plasmon excitation as shown by Sychugov et al [7]. It does not by far reach the 100% predicted in a lossless metal because much of the power is lost by the plasmon collective oscillation of electrons. More remarkable effects occur if the undulated metal is a thin film as currently used in plasmon based bio-sensors [8] instead of a semi-infinite substrate. Two plasmon modes propagate with field maxima at the film surfaces. If the metal film is embedded in a medium of homogeneous refractive index, the longitudinal electric field of the second order symmetrical plasmon mode is zero in the middle of the metal film, consequently its propagation losses are small; this mode is called LRSP - Long Range Surface Plasmon. The most remarkable effect of such continuous thin undulated film is that the accumulation of the incident field in the LRSP results in a resonant transmission trough the metal wall [9]. This effect must not be confused with resonant transmission in narrow trenches or holes pierced through a metal film [10] which relies upon plasmon propagation down the apertures. The Chandezon method [11] easily models this tunneling effect. Figure 1 shows the TM transmission spectrum expected from a gold film of 30 nm thickness undulated sinusoidally at a spatial frequency of 1886 lines/mm with 70 nm peak to trough amplitude; it also shows the transmission of the TE polarization which remain under 10%. 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 700 750 800 850 900 950 1000 1050 1100 Wavelength (nm) Transmitted power (%) TE Metal TM Lossless gold Fig.1. TE and TM polarisation transmission trough a gold layer of 30 nm versus wavelength The TM transmission curve exhibits interesting features: at the short wavelength side the transmission zero corresponds to the condition of abnormal reflection if the metal would be lossless (dashed curve); the 90% transmission peak is that of the LRSP whereas one discerns a shoulders which is the effect of the short range plasmon mode as evidenced in the lossless spectrum. 3. The experimental results An easy way to demonstrate this tunneling effect, which is also the first step of a replication process, is to print an interferogram in a pre-exposed photoresist layer deposited on a glass substrate, to deposit the prescribed metal layer, and finally to overcoat the latter by a thick layer of photoresist. The transmission spectrum measured under normal incidence is shown in Fig. 2. M C 14th MICROOPTICS CONFERENCE Co-located with ECOC 2008 Brussels - Belgium Diamant Conference and Business Centre ‘ 08 Sept. 26 (Fr.) Sept. 27 (Sat.) Sept. 25 (Thu.)