Friedrich-Alexander-Universität Erlangen-Nürnberg

Metallic metamaterials with highly modified transmission

Metallic metamaterials with highly modified transmission

A lot of applications in modern optics would highly benefit from an extension of the spectrum of available materials. The index of refraction and the absorption coefficient of real materials is limited to a narrow band restraining theoretically predicted effects, like microscopy with super resolution by using materials with negative index of refraction.

Optical lithography being the fundamental technique in modern chip fabrication is facing the problem of constantly decreasing lateral structure sizes necessary for the state-of-the-art technology. As a result, the standard masks are constantly harder to fabricate. On the one hand keeping the same high aspect ratios is crucial and therefore thinner masks have to be used. On the other hand increasing the resolution by using smaller imaging wavelengths and therefore higher photon energies leads to a decrease in absorption. Hence, in order to ensure a high enough contrast the blocking areas of the mask cannot be fabricated thinner. Metamaterials with reduced transmission could help to solve this problem. There is a justified hope, that these artificial materials can be generated by nanostructuring of conventional materials, which is a goal of this project.

Metallic films are used for lithographic masks because of their negligible transmission. Ultrathin films below 50 nm show a noticeable transmission and sub-30nm-thickness leads to a nearly transparent film. According to numerical simulations the transmission of the metal film can be suppressed a few orders of magnitude by sub-wavelength patterning. The build quadratic metal islands excited resonantly show an enhanced back-reflection and a total suppression of transmission. 

Fig. 1 (left) Metamaterial structure (substrate: glass, film: silver), (center) SEM image of sample structure. (right) Simulation results for Trel=(Tbulk-T)/Tbulk+T) (T: transmission of metamaterial, Tbulk: transmission metal film), Trel=1 correspond to totally suppressed transmission (thickness of the metal film: 30nm, wavelength 633 nm, fill factor=mesa/pitch)



[1] D. Reibold, F. Shao, A. Erdmann, and U. Peschel, “Extraordinary low transmission effects for ultra-thin patterned metal films,” Optics Express 17, 544–551 (2009).




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