Volume 31 Issue 8 - April 27, 2018 PDF
Waveguide plasmon resonance of arrayed metallic nanostructures patterned on a soft substrate by direct contact printing lithography
Wei-Xiang Su, Chun-Ying Wu, and Yung-Chun Lee*
Department of Mechanical Engineering, National Cheng-Kung University
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【106 MOST Outstanding Research Award】Special Issue

Here we present a direct contact printing method to obtain arrayed metallic nanostructures on a soft polymer substrate. It utilizes a polydimethylsiloxane (PDMS) mold replicated from silicon molds to transfer metallic nano-patterns to a polymer substrate based on difference in interfacial bonding energy. Figure 1 shows schematically the fabrication processes, which starts with (a) replicating a PDMS mold for a silicon mold, (b) evaporating a thin metal layer on PDMS mold surface, (c) and (d) contact transferring the metal patterns to a PTE film, and finally (e) spin-coating an over layer on top of the patterned metal nano-structures.
Fig. 1. Schematic diagrams of direct contact printing lithography for preparing arrayed metallic nanostructures with a top coating layer.

Experimentally, arrayed metallic nano-disks with a disk diameter down to 180 nm and a center-to-center pitch around 400 nm are patterned on a PET substrate. The patterned metallic nanostructures are then spin-coated with a polymer layer, which mechanically secures the patterned nanostructures and optically allows waveguide plasmon resonance being excited by incident EM waves. Both experimental works and theoretical modeling are given to illustrate the behaviors of different types of plasmon resonance. As shown in Fig. 2, both localized surface plasmon resonance (LSPR) and wave guided plasmon resonance are observed. Figure 3(a) and 3(b) show the experimental and simulated optical transmittance of various samples with different metallic nano-structures. Good agreement is observed and strong resonance is induced at different wavelength or frequeny.
Fig. 2. Simulated electric field intensity for (a) LSPR mode and (b) wave-guided mode.

Fig. 3. (a) Experimental and (b) simulated optical transmittance of metallic nano-disk arrays on PET substrates with a center-to-center pitch of 400 nm and three different disk diameters of 180, 250, and 300 nm.

To conclude, we demonstrate a rapid, large-area, easily implemented, and low-cost fabrication method to obtain arrayed metallic nanostructures on soft polymer substrates. The smallest feature size and the largest patterning area size that can be achieved by this method are mostly determined by the silicon mold prepared from standard photolithograph method. In this work, the silicon mold is an eight-inch wafer with the capability of 130 nm in linewidth. Both experimental works and theoretical modeling are given in this paper to illustrate the behaviors of different types of plasmon resonance. It is conceivable that such a soft and polymer-based metallic nano-arrays and their tunable optical characteristics can find many applications in soft and wearable optoelectronics devices in the future.

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