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Scanning electron microscope image of nanogrooves etched in silicon. Design and ... allows one to reuse the etched silicon molds and fabricate multiple ...
Design and fabrication of plasmonic nanostructures for spectroscopic applications Aleksandr Kravchenko, Roman Khakimov, Andriy Shevchenko, Arri Priimagi, Matti Kaivola Department of Applied Physics Aalto University P.O. Box 13500, FI-00076 AALTO, Finland [email protected] Abstract— This work is focused on the design and fabrication of plasmonic nanostructures for spectroscopic applications. Our goal is to fabricate large-surface-area plasmonic nanostructures that provide local enhancement of electromagnetic fields for applications in fluorescence and Raman scattering spectroscopy. Periodic two-dimensional arrays of nanogrooves and nanopillars are obtained by holographic photolithography that makes use of surface relief gratings on azo-polymer films. Numerical calculations are performed to optimize the geometrical and material parameters of the structures in order to maximize the field enhancement factor.

I.

INTRODUCTION

It is a well known fact that metal nanostructures exhibiting surface plasmon resonances can be used to enhance electromagnetic fields for spectroscopic applications. Surface enhanced Raman scattering (SERS) spectroscopy is one of these applications [1-3]. While the theoretical principles underlying SERS are still a matter of debate, for metals SERS is primarily the result of excitation of localized surface plasmons and it strongly depends on the nanostructures involved. In this work we present our results on developing periodic two-dimensional (2D) arrays of nanogrooves and nanopillars for SERS spectroscopy.

Viktor Ovchinnikov Micronova Nanofabrication Centre Aalto University Micronova, P.O. Box 13500, FI-00076 AALTO, Finland

calculated from the field distribution on the surface of the structure [4] by using the commercial software COMSOL Multiphysics. For the optimized nanogroove geometries (Fig. 1) the intensity enhancement factor can reach a level on the order of 40. III.

TECHNOLOGICAL REALIZATION

The nanogroove arrays (Fig. 2) are fabricated by etching a silicon wafer through an azobenzene-containing polymer (azopolymer) mask and a stack of intermediate layers. The mask is prepared by inscribing a periodic pattern on the azo-polymer film using holographic photolithography [5]. This allows obtaining nanoscale features under large-surface-area exposure. Since the azo-polymer film is sensitive rather to the light polarization than to the intensity, periodic patterns can be inscribed in it at regular laboratory lightning [6]. Two subsequent exposures at right angle can be used to obtain a periodic nanopillar array (Fig. 3). The etched silicon substrate is then coated with gold or silver. Alternatively, the etched silicon patterns can be oxidized and covered with metal. Then the metal can be separated from the mold and used as a nanostructured plasmonic active material. The latter approach allows one to reuse the etched silicon molds and fabricate multiple identical SERS substrates. IV.

II.

NUMERICAL OPTIMIZATION OF STRUCTURE GEOMETRIES

We consider periodic 2D arrays of nanogrooves made of a dielectric material covered with silver or gold. The geometrical parameters of the nanogroove arrays such as the groove period, depth and width, and the thickness of the coating are numerically optimized and the field enhancement factor is

Figure 1. Electric field amplitude for an optimized metallic nanogroove geometry at 532 nm wavelength. The incident field amplitude is 1 V/m.

SUMMARY

We present our results on the optimization and fabrication of periodic nanogroove and nanopillar arrays. The dimensions of the fabricated nanogrooves in silicon are as follows: period –

Figure 2. Scanning electron microscope image of nanogrooves etched in silicon.

978-1-4244-8227-6/10/$26.00 ©2010 IEEE

[1]

[2] [3]

[4]

[5]

[6] Figure 3. Scanning electron microscope image of azo-polymer nanopillars.

380 nm, width – 200 nm, depth – 300 nm. Azopolymer nanopillars used as an etching mask have diameters of 200 nm and a period of 500 nm. We believe that this approach will allow creating large-area nanostructured plasmonic devices using a fast and inexpensive fabrication technique.

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