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Metallic Nanostructures: Findings

In this seed project, theoretical simulations were performed to investigate the extraordinary optical transmission (EOT) in the visible and infrared regions. Finite-difference time-domain (FDTD) method with software MEEP and finite element method (FEM) with software COMSOL are applied. The goal is to find out the ideal parameters for the desired transmission spectrum of the nanostructures.

The calculated are coefficients of reflection and transmission as a function of the diameters of the hole, the periodicities of the hole array, the thicknesses of the metal film, and incident angles and polarizations for the light of different wavelengths.

These simulations show that: 1) the transmittance spectrums are dependent on the period of the structure (see Figure 1). The peaks of the transmittance spectrum are raised from 4μm and 5μm for a radius of 1μm circular open on a 200nm metal film with 4μm*4μm and 5μm*5μm, respectively; 2) the bandwidth of the transmittance peak are dependent on the ratio of the open area to the unit area. In order to investigate the property of the transmittance spectrum as the function of the unit cell’s periodicity, we did the simulation for a metal film structure with a square open, in this simulation, we keep the ratio of the width of the square open to the periodicity as a constant of 1/3. The simulation results are show in Figure2. The simulations of more complicated structure of annular circular open shown in Figure3 have the similar transmittance properties. These simulation results match very well with the research literature’s reported.


Figure1. Transmittance spectrum of a circular open on a 200nm thick metal film., Trace1 (red) is for the period of 4μm*4μm; and Trace2 (dark blue) is for the period of 5μm*5μm.


Figure 2. Transmittance spectrum of a square open on a 200nm thick metal film, Trace1 (blue) is for the open of 1μm*1μm square with period of 3μm; and Trace2 (green) is for open of 1.5μm*1.5μm with period of 4.5μm; and trace3 (dark red) is for open of 2μm* 2μm with period of 6μm.

The metallic nanostructures’ fabrication procedure can be briefly described as:

  • photoresist processing: clean a 2” silicon wafer, apply PMMA on it with the spin coater;
  • electron beam lithography (EBL) patterning: use CAD software to create the desired nanostructure, setting up working parameters: current, exposure time, etc.
  • pattern develop processing: following the EBL exposure, develop in the MIBK:IPA solution;
  • electron beam metal evaporator processing: put the patterned the wafer in the e-beam evaporator to deposit the thin film of metal (gold);
  • metal lift-off: immerse the sample in the Acetone to lift off.
  • Figure 3 shows the nanostructures of our preliminary fabrication results. Optical characterization will be carried by Monochromator or FTIR.