We will calculate the transmission and reflection spectrum from an array of nanoholes in a metallic film using DGTD solver. We will also visualize the near field profiles at the surface of the film and the local field enhancements. In addition, the application of this array as a simple bio-sensor will be demonstrated. It is expected that when the refractive index of an analyte which surrounds the nanohole array changes, the resonance peaks in reflection and transmission of the array will be shifted in wavelength.
Open nanoarray_dgtd.ldev which contains the mentioned nanohole array. Alternatively, the script make_nanoarray.lsf will generate a project with the nanohole array set up in the simulation file. Holes are radius 100nm and pitch 400nm in a layer of gold 100nm thick, using the "Au (Gold) -CRC" model included in the default material database. For biosensor application, the holes and region above are assumed filled with analyte, which is modeled as a dielectric. The project includes a parameter sweep for the refractive index of the analyte, recording transmission and reflection spectra. Initially, assuming that no analyte is present, the analyte refractive index is set to 1.0 (representing air) and the initial simulation runs in only a few seconds.
After running the simulation for default analyte refractive index of 1.0, run plot_nanoarray.lsf to generate the figures shown below. We can see the strongest resonance is at approximately 700nm.
Transmission and reflection vs wavelength
Near field plots
Following images show the E-field intensity |E|^2, 10nm from the surface of the gold layer on the transmitted and reflected side. There is significant local near field enhancement because the incident field intensity is 1 V/m. For aesthetic purposes, these plots were generated after applying mesh constraints to new 2d surfaces at z=+/-60nm. This is not recommended for typical use, since the resulting finite element mesh is much finer and forces a short time step without affecting the transmission or reflection profiles.
The cross section in the x-z plane shown below has the colorbar adjusted to a range of 1 to 10. This makes it possible to easily see the region where the near field intensity enhancement is at least 10.
Run the sweep "analyte_index_sweep" which sweeps the refractive index of the analyte material from 1 to 1.5. The entire sweep may take a few minutes to complete. After completing the sweep, run plot_nanoholes_sweep.lsf to generate lower resolution versions of the figures shown below.
The shift in the position of reflection, transmission and absorption spectra resonant peaks when the analyte refractive index is changing is obvious from the figures above. This implies that the nanohole array can be used to identify various analytes as an effective bio-sensor.