Periodic modulation of extraordinary optical transmission through subwavelength hole arrays using surrounding Bragg mirrors

The enhanced light transmission through an array of subwavelength holes surrounded by Bragg mirrors is studied, showing that the mirrors act to confine the surface plasmons associated with the extraordinary optical transmission effect, forming a surface resonant cavity. The overall effect is increased light transmission intensity by more than a factor of 3 beyond the already enhanced transmission, independent of whether the Bragg mirrors are on the input or the output side of the incident light. The geometry of the Bragg mirror structures controls the enhancement and can even reduce the transmission by half. By varying these geometric parameters, we were able to periodically modulate the transmission of light for specific wavelengths, consistent with the propagation and interference of surface plasmon waves in a resonant cavity. Finite difference time domain simulations and a wave propagation model verify this effect.

Figure 1

A scanning ion beam image of the nanohole array surrounded by narrow Bragg mirror grooves on a 100-nm-thick gold film on a glass substrate. The nanohole array has a periodicity of $635\mathrm{nm}$, and the holes have a diameter of $200\mathrm{nm}$. The periodic nanohole array couples incident light into SPs, and the surrounding Bragg mirrors, with a pitch of $300\mathrm{nm}$ and width of $60\mathrm{nm}$, confine the SPs within the nanohole array, enhancing light transmission. The diagram shows a two-dimensional profile, an important geometric parameter being the cavity width. By adjusting this distance, it is possible to periodically modulate the extraordinary optical transmission through the nanohole array.

Figure 2

Plots of the normalized transmitted spectra for nanohole arrays both with surrounding Bragg mirrors (black lines) and without (gray lines). The plots also include an unflipped sample, where the grooves are directly illuminated, and a flipped sample, where the grooves are not directly illuminated. The 7-by-7 nanohole array without surrounding Bragg mirrors shows two peaks, corresponding to the (1,1) and (1,0) EOT transmission maxima. The transmitted spectra with Bragg mirrors depend strongly on the cavity width as it is varied from 4450 to $5650\mathrm{nm}$. Graphs A, B, and C show the progression through one period of modulation, with A and C corresponding to a transmission maximum at $700\mathrm{nm}$, and B corresponding to a transmission minimum. One period of modulation is completed when the cavity width changes by $635\mathrm{nm}$. This modulation, near the (1,0) EOT peak at $700\mathrm{nm}$, is independent of which side, top or bottom, of the device is illuminated.

Figure 3

(A) Plot of the enhancement factor $F_E$ (transmission with Bragg mirrors normalized to transmission without) of the (1,0) EOT peak versus cavity width. Tracking $F_E$ shows a periodic modulation, with a period of $\approx 635\mathrm{nm}$, as the cavity width is varied from 4450 to $5650\mathrm{nm}$. The data points correspond to individual samples for each $50\mathrm{nm}$ step. The maximum enhancement factor is $F_E$ = 3.0, whereas the minimum corresponds to a reduction in transmission with $F_E$ = 0.5. (B) Plot showing the influence of the number of holes on the resonant cavity effect, with the optimal number showing $F_E$ approximately equal to 4.

Figure 4

(Color online) FDTD simulation results showing the time-averaged intensity of the evanescent surface plasmon (SP) field for three configurations: (A) a bare nanohole array, (B) a maximally resonant SP cavity (cavity $\mathrm{width}=4900\mathrm{nm}$), and (C) a SP cavity with destructive interference (cavity $\mathrm{width}=5200\mathrm{nm}$). All plots are shown with the same intensity scaling for comparison, demonstrating that the SPs are well confined in a lateral resonant cavity across the entire array and either enhanced or suppressed by the surrounding Bragg mirrors. The intensity on the lower output side of the film is the highest for the constructive interference effect, showing increased transmission.

Figure 5

Plot of the enhancement factor $F_E$ of the (1,0) EOT peak versus position of the nanohole array within the cavity. By shifting the nanohole array within the surrounding Bragg mirrors, $F_E$ is again modulated, this time completing one period every $\approx 315\mathrm{nm}$. The scanning ion beam images show two different samples where the nanohole array is shifted from one side of the surrounding Bragg mirrors to the other by $\pm 300\mathrm{nm}$. The maximum enhancement factor seen here is $F_E=3.2$, whereas the minimum is again a reduction in transmission with $F_E=0.75$.