This paper investigates the electromagnetic transmission through the sub-wavelength slot model on a metal film for TM- and TE-polarized light. The influence of several parameters such as the slot width, the metal film thickness and the polarization of the incident field is investigated using FDTD method. The FDTD simulation’s results have shown that the sub-wavelength slot in the metal films has extraordinary optical transmission (EOT) properties for TM-polarized light. The EOT has been observed as being symmetrically punctured and characterized by the appearance of a series of transmission peaks and dips in the transmission field. By varying the slot thickness we have investigated the effect of the Fabry-Pérot like resonance in the sub wavelength slot. This component can be a key element in many applications. High-spatial-resolution imaging and information and communications technologies and sensing with high spectral and spatial precision, enhanced solar cells, efficient optical sources and detectors, disease treatment, are such examples.
Unluckily, the fundamental laws of diffraction limit the size of photonics, while the interconnect RC delay-time issues limit the speed of semiconductor electronics. Over the last two decades, it has gradually become clear that the plasmonic devices can play in the future technologies to complement conventional photonics and electronics. Plasmonics offers exactly what electronics and photonics do not have, i.e. the size of electronics and the speed of photonics [
Plasmonics technologies have grown rapidly over the last two decades due to the exciting physical properties of Surface Plasmon Polaritons (SPPs). SPPs are quantized charge density oscillations occurring at the interface between the free electron gas of a metal and a dielectric material. The nature of these quasi-particles is that of an electromagnetic wave trapped at the surface between such two media. Motions of the charge density drive the electromagnetic wave, which can propagate for relatively large distances across the surface of the interface (in the order of micrometers, or even millimeters relying upon the materials and the frequency of operation). Even though surface-enhanced Raman spectroscopy (SERS), being the first applications of surface Plasmon metallic nanostructures, was discovered in the 1970s [
In the next sections we will study the conditions of propagation inside an infinitely long metallic slot. A schematic of the system is represented in
є ( ω ) = є ∞ − ω p 2 / ( ω 2 + I ^ ω γ ) (1)
where ω is the angular frequency of the incident light, є ∞ the dielectric constant at an infinite angular frequency and equals 3.7, ω p the bulk plasma frequency and equals 1.3709 ∗ 10 16 rad / s , and γ the electron collision frequency equals 4.059 ∗ 10 13 rad / s [
In this section, we investigate the nano slot properties of the nano slot waveguide in a two dimensional (2D) plasmonic waveguide with Au-air-Au interfaces using a finite-difference-time-domain (FDTD) method [
This system is made up of a 2D box (along the x & y-axes) with propagation along the y-axis. UPML boundary conditions are applied at both boundaries in y and x of the box in order to eliminate the reflection of outgoing waves [
The wavelength of the incident light, λo = 830 nm, which lies in the near-infrared range.
After the presentation of our analysis for the propagation characteristics of the electromagnetic waves throw the nano slots. The structure presented in
The structure display in
The electric field at the top metal surface is approximately zero (see the |Ez| field distribution in
As seen in
When W < 0.5 λo, very little light transmits through the sub-wavelength slot and the incident EM wave is reflected back. These simulations for TE-polarized incident light revealed the existence of a cut-off at around W ≈ 0.5λo for TE-polarized waves through the sub-wavelength slot. In contrast, as shown in
In
The simulations described in this section show the demonstration of a cut-off for TM-polarized incident light with extraordinary light transmission, even for an aperture as narrow as W = 0.01λo. Transmission is seen to be the result of strong electric and magnetic fields that propagate along the sub-wavelength slot walls,
being supported by the appropriates distribution of surface charges on these walls.
As for TE illumination, very little Ex is needed on the top metal surface to sustain the Jx surface current which supports the magnetic field Hz immediately above the surface. The reflected Ex and Hz interfere with the incident fields to produce standing waves above the top surface. The surface current stops at the edges of the slot, giving rise to accumulated charges at the slot corners (see |Ey| field distribution in
For the sub-wavelength slot with thickness of t = 500 nm, only one strong dipole is observed at the bottom of the slot (see |Ey| distribution field in
The charges that produce the dipole at the top of the slot have diminished, and the transmission efficiency is substantially reduced compared with the 300 nm film thickness. It appears that the distractive interference between two counter-propagating beams within the slot is responsible for the reduced transmission efficiency in this case [
nm strong electric dipoles once again at the sharp edges of the slot (see |Ey| and |Ex| fields allocation in
As shown in the |Ex| and |Ey| fields distribution in (
In
In
The normalized transmitted power throughput from the sub-wavelength slot oscillates periodically as the Au thickness increases shows series of Fabry-Pérot like resonance. The transmission is maximal due to constructive interference when thickness t gives a length phase of even integer of π/2 and is minimal when thickness t corresponds to a length phase of odd integer of π/2. As seen from
enhanced through the SPP waves which contribute to the final transmission power.
In
waves at wavelength 800 nm will not transmit, while it will transmit for width 500 nm.
The transmission through the sub-wavelength slot model on a metal film, namely gold, for TM- and TE-polarized light has been investigated in this paper. The FDTD simulation’s results have shown that the sub-wavelength slot in the metal films has extraordinary optical transmission (EOT) properties for TM-polarized light. The effect of different parameters has been studied such as width, thickness, and polarization of the incident wave. In these interpretations, we considered the slot as a waveguide. Each waveguide has a dominant cutoff wavelength. The EOT has been observed as being symmetrically perforated and characterized by the appearance of a series of transmission peaks and dips in the transmission field. By varying the slot thickness we have investigated the effect of the Fabry-Pérot like resonance in the sub wavelength slot. This component can be a key element in many applications such as high-spatial-resolution imaging and information and communications technologies and sensing with high spectral and spatial precision, enhanced solar cells, efficient optical sources and detectors, disease treatment which are such examples.
The authors declare no conflicts of interest regarding the publication of this paper.
Fouad, M.S., Nady, M. and Shaalan, A.A. (2019) Extraordinary Optical Transmission through Single Sub- Wavelength Slot Nano Antennas. Optics and Photonics Journal, 9, 112-125. https://doi.org/10.4236/opj.2019.97011