Periodic structures are essential for designing antennas, filters, and other wave-generating and controlling equipment for various frequency regimes, from microwaves to optics. Frequency-selective surfaces, which are currently being actively researched under considerably more sophisticated metasurfaces, first arose in the microwave research community in the past century as a distributed version of passband or bandstop electrical devices. These structures use the interplay of resonating components (such as patches or apertures) to shape wave propagation and radiation, resulting in integrated lensing/radiating devices and metasurface antennas.
Extraordinary Optical Transmission (EOT)
A new era of optical uses for periodic surfaces was disclosed with the discovery of extraordinary optical transmission (EOT). When a periodic arrangement of tiny apertures was appropriately tuned, it permitted an extremely narrow transmission band.
A particular type of resonance, known as extraordinary resonance, was demonstrated at frequencies well below such natural resonance when the periodicity of the array was greater than twice the electrical length of the aperture. This resonance is in addition to the commonly used natural resonance of apertures when their electrical length corresponds to half of the operating wavelength.
In other words, given an array with constant periodicity, the resonance frequency rises (as expected for l/2 resonances) as the electrical length of the aperture lowers until it hits the beginning of the first set of grating lobes.
Due to the complexity of their modeling, aperiodic arrays (those with variable or even random separations between scatterers) have received much less attention than periodic exceptional transmission devices. Only smooth fluctuations of geometrical parameters are employed to restrict the consequences of aperiodicity in practical applications since local periodicity is expected to exist. However, the topological photonics and acoustics groups have become very interested in aperiodic structures because of their potential for a new edge and bandgap modes.
Extraordinary Transmission in Disorganized Slot Arrays
This study focuses on extraordinary transmission in disorganized slot arrays in perfectly conducting screens. Researchers looked at the initial row-by-row, one-dimensionally periodic slot array and introduced several sorts of disorder, some relating to the layout of the slots and others to the scatterers of the slots. Although they all violate the structure's strict periodicity, their effects are very different from one another.
The researchers used the commercially available Ansys HFSS software to simulate two slots of varied sizes and orientations that make up a square unit cell to test the numerical approach used for the generation of the entries of the matrix of the Method of Moments.
Using a computational technique, they examined the transmission spectra of two different slot apertures. They discovered that five basis functions are enough to mimic the output of the commercial program. They discovered a quick convergence in the computation of the MoM matrix's inter-slot coupling components with errors under 5% and just seven quadrature points per integral, resulting in 2401 evaluations.
It was crucial to minimize the number of assessments needed since the CPU time required for these valuations grew to the fourth power due to the nested structure of these valuations. The results reveal that the MoM implementation shown here beats the commercial software in terms of CPU time usage by a factor of 200, proving the creation of this technology to be worthwhile.
Significant Findings of the Study
The effects of adding disorder to extraordinary transmission across a limited chain of slots carved out of a perfectly conducting screen are examined in this research. The researchers discovered that a finite chain congeals to a maximum transmission for only 50 slots, whereas 25 already provide a nearly complete transmission.
Random individual rotation of the slots reduces the transmission of the Wood's anomaly and the low-frequency tail while allowing for a controlled lowering of the peak transmission level when the disorder is introduced.
It is possible to preserve the low frequency and Wood's anomaly levels while lowering the peak transmission value and expanding the relative bandwidth by randomly changing the slot lengths. The best way to recreate the transmission characteristics of a single isolated slot is to introduce random displacements along the x-axis.
The addition of random displacements along the array's perpendicular direction also permits the introduction of a second, higher frequency peak, whose level rises with the standard deviation of the displacement distribution. In contrast, the extraordinary optical transmission peak's level falls and the levels of the low frequency and Wood's anomaly remain constant.
In future, these findings could open the door for optical communications and extraordinary optical transmission filtering equipment.
Miguel Camacho , Armando Fernández-Prieto, Rafael R. Boix and Francisco Medina (2022) Disorder Effects in One-Dimensionally Periodic Extraordinary Transmission Structures. Electronics. https://www.mdpi.com/2079-9292/11/18/2830/htm