Take a very thin sheet of metal and drill it with tiny holes in a regular rectangular pattern. Ordinarily, if you shine light with wavelength that is larger than the holes, it wouldn't get through—the metal would be opaque. However, in the case of this particular pattern of holes, a lot of the light gets through the sheet anyway, a phenomenon known as extraordinary optical tranmission (EOT). Since the discovery of EOT, the effect has been harnessed in a number of optical and biophysical devices. A full theoretical understanding of the phenomenon proved elusive, however, which could hamper further device development.
A systematic exploration of hole spacing may help elucidate the mechanism behind EOT. Frerik van Beijnum and colleagues demonstrated that electrons on the metal's surface have two properties that contribute to EOT, with different strengths depending on the hole density and configuration. These results enabled the researchers to determine the physical parameters that dictate EOT, potentially leading to new device designs.
Ordinarily, light can pass through an opaque barrier only if the barrier is pierced with openings larger than the light's wavelength. (This also applies to all manner of waves, including sound and water waves.) That's why EOT is fascinating: the holes are smaller than the wavelength, yet a substantial amount of light still gets through something that would ordinarily be opaque. Oddly, making the material thinner—and therefore more transparent—decreases the EOT effect.
However, prior experiments showed that surface plasmon polaritons by themselves cannot explain EOT. Researchers proposed quasi-cylindrical waves (QCW) as a secondary mechanism; these are waves in the electrons excited by the light striking the metal, centered at the holes. These waves also occur in non-metals, so they cannot be the sole driver of EOT either—but in combination with surface plasmon polaritons, they could successfully describe the entire anomalous transmission.
To test the effect of positioning of holes on EOT, the researchers created seven configurations laid out in a rectangular pattern. Each configuration kept the vertical spacing of the holes fixed but gradually increased the separation between the holes in the horizonal direction. To make the holes, the researchers started with a piece of glass with regularly spaced protrusions and overlaid it with thin gold and chrome foils. When they etched the protrusions in the glass away, it left evenly distributed perforations in the metal layers. For each of these configurations, they used the same wavelength of infrared light, known to produce EOT in these hole sizes.
As the new study showed, the positioning of the holes is essential for both the polariton waves and the quasi-cylindrical waves. The holes seem to provide a kind of artificial crystal lattice for the plasmons, and the farther apart the holes are placed, the smaller the quasi-cylindrical wave effect got. This is because cylindrical waves decrease in amplitude over distance, much as ripples in a pond get smaller the farther from the place where the rock fell in. They determined that, for widely spaced holes, EOT was dominated by surface plasmon polaritons, but for the more closely packed configuration, the QCW effects became significant.
Because they kept the vertical spacing of the holes constant, the researchers were able to separate the contributions from both factors in a clear way—a significant measurement in its own right. Understanding the phenomena driving EOT could help design of future devices and refinement of existing technology, which can have applications in diode lasers and photovoltaic solar cells.
Nature, 2012. DOI: 10.1038/nature11669 (About DOIs).
Note:
A plasmon is a quasiparticle, a particle-like entity, created by the collective activity of electrons inside a plasma or metal. (Plasmas are materials, usually gases, in which electrons separate from their atoms, creating an electrically neutral substance. Metals are effectively solid plasmas.) Polaritons are another type of particle-like excitation, this time arising from the interaction between electrons and light. Thus, a surface plasmon polariton is a quasiparticle generated when light strikes a metal surface; it acts much like a particle, being indivisible and having specific properties like electric charge, spin, and so forth.
The type of material makes a difference, however. So far, EOT is a phenomenon confined to metals, materials in which the outer layer of electrons in the atoms forms a more-or-less free-flowing fluid. That property explains metals' excellent electrical conductivity. Electron mobility also contributes to surface plasmon polaritons, in which the light directly excites the electrons into motion, creating waves that cross the surface of the metal. These waves are what transfer the light involved in EOT across the material—the light itself never passes through the holes.
