Transparent gold unravelled
A thin gold layer is almost opaque but with a grating of minute holes such a gold layer allows a surprising amount of light to pass through it. The mechanism underlying this has been debated for almost a decade but now researchers from Leiden University, FOM and FOM Institute AMOLF have unravelled it. Together with colleagues from China and France they published their discovery yesterday in the leading scientific journal Nature.
Making a grating of miniscule holes in an opaque layer of gold dramatically changes its optical properties: the gold is no longer gold-coloured and becomes far more transparent. This is also a fascinating phenomenon for scientists, according to FOM workgroup leader Dr Martin van Exter: “The holes are so small, roughly 500 times thinner than a hair, that you would not expect the gold to become transparent. This implies that the holes in the gold layer 'collaborate ', as a result of which they can transmit more light through the gold. FOM PhD student Frerik van Beijnum and his co-authors have now unravelled exactly how this collaboration works."
The holes in the gold collaborate because the light that falls on a hole is not only allowed through but is also diffracted. A large part of the diffracted light subsequently travels across the gold surface. If this travelling light encounters another hole then it is still transmitted through the gold layer here and consequently the gold becomes transparent. Previous experiments had raised the suspicion that two processes are involved in this phenomenon. Van Exter: "In the first process, light in the form of a 'surface wave', only spreads in the two dimensions of the surface. In the second process, light is not only spread out across the surface but also in the third dimension: a so-called quasi-cylindrical wave. Over the past few years there has been a lot of discussion in this research field as to whether this quasi-cylindrical wave contributes to the gold with holes becoming transparent."
Innovative
Van Beijnum and his colleagues have demonstrated that this quasi-cylindrical wave definitely does contribute to the transparency of the gold layer with holes. The researchers achieved this by varying the hole density of the gratings in an innovative manner, partly based on new theoretical insights. "Surprisingly these experiments had always been carried out on square gratings," says Van Exter. "However a new theory from our Chinese and French colleagues took chains of holes as the starting point. Based on this theory, we only changed the gratings in a single direction. In effect, we only changed the distance between chains of holes and examined the influence of the hole density on the transparency of the gold."
For each hole density, the researchers observed an improved transparency of the gold. This means that the holes can also collaborate at a greater distance. If the density subsequently becomes higher, the transparency increases slowly at first. Yet if the holes are just a single wavelength apart then the transparency suddenly increases dramatically. "This sudden increase revealed that the quasi-cylindrical wave definitely plays a role in this process," says Van Exter.
Even more exciting
The gratings studied are an example of how the structure of metals can be altered at the nanoscale to create new optical properties. Van Exter: "This experiment provides insights that will contribute to the production of new structures in the future with far more exciting properties. Ultimately variations in these gratings could be used to produce more efficient solar cells or sensors that can detect viruses."
Note for editors
Contact
Martin van Exter
+31 (0)71 527 59 27
Reference
Quasi-cylindrical wave contribution in experiments on extraordinary optical transmission, Frerik van Beijnum, Chris Rétif, Chris B. Smiet, Haitao T. Liu, Philippe Lalanne and Martin P. van Exter