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Unexpected properties of nanostructures: When holes obscure the view
11 November 2009
University of Stuttgart
Metals are opaque: they reflect light almost completely. For that reason they are utilized as mirrors; as films deposited onto a glassyou find them in any bathroom. If the metal film is very thin, the mirror is semitransparent. These half-silvered mirrors help to hide surveillance video cameras, for instance. One might think that holes in a metal film enhance the view. Exactly the opposite is true. Physicists at the University Stuttgart discovered that tiny holes make the metal opaque. These findings are reported in the recent issue of Physical Review Letters).
Already ten years ago physicists found a strange phenomenon in this regard: They drilled tiny holes in a thick metal layer, which usually is non-transparent. Since the holes were much smaller than the wavelength, classical optics tells us that the light should not be transmitted but completely reflected. Surprisingly more light passed through the holes than expected. This triggered worldwide research activities that last till today with exciting new finding every year.
In general, materials are not optically transparent and electrically conducting at the same time. The scientists at the 1. Physikalisches Institut of Universität Stuttgart tried to perforate a very thin metal film by tiny holes in such a way that it still conducts electricity and transmits light unimpeded. To their large surprise the opposite effect was observed. Perforating a semi-transparent metal film by a periodic array of tiny holes does not lead to enhanced transmission, but significantly less light passes through the film; although almost half of the film consists of holes. As a matter of fact, the holes obscure the view.
In the present case the physicists at Stuttgart measured the optical transmission of a 20 nanometer gold film, just a few dozen atoms thick. The film was perforated by 200 nm holes arranged in a regular fashion 100 nm apart. These layers are produced by lithographic methods which are standard in semiconductor industry and can be used on a large scale. In a certain spectral range in the infrared and visible, the films exhibit strong absorption. Normally metals do not show these absorption features and they are a direct consequence of the period arrangement of the holes. In mainly depends on the periodicity (300 nm in the present case) and not on the size of the holes. Due this periodic structure collective excitations of the metallic charge are possible, so-called plasmons. The particular feature of the plasmons is that the excitation depends on the angle of the incident light. Turning the film slightly changes the color of the plasmons. This is exactly what was observed in the experiment. There are already ideas how to utilize these unexpected optical properties of the metallic nanostructures in future applications, and there is more to come.