Artistic representation of two gold nanorods with a strongly localized optical field in an atomic-scale air gap.
Graphics Thorsten Feichtner
"Unfortunately, the spatial concentration of light in free space has its natural limits set by diffraction effects," says Professor Bert Hecht. "The spatial resolution in microscopy and the storage density of optical media are limited by diffraction if only conventional components, such as lenses or mirrors, are used." Therefore, the physicist and his study group at the Department of Experimental Physics 5 have long been seeking new ways of confining light to the smallest possible dimensions. They have now achieved a breakthrough, working together with colleagues from engineering physics.Brave the (atomic) gap
As reported in the current online edition of the prestigious journal "NanoLetters", the study group of Bert Hecht managed for the first time to localize such surface-bound fields accurately in an experiment in the extremely small gap between two adjacent plasmonic gold nanostructures. The relevant gap has the smallest possible width, which corresponds approximately to the distance of two atoms in a gold crystal. This corresponds to a light spot that is a thousand times smaller than the respective wavelength of the light.
The nanostructures required for this experiment were created by the physicists in a remarkably simple process. The scientists used chemically grown gold rods, each of them only about 30 nanometers in diameter and about 70 nanometers in length – a nanometer is equal to one millionth of a millimeter. These rods were dissolved in water, droplets of which were then applied to a glass carrier. Due to an effect that is also present in the formation of coffee rings, pairs of laterally aligned gold nanorods assemble themselves automatically at the edge of the droplet, moving closer to each other during the evaporation of the liquid until no more than an atomic-scale gap is left.
In their experiments, the Würzburg researchers irradiated these rod pairs with white light and examined the colors of the scattered light. From the characteristic spectral position of the color components in the scattered light, the researchers were able to infer the resonant oscillation states of the electrons, thus deducing a concentration of the light in the gap between the gold rods.
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