Monday, 15 May, 2017

Saving energy with a tiny spot of silver

Nanophysics

 

In the future, computers are expected to run on light particles instead of electrons. To that end, researchers are testing the use of gold nanoparticle chains as light conductors. LMU scientists now demonstrate how a tiny spot of silver could save enormous amounts of energy in light computation.

Today’s computers are faster and smaller than ever before. The latest generation of transistors will have structural features of only ten nanometers. If computers are to become even faster and at the same time more energy efficient in these minuscule dimensions, they will probably soon be processing information using light particles instead of electrons. This is referred to as “optical computing”.

Fiber-optic networks already use light to transport data over long distances at high speed and with minimum loss. The diameters of the thinnest cables, however, are in the micrometer range as the light waves with a wavelength of around one micrometer must be able to oscillate unhindered. In order to process data on a micro- or even nanochip, an entirely new system is therefore required. 

One possibility would be to conduct light signals via so-called plasmon oscillations. This involves a light particle (photon) exciting the electron cloud of a gold nanoparticle so that it starts oscillating. These waves will then travel across a chain of nanoparticles at approx. 10 percent of the speed of light. This meets two goals: nanometer-scale dimension and enormous speed. What remains, however, is the energy consumption. In a chain composed purely of gold it would be almost as high as in conventional transistors, due to the considerable heat development in the gold particles.

A tiny spot of silver

CeNS scientist Professor Tim Liedl (LMU Munich), together with colleagues from Ohio University, now published an article in the journal Nature Physics, describing how silver nanoparticles can significantly reduce the energy consumption. The physicists built a sort of miniature test track with a length of around 100 nanometers, composed of three nanoparticles: one gold nanoparticle at the beginning and one at the end, and a silver nanoparticle right in the middle.

The silver serves as a kind of intermediary between the gold particles while not dissipating energy. To make the silver particle’s plasmon oscillate, more excitation energy would be required than for gold.  Therefore, the energy just flows “around” the particle. “Transport is mediated via the coupling of the electromagnetic fields around the so-called hot spots which are created between each of the two gold particles and the silver particle,” explains Tim Liedl. “This allows the energy to be transported with almost no loss, and that at a femtosecond time scale.” 

Textbook quantum model

The decisive precondition for the experiments was the fact that Tim Liedl and his colleagues are experts in placing nanostructures to the point. This is done by using the DNA origami method, which allows different crystalline nanoparticles to be placed in a precisely defined nano-distance next to each other. Previous similar experiments had been conducted using conventional lithography techniques, which do not provide the required spacial precision to place in particular different types of metals next to each other.

Parallel to the experiments, the physicists simulated the test on the computer – and had their results confirmed. Apart from classical electrodynamic simulations, Professor Govorov was also able to establish a simple quantum-mechanical model: “In this model, the classical and the quantum-mechanical pictures match very well, which qualifies it as a potential example for the textbooks.” (NIM/LMU)

Publication (Link):

Hotspot-mediated non-dissipative and ultrafast plasmon passage. Eva-Maria Roller, Lucas V. Besteiro, Claudia Pupp, Larousse Khosravi Khorashad, Alexander O. Govorov, Tim Liedl. Nature Physics 2017 (Published online 15 May 2017), doi:10.1038/nphys4120

Pressemitteilung NIM/LMU (German)