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Posted: May 29, 2012
Classical and quantum physics hand in hand with optical nano-antennas
(Nanowerk News) Researchers at the Physics of Materials Centre (CSIC-UPV/University of the Basque Country) and the Donostia International Physics Center (DIPC), together with others from the Orsay Institute of Molecular Sciences (France) and from the Nanophotonics Laboratory at Houston in the United States, have developed a new theoretical framework which includes quantum effects to describe the sub-nanometric properties of optical nano-antennas. The new model resolves this operating range wherein quantum effects are noted and descriptions based on classical physics equations do not work. The research was recently published in the Nature Communications journal ("Bridging quantum and classical plasmonics with a quantum-corrected model").
Metallic nanoparticles act as optical antennas, enhancing the reception, control and emission of optical radiation on a nanoscale. The antenna effect is achieved by the collective excitation of the free electrons in the metal, giving rise to oscillating plasma, commonly known as surface plasmons. To date the description of nanoantennas has only been within the framework of classical electromagnetics, as established by the equations of Maxwell more than one century ago.
With the advance of technology, the sizes and separation distances between interacting metallic nanoparticles have decreased, thus acting as nanoantennas. This progressive reduction in nanoanntena distances gives rise to new properties that have hitherto been impossible to describe with classical physics, such as electron tunneling, based on the probability that such electrons can cross the vacuum between the nanoparticles, jumping from one to another. Electron tunneling at a macroscopic scale would be equivalent to an object being capable of passing through a wall.
Mechanical calculation equations would be capable of describing these phenomena, but millions or billions of electrons involved in real nanoantennas – those uses in experiments – make these calculations practically unfeasible.
The lead researcher of this work at the Physics of Materials Centre (CFM) and the DIPC, Javier Aizpurua, stated that "to date, for real systems, these properties could only be described in a very approximate manner, when the interaction distances started to have subnanometric values". The model put forward by Mr Aizpurua's team neatly takes on "the enormous quantity of electrons involved in the optical response of a nanostructure and the quantum effects that appear at subnanometric distances", he added. To this end they incorporated a quantum correction within the framework of a classical description.
According to the CFM researcher, "the model opens up a new path for the calculation and design of the quantum effects in classical optical antennas and in optoelectronic devices". Likewise, "it will enable tackling problems in a variety of disciplines such as biochemistry, molecular electronics and optical communications, where all these quantum effects are of great importance", concluded Mr Aizpurua.