Quantum kisses

Print edition : December 28, 2012

FOR the first time, using optical methods, the quantum regime in the interaction between nano-sized spheres of gold has been observed. This regime can be identified through a change of colour of the gap, or empty space, between the particles when they are at distances of less than one nanometre, according to researchers. This work, published in Nature, enables one to literally see quantum mechanics in action between nanoparticles.

The researchers are from the Donostia International Physics Centre and the Centro de Fsica de Materiales, Donostia-San Sebastin, Spain, in collaboration with researchers from the Universities of Cambridge and Paris-Sud. The work confirms that electrons accumulated on gold surfaces around an illuminated gap between the spheres can jump from one to the other owing to the quantum tunnel effect, thus reducing the accumulated charge on the surface of each of these spheres. This alters the colour of the gap from red to blue (blueshifting).

When two metallic spheres at a sub-nanometre separation are illuminated with white light, the beam of light pushes the electrons and makes them oscillate, which gives the gap a red colour. As the spheres get closer, the electron charge increases and the red colour intensifies. When the distance between them is under 0.35 nanometre, the accumulation of charge can be seen to drop because electrons can jump across the gap by quantum tunnelling, without the spheres coming into contact with each other. The change in colour of the gap is the chromatic fingerprint that identifies the scale of the quantum regime. This result establishes a fundamental quantum limit for the minimum dimensions within which white light can be trapped. Moreover, this reinterpretation of the interaction between light and matter at the sub-nanometric scale could provide new ways of describing and measuring the atomic-scale world and open doors to new strategies for engineering even smaller optoelectric technological devices and access new limits of resolution in photochemistry.

Compiled by R. Ramachandran
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