For centuries, scientists believed that light couldn’t be focussed down smaller than its wavelength, just under a millionth of a metre. Now researchers from the Center of Materials Physics in San Sebastián (CSIC-UPV/EHU) and DIPC, in collaboration with the University of Cambridge, have created the world’s smallest magnifying-glass which focuses light a billion times more tightly, down to the scale of single atoms.
“Our theoretical models suggested that atoms might act as tiny lightning rods that could localize light to the atomic scale”, continues Prof. Javier Aizpurua who led the theoretical effort of this work, at the Center for Materials Physics in San Sebastian, that predicted the confinement and interaction of light on such tiny length scales.
The experimental team in Cambridge used highly conductive gold nanoparticles to make the world’s tiniest optical cavity, so small that only a single molecule can fit within it, opening up new ways to study the interaction of light and matter. The cavity –called a ‘pico-cavity’ by the researchers – consists of a bump in a gold nanostructure the size of a single atom, and confines light to less than a billionth of a metre. The results are reported in the journal Science.
In the same way as a hand plucks the strings of a guitar, the energy of light can activate the vibrations of a particular bond in a molecule. This phenomenon is called optomechanical interaction, and in this work, the researchers have achieved that light localized at the picocavity can “pluck” the vibrations of a nearby molecule. This can be understood as the tiniest guitar in the world, a “molecular guitar” activated by light. This molecular optomechanical interaction can be used to switch optical signal, i.e. to “play” particular notes in the molecular “guitar”: certain light plays some notes, and others are not capable to activate them, thus allowing for switching the molecular signal with light at the tiniest scale: the atomic scale.
Building nanostructures with single atom control is extremely challenging, and it required cooling the samples to -260°C in order to freeze the scurrying gold atoms. The researchers shone laser light on the sample to build the pico-cavities, allowing them to watch single atom movement in real time.
Single gold atoms behave just like tiny metallic basketballs that trap light thanks to the behaviour of their electrons roaming around. They used this to blend light together with mechanical springs based on single vibrating bonds, which allows molecular motion to be used for tiny switches. This has the potential to open a whole new field of light-catalysed chemical reactions, allowing complex molecules to be built from smaller components, as well as make new opto-mechanical devices.