When Optics goes Atomic
The smallest lens in the world, capable to concentrate light down to dimensions of an atom, has been created.
We have used gold nanoparticles as focusing lenses that allow to flex individual chemical bonds in molecules.
Artistic composition that shows the effect of light focusing from the daily dimensions of a bulb, down to the atomic dimensions where, once localized, it can flex the bond of a single molecule nearby. (Credit: Univ. Cambridge/Bart de Nijs)
For centuries, scientists believed that light couldn’t be focussed down smaller than its wavelength, just under a millionth of a metre. Now our group, 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.
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, we 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. When the gold nanoparticles are illuminated with laser light, a few isolated atoms move around forming the picocavity. In that very same instant, the light focused in this picocavity activates the vibration of the molecule, a process which is monitored in real time.
Single gold atoms behave just like tiny metallic baskets that trap light, and potentially open new perspectives for controlling light-catalysed chemical reactions at the nanoscale, allowing complex molecules to be built from smaller components for instance, as well as make new opto-mechanical devices.
Calculation of the light localized around a picocavity formed by several atoms in a metallic nanoparticle, calculated within a quantum framework based on Time-dependent Density Functional Theory (TDDFT). The chemical bond of a molecule nearby is excited by the light.
This research is funded in part by the excellence scientific and technological research program of the Spanish Ministry of Economy and Competitiveness (MINECO, FIS2013-41184-P), the UK Engineering and Physical Sciences Research Council (EPSRC) investment in the Cambridge NanoPhotonics Centre, as well as the European Research Council (ERC), and the Winton Programme for the Physics of Sustainability.
Single-molecule optomechanics in ‘pico-cavities’, Felix Benz, Mikolaj K. Schmidt, Alexander Dreismann, Rohit Chikkaraddy, Yao Zhang, Angela Demetriadou, Cloudy Carnegie, Hamid Ohadi, Bart de Nijs, Ruben Esteban, Javier Aizpurua, Jeremy J. Baumberg. Science 354, 725-728 (2016).