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Optical technique sorts nanodiamonds by their quantum properties

Nanotechnology Briefing 2021

Physics World

 
Nanodiamonds Physics World  March 2021

Optical technique sorts nanodiamonds by their quantum properties

Dazzle An optical technique has been developed to sort diamonds with NV centres. (Courtesy: iStock/Gizmo)

A method of optically selecting and sorting nanoparticles according to their quantum mechanical properties has been developed by researchers in Japan. The method could prove a crucial tool for manufacturing nanostructures for quantum sensing, biological imaging and quantum information technology (Sci. Adv. 7 eabd9551).

Scientists have several ways of manipulating and positioning tiny objects without touching them. Optical tweezers, for example, use a highly focused laser beam to generate optical forces that hold and move objects in the beam’s trajectory. However, such tweezers struggle to grasp nanoparticles because these tiny objects are much smaller than the wavelength of the laser light used.

Now, a team led by Hajime Ishihara of Osaka University and Keiji Sasaki at Hokkaido University has developed a way of using light to sort nanodiamonds. These are tiny pieces of semiconductor with very useful optoelectronic properties that derive from bulk diamond as well as certain defects such as nitrogen-vacancy (NV) centres.

When illuminated by a laser, the nanodiamonds scatter light while their NV centres (if they have any) absorb it. The combination of light scattering and absorption transfers momentum from the photons to the nanoparticles, and the different momentum transfers experienced by nanodiamonds with and without NV centres could be used to differentiate them – at least in principle.

“While these two effects [scattering and absorption] produce optical forces that can be used to move the particles on the macroscopic scale, it is difficult to select nanodiamonds that contain NV centres from surrounding pristine nanodiamonds that don’t contain these defects,” Ishihara and Sasaki explain. “This is because the scattering optical force of bulk diamond is much stronger than the optical force coming from light absorbed by NVs.”

The researchers’ solution was to balance out the larger scattering force so that they could distinguish the absorbing force. To do this, they sent two different-coloured laser beams propagating in opposite directions along a nanofibre. An intense evanescent light field forms around this fibre. This field, explain Ishihara and Sasaki, allows light to propagate for long distances while remaining as a tightly focused beam, thus restricting the motion of nanoparticles trapped within it to one dimension.

Within such a waveguide, the momentum of the photons is constant, making the setup ideal for analysing the optical forces exerted on the nanoparticles. By balancing the absorption and scattering forces induced by the two laser beams along the nanofibre, the researchers were able to transport single nanoparticles according to whether NV centres were present.

Isabelle Dumé