Sorting nanodiamonds with fluorescent centers

Scientists have long worked to improve their ability to use lasers to move small objects without touching them. This method of “optical trapping and manipulation” is already used in optics, biological sciences and chemistry. But objects become much more difficult to control once they reach nanoscale.

Now, a team of scientists including Keiji Sasaki from Hokkaido University and Osaka Prefecture University and Hajime Ishihara from Osaka University have found a way to displace approximately 50 diamond nanoparticles. nanometers in diameter, using opposing lasers. Their experiences, published in the journal Scientific progress, aim to deepen research on the development of applications in fields such as biological imaging and quantum computing.

“We believe that our approach may enable a new class of optical force methodologies to study the characteristics of advanced nanomaterials and quantum materials and to develop advanced nanodevices,” says Sasaki.

Nanodiamonds have arrays of carbon atoms that sometimes contain an imperfection in which two neighboring carbon atoms are replaced by a nitrogen atom and a gap (fluorescent center), which affect their quantum mechanical properties; nanoparticles react differently to light depending on their quantum mechanical properties. Nanodiamonds with this fluorescent center (resonant nanodiamonds) absorb green light and emit red fluorescence and are being studied for applications in biological imaging, detection and single-photon sources. Nanodiamonds without fluorescent centers do not resonate.

Sasaki and his colleagues soaked an optical nanofiber in solutions of nanodiamonds with and without fluorescent centers. Shining a green laser through one end of the nanofiber trapped a single nanodiamond with fluorescent centers and carried it away from the laser.

Resonant and non-resonant nanodiamonds moving in opposite directions

Scientists demonstrated that when a green laser and a red laser shine on nanodiamonds on opposite sides of the optical nanofiber, the movement of resonant and non-resonant nanodiamonds can be controlled independently: for non-resonant nanodiamonds, the red laser pushes them more strongly than the green laser; however, the resonants absorb red laser light and are therefore pushed more strongly by the green laser. Thus, they could be sorted according to their optical properties. In addition, the number of fluorescent centers in resonant nanodiamonds could be quantified by observing their movements under these conditions.

By using this technique to trap and manipulate nanodiamonds, scientists have demonstrated a proof of concept. Their next step would be to apply it to nanoparticles doped with organic dyes, which can be used as nanoprobes in biodetection systems.