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Optical selection and sorting of nanoparticles according to quantum mechanical properties

 On January 14, 2021, Hokkaido University, Hokkai-Gakuen University, Osaka Prefecture University, and Osaka University announced that researchers from these universities succeeded in selecting and sorting nanoparticles by optical forces from optical fiber with diameter of several hundred nanometers. This work was supported by JSPS KAKENHI, and details were published in Science Advances* with the senior author Prof. Keiji Sasaki at Research Institute for Electronic Science, Hokkaido University.

 Nanoparticles are attracting great attentions due to their unique properties to be used in such as medicine, solar cells, and photocatalysts. The selection and sorting of single nanoparticles according to their characteristics are essential for their applications. Although optical tweezers successfully manipu-late biomaterials, optical manipulation of quantum mechanical nanoparticles has been still challenging.

 Researchers noticed that there are three types of optical forces: (i) gradient force arising from the in-homogeneous electric field, (ii) dissipative scattering force, and (iii) quantum resonant absorption force on nanomaterials. Then, they have succeeded in demonstrating selective transportation of nanodia-monds with and without nitrogen- vacancy centers (r-ND and n-ND) by balancing resonant absorption and scattering forces induced by two different-colored lasers counterpropagating along a nanofiber.
 
 A commercially available single-mode optical fiber was stretched at both ends to make a waist with diameter of 400nm. Nanodiamonds (r-ND and n-ND) of 50nm in size were dispersed around the waist(nanofiber)and continuous-waves of green (532nm) and near-infrared (1064nm) diode lasers were introduced from both ends of the fabricated nanofiber. Resonant absorption of green light by r-ND caused selective unbalance of optical forces to move r-ND, while n-ND kept stationary position.

*Hideki Fujiwara, Kyosuke Yamauchi, Takudo Wada, Hajime Is*hihara, and Keiji Sasaki, "Optical se-lection and sorting of nanoparticles according to quantum mechanical properties", Science Advances  13 Jan 2021: Vol. 7, no. 3, eabd9551, DOI: 10.1126/sciadv.abd9551