[Optics Seminars OIST] by Prof. Keiji Sasaki

Speaker: Prof. Keiji Sasaki
Research Institute for Electronic Science, Hokkaido University, Japan 

Title: Nano-Material Optical Manipulation and Structural Order Control

Abstract:
Optical trapping and manipulation based on optical forces are promising tools for positioning, transporting, and aligning fine particles without mechanical contacts. We aim to realize the ultimate performance of nano-material optical manipulation. We are challenging how small particles and molecules can be trapped and selectively positioned, and how high spatial-resolution and precision can be achieved for the manipulation and motion-control of nanomaterials. For this purpose, we are trying to design optical forces exerted on nanoparticles and molecules by shaping the amplitude, phase, and polarization of the optical electric field on single-nanometer scale. Here, we present our research progresses on nano-material manipulation using optical nanofiber and plasmonic nanostructures.

1. Optical selection and sorting of nanoparticles according to quantum mechanical properties
We experimentally demonstrated selective transportation of nanodiamonds (NDs) with and without nitrogen-vacancy centers (NVCs). Quantum resonant absorption of the NVCs induces the pushing optical force, while the ND having high refractive index causes the scattering force. We prepared a tapered optical fiber with ~400-nm diameter, that has characteristics of single-mode propagation with diffraction-limited cross-section over millimeter length. Two laser beams with 532-nm (in resonance of NVC) and 1064-nm (out of resonance of NVC) wavelengths were introduced into the nanofiber from both ends, so that the absorption force can be extracted by balancing the scattering forces with the counter-propagating beams. 50-nm NDs were attracted by the gradient force of the evanescent field around the nanofiber, and moved along the fiber by the absorption and scattering forces. By adjusting the powers of two lasers, we succeeded in selective transportation of NDs, where NDs with NVCs slowly move in one direction whereas the NDs without NVC move in the opposite direction. Furthermore, we also propose a methodology for precisely determining the absolute values of absorption cross-sections for single nanoparticles by monitoring the optically driven motion, called as “optical force spectroscopy”. Optical force spectroscopy sensitively measures the interaction between light and nanoparticles separately from the scattering effects, based on the photon momentum change and not the energy change.

2. Nano-space manipulation of nanoparticles with designed plasmonic fields
Recently, we propose a novel approach to forming the single-nanometer-scale localized fields of optical vortex (Laguerre-Gaussian mode) by employing the whispering gallery mode plasmonic nano-cavities. We designed the tailored plasmonic structure consisting of metal multimer surrounding a nano-gap. This metal structure makes it possible to localize the optical vortex field into the gap space with conserving the high-order orbital and spin angular momenta (OAM and SPM). The transfer of the angular momenta from this nano-vortex fields to molecules or nanoparticles induces rotational radiation pressure, i.e., optical torque, and gradient force directed to the center, which causes nano-vortex flow of molecules/particles and may lead to chiral structuring of molecule/particle assemblies. We succeeded in rotational manipulation of a polymer nano-bead with a gold triangle trimer structure. The plasmonic nanostructure was illuminated with a circularly polarized beam of a near-infrared laser, so that the nano-sized field with the OAM and SPM is formed within the gap. The motion analysis of the nanoparticle indicated the orbital rotation with <50-nm diameter. We will explain detailed analyses of the rotational motions and their relations to the chirality of the plasmonic fields.

References
[1] Y. Tanaka, K. Sasaki, et al., Nano Lett. 15, 7086–7090 (2015).
[2] X. Shi, K. Sasaki, et al., Nat. Nanotechnol. 13, 953–958 (2018).
[3] H. Fujiwara, K. Sasaki, et al., Nano Lett. 20, 389–394 (2019).
[4] T. Arikawa, K. Sasaki, et al., Sci. Adv. 6, eaay1977 (2020).
[5] A.-C. Cheng, K. Sasaki, et al., J. Phys. Chem. Lett. 11, 4422-4426 (2020).
[6] H. Fujiwara, K. Sasaki, et al., Sci. Adv. in press.

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