Tracking nanoparticles in self-consistent light-matter swarms

Since the pioneering work of Ashkin in1986, optical trapping has been used in various research fields (e.g., biology, chemistry, physics, and material sciences) for three-dimensional trapping and manipulation of micro- and nano-scale objects (e.g., nanoparticles (NPs), live cells, proteins, DNA, or small molecules).[1-4] Inside an optical field, three different optical (scattering, gradient, and absorption) forces are imposed on  particles. While inside bulk solution, a stable trapping spot is only generated at the laser focus when the gradient force is larger than the scattering force, completely different phenomena occur at an interface, where all the optical forces contribute to stably trapping and gathering the objects.[5] As result, so called “dynamic evolving assemblies or self-consistent light-matter swarms” have been recently reported using different types of objects (metallic and polymeric particles, polymers, and proteins).[6-8] These assemblies can gather more than hundreds of objects outside the irradiated area, which can only be explained by an expansion of the optical potential, most likely through multiple scattering processes.[5] In this PhD project, we will further investigate the unexplored phenomenon of optical binding outside of the irradiated area, which has the potential to generate new, sub-millimeter-sized colloidal assemblies. Our initial working hypothesis considers that the NPs are optically bound outside the focal spot by the back-scattered light and multi-channel light scattering, forming a dynamic optical binding network. The PhD candidate will explore the different experimental conditions which yield optical binding outside the irradiated area, from both optical (e.g., laser beam mode and pattern, laser polarization, number of laser beams, etc.) and material (e.g., size, shape, metallic vs dielectric vs hybrid materials, surface decoration, etc.) viewpoints. Another interesting aspect to consider is the chirality of the system, and how chiral particles can modify the optical binding properties.

References
[1] Ashkin, A.; Dziedzic, J. M. Optical Trapping and Manipulation of Viruses and Bacteria. Science 1987, 235, 1517–1520.
[2] Tsuboi, Y. A Long Arm and a Tight Grip. Nat. Nanotechnol. 2016, 11, 5-6.
[3] Ito, S.; Mitsuishi, M.; Setoura, K.; Tamura, M.; Iida, T.; Morimoto, M.; Irie, M.; Miyasaka, H. Mesoscopic Motion of Optically Trapped Particle Synchronized with Photochromic Reactions of Diarylethene Derivatives. J. Phys. Chem. Lett. 2018, 9, 2659–2664.
[4] Fujiwara, H.; Yamauchi, K.; Wada, T.; Ishihara, H.; Sasaki, K. Optical Selection and Sorting of Nanoparticles According to Quantum Mechanical Properties. Sci. Adv. 2021, 7, eabd9551.
[5] Masuhara, H.; Yuyama, K.-I. Optical Force-Induced Chemistry at Solution Surfaces. Annu. Rev. Phys. Chem. 2021, 72, 565−589.
[6] Kudo, T.; Wang, S.-F.; Yuyama, K.-I.; Masuhara, H. Optical Trapping-Formed Colloidal Assembly with Horns Extended to the Outside of a Focus through Light Propagation. Nano Lett. 2016, 16, 3058−3062.
[7] Kudo, T.; Yang, S. J.; Masuhara, H. A Single Large Assembly with Dynamically Fluctuating Swarms of Gold Nanoparticles Formed by Trapping Laser. Nano Lett. 2018, 18, 5846–5853.