Seminar: Micro- and Nanomotors Driven by Optical Forces and Torques

Title:  Micro- and Nanomotors Driven by Optical Forces and Torques

Speaker: Dr Mikael Käll, Professor, Department of Physics, Chalmers University of Technology,  Sweden

Abstract:
Almost all engines and motors that we utilize in our macroscopic world are driven by light energy, in the form of fuels derived from photosynthesis or electricity generated by photovoltaics. In the micro and nanoworld, however, it is also possible to utilize light momentum to generate mechanical movement. In this talk, I will discuss some of our recent works in this field with a focus on two different systems. In the first case, we investigate plasmonic rotary nanomotors, which can rotate at kHz frequencies in water due to spin angular momentum transfer [1]. Surprisingly, we recently found that these tiny objects can optically self-organize into regular arrays and synchronize their rotational movement in a manner that is essentially captured by the famous Kuramoto model of synchronization behaviors [2]. The second system consists of micromotors that utilize photon recoil in optical metasurfaces to generate translation and rotation. We constructed microscopic "metavehicles" able to navigate across a surface in water under plane-wave illumination, while being steered through the incident polarization [3], as well as “metaspinners” [4] and “metarotors” [5] that rotate because they generate orbital angular momentum from unstructured incident light. Although these works are mostly motivated by curiosity, there might also be an application potential. For example, rotary nanomotors can be used as ultrasensitive rheological sensors [1]; metavehicles can be used as transporters of microscopic cargo [3], and metarotors can be used to rotate hundreds of passive microparticles in solution [5]. It is even possible to transform metaspinners into tiny cogwheels for optically driven micromachinery [6].

[1] L. Shao et al, Adv. Funct. Mat. (2018), https://doi.org/10.1002/adfm.201706272
[2] X. Cui et al, Science Advances (2024), DOI: 10.1126/sciadv.adn3485
[3] Andrén et al, Nature Nanotechnology (2021). https://doi.org/10.1038/s41565-021-00941-0
[4] Engay et al., Light: Science & Applications (2025). https://doi.org/10.1038/s41377-024-01720-x
[5] Shanei et al, Nano Letters (2025). https://pubs.acs.org/doi/10.1021/acs.nanolett.4c06410
[6] Wang et al., arXiv preprint (2025). arXiv:2409.17284

Bio:
Mikael Käll is an experimental physicist focusing on fundamental and applied nanooptics and biophotonics. His current research projects deal with the physics and applications of optical metasurfaces and optical forces. He has previously worked with topics like surface-enhanced Raman scattering, plasmonic biosensors, optical antennas, bioimaging, and strongly correlated electron systems. He has co-authored more than 200 journal papers that have been cited more than 30000 times to date.