Metasurfaces-Empowered Optical Micromanipulation (Invited)

Significance Optical micromanipulation utilizes optical force to dynamically control particles, which has the characteristics of non-contact and can be operated in a vacuum environment. Since the invention of optical tweezers in the 1980s, the field has experienced rapid development and has given rise to many emerging research directions, such as holographic optical tweezers, near-field evanescent wave optical tweezers, fiber optic tweezers, optoelectronic tweezers, and photo-induced temperature field optical tweezers, providing rich and powerful tools for fields such as biology, chemistry, nanoscience, and quantum technology. These methods can not only capture, separate, and transport small objects but also allow more precise manipulation, such as the rotation of small objects. However, traditional manipulation methods rely on tightly focused local light, greatly limiting the action range of optical force. In addition, in order to generate a structured light field, larger optical components such as spatial light modulators are usually required, making it difficult to miniaturize and integrate the optical manipulation system. In recent years, metasurfaces have emerged as integrated devices composed of subwavelength nanoantennas, promising new opportunities for optical micromanipulation. This ultra-thin artificial microstructure device can flexiblely control multiple degrees of freedom such as amplitude, phase, and polarization of light, by specially designing the geometric shape, size, and material of its own micro/nanostructure. Compared with traditional optical components such as liquid crystal spatial light modulators, gratings, and lenses, metasurfaces exhibit higher operating bandwidth, structural compactness, and integration. With the merits of miniaturization, integration, and excellent performance in light tailoring, optical metasurfaces have been extensively incorporated into the realm of optical micromanipulation. Especially, owing to their peculiar photomechanical properties, the metasurfaces hold the ability to be actuated by light fields, paving the way to the next generation of light-driven artificial micro-robots. The fast development of this subject indicates that the time is now ripe to overview recent progress in this cross-field. Progress We summarized principles of optical micromanipulation and metasurfaces (Fig. 1) and overviewed meta-manipulation devices, including metasurface-based optical tweezers (Fig. 2), tractor beams (Fig. 5), multifunctional micromanipulation systems (Fig. 3), and metamachines (Figs. 7 and 8). Furthermore, we provided a detailed discussion of novel mechanical effects, such as topological light manipulation, which stems from the topological characteristics of nanostructures (Fig. 6). Conclusions and Prospects We review the cutting-edge developments in the field of optical micromanipulation based on metasurfaces. The metasurface-based micromanipulation technology is expected to evolve toward higher temporal resolution, higher spatial accuracy, and lower manipulation power. To this end, more urgent requirements have been imposed on the underlying design scheme and experimental preparation standards of the metasurface. Although the introduction of metasurfaces has benefited micromanipulation systems and significantly reduced their sizes, there is still much room for further development and improvement in wide bands, multi-dimensional responses, and device thresholds. In terms of micromanipulation systems, the subwavelength-scale structure of metasurfaces will continue to be a key focus of research. Especially in the field of topological light manipulation, it is expected to further expand its research scope, combining non-Abelitan, non-Hermitian, and nonlinear effects to discover new physical phenomena. In the fields of biology and chemistry, metasurface technology is expected to be flexibly applied on smaller scales, even achieving manipulation of single molecule-level objects. This technology is expected to be further applied to the fields such as battery quality inspection and targeted therapy, bringing changes to the basic research and practical applications of energy and life sciences. Specifically, in the development of ultrafast optics, metasurfaces are gradually exhibiting unique advantages. Nanoscale superlattice enables high-resolution spectral measurements, and the design of nonlinear superlattice surfaces can be used to enhance nonlinear effects or generate high-order harmonics, making high time resolution transient micromanipulation technology possible. Overall, the technological evolution from traditional optical micromanipulation to meta-manipulation will continue to drive the vigorous development of nanophotonics. This technological paradigm not only meets the needs of various basic research but also arouses more innovative applications, opening up new prospects for branched sciences and technologies.