Microscopic metavehicles powered and steered by embedded optical metasurfaces

Shining Light in Mechanobiology: Optical Tweezers, Scissors, and Beyond

Mechanobiology helps us to decipher cell and tissue functions by looking at changes in their mechanical properties that contribute to development, cell differentiation, physiology, and disease. Mechanobiology sits at the interface of biology, physics and engineering. One of the key technologies that enables characterization of properties of cells and tissue is microscopy. Combining microscopy with other quantitative measurement techniques such as optical tweezers and scissors, gives a very powerful tool for unraveling the intricacies of mechanobiology enabling measurement of forces, torques and displacements at play. We review the field of some light based studies of mechanobiology and optical detection of signal transduction ranging from optical micromanipulation-optical tweezers and scissors, advanced fluorescence techniques and optogenentics. In the current perspective paper, we concentrate our efforts on elucidating interesting measurements of forces, torques, positions, viscoelastic properties, and optogenetics inside and outside a cell attained when using structured light in combination with optical tweezers and scissors. We give perspective on the field concentrating on the use of structured light in imaging in combination with tweezers and scissors pointing out how novel developments in quantum imaging in combination with tweezers and scissors can bring to this fast growing field.

Synchronization of optically self-assembled nanorotors

Self-assembly of nanoparticles by means of interparticle optical forces provides a compelling approach toward contact-free organization and manipulation of nanoscale entities. However, exploration of the rotational degrees of freedom in this process has remained limited, primarily because of the predominant focus on spherical nanoparticles, for which individual particle orientation cannot be determined. Here, we show that gold nanorods, which self-assemble in water under the influence of circularly polarized light, exhibit synchronized rotational motion at kilohertz frequencies. The synchronization is caused by strong optical interactions and occurs despite the presence of thermal diffusion. Our findings elucidate the intricate dynamics arising from the transfer of photon spin angular momentum to optically bound matter and hold promise for advancing the emerging field of light-driven nanomachinery.

Creating tunable lateral optical forces through multipolar interplay in single nanowires

The concept of lateral optical force (LOF) is of general interest in optical manipulation as it releases the constraint of intensity gradient in tightly focused light, yet such a force is normally limited to exotic materials and/or complex light fields. Here, we report a general and controllable LOF in a nonchiral elongated nanoparticle illuminated by an obliquely incident plane wave. Through computational analysis, we reveal that the sign and magnitude of LOF can be tuned by multiple parameters of the particle (aspect ratio, material) and light (incident angle, direction of linear polarization, wavelength). The underlying physics is attributed to the multipolar interplay in the particle, leading to a reduction in symmetry. Direct experimental evidence of switchable LOF is captured by polarization-angle-controlled manipulation of single Ag nanowires using holographic optical tweezers. This work provides a minimalist paradigm to achieve interface-free LOF for optomechanical applications, such as optical sorting and light-driven micro/nanomotors.

Achiral nanoparticle trapping and chiral nanoparticle separating with quasi-BIC metasurface

Dielectric metasurfaces based on quasi-bound states in the continuum (quasi-BICs) are a promising approach for manipulating light-matter interactions. In this study, we numerically demonstrate the potential of silicon elliptical tetramer dielectric metasurfaces for achirality nanoparticle trapping and chiral nanoparticle separation. We first analyze a symmetric tetramer metasurface, which exhibits dual resonances (P1 and P2) with high electromagnetic field intensity enhancement and a high-quality factor (Q-factor). This metasurface can trap achiral nanoparticles with a maximum optical trapping force of 35 pN for 20 nm particles at an input intensity of 100 mW. We then investigate an asymmetric tetramer metasurface, which can identify and separate enantiomers under the excitation of left-handed circularly polarized (LCP) light. Results show that the chiral optical force can push one enantiomer towards regions of the quasi-BIC system while removing the other. In addition, the proposed asymmetric tetramer metasurface can provide multiple Fano resonances (ranging from R1 to R5) and high trap potential wells of up to 33 kBT. Our results demonstrate that the proposed all-dielectric metasurface has high performance in nanoparticle detection, with potential applications in biology, life science, and applied physics.

Plasmonic Nanostructure Engineering with Shadow Growth

Physical shadow growth is a vacuum deposition technique that permits a wide variety of 3D-shaped nanoparticles and structures to be fabricated from a large library of materials. Recent advances in the control of the shadow effect at the nanoscale expand the scope of nanomaterials from spherical nanoparticles to complex 3D shaped hybrid nanoparticles and structures. In particular, plasmonically active nanomaterials can be engineered in their shape and material composition so that they exhibit unique physical and chemical properties. Here, the recent progress in the development of shadow growth techniques to realize hybrid plasmonic nanomaterials is discussed. The review describes how fabrication permits the material response to be engineered and highlights novel functions. Potential fields of application with a focus on photonic devices, biomedical, and chiral spectroscopic applications are discussed.

Intelligent indoor metasurface robotics

Intelligent indoor robotics is expected to rapidly gain importance in crucial areas of our modern society such as at-home health care and factories. Yet, existing mobile robots are limited in their ability to perceive and respond to dynamically evolving complex indoor environments because of their inherently limited sensing and computing resources that are, moreover, traded off against their cruise time and payload. To address these formidable challenges, here we propose intelligent indoor metasurface robotics (I2MR), where all sensing and computing are relegated to a centralized robotic brain endowed with microwave perception; and I2MR's limbs (motorized vehicles, airborne drones, etc.) merely execute the wirelessly received instructions from the brain. The key aspect of our concept is the centralized use of a computation-enabled programmable metasurface that can flexibly mold microwave propagation in the indoor wireless environment, including a sensing and localization modality based on configurational diversity and a communication modality to establish a preferential high-capacity wireless link between the I2MR's brain and limbs. The metasurface-enhanced microwave perception is capable of realizing low-latency and high-resolution three-dimensional imaging of humans, even around corners and behind thick concrete walls, which is the basis for action decisions of the I2MR's brain. I2MR is thus endowed with real-time and full-context awareness of its operating indoor environment. We implement, experimentally, a proof-of-principle demonstration at ∼2.4 GHz, in which I2MR provides health-care assistance to a human inhabitant. The presented strategy opens a new avenue for the conception of smart and wirelessly networked indoor robotics.

Metasurfaces-Driven Hyperspectral Imaging via Multiplexed Plasmonic Resonance Energy Transfer

Obtaining single-molecular-level fingerprints of biomolecules and electron-transfer dynamic imaging in living cells are critically demanded in postgenomic life sciences and medicine. However, the possible solution called plasmonic resonance energy transfer (PRET) spectroscopy remains challenging due to the fixed scattering spectrum of a plasmonic nanoparticle and limited multiplexing. Here, multiplexed metasurfaces-driven PRET hyperspectral imaging, to probe biological light-matter interactions, is reported. Pixelated metasurfaces with engineered scattering spectra are first designed over the entire visible range by the precision nanoengineering of gap plasmon and grating effects of metasurface clusters. Pixelated metasurfaces are created and their full dark-field coloration is optically characterized with visible color palettes and high-resolution color printings of the art pieces. Furthermore, three different biomolecules (i.e., chlorophyll a, chlorophyll b, and cytochrome c) are applied on metasurfaces for color palettes to obtain selective molecular fingerprint imaging due to the unique biological light-matter interactions with application-specific biomedical metasurfaces. This metasurface-driven PRET hyperspectral imaging will open up a new path for multiplexed real-time molecular sensing and imaging methods.

Perspectives on adaptive dynamical systems

Adaptivity is a dynamical feature that is omnipresent in nature, socio-economics, and technology. For example, adaptive couplings appear in various real-world systems, such as the power grid, social, and neural networks, and they form the backbone of closed-loop control strategies and machine learning algorithms. In this article, we provide an interdisciplinary perspective on adaptive systems. We reflect on the notion and terminology of adaptivity in different disciplines and discuss which role adaptivity plays for various fields. We highlight common open challenges and give perspectives on future research directions, looking to inspire interdisciplinary approaches.

Light, Matter, Action: Shining Light on Active Matter

Light carries energy and momentum. It can therefore alter the motion of objects on the atomic to astronomical scales. Being widely available, readily controllable, and broadly biocompatible, light is also an ideal tool to propel microscopic particles, drive them out of thermodynamic equilibrium, and make them active. Thus, light-driven particles have become a recent focus of research in the field of soft active matter. In this Perspective, we discuss recent advances in the control of soft active matter with light, which has mainly been achieved using light intensity. We also highlight some first attempts to utilize light's additional properties, such as its wavelength, polarization, and momentum. We then argue that fully exploiting light with all of its properties will play a critical role in increasing the level of control over the actuation of active matter as well as the flow of light itself through it. This enabling step will advance the design of soft active matter systems, their functionalities, and their transfer toward technological applications.

Polarization-Dependent Forces and Torques at Resonance in a Microfiber-Microcavity System

Spin-orbit interactions in evanescent fields have recently attracted significant interest. In particular, the transfer of the Belinfante spin momentum perpendicular to the propagation direction generates polarization-dependent lateral forces on particles. However, it is still elusive as to how the polarization-dependent resonances of large particles synergize with the incident light's helicity and resultant lateral forces. Here, we investigate these polarization-dependent phenomena in a microfiber-microcavity system where whispering-gallery-mode resonances exist. This system allows for an intuitive understanding and unification of the polarization-dependent forces. Contrary to previous studies, the induced lateral forces at resonance are not proportional to the helicity of incident light. Instead, polarization-dependent coupling phases and resonance phases generate extra helicity contributions. We propose a generalized law for optical lateral forces and find the existence of optical lateral forces even when the helicity of incident light is zero. Our work provides new insights into these polarization-dependent phenomena and an opportunity to engineer polarization-controlled resonant optomechanical systems.

Photoacoustic 2D actuator via femtosecond pulsed laser action on van der Waals interfaces

Achieving optically controlled nanomachine engineering can satisfy the touch-free and non-invasive demands of optoelectronics, nanotechnology, and biology. Traditional optical manipulations are mainly based on optical and photophoresis forces, and they usually drive particles in gas or liquid environments. However, the development of an optical drive in a non-fluidic environment, such as on a strong van der Waals interface, remains difficult. Herein, we describe an efficient 2D nanosheet actuator directed by an orthogonal femtosecond laser, where 2D VSe₂ and TiSe₂ nanosheets deposited on sapphire substrates can overcome the interface van der Waals forces (tens and hundreds of megapascals of surface density) and move on the horizontal surfaces. We attribute the observed optical actuation to the momentum generated by the laser-induced asymmetric thermal stress and surface acoustic waves inside the nanosheets. 2D semimetals with high absorption coefficient can enrich the family of materials suitable to implement optically controlled nanomachines on flat surfaces.

Reversible lateral optical force on phase-gradient metasurfaces for full control of metavehicles

Photonics is currently undergoing an era of miniaturization thanks in part to two-dimensional (2D) optical metasurfaces. Their ability to sculpt and redirect optical momentum can give rise to an optical force, which acts orthogonally to the direction of light propagation. Powered by a single unfocused light beam, these lateral optical forces (LOFs) can be used to drive advanced metavehicles and are controlled via the incident beam's polarization. However, the full control of a metavehicle on a 2D plane (i.e. forward, backward, left, and right) with a sign-switchable LOF remains a challenge. Here we present a phase-gradient metasurface route for achieving such full control while also increasing efficiency. The proposed metasurface is able to deflect a normally incident plane wave in a traverse direction by modulating the plane wave's polarization, and results in a sign-switchable recoil LOF. When applied to a metavehicle, this LOF enables a level of motion control that was previously unobtainable.

Light-driven transport of microparticles with phase-gradient metasurfaces

Optical tweezers have opened numerous possibilities for precise control of microscopic particles for applications in life science and soft matter research and technology. However, traditional optical tweezers employ bulky conventional optics that prevents construction of compact optical manipulation systems. As an alternative, we present an ultrathin silicon-based metasurface that enables simultaneous confinement and propulsion of microparticles based on a combination of intensity and phase-gradient optical forces. The metasurface is constructed as a water-immersion line-focusing element that enables trapping and transport of 2μm particles over a wide area within a thin liquid cell. We envisage that the type of multifunctional metasurfaces reported herein will play a central role in miniaturized optical sensing, driving, and sorting of microscopic objects, such as cells or other biological entities.

Exploiting extraordinary topological optical forces at bound states in the continuum

Polarization singularities and topological vortices in photonic crystal slabs centered at bound states in the continuum (BICs) can be attributed to zero amplitude of polarization vectors. We show that such topological features are also observed in optical forces within the vicinity of BIC, around which the force vectors wind in the momentum space. The topological force carries force topological charge and can be used for trapping and repelling nanoparticles. By tailoring asymmetry of the photonic crystal slab, topological force will contain spinning behavior and shifted force zeros, which can lead to three-dimensional asymmetric trapping. Several off-Γ BICs generate multiple force zeros with various force distribution patterns. Our findings introduce the concepts of topology to optical force around BICs and create opportunities to realize optical force vortices and enhanced reversible forces for manipulating nanoparticles and fluid flow.

Creating stable trapping force and switchable optical torque with tunable phase of light

Light-induced rotation of microscopic objects is of general interest as the objects may serve as micromotors. Such rotation can be driven by the angular momentum of light or recoil forces arising from special light-matter interactions. However, in the absence of intensity gradient, simultaneously controlling the position and switching the rotation direction is challenging. Here, we report stable optical trapping and switchable optical rotation of nanoparticle (NP)-assembled micromotors with programmed phase of light. We imprint customized phase gradients into a circularly polarized flat-top (i.e., no intensity gradient) laser beam to trap and assemble metal NPs into reconfigurable clusters. Modulating the phase gradients allows direction-switchable and magnitude-tunable optical torque in the same cluster under fixed laser wavelength and handedness. This work provides a valuable method to achieve reversible optical torque in micro/nanomotors, and new insights for optical trapping and manipulation using the phase gradient of a spatially extended light field.

Stable Negative Optical Torque in Optically Bound Nanoparticle Dimers

Negative optical torque is a counterintuitive optomechanical phenomenon that can emerge in light-assembled nanoparticle (NP) clusters (i.e., optical matter) under circular polarization. However, in experiments, stable negative torque was limited to optical matter with 3 or more NPs. Here, we show that by increasing the particle size, the sign of optical torque can be reversed in optical matter dimers, where stable negative torque arises in dimers of 300 nm diameter Au or 490 nm diameter polystyrene NPs. Our computational analysis reveals that the multipolar resonances in large NPs can enhance the forward scattering along the spin angular momentum (SAM) direction of light, creating a recoil negative torque due to momentum conservation. The observation of stable negative torque in dimers pushes the limit to the smallest optical matter, demonstrating the universal existence of negative torque in such a system. The underlying principle also provides new strategies for making light-driven nanomotors.

Optically Triggered Nanoscale Plasmonic Dynamite

Photons as energy carriers are clean and abundant, which can be conveniently applied for nanoactuation but the response is usually slow with very low energy efficiency/density. Here, we underpin the concept of robust nanoscale plasmonic dynamite by incorporating fullerene (C₆₀). The Au@C₆₀ core-shell nanoparticles can be triggered to explode in nanoscale with synergy of plasmon-enhanced photochemical and photothermal effects. It is suggested that a sensible amount of CO₂ was generated and pressurized in nanometric volume in an extremely short time scale (∼ns), which triggers the nanoexplosion, rendering the ejection of Au NPs at the speed over 300 m/s. The ejection generates extremely large local forces (∼1 μN) with thermomechanical energy efficiency up to ∼30%, which is demonstrated as a powerful nanoengine for controlled mobilization of micro-objects on solid surfaces. Such nanoscale plasmonic dynamite is highly exploitable for different types of nanomachines, which provides a powerful energy source for nanoactuation and nanomigration.

Tailoring matter orbitals mediated using a nanoscale topographic interface for versatile colloidal current devices

Conventional micro-particle manipulation technologies have been used for various biomedical applications using dynamics on a plane without vertical movement. In this case, irregular topographic structures on surfaces could be a factor that causes the failure of the intended control. Here, we demonstrated a novel colloidal particle manipulation mediated by the topographic effect generated by the "micro hill" and "surface gradient" around a micro-magnet. The magnetic landscape, matter orbital, created by periodically arranged circular micro-magnets, induces a symmetric orbit of magnetic particle flow under a rotating magnetic field. The topographic effect can break this symmetry of the energy distribution by controlling the distance between the source of the driving force and target particles by several nanometers on the surface morphology. The origin symmetric orbit of colloidal flow can be distorted by modifying the symmetry in the energy landscape at the switching point without changing the driving force. The enhancement of the magnetic effect of the micro-magnet array can lead to the recovery of the symmetry of the orbit. Also, this effect on the surfaces of on-chip-based devices configured by symmetry control was demonstrated for selective manipulation, trapping, recovery, and altering the direction using a time-dependent magnetic field. Hence, the developed technique could be used in various precise lab-on-a-chip applications, including where the topographic effect is required as an additional variable without affecting the existing control method.

Programmable Multimodal Optothermal Manipulation of Synthetic Particles and Biological Cells

Optical manipulation of tiny objects has benefited many research areas ranging from physics to biology to micro/nanorobotics. However, limited manipulation modes, intense lasers with complex optics, and applicability to limited materials and geometries of objects restrict the broader uses of conventional optical tweezers. Herein, we develop an optothermal platform that enables the versatile manipulation of synthetic micro/nanoparticles and live cells using an ultralow-power laser beam and a simple optical setup. Five working modes (i.e., printing, tweezing, rotating, rolling, and shooting) have been achieved and can be switched on demand through computer programming. By incorporating a feedback control system into the platform, we realize programmable multimodal control of micro/nanoparticles, enabling autonomous micro/nanorobots in complex environments. Moreover, we demonstrate in situ three-dimensional single-cell surface characterizations through the multimodal optothermal manipulation of live cells. This programmable multimodal optothermal platform will contribute to diverse fundamental studies and applications in cellular biology, nanotechnology, robotics, and photonics.

Light-driven microdrones

When photons interact with matter, forces and torques occur due to the transfer of linear and angular momentum, respectively. The resulting accelerations are small for macroscopic objects but become substantial for microscopic objects with small masses and moments of inertia, rendering photon recoil very attractive to propel micro- and nano-objects. However, until now, using light to control object motion in two or three dimensions in all three or six degrees of freedom has remained an unsolved challenge. Here we demonstrate light-driven microdrones (size roughly 2 μm and mass roughly 2 pg) in an aqueous environment that can be manoeuvred in two dimensions in all three independent degrees of freedom (two translational and one rotational) using two overlapping unfocused light fields of 830 and 980 nm wavelength. To actuate the microdrones independent of their orientation, we use up to four individually addressable chiral plasmonic nanoantennas acting as nanomotors that resonantly scatter the circular polarization components of the driving light into well-defined directions. The microdrones are manoeuvred by only adjusting the optical power for each motor (the power of each circular polarization component of each wavelength). The actuation concept is therefore similar to that of macroscopic multirotor drones. As a result, we demonstrate manual steering of the microdrones along complex paths. Since all degrees of freedom can be addressed independently and directly, feedback control loops may be used to counteract Brownian motion. We posit that the microdrones can find applications in transport and release of cargos, nanomanipulation, and local probing and sensing of nano and mesoscale objects.

Negative radiation pressure in metamaterials explained by light-driven atomic mass density rarefication waves

The momentum and radiation pressure of light in negative-index metamaterials (NIMs) are commonly expected to reverse their direction from what is observed for normal materials. The negative refraction and inverse Doppler effect of light in NIMs have been experimentally observed, but the equally surprising phenomenon, the negative radiation pressure of light, still lacks experimental verification. We show by simulating the exact position- and time-dependent field-material dynamics in NIMs that the momentum and radiation pressure of light in NIMs can be either positive or negative depending on their subwavelength structure. In NIMs exhibiting negative radiation pressure, the negative total momentum of light is caused by the sum of the positive momentum of the electromagnetic field and the negative momentum of the material. The negative momentum of the material results from the optical force density, which drives atoms backward and reduces the local density of atoms at the site of the light field. In contrast to earlier works, light in NIMs exhibiting negative radiation pressure has both negative total momentum and energy. For the experimental discovery of the negative radiation pressure, one must carefully design the NIM structure and record the joint total pressure of the field and material momentum components.

Tunable and enhanced optical force with bound state in the continuum

Light-actuated motors, vehicles, and even space sails have drawn tremendous attention for basic science and applications in space, biomedical, and sensing domains. Optical bound states in the continuum (BIC) are topological singularities of the scattering matrix, known for their unique light-trapping capability and enhanced light-matter interaction. We show that BIC modes enable the generation of enhanced and tunable optical forces and torques. A sharp and controllable line shape is observed in force and torque spectra when approaching high-Q resonance BIC modes. Wavelength and polarization tunability are presented as an effective method to control forces on BIC enclosed structures. Finally finite-size simulations are performed to evaluate the practical applications for a BIC-assisted metavehicle.

Optical Pulling Using Chiral Metalens as a Photonic Probe

Optical pulling forces, which can pull objects in the source direction, have emerged as an intensively explored field in recent years. Conventionally, optical pulling forces exerted on objects can be achieved by tailoring the properties of an electromagnetic field, the surrounding environment, or the particles themselves. Recently, the idea of applying conventional lenses or prisms as photonic probes has been proposed to realize an optical pulling force. However, their sizes are far beyond the scope of optical manipulation. Here, we design a chiral metalens as the photonic probe to generate a robust optical pulling force. The induced pulling force exerted on the metalens, characterized by a broadband spectrum over 0.6 μm (from 1.517 to 2.117 μm) bandwidth, reached a maximum value of −83.76 pN/W. Moreover, under the illumination of incident light with different circular polarization states, the longitudinal optical force acting on the metalens showed a circular dichroism response. This means that the longitudinal optical force can be flexibly tuned from a pulling force to a pushing force by controlling the polarization of the incident light. This work could pave the way for a new advanced optical manipulation technique, with potential applications ranging from contactless wafer-scale fabrication to cell assembly and even course control for spacecraft.