Optical trapping and manipulation of nanostructures

Reconfigurable optical matter of polystyrene microparticles fabricated by optical trapping at solution surface

Light-matter interactions are fundamental in materials fabrication and property control, with significant applications in broad fields. A notable phenomenon arises when optical forces are exerted among nanoparticles and microparticles, in which optical binding leads to the development of optical matters with a well-patterned structure. This work explores a dynamic optical matter of polystyrene microparticles (PS MPs) prepared at the air/solution interface under optical trapping, specifically focusing on the interactions between 1-µm and 20-µm PS MPs. We report the formation of an unprecedented three-dimensional (3D) bulky assembly where smaller particles form a necklace and belt assembly around the larger particle, created by multiple light scattering. The resulting assembly, which can exceed 50 µm in diameter, elongates light-matter interaction lengths and exhibits reconfigurability tuning optical conditions, forming a unique dynamic optical matter with a random and disordered structure. As a result, we demonstrate amplified spontaneous emissions in the 3D bulky assembly thanks to the feature of randomness and multiple light scattering. These findings present a new approach to the study of reconfigurable and tunable optical matter, opening avenues for novel applications in disordered photonics and material science.

Harnessing optical forces with advanced nanophotonic structures: principles and applications

Non-contact mechanical control of light has given rise to optical manipulation, facilitating diverse light-matter interactions and enabling pioneering applications like optical tweezers. However, the practical adoption of versatile optical tweezing systems remains constrained by the complexity and bulkiness of their optical setups, underscoring the urgent requirement for advancements in miniaturization and functional integration. In this paper, we present innovations in optical manipulation within the nanophotonic domain, including fiber-based and metamaterial tweezers, as well as their emerging applications in manipulating cells and artificial micro-nano robots. Furthermore, we explore interdisciplinary on-chip devices that integrate photonic crystals and optofluidics. By merging optical manipulation with the dynamism of nanophotonics and metamaterials, this work seeks to chart a transformative pathway for the future of optomechanics and beyond.

Differential Elasticity Affects Lineage Segregation of Embryonic Stem Cells

The question of what guides lineage segregation is central to development, where cellular differentiation leads to segregated cell populations destined for specialized functions. Here, using optical tweezers measurements of mouse embryonic stem cells, we reveal a mechanical mechanism based on differential elasticity in the second lineage segregation of the embryonic inner cell mass into epiblast (EPI) cells, which will develop into the fetus, and primitive endoderm (PrE), which will form extraembryonic structures such as the yolk sac. Remarkably, we find that these mechanical differences already occur during priming, not just after a cell has committed to differentiation. Specifically, we show that PrE-primed cells exhibit significantly higher elasticity than EPI-primed cells, characterized by lower power spectrum scaling exponents, higher Young's modulus, and lower loss tangent. Using a model of two cell types differing only in elasticity, we show that differential elasticity alone is sufficient to lead to segregation between cell types, suggesting that the mechanical attributes of the cells contribute to the segregation process. Importantly, we find that this process relies on cellular activity. Our findings present differential elasticity as a previously unknown mechanical contributor to lineage segregation during embryo morphogenesis.

Unified Simulation Platform for Optical Tweezers and Optofluidic Force Induction

Optical tweezers utilize the forces exerted by focused laser beams to trap particles. In optofluidic force induction (OF2i), the forces exerted by a weakly focused laser beam trap particles in the transverse directions and push them in the laser propagation direction, which can be utilized for optical nanoparticle characterization with single-particle sensitivity. Here, we present a unified approach for the simulation of nanoparticles propagating in the presence of fluidic and optical forces, which can be used for both optical tweezers and OF2i simulations. We demonstrate the working principle at a number of selected examples and provide the simulation software as an add-on to our generic Maxwell solver NANOBEM that is based on a boundary element method approach.

Axial electrokinetic trapping of label-free nanoparticles using evanescent field scattering

Anti-Brownian electrokinetic trapping enables the confinement of individual nanoparticles in liquids by applying electric fields. This technique facilitates the long-term observation of nanoscopic objects, allowing for detailed studies of their physical, chemical, and biomolecular properties. However, this method has been largely restricted to nanoparticles that can be visualized by photoluminescence. While some techniques avoid fluorescent labeling by using dark-field or interferometric scattering microscopy, they are limited to two-dimensional particle trapping and lack control over the axial direction. Here, we demonstrate the axial electrokinetic trapping of fluorescence-free nanoparticles that scatter the evanescent field induced by total internal reflection. The distance between the particle and the glass surface is directly related to the intensity of scattered light, and controlled by an applied electric field. Consequently, nanoparticles can be trapped and monitored in response to applied voltages at kilohertz rates without the need for fluorescent labeling. In addition, we utilize this approach to investigate how surface proximity impacts the diffusion and mobility of the trapped nanoparticles. Our method paves a new way to study a broad range of nano-objects that can be trapped at the single-particle level, relying solely on their light-scattering properties, which offers significant potential for advancing research in surface chemistry, single-molecule biophysics, and cell membrane biology.

Dynamics of dual-orbit rotations of nanoparticles induced by spin-orbit coupling

Spin-orbit coupling (SOC) in tightly focused optical fields offers a powerful mechanism for manipulating the complex motion of particles. However, to date, such a mechanism has only been applied to the single-orbit motion for particles, while multi-orbital dynamics have not yet been experimentally demonstrated. Here, the theoretical and experimental realization of dual-orbit rotational dynamics of nanoparticles in a tightly focused circularly polarized Laguerre-Gaussian beam is reported. Analyses reveal that the dual-orbit rotation of nanoparticles originates from SOC in a tightly focused vortex beam, with the motion velocity and direction determined by the topological charge of the beam. Experimentally, the dual-orbit rotation of polystyrene nanoparticles was observed for the first time using an inverted optical tweezer. In addition, the rotation velocity showed a clear linear dependence on the topological charge of the incident beam. This work reveals the pivotal role of SOC in enabling precise dual-orbit control at the nanoscale, paving the way for applications in optical sorting, grinding and delivery of microparticles.

Collective transport of particles and cells enabled by wavelength-division multiplexing in microcavity cascade optical tweezers

This study presents a microcavity cascade optical tweezer (MCOT) system incorporating wavelength-division multiplexing for collective transport of particles and cells in biomedical applications. The MCOT system traps and transports yeast cells (5 μm) and silica microspheres using 980 nm and 1550 nm lasers, with a maximum capacity of six particles. Under 980 nm laser illumination, capillary microflow force surpasses optical forces, stably trapping particles and cells in the microcavity. At 1550 nm, significant heat absorption excites thermophoretic forces, which, combined with optical forces, enhance particle transport. Experimental results closely match simulations, confirming the system's potential for efficient particle and cell transport, especially for drug and cell delivery applications.

Solving non-Hermitian physics for optical manipulation on a quantum computer

Intense laser light, with its ability to trap small particles, is providing us unprecedented access to the microscopic world. Nevertheless, owing to its open nature, optical force is nonconservative and can only be described by a non-Hermitian theory. This non-Hermiticity sets such system apart from conventional systems and has offered rich physics, such as the possession of the exceptional points. Consequently, analyzing and demonstrating the dynamics of large optically-bound clusters becomes an intricate challenge. Here, we developed a scalable quantum approach that allows us to predict the trajectories of optically trapped particles and tackle the associated non-Hermitian physics. This approach is based on the linear combination of unitary operations. With this, we experimentally revealed the non-Hermiticity and exceptional point for a single or multiple particles trapped by optical force fields, using a nuclear magnetic resonance quantum processor. Our method's scalability and stability have offering a promising path for large-scale optical manipulation with non-Hermitian dynamics.

Electrostatic All-Passive Force Clamping of Charged Nanoparticles

In the past decades, many techniques have been explored for trapping microscopic and nanoscopic objects, but the investigation of nano-objects under arbitrary forces and conditions remains nontrivial. One fundamental case concerns the motion of a particle under a constant force, known as force clamping. Here, we employ metallic nanoribbons embedded in a glass substrate in a capacitor configuration to generate a constant electric field on a charged nanoparticle in a water-filled glass nanochannel. We estimate the force fields from Brownian trajectories over several micrometers and confirm the constant behavior of the forces both numerically and experimentally. Furthermore, we manipulate the diffusion and relaxation times of the nanoparticles by tuning the charge density on the electrode. Our highly compact and controllable setting allows for the trapping and force-clamping of charged nanoparticles in a solution, providing a platform for investigating nanoscopic diffusion phenomena.

Synergizing microfluidics and plasmonics: advances, applications, and future directions

In the past decade, interest in nanoplasmonic structures has experienced significant growth, owing to rapid advancements in materials science and the evolution of novel nanofabrication techniques. The activities in the area are not only leading to remarkable progress in specific applications in photonics, but also permeating to and synergizing with other fields. This review delves into the symbiosis between nanoplasmonics and microfluidics, elucidating fundamental principles on nanophotonics centered on surface plasmon-polaritons, and key achievements arising from the intricate interplay between light and fluids at small scales. This review underscores the unparalleled capabilities of subwavelength plasmonic structures to manipulate light beyond the diffraction limit, concurrently serving as fluidic entities or synergistically combining with micro- and nanofluidic structures. Noteworthy phenomena, techniques and applications arising from this synergy are explored, including the manipulation of fluids at nanoscopic dimensions, the trapping of individual nanoscopic entities like molecules or nanoparticles, and the harnessing of light within a fluidic environment. Additionally, it discusses light-driven fabrication methodologies for microfluidic platforms and, contrariwise, the use of microfluidics in the fabrication of plasmonic nanostructures. Pondering future prospects, this review offers insights into potential future developments, particularly focusing on the integration of two-dimensional materials endowed with exceptional optical, structural and electrical properties, such as goldene and borophene, which enable higher carrier densities and higher plasmonic frequencies. Such advancements could catalyze innovations in diverse applications, including energy harvesting, advanced photothermal cancer therapies, and catalytic processes for hydrogen generation and CO2 conversion.

Manipulating π-π Interactions between Single Molecules by Using Antenna Electrodes as Optical Tweezers

Via conductance measurements of thousands of single-molecule junctions, we report that the π-π coupling between neighboring aromatic molecules can be manipulated by laser illumination. We reveal that this optical manipulation originates from the optical plasmonic gradient force generated inside the nanogaps, in which the gapped antenna electrodes act as optical tweezers pushing the neighboring molecules closer together. These findings offer a nondestructive approach to regulate the interaction of the molecules, deepening the understanding of the mechanism of π-π interaction, and open an avenue to manipulate the relative position of extremely small objects down to the scale of single molecules.

Construction of Chirality-Sorting Optical Force Pairs

Chiral objects are abundant in nature, and although the enantiomers have almost identical physical properties apart from their handedness, they can exhibit significantly different chemical properties and biological functions. This underscores the importance of sorting chiral substances. In this Letter, we demonstrate that chirality-sorting optical force pairs can be inversely generated in a tightly focused Gaussian beam by tailoring the input polarization state. We provide a detailed method for constructing the polarization state of the incident light to create the desired chiral optical field that generates the chirality-sorting optical force pairs. These force pairs precisely trap two opposite enantiomers at distinct predetermined positions within the same equilibrium plane, enabling their simultaneous identification and separation. Notably, the trapping positions and separation distances can be freely adjusted by altering the incident polarization parameters.

Dynamic control and manipulation of near-fields using direct feedback

Shaping and controlling electromagnetic fields at the nanoscale is vital for advancing efficient and compact devices used in optical communications, sensing and metrology, as well as for the exploration of fundamental properties of light-matter interaction and optical nonlinearity. Real-time feedback for active control over light can provide a significant advantage in these endeavors, compensating for ever-changing experimental conditions and inherent or accumulated device flaws. Scanning nearfield microscopy, being slow in essence, cannot provide such a real-time feedback that was thus far possible only by scattering-based microscopy. Here, we present active control over nanophotonic near-fields with direct feedback facilitated by real-time near-field imaging. We use far-field wavefront shaping to control nanophotonic patterns in surface waves, demonstrating translation and splitting of near-field focal spots at nanometer-scale precision, active toggling of different near-field angular momenta and correction of patterns damaged by structural defects using feedback enabled by the real-time operation. The ability to simultaneously shape and observe nanophotonic fields can significantly impact various applications such as nanoscale optical manipulation, optical addressing of integrated quantum emitters and near-field adaptive optics.

Optical tweezing of microparticles and cells using silicon-photonics-based optical phased arrays

Integrated optical tweezers have the potential to enable highly-compact, low-cost, mass-manufactured, and broadly-accessible optical manipulation when compared to standard bulk-optical tweezers. However, integrated demonstrations to date have been fundamentally limited to micron-scale standoff distances and, often, passive trapping functionality, making them incompatible with many existing applications and significantly limiting their utility, especially for biological studies. In this work, we demonstrate optical trapping and tweezing using an integrated OPA for the first time, increasing the standoff distance of integrated optical tweezers by over two orders of magnitude compared to prior demonstrations. First, we demonstrate trapping of polystyrene microspheres 5 mm above the surface of the chip and calibrate the trap force. Next, we show tweezing of polystyrene microspheres in one dimension by non-mechanically steering the trap by varying the input laser wavelength. Finally, we use the OPA tweezers to demonstrate, to the best of our knowledge, the first cell experiments using single-beam integrated optical tweezers, showing controlled deformation of mouse lymphoblast cells. This work introduces a new modality for integrated optical tweezers, significantly expanding their utility and compatibility with existing applications, especially for biological experiments.

Direction-switchable transverse optical torque on a dipolar phase-change nanoparticle

We propose that a transition from positive optical torque (OT) to negative OT occurs in a dipolar nanoparticle subjected to a simple optical field composed of two circularly polarized plane waves. This phenomenon can be observed in a phase-change nanoparticle comprising insulating and metallic phases. The analytical expression based on the multipole expansion theory reveals that the positive OT in the metallic phase originates from the electric response during light-matter interaction. However, in the insulating phase, the magnetic response is excited, leading to a significant negative OT due to the contribution of the magnetic field-magnetic dipole interaction. It is noted that the phenomenon of reversible transverse OT is robust to the angle between two constituent plane waves, ensuring its practical application.

Fundamentals and Applications of Raman-Based Techniques for the Design and Development of Active Biomedical Materials

Raman spectroscopy is an analytical method based on light-matter interactions that can interrogate the vibrational modes of matter and provide representative molecular fingerprints. Mediated by its label-free, non-invasive nature, and high molecular specificity, Raman-based techniques have become ubiquitous tools for in situ characterization of materials. This review comprehensively describes the theoretical and practical background of Raman spectroscopy and its advanced variants. The numerous facets of material characterization that Raman scattering can reveal, including biomolecular identification, solid-to-solid phase transitions, and spatial mapping of biomolecular species in bioactive materials, are highlighted. The review illustrates the potential of these techniques in the context of active biomedical material design and development by highlighting representative studies from the literature. These studies cover the use of Raman spectroscopy for the characterization of both natural and synthetic biomaterials, including engineered tissue constructs, biopolymer systems, ceramics, and nanoparticle formulations, among others. To increase the accessibility and adoption of these techniques, the present review also provides the reader with practical recommendations on the integration of Raman techniques into the experimental laboratory toolbox. Finally, perspectives on how recent developments in plasmon- and coherently-enhanced Raman spectroscopy can propel Raman from underutilized to critical for biomaterial development are provided.

Spatiotemporal formation of a single liquid-like condensate and amyloid fibrils of α-synuclein by optical trapping at solution surface

Liquid-like protein condensates have recently attracted much attention due to their critical roles in biological phenomena. They typically show high fluidity and reversibility for exhibiting biological functions, while occasionally serving as sites for the formation of amyloid fibrils. To comprehend the properties of protein condensates that underlie biological function and pathogenesis, it is crucial to study them at the single-condensate level; however, this is currently challenging due to a lack of applicable methods. Here, we demonstrate that optical trapping is capable of inducing the formation of a single liquid-like condensate of α-synuclein in a spatiotemporally controlled manner. The irradiation of tightly focused near-infrared laser at an air/solution interface formed a condensate under conditions coexisting with polyethylene glycol. The fluorescent dye-labeled imaging showed that the optically induced condensate has a gradient of protein concentration from the center to the edge, suggesting that it is fabricated through optical pumping-up of the α-synuclein clusters and the expansion along the interface. Furthermore, Raman spectroscopy and thioflavin T fluorescence analysis revealed that continuous laser irradiation induces structural transition of protein molecules inside the condensate to β-sheet rich structure, ultimately leading to the condensate deformation and furthermore, the formation of amyloid fibrils. These observations indicate that optical trapping is a powerful technique for examining the microscopic mechanisms of condensate appearance and growth, and furthermore, subsequent aging leading to amyloid fibril formation.

Airborne Acoustic Vortex End Effector-based Contactless, Multi-mode, Programmable Control of Object Surfing

Tweezers based on optical, electric, magnetic, and acoustic fields have shown great potential for contactless object manipulation. However, current tweezers designed for manipulating millimeter-sized objects such as droplets, particles, and small animals, exhibit limitations in translation resolution, range, and path complexity. Here, we introduce a novel acoustic vortex tweezers system, which leverages a unique airborne acoustic vortex end effector integrated with a three degree-of-freedom (DoF) linear motion stage, for enabling contactless, multi-mode, programmable manipulation of millimeter-sized objects. The acoustic vortex end effector utilizes a cascaded circular acoustic array, which is portable and battery-powered, to generate an acoustic vortex with a ring-shaped energy pattern. The vortex applies acoustic radiation forces to trap and spin an object at its center, simultaneously protecting this object by repelling other materials away with its high-energy ring. Moreover, our vortex tweezers system facilitates contactless, multi-mode, programmable object surfing, as demonstrated in experiments involving trapping, repelling, and spinning particles, translating particles along complex paths, guiding particles around barriers, translating and rotating droplets containing zebrafish larvae, and merging droplets. With these capabilities, we anticipate that our tweezers system will become a valuable tool for the automated, contactless handling of droplets, particles, and bio-samples in biomedical and biochemical research.

Chiral whispering gallery modes and chirality-dependent symmetric optical force induced by a spin-polarized surface wave of photonic Dirac semimetal

Dirac degeneracy is a fourfold band crossing point in a three-dimensional momentum space, which possesses Fermi-arc-like surface states, and has extensive application prospects. In this work, we systematically study the exceptional effects of the robust chiral surface wave supported by photonic Dirac semimetal acts on the dielectric particles. Theoretical results show that orthogonal electromagnetic modes and helical or chiral whispering gallery modes (WGMs) of dielectric particles can be efficiently excited by the unidirectional spin-polarized surface wave. More importantly, optical forces exerted by the spin-polarized surface wave exhibit chirality-dependent symmetric behavior and high chiral Q factor with precise size selectivity. Our findings may provide potential applications in the area of chiral microcavity, spin optical devices, and optical manipulations.

Light-Driven, Dynamic Assembly of Micron-To-Centimeter Parts, Micromachines and Microbot Swarms

This work describes light-driven assembly of dynamic formations and functional particle swarms controlled by appropriately programmed light patterns. The system capitalizes on the use of a fluidic bed whose low thermal conductivity assures that light-generated heat remains "localized" and sets strong convective flows in the immediate vicinity of the particles being irradiated. In this way, even low-power laser light or light from a desktop slide projector can be used to organize dynamic formations of objects spanning four orders of magnitude in size (from microns to centimeters) and over nine orders of magnitude in terms of mass. These dynamic assemblies include open-lattice structures with individual particles performing intricate translational and/or rotational motions, density-gradient particle arrays, nested architectures of mechanical components (e.g., planetary gears), or swarms of light-actuated microbots controlling assembly of other objects.

Modular Assembly of Metamaterials Using Light Gradients

This is a report on a pilot study that tests the feasibility of assembling photonic metamaterials (PMs) using light gradient forces. Following a strategy that works like modular construction, light gradient forces, produced by a tightly focused, 1D standing wave optical trap, time-multiplexed across a 2D lattice are used to assemble voxels consisting of prefabricated, monodispersed nanoparticles (NPs) with radii ranging from 30 to 500 nm into 3D structures on a hydrogel scaffold. Hundreds of NPs can be manipulated concurrently into a complex heterogeneous voxel this way, and then the process can be repeated by stitching together voxels to form a metamaterial of any size, shape, and constituency although imperfectly. Imperfections introduce random phase shifts and amplitude variations that can have an adverse effect on the band structure. Regardless, PMs are created this way using two different dielectric NPs, polystyrene and rutile, and then the near-infrared performance for each is analyzed with angle-, wavelength-, and polarization-dependent reflection spectroscopy. The cross-polarized spectra show evidence of a resonance peak. Interestingly, whereas the line shape from the polystyrene array is symmetric, the rutile array is not, which may be indicative of Fano resonance. So, even with the structural defects, reflection spectroscopy reveals a resonance.

Photovoltaic Rotation and Transportation of a Fragile Fluorescent Microrod Toward Assembling a Tunable Light-Source System

Continuous rotation of a fragile, photosensitive microrod in a safe, flexible way remains challenging in spite of its importance to microelectro-mechanical systems. We propose a photovoltaic strategy to continuously rotate a fragile, fluorescent microrod on a LiNbO3/Fe (LN/Fe) substrate using a continuous wave visible (473 nm) laser beam with an ultralow power (few tens of μW) and a simple structure (Gaussian profile). This strategy does not require the laser spot to cover the entire microrod nor does it result in a sharp temperature rise on the microrod. Both experiments and simulation reveal that the strongest photovoltaic field generated beside the laser spot firmly traps one corner of the microrod and the axisymmetric photovoltaic field exerts an electrostatic torque on the microrod driving it to rotate continuously around the laser spot. The dependence of the rotation rate on the laser power indicates contributions from both deep and shallow photovoltaic centers. This rotation mode, combined with the transportation mode, enables the controllable movement of an individual microrod along any complex trajectory with any specific orientation. The tuning of the end-emitting spectrum and the photothermal cutting of the fluorescent microrod are also realized by properly configuring the laser illumination. By taking a microrod as the emitter and a polystyrene microsphere as the focusing lens, we demonstrate the photovoltaic assembly of a microscale light-source system with both spectrum and divergence-angle tunabilities, which are realized by adjusting the photoexcitation position along the microrod and the geometry relationship in the system, respectively.

Photon-efficient optical tweezers via wavefront shaping

Optical tweezers enable noncontact trapping of microscale objects using light. It is not known how tightly it is possible to three-dimensionally (3D) trap microparticles with a given photon budget. Reaching this elusive limit would enable maximally stiff particle trapping for precision measurements on the nanoscale and photon-efficient tweezing of light-sensitive objects. Here, we customize the shape of light fields to suit specific particles, with the aim of optimizing trapping stiffness in 3D. We show, theoretically, that the confinement volume of microspheres held in sculpted optical traps can be reduced by one to two orders of magnitude. Experimentally, we use a wavefront shaping-inspired strategy to passively suppress the Brownian fluctuations of microspheres in every direction concurrently, demonstrating order-of-magnitude reductions in their confinement volumes. Our work paves the way toward the fundamental limits of optical control over the mesoscopic realm.

Nanoalignment by critical Casimir torques

The manipulation of microscopic objects requires precise and controllable forces and torques. Recent advances have led to the use of critical Casimir forces as a powerful tool, which can be finely tuned through the temperature of the environment and the chemical properties of the involved objects. For example, these forces have been used to self-organize ensembles of particles and to counteract stiction caused by Casimir-Liftshitz forces. However, until now, the potential of critical Casimir torques has been largely unexplored. Here, we demonstrate that critical Casimir torques can efficiently control the alignment of microscopic objects on nanopatterned substrates. We show experimentally and corroborate with theoretical calculations and Monte Carlo simulations that circular patterns on a substrate can stabilize the position and orientation of microscopic disks. By making the patterns elliptical, such microdisks can be subject to a torque which flips them upright while simultaneously allowing for more accurate control of the microdisk position. More complex patterns can selectively trap 2D-chiral particles and generate particle motion similar to non-equilibrium Brownian ratchets. These findings provide new opportunities for nanotechnological applications requiring precise positioning and orientation of microscopic objects.

Mass measurement under medium vacuum in optically levitated nanoparticles based on Maxwell speed distribution law

As one of the directions of optical levitation technology, the mass measurement of micro-nano particles has always been a research hotspot in extremely weak mechanical measurements. When nanoscale particles are trapped in an optical trap, parameters such as density, diameter, and shape are unknown. Here we propose what we believe to be a new method to measure mass by fitting particle motion information to the Maxwell speed distribution law, with an accuracy better than 7% at 10 mbar. This method has the characteristics of requiring no external driving force, no precise natural frequency, no prior information such as density, and non-destructive testing within the medium vacuum range. With the increasing iterations, the uncertainty of mass measurement is reduced, and the accuracy of mass measurement of levitated particles is verified under multiple air pressures. It provides what we believe is a new method for the future non-destructive testing of nanoscale particles, and provides an apparently new way for the sensing measurement and metrology application fields of levitation dynamics systems.

Background-free imaging of cold atoms in optical traps

Optical traps, including those used in atomic physics, cold chemistry, and quantum science, are widely used in the research on cold atoms and molecules. Owing to their microscopic structure and excellent operational capability, optical traps have been proposed for cold atom experiments involving complex physical systems, which generally induce violent background scattering. In this study, using a background-free imaging scheme in cavity quantum electrodynamics systems, a cold atomic ensemble was accurately prepared below a fiber cavity and loaded into an optical trap for transfer into the cavity. By satisfying the demanding requirements for the background-free imaging scheme in optical traps, cold atoms in an optical trap were detected with a high signal-to-noise ratio while maintaining atomic loading. The cold atoms were then transferred into the fiber cavity using an optical trap, and the vacuum Rabi splitting was measured, facilitating relevant research on cavity quantum electrodynamics. This method can be extended to related experiments involving cold atoms and molecules in complex physical systems using optical traps.

Acoustic manipulation of multi-body structures and dynamics

Sound can exert forces on objects of any material and shape. This has made the contactless manipulation of objects by intense ultrasound a fascinating area of research with wide-ranging applications. While much is understood for acoustic forcing of individual objects, sound-mediated interactions among multiple objects at close range gives rise to a rich set of structures and dynamics that are less explored and have been emerging as a frontier for research. We introduce the basic mechanisms giving rise to sound-mediated interactions among rigid as well as deformable particles, focusing on the regime where the particles' size and spacing are much smaller than the sound wavelength. The interplay of secondary acoustic scattering, Bjerknes forces, and micro-streaming is discussed and the role of particle shape is highlighted. Furthermore, we present recent advances in characterizing non-conservative and non-pairwise additive contributions to the particle interactions, along with instabilities and active fluctuations. These excitations emerge at sufficiently strong sound energy density and can act as an effective temperature in otherwise athermal systems.

Profiling native pulmonary basement membrane stiffness using atomic force microscopy

Mammalian cells sense and react to the mechanics of their immediate microenvironment. Therefore, the characterization of the biomechanical properties of tissues with high spatial resolution provides valuable insights into a broad variety of developmental, homeostatic and pathological processes within living organisms. The biomechanical properties of the basement membrane (BM), an extracellular matrix (ECM) substructure measuring only ∼100-400 nm across, are, among other things, pivotal to tumor progression and metastasis formation. Although the precise assignment of the Young's modulus E of such a thin ECM substructure especially in between two cell layers is still challenging, biomechanical data of the BM can provide information of eminent diagnostic potential. Here we present a detailed protocol to quantify the elastic modulus of the BM in murine and human lung tissue, which is one of the major organs prone to metastasis. This protocol describes a streamlined workflow to determine the Young's modulus E of the BM between the endothelial and epithelial cell layers shaping the alveolar wall in lung tissues using atomic force microscopy (AFM). Our step-by-step protocol provides instructions for murine and human lung tissue extraction, inflation of these tissues with cryogenic cutting medium, freezing and cryosectioning of the tissue samples, and AFM force-map recording. In addition, it guides the reader through a semi-automatic data analysis procedure to identify the pulmonary BM and extract its Young's modulus E using an in-house tailored user-friendly AFM data analysis software, the Center for Applied Tissue Engineering and Regenerative Medicine processing toolbox, which enables automatic loading of the recorded force maps, conversion of the force versus piezo-extension curves to force versus indentation curves, calculation of Young's moduli and generation of Young's modulus maps, where the pulmonary BM can be identified using a semi-automatic spatial filtering tool. The entire protocol takes 1-2 d.

Wide-Size Range and High Robustness Self-Assembly Micropillars for Capturing Microspheres

Capillary force driven self-assembly micropillars (CFSA-MP) holds immense promise for the manipulation and capture of cells/tiny objects, which has great demands of wide size range and high robustness. Here, we propose a novel method to fabricate size-adjustable and highly robust CFSA-MP that can achieve wide size range and high stability to capture microspheres. First, we fabricate a microholes template with an adjustable aspect ratio using the spatial-temporal shaping femtosecond laser double-pulse Bessel beam-assisted chemical etching technique, and then the micropillars with adjustable aspect ratio are demolded by polydimethylsiloxane (PDMS). We fully demonstrated the advantages of the Bessel optical field by using the spatial-temporal shaping femtosecond laser double-pulse Bessel beams to broaden the height range of the micropillars, which in turn expands the size range of the captured microspheres, and finally achieving a wide range of capturing microspheres with a diameter of 5-410 μm. Based on the inverted mold technology, the PDMS micropillars have ultrahigh mechanical robustness, which greatly improves the durability. CFSA-MP has the ability to capture tiny objects with wide range and high stability, which indicates great potential applications in the fields of chemistry, biomedicine, and microfluidics.

Optical Force Effects of Rayleigh Particles by Cylindrical Vector Beams

High-order cylindrical vector beams possess flexible spatial polarization and exhibit new effects and phenomena that can expand the functionality and enhance the capability of optical systems. However, building a general analytical model for highly focused beams with different polarization orders remains a challenge. Here, we elaborately develop the vector theory of high-order cylindrical vector beams in a high numerical aperture focusing system and achieve the vectorial diffraction integrals for describing the tight focusing field with the space-variant distribution of polarization orders within the framework of Richards-Wolf diffraction theory. The analytical formulae include the exact three Cartesian components of electric and magnetic distributions in the tightly focused region. Additionally, utilizing the analytical formulae, we can achieve the gradient force, scattering force, and curl-spin force exerted on Rayleigh particles trapped by high-order cylindrical vector beams. These results are crucial for improving the design and engineering of the tightly focused field by modulating the polarization orders of high-order cylindrical vector beams, particularly for applications such as optical tweezers and optical manipulation. This theoretical analysis also extends to the calculation of complicated optical vortex vector fields and the design of diffractive optical elements with high diffraction efficiency and resolution.

Magneto-optical-like effect in tight focusing of azimuthally polarized sine-Gaussian beams

Magneto-optical effects, which have been known for over a century, are among the most fundamental phenomena in physics and describe changes in the polarization state of light when it interacts with magnetic materials. When a polarized plane wave propagates in or through a homogeneous and isotropic transparent medium, it is generally accepted that its transverse polarization structure remains unchanged. However, we show that a strong radial polarization component can be generated when an azimuthally polarized sine-Gaussian plane wave is tightly focused by a high numerical aperture lens, resulting in a magneto-optical-like effect that does not require external magnetic field or magnetic medium. Calculations show that the intensity structure and polarization distribution of the highly confined electric field strongly depend on the parameters m and φ0 in the sinusoidal term, where m can be used to control the number of the multifocal spots and φ0 can be used to control the position of each focal spot. Finally, we show that this peculiar electric field distribution can be used to realize multiple particles trapping with controllable numbers and locations.

Quadrupole excitation of atoms with tightly focused Laguerre-Gaussian beams

This article investigates the quadrupole excitation of a trapped atom exposed to the tightly focused Laguerre-Gaussian (LG) beams with parallel and antiparallel spin angular momentum (SAM) and orbital angular momentum (OAM) under nonparaxial conditions. The Rabi frequency profile of allowed quadrupole transition channels, modified by SAM and OAM interaction, in the focal plane is provided. In the case of antiparallel SAM and OAM, the excitation probability undergoes substantial modification due to the considerable contribution of longitudinal intensity variations in tightly focused condition. The findings offer insights into controlling localized atom transition, including OAM transfer, with potential applications in qudit-based technologies.

Three-Dimensional Optothermal Manipulation of Light-Absorbing Particles in Phase-Change Gel Media

Rational manipulation and assembly of discrete colloidal particles into architected superstructures have enabled several applications in materials science and nanotechnology. Optical manipulation techniques, typically operated in fluid media, facilitate the precise arrangement of colloidal particles into superstructures by using focused laser beams. However, as the optical energy is turned off, the inherent Brownian motion of the particles in fluid media impedes the retention and reconfiguration of such superstructures. Overcoming this fundamental limitation, we present on-demand, three-dimensional (3D) optical manipulation of colloidal particles in a phase-change solid medium made of surfactant bilayers. Unlike liquid crystal media, the lack of fluid flow within the bilayer media enables the assembly and retention of colloids for diverse spatial configurations. By utilizing the optically controlled temperature-dependent interactions between the particles and their surrounding media, we experimentally exhibit the holonomic microscale control of diverse particles for repeatable, reconfigurable, and controlled colloidal arrangements in 3D. Finally, we demonstrate tunable light-matter interactions between the particles and 2D materials by successfully manipulating and retaining these particles at fixed distances from the 2D material layers. Our experimental results demonstrate that the particles can be retained for over 120 days without any change in their relative positions or degradation in the bilayers. With the capability of arranging particles in 3D configurations with long-term stability, our platform pushes the frontiers of optical manipulation for distinct applications such as metamaterial fabrication, information storage, and security.

Levitated 2D manipulation on dielectric metasurface by the tuning of polarization states

In this Letter, we have proposed a particle manipulation system based on a polarization-dependent dielectric metasurface (PDM), which enables far-field trapping and 2D arbitrary transporting. Based on flexible phase manipulation, by tuning the size and angle of meta-atoms, polarization-selective focusing in different modules of the metasurface can be realized. Then, when those regional focuses are continuously lighted in a relay way, the trapped particle at the focus could be delivered to the next one. When six different characteristic polarization states are tuned in order, the trapped particle could be transported to any adjacent hot spots so that 2D manipulation can be realized in an extended range. With the consideration of the Brownian motion, our simulation results show that the success rate of the particle transport can reach more than 96.0%, even after 20 periods when excited at the wavelength of 1064 nm with a power density of 0.15 mW/µm2. We believe that our research provides a new and promising method for particle manipulation and furthers on-chip optofluidic applications.

Key Role of Asymmetric Photothermal Effect in Selectively Chiral Switching of Plasmonic Dimer Driven by Circularly Polarized Light

Strong interaction between circularly polarized light and chiral plasmonic nanostructures can enable controllable asymmetric photophysical processes, such as selective chiral switching of a plasmonic nanorod-dimer. Here, we uncover the underlying physics that governs this chiral switching by theoretically investigating the interplay between asymmetric photothermal and optomechanical effects. We find that the photothermally induced local temperature rises could play a key role in activating the dynamic chiral configurations of a plasmonic dimer due to the temperature-sensitive molecular linkages located at the gap region. Importantly, different temperature rises caused by the opposite handedness of light could facilitate selective chiral switching of the plasmonic dimer driven by asymmetric optical torques. Our analyses on the wavelength-dependent selectively chiral switching behaviors are in good agreement with the experimental observations. This work contributes to a comprehensive understanding of the physical mechanism involved in the experimentally designed photoresponsive plasmonic nanosystems for practical applications.

Switchable rotation of metal nanostructures in an intensity chirality-invariant focus field

Light-induced rotation is a fundamental motion form that is of great significance for flexible and multifunctional manipulation modes. However, current optical rotation by a single optical field is mostly unidirectional, where switchable rotation manipulation is still challenging. To address this issue, we demonstrate a switchable rotation of non-spherical nanostructures within a single optical focus field. Interestingly, the intensity of the focus field is chiral invariant. The rotation switch is a result of the energy flux reversal in front and behind the focal plane. We quantitatively analyze the optical force exerted on a metal nanorod at different planes, as well as the surrounding energy flux. Our experimental results indicate that the direct switchover of rotational motion is achievable by adjusting the relative position of the nanostructure to the focal plane. This result enriches the basic motion mode of micro-manipulation and is expected to create potential opportunities in many application fields, such as biological cytology and optical micromachining.

Scattering of a radially polarized Bessel beam by a PEMC sphere: photonic nanojet and bottle beam formation

The scattering of a radially polarized (r p) Bessel vortex and nonvortex beam by a perfect electromagnetic conductor (PEMC) sphere is studied based on the generalized Lorenz-Mie theory. The electric and magnetic fields of the incident arbitrary-shaped polarized beams are constructed using vector spherical wave functions (VSWFs) and beam shape coefficients. The analytical expression of the scattered field is expanded using VSWFs and scattering coefficients, which are derived by considering PEMC boundary conditions. The expression of the normalized dimensionless far-field scattering intensity (NDFSI) is also defined and derived. The photonic nanojet (PNJ) and the "bottle beam" generated by the interaction between the PEMC sphere and the vortex and nonvortex Bessel beam under r p are emphasized in this paper. Moreover, the intensity and directivity of NDFSI are also considered. It has been found that the generation of the PNJ and the "bottle beam" is determined by the half-cone angle α 0 of the r p Bessel beam and admittance parameter M of the PEMC sphere. Furthermore, the influence of M, α 0, and integer order l of the Bessel beam on the intensity and distribution of NDFSI is also discussed. The findings are important in the research on meta-materials and promising prospects in microwave engineering, antenna engineering, imaging, subwavelength focusing, optical radiation force, and torque.

Carbon Dots for Electroluminescent Light-Emitting Diodes: Recent Progress and Future Prospects

Carbon dots (CDs), as emerging carbon nanomaterials, have been regarded as promising alternatives for electroluminescent light-emitting diodes (LEDs) owing to their distinct characteristics, such as low toxicity, tuneable photoluminescence, and good photostability. In the last few years, despite remarkable progress achieved in CD-based LEDs, their device performance is still inferior to that of well-developed organic, heavy-metal-based QDs, and perovskite LEDs. To better exploit LED applications and boost device performance, in this review, a comprehensive overview of currently explored CDs is presented, focusing on their key optical characteristics, which are closely related to the structural design of CDs from their carbon core to surface modifications, and to macroscopic structural engineering, including the embedding of CDs in the matrix or spatial arrangement of CDs. The design of CD-based LEDs for display and lighting applications based on the fluorescence, phosphorescence, and delayed fluorescence emission of CDs is also highlighted. Finally, it is concluded with a discussion regarding the key challenges and plausible prospects in this field. It is hoped that this review inspires more extensive research on CDs from a new perspective and promotes practical applications of CD-based LEDs in multiple directions of current and future research.

Formation of Two-Dimensional Magnetically Responsive Clusters Using Hematite Particles Self-Assembled via Particle-Induced Heating at an Interface

Hematite particles, which exhibit a high magnetic moment, are used to apply large forces on physical and biological systems under magnetic fields to investigate various phenomena, such as those of rheology and micromanipulation. However, the magnetic confinement of these particles requires complicated field configurations. On the other hand, laser-assisted optical confinement of single hematite particles results in thermophoresis and subsequent ejection of the particle from the laser spot. Herein, we explore an alternative strategy to induce the self-assembly of hematite. In this strategy, with indirect influence from an optically confined and heated upconverting particle (UCP) at an air-water interface, there is the generation of convection currents that facilitate assembly. We also show that the assembly remains at the interface even after removal of the laser light. The hematite particle assemblies can then be moved using magnetic fields and employed to perform interfacial rheology.

Nonlinear effects in optical trapping of titanium dioxide and diamond nanoparticles

Optical trapping in biophysics typically uses micron-scale beads made of materials like polystyrene or glass to probe the target of interest. Using smaller beads made of higher-index materials could increase the time resolution of these measurements. We characterized the trapping of nanoscale beads made of diamond and titanium dioxide (TiO2) in a single-beam gradient trap. Calculating theoretical expectations for the trapping stiffness of these beads, we found good agreement with measured values. Trap stiffness was significantly higher for TiO2 beads, owing to notable enhancement from nonlinear optical effects, not previously observed for continuous-wave trapping. Trap stiffness was over 6-fold higher for TiO2 beads than polystyrene beads of similar size at 70 mW laser power. These results suggest that diamond and TiO2 nanobeads can be used to improve time resolution in optical tweezers measurements.

A Review of Single-Cell Microrobots: Classification, Driving Methods and Applications

Single-cell microrobots are new microartificial devices that use a combination of single cells and artificial devices, with the advantages of small size, easy degradation and ease of manufacture. With externally driven strategies such as light fields, sound fields and magnetic fields, microrobots are able to carry out precise micromanipulations and movements in complex microenvironments. Therefore, single-cell microrobots have received more and more attention and have been greatly developed in recent years. In this paper, we review the main classifications, control methods and recent advances in the field of single-cell microrobot applications. First, different types of robots, such as cell-based microrobots, bacteria-based microrobots, algae-based microrobots, etc., and their design strategies and fabrication processes are discussed separately. Next, three types of external field-driven technologies, optical, acoustic and magnetic, are presented and operations realized in vivo and in vitro by applying these three technologies are described. Subsequently, the results achieved by these robots in the fields of precise delivery, minimally invasive therapy are analyzed. Finally, a short summary is given and current challenges and future work on microbial-based robotics are discussed.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Analytical Models for Measuring the Mechanical Properties of Yeast

The mechanical properties of yeast play an important role in many biological processes, such as cell division and growth, maintenance of internal pressure, and biofilm formation. In addition, the mechanical properties of cells can indicate the degree of damage caused by antifungal drugs, as the mechanical parameters of healthy and damaged cells are different. Over the past decades, atomic force microscopy (AFM) and micromanipulation have become the most widely used methods for evaluating the mechanical characteristics of microorganisms. In this case, the reliability of such an estimate depends on the choice of mathematical model. This review presents various analytical models developed in recent years for studying the mechanical properties of both cells and their individual structures. The main provisions of the applied approaches are described along with their limitations and advantages. Attention is paid to the innovative method of low-invasive nanomechanical mapping with scanning ion-conductance microscopy (SICM), which is currently starting to be successfully used in the discovery of novel drugs acting on the yeast cell wall and plasma membrane.

All-Optical Rapid Formation, Transport, and Sustenance of a Sessile Droplet in a Two-Dimensional Slit with Few-Micrometer Separation

We present a sessile droplet manipulation platform that enables the formation and transport of a droplet on a light-absorbing surface via local laser-beam irradiation. The mechanism relies on solutocapillary Marangoni flow arising from a concentration gradient in a binary mixture liquid. Because the mixture is strongly confined in a two-dimensional slit with a spacing of a few micrometers, the wetting film is stably sustained, enabling the rapid formation, deformation, and transport of a sessile droplet. In addition, to sustain the droplet in the absence of laser irradiation, we developed a method to bridge the droplet between the top and bottom walls of the slit. The bridge is stably sustained because of the hydrophilicity of the slit wall. Splitting and merging of the droplet bridges are also demonstrated.

Nonlinearity-Induced Optical Torque

Optically induced mechanical torque driving rotation of small objects requires the presence of absorption or breaking cylindrical symmetry of a scatterer. A spherical nonabsorbing particle cannot rotate due to the conservation of the angular momentum of light upon scattering. Here, we suggest a novel physical mechanism for the angular momentum transfer to nonabsorbing particles via nonlinear light scattering. The breaking of symmetry occurs at the microscopic level manifested in nonlinear negative optical torque due to the excitation of resonant states at the harmonic frequency with higher projection of angular momentum. The proposed physical mechanism can be verified with resonant dielectric nanostructures, and we suggest some specific realizations.

Nanoparticles for Interrogation of Cell Signaling

Cell functions rely on signal transduction-the cascades of molecular interactions and biochemical reactions that relay extracellular signals to the cell interior. Dissecting principles governing the signal transduction process is critical for the fundamental understanding of cell physiology and the development of biomedical interventions. The complexity of cell signaling is, however, beyond what is accessible by conventional biochemistry assays. Thanks to their unique physical and chemical properties, nanoparticles (NPs) have been increasingly used for the quantitative measurement and manipulation of cell signaling. Even though research in this area is still in its infancy, it has the potential to yield new, paradigm-shifting knowledge of cell biology and lead to biomedical innovations. To highlight this importance, we summarize in this review studies that pioneered the development and application of NPs for cell signaling, from quantitative measurements of signaling molecules to spatiotemporal manipulation of cell signal transduction.

Creating multiple ultra-long longitudinal magnetization textures by strongly focusing azimuthally polarized circular Airy vortex beams

We come up with a simple feasible scheme for the creation of multiple ultra-long longitudinal magnetization textures. This is realized by directly strongly focusing azimuthally polarized circular Airy vortex beams onto an isotropic magneto-optical medium based on the vectorial diffraction theory and the inverse Faraday effect. It is found that, by jointly tuning the intrinsic parameters (i. e. the radius of main ring, the scaling factor, and the exponential decay factor) of the incoming Airy beams and the topological charges of the optical vortices, we are able to garner not only super-resolved scalable magnetization needles as usual, but also steerable magnetization oscillations and nested magnetization tubes with opposite polarities for the first time. These exotic magnetic behaviors depend on the extended interplay between the polarization singularity of multi-ring structured vectorial light fields and the additional vortex phase. The findings demonstrated are of great interest in opto-magnetism and emerging classical or quantum opto-magnetic applications.

Opto-thermoelectric trapping of fluorescent nanodiamonds on plasmonic nanostructures

Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this Letter, we show how chemically prepared gold nanoparticles and silver nanowires can facilitate an opto-thermoelectric force to trap individual entities of FNDs using a long working distance lens, low power-density illumination (532-nm laser, 12 µW/µm²). Our trapping configuration combines the thermoplasmonic fields generated by individual plasmonic nanoparticles and the opto-thermoelectric effect facilitated by the surfactant to realize a nano-optical trap down to a single FND that is 120 nm in diameter. We use the same trapping excitation source to capture the spectral signatures of single FNDs and track their position. By tracking the FND, we observe the differences in the dynamics of the FND around different plasmonic structures. We envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells.

Single-Photon Emitting Arrays by Capillary Assembly of Colloidal Semiconductor CdSe/CdS/SiO2 Nanocrystals

The controlled placement of colloidal semiconductor nanocrystals (NCs) onto planar surfaces is crucial for scalable fabrication of single-photon emitters on-chip, which are critical elements of optical quantum computing, communication, and encryption. The positioning of colloidal semiconductor NCs such as metal chalcogenides or perovskites is still challenging, as it requires a nonaggressive fabrication process to preserve the optical properties of the NCs. In this work, periodic arrays of 2500 nanoholes are patterned by electron beam lithography in a poly(methyl methacrylate) (PMMA) thin film on indium tin oxide/glass substrates. Colloidal core/shell CdSe/CdS NCs, functionalized with a SiO₂ capping layer to increase their size and facilitate deposition into 100 nm holes, are trapped with a close to optimal Poisson distribution into the PMMA nanoholes via a capillary assembly method. The resulting arrays of NCs contain hundreds of single-photon emitters each. We believe this work paves the way to an affordable, fast, and practical method for the fabrication of nanodevices, such as single-photon-emitting light-emitting diodes based on colloidal semiconductor NCs.

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.