Nanophotonic Array-Induced Dynamic Behavior for Label-Free Shape-Selective Bacteria Sieving

Nanoparticle Deep-Subwavelength Dynamics Empowered by Optical Meron-Antimeron Topology

Optical meron is a type of nonplanar topological texture mainly observed in surface plasmon polaritons and highly symmetric points of photonic crystals in the reciprocal space. Here, we report Poynting-vector merons formed at the real space of a photonic crystal for a Γ-point illumination. Optical merons can be utilized for subwavelength-resolution manipulation of nanoparticles, resembling a topological Hall effect on electrons via magnetic merons. In particular, staggered merons and antimerons impose strong radiation pressure on large gold nanoparticles (AuNPs), while focused hot spots in antimerons generate dominant optical gradient forces on small AuNPs. Synergistically, differently sized AuNPs in a still environment can be trapped or orbit in opposite directions, mimicking a coupled galaxy system. They can also be separated with a 10 nm precision when applying a flow velocity of >1 mm/s. Our study unravels a novel way to exploit topological textures for optical manipulation with deep-subwavelength precision and switchable topology in a lossless environment.

A microfluidic hemostatic diagnostics platform: Harnessing coagulation-induced adaptive-bubble behavioral perception

Clinical viscoelastic hemostatic assays, which have been used for decades, rely on measuring biomechanical responses to physical stimuli but face challenges related to high device and test cost, limited portability, and limited scalability.. Here, we report a differential pattern using self-induced adaptive-bubble behavioral perception to refresh it. The adaptive behaviors of bubble deformation during coagulation precisely describe the transformation of viscoelastic hemostatic properties, being free of the precise and complex physical devices. And the integrated bubble array chip allows microassays and enables multi-bubble tests with good reproducibility. Recognition of the developed bubble behaviors empowers automated and user-friendly diagnosis. In a prospective clinical study (clinical model development [n = 273]; clinical assay [n = 44]), we show that the diagnostic accuracies were 99.1% for key viscoelastic hemostatic assay indicators (reaction time [R], kinetics time [K], alpha angle [Angle], maximum amplitude [MA], lysis at 30 min [LY30]; n = 220) and 100% (n = 44) for hypercoagulation, healthy, and hypocoagulation diagnoses. This should provide fresh insight into existing paradigms and help more clinical needs.

Acoustic quasi-periodic bioassembly based diverse stem cell arrangements for differentiation guidance

Arrangement patterns and geometric cues have been demonstrated to influence cell function and fate, which calls for efficient and versatile cell patterning techniques. Despite constant achievements that mainly focus on individual cells and uniform cell patterns, simultaneously constructing cellular arrangements with diverse patterns and positional relationships in a flexible and contact-free manner remains a challenge. Here, stem cell arrangements possessing multiple geometries and structures are proposed based on powerful and diverse pattern-building capabilities of quasi-periodic acoustic fields, with advantages of rich patterns and structures and flexibility in structure modulation. Eight-fold waves' interference produces regular potentials that result in higher rotational symmetry and more complex arrangement of geometric units. Moreover, through flexible modulation of the phase relations among these wave vectors, a wide variety of cellular pattern units are arranged in this potential, such as circular-, triangular- and square-shape, simultaneously. It is proved that these diverse cellular patterns conveniently build human mesenchymal stem cell (hMSC) models, for research on the effect of cellular arrangement on stem cell differentiation. This work fills the gap of acoustic cell patterning in quasi-periodic patterns and shows promising potential in tissue engineering and regenerative medicine.

Optofluidic Tweezers: Efficient and Versatile Micro/Nano-Manipulation Tools

Optical tweezers (OTs) can transfer light momentum to particles, achieving the precise manipulation of particles through optical forces. Due to the properties of non-contact and precise control, OTs have provided a gateway for exploring the mysteries behind nonlinear optics, soft-condensed-matter physics, molecular biology, and analytical chemistry. In recent years, OTs have been combined with microfluidic chips to overcome their limitations in, for instance, speed and efficiency, creating a technology known as "optofluidic tweezers." This paper describes static OTs briefly first. Next, we overview recent developments in optofluidic tweezers, summarizing advancements in capture, manipulation, sorting, and measurement based on different technologies. The focus is on various kinds of optofluidic tweezers, such as holographic optical tweezers, photonic-crystal optical tweezers, and waveguide optical tweezers. Moreover, there is a continuing trend of combining optofluidic tweezers with other techniques to achieve greater functionality, such as antigen-antibody interactions and Raman tweezers. We conclude by summarizing the main challenges and future directions in this research field.

Microfluidic systems for particle capture and release: A review

Microfluidic technology has emerged as a promising tool in various applications, including biosensing, disease diagnosis, and environmental monitoring. One of the notable features of microfluidic devices is their ability to selectively capture and release specific cells, biomolecules, bacteria, and particles. Compared to traditional bulk analysis instruments, microfluidic capture-and-release platforms offer several advantages, such as contactless operation, label-free detection, high accuracy, good sensitivity, and minimal reagent requirements. However, despite significant efforts dedicated to developing innovative capture mechanisms in the past, the release and recovery efficiency of trapped particles have often been overlooked. Many previous studies have focused primarily on particle capture techniques and their efficiency, disregarding the crucial role of successful particle release for subsequent analysis. In reality, the ability to effectively release trapped particles is particularly essential to ensure ongoing, high-throughput analysis. To address this gap, this review aims to highlight the importance of both capture and release mechanisms in microfluidic systems and assess their effectiveness. The methods are classified into two categories: those based on physical principles and those using biochemical approaches. Furthermore, the review offers a comprehensive summary of recent applications of microfluidic platforms specifically designed for particle capture and release. It outlines the designs and performance of these devices, highlighting their advantages and limitations in various target applications and purposes. Finally, the review concludes with discussions on the current challenges faced in the field and presents potential future directions.

Stable optical lateral forces from inhomogeneities of the spin angular momentum

Transverse spin momentum related to the spin angular momentum (SAM) of light has been theoretically studied recently and predicted to generate an intriguing optical lateral force (OLF). Despite extensive studies, there is no direct experimental evidence of a stable OLF resulting from the dominant SAM rather than the ubiquitous spin-orbit interaction in a single light beam. Here, we theoretically unveil the nontrivial physics of SAM-correlated OLF, showing that the SAM is a dominant factor for the OLF on a nonabsorbing particle, while an additional force from the canonical (orbital) momentum is exhibited on an absorbing particle due to the spin-orbit interaction. Experimental results demonstrate the bidirectional movement of 5-μm-diameter particles on both sides of the beam with opposite spin momenta. The amplitude and sign of this force strongly depend on the polarization. Our optofluidic platform advances the exploitation of exotic forces in systems with a dominant SAM, facilitating the exploration of fascinating light-matter interactions.

Recent Progress on Optical Micro/Nanomanipulations: Structured Forces, Structured Particles, and Synergetic Applications

Optical manipulation has achieved great success in the fields of biology, micro/nano robotics and physical sciences in the past few decades. To date, the optical manipulation is still witnessing substantial progress powered by the growing accessibility of the complex light field, advanced nanofabrication and developed understandings of light-matter interactions. In this perspective, we highlight recent advancements of optical micro/nanomanipulations in cutting-edge applications, which can be fostered by structured optical forces enabled with diverse auxiliary multiphysical field/forces and structured particles. We conclude with our vision of ongoing and futuristic directions, including heat-avoided and heat-utilized manipulation, nonlinearity-mediated trapping and manipulation, metasurface/two-dimensional material based optical manipulation, as well as interface-based optical manipulation.

Nanoparticle manipulation using plasmonic optical tweezers based on particle sizes and refractive indices

As an effective tool for micro/nano-scale particle manipulation, plasmonic optical tweezers can be used to manipulate cells, DNA, and macromolecules. Related research is of great significance to the development of nanoscience. In this work, we investigated a sub-wavelength particle manipulation technique based on plasmonic optical tweezers. When the local plasmonic resonance is excited on the gold nanostructure arrays, the local electromagnetic field will be enhanced to generate a strong gradient force acting on nanoparticles, which could achieve particle sorting in sub-wavelength scale. On this basis, we explored the plasmonic enhancement effect of the sorting device and the corresponding optical force and optical potential well distributions. Additionally, the sorting effect of the sorting device was investigated in statistical methods, which showed that the sorting device could effectively sort particles of different diameters and refractive indices.

Metasurface Micro/Nano-Optical Sensors: Principles and Applications

Metasurfaces are 2D artificial materials consisting of arrays of metamolecules, which are exquisitely designed to manipulate light in terms of amplitude, phase, and polarization state with spatial resolutions at the subwavelength scale. Traditional micro/nano-optical sensors (MNOSs) pursue high sensitivity through strongly localized optical fields based on diffractive and refractive optics, microcavities, and interferometers. Although detections of ultra-low concentrations of analytes have already been demonstrated, the label-free sensing and recognition of complex and unknown samples remain challenging, requiring multiple readouts from sensors, e.g., refractive index, absorption/emission spectrum, chirality, etc. Additionally, the reliability of detecting large, inhomogeneous biosamples may be compromised by the limited near-field sensing area from the localization of light. Here, we review recent advances in metasurface-based MNOSs and compare them with counterparts using micro-optics from aspects of physics, working principles, and applications. By virtue of underlying the physics and design flexibilities of metasurfaces, MNOSs have now been endowed with superb performances and advanced functionalities, leading toward highly integrated smart sensing platforms.

Inverse Optical Torques on Dielectric Nanoparticles in Elliptically Polarized Light Waves

Elliptically polarized light waves carry the spin angular momentum (SAM), so they can exert optical torques on nanoparticles. Usually, the rotation follows the same direction as the SAM due to momentum conservation. It is counterintuitive to observe the reversal of optical torque acting on an ordinary dielectric nanoparticle illuminated by an elliptically or circularly polarized light wave. Here, we demonstrate that negative optical torques, which are opposite to the direction of SAM, can ubiquitously emerge when elliptically polarized light waves are impinged on dielectric nanoparticles obliquely. Intriguingly, the rotation can be switched between clockwise and counterclockwise directions by controlling the incident angle of light. Our study suggests a new playground to harness polarization-dependent optical force and torque for advancing optical manipulations.

Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles

Circularly polarized light carries spin angular momentum, so it can exert an optical torque on the polarization-anisotropic particle by the spin momentum transfer. Here, we show that giant positive and negative optical torques on Mie-resonant (gain) particles arise from the emergence of superhybrid modes with magnetic multipoles and electric toroidal moments, excited by linearly polarized beams. Anomalous positive and negative torques on particles (doped with judicious amount of dye molecules) are over 800 and 200 times larger than the ordinary lossy counterparts, respectively. Meanwhile, a rotational motor can be configured by switching the s- and p-polarized beams, exhibiting opposite optical torques. These giant and reversed optical torques are unveiled for the first time in the scattering spectrum, paving another avenue toward exploring unprecedented physics of hybrid and superhybrid multipoles in metaoptics and optical manipulations.

Reconfigurable label-free shape-sieving of submicron particles in paired chalcogenide waveguides

Up-to-date particle sieving schemes face formidable challenges for sieving label-free submicron molecules with similar sizes and dielectric constants but diverse shapes. Herein, optical sorting of polystyrene particles with various shapes is illustrated in optofluidic nanophotonic paired waveguide (ONPW) composed of chalcogenide semiconductor Sb₂Se₃. The Sb₂Se₃-ONPW creates the coupling length (C_L) between the neighboring hot spots that can be actively modulated via the transition of Sb₂Se₃ between amorphous (AM) and crystalline (CR) phases. Submicron particles interfere with the coupled hotspots, which can exert various optical torques on the particles according to their profiles. In the model system, spherical (diameter of 0.5 μm) and rod-shaped (diameter of 0.5 μm, length of 1.5 μm) polystyrene particles were employed to mimic two types of bacteria, namely, Staphylococcus aureus and rod-shaped Escherichia coli, respectively. For the AM state, the C_L value is ∼7.0 μm, enabling the structure to trap the sphere stably in the hot spots. For the CR state, the C_L value becomes ∼25 μm, leading to stable trapping of the rod-shaped particle. In this work, the working wavelength was fixed at 1.55 μm at which both AM- and CR-Sb₂Se₃ are transparent. Our scheme may offer a paradigm shift in shape-selective sieving of biomolecules and fulfill the requirements of the new-generation lab-on-chip techniques, where the integrated manipulation system must be much more multifunctional and flexible.

Quantifying the force in flow-cell based single-molecule stretching experiments

The flow-cell based single-molecule manipulation technique has found many applications in the study of DNA mechanics and protein-DNA interactions. However, the force in these experiments has not been fully characterized and is usually limited to a moderate force regime (<25 pN). In this work, using the "tethered-bead" assay, the hydrodynamic drag of DNA has been quantitatively evaluated based on a "bead-spring chain" model. The force derived from the Brownian motion of the bead thus contains both contributions from this equivalent hydrodynamic drag of DNA and the pulling force from the tethered bead. Next, using flow-cell based DNA pulling experiments, the linear relationship between the flow rate and total hydrodynamic force on the bead-DNA system has been demonstrated to be valid over a wide force range (0-110 pN). Consequently, the force can be directly converted from the flow rate by a linear factor that can be calibrated either by the bead's Brownian motion at low flow rates or using DNA overstretching transition. Furthermore, the hydrodynamic force and torque due to the shear flow on the bead as well as the equivalent stretching force on DNA are calculated based on theoretical models with the hydrodynamic drag on DNA also considered. The calculated force-extension curves show a good agreement with the measured ones. These results offer important insights into the force in flow-cell based single-molecule stretching experiments and provide a foundation for establishing flow-cells as a simple, low-cost, yet flexible and precise tool for single-molecule force measurements over a wide force range.

Trapping and Detection of Single Viruses in an Optofluidic Chip

Accurate single virus detection is critical for disease diagnosis and early prevention, especially in view of current pandemics. Numerous detection methods have been proposed with the single virus sensitivity, including the optical approaches and immunoassays. However, few of them hitherto have the capability of both trapping and detection of single viruses in the microchannel. Here, we report an optofluidic potential well array to trap nanoparticles stably in the flow stream. The nanoparticle is bound with single viruses and fluorescence quantum dots through an immunolabeling protocol. Single viruses can be swiftly captured in the microchannel by optical forces and imaged by a camera. The number of viruses in solution and on each particle can be quantified via image processing. Our method can trap and detect single viruses in the 1 mL serum or water in 2 h, paving an avenue for the advanced, fast, and accurate clinical diagnosis, as well as the study of virus infectivity, mutation, drug inhibition, etc.

Plasmonic Optical Tweezers for Particle Manipulation: Principles, Methods, and Applications

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed.

On-Chip Optical Detection of Viruses: A Review

The current outbreak of the coronavirus disease-19 (COVID-19) pandemic worldwide has caused millions of fatalities and imposed a severe impact on our daily lives. Thus, the global healthcare system urgently calls for rapid, affordable, and reliable detection toolkits. Although the gold-standard nucleic acid amplification tests have been widely accepted and utilized, they are time-consuming and labor-intensive, which exceedingly hinder the mass detection in low-income populations, especially in developing countries. Recently, due to the blooming development of photonics, various optical chips have been developed to detect single viruses with the advantages of fast, label-free, affordable, and point of care deployment. Herein, optical approaches especially in three perspectives, e.g., flow-free optical methods, optofluidics, and surface-modification-assisted approaches, are summarized. The future development of on-chip optical-detection methods in the wave of emerging new ideas in nanophotonics is also briefly discussed.

Microfluidic channel integrated with a lattice lightsheet microscopic system for continuous cell imaging

In this study, a continuous cell-imaging system with subcellular resolution was developed by integrating a microfluidic platform with lattice lightsheet microscopy (LLSM). To reduce aberrations of the lightsheet propagating into the device, a microfluidic channel sealed with a water refractive index-matched thin film was fabricated. When the lightsheet emerged from the water-immersed objectives and penetrated through the water refractive-matched thin film into the microfluidic channel at an incident angle, less light scattering and fewer aberrations were found. Suspended cells flowed across the lattice lightsheet, and an imaging system with the image plane perpendicular to the lightsheet was used to sequentially acquire cell images. By applying a thinner lattice lightsheet, higher-resolution, higher-contrast images were obtained. Furthermore, three-dimensional cell images could be achieved by reconstructing sequential two-dimensional cell images.

Microfluidic Isolation and Enrichment of Nanoparticles

Over the past decades, nanoparticles have increased in implementation to a variety of applications ranging from high-efficiency electronics to targeted drug delivery. Recently, microfluidic techniques have become an important tool to isolate and enrich populations of nanoparticles with uniform properties (e.g., size, shape, charge) due to their precision, versatility, and scalability. However, due to the large number of microfluidic techniques available, it can be challenging to identify the most suitable approach for isolating or enriching a nanoparticle of interest. In this review article, we survey microfluidic methods for nanoparticle isolation and enrichment based on their underlying mechanisms, including acoustofluidics, dielectrophoresis, filtration, deterministic lateral displacement, inertial microfluidics, optofluidics, electrophoresis, and affinity-based methods. We discuss the principles, applications, advantages, and limitations of each method. We also provide comparisons with bulk methods, perspectives for future developments and commercialization, and next-generation applications in chemistry, biology, and medicine.

Machine Learning-Based Pipeline for High Accuracy Bioparticle Sizing

High accuracy measurement of size is essential in physical and biomedical sciences. Various sizing techniques have been widely used in sorting colloidal materials, analyzing bioparticles and monitoring the qualities of food and atmosphere. Most imaging-free methods such as light scattering measure the averaged size of particles and have difficulties in determining non-spherical particles. Imaging acquisition using camera is capable of observing individual nanoparticles in real time, but the accuracy is compromised by the image defocusing and instrumental calibration. In this work, a machine learning-based pipeline is developed to facilitate a high accuracy imaging-based particle sizing. The pipeline consists of an image segmentation module for cell identification and a machine learning model for accurate pixel-to-size conversion. The results manifest a significantly improved accuracy, showing great potential for a wide range of applications in environmental sensing, biomedical diagnostical, and material characterization.

Optical Forces: From Fundamental to Biological Applications

Optical forces, generally arising from changes of field gradients or linear momentum carried by photons, form the basis for optical trapping and manipulation. Advances in optical forces help to reveal the nature of light-matter interactions, giving answers to a wide range of questions and solving problems across various disciplines, and are still yielding new insights in many exciting sciences, particularly in the fields of biological technology, material applications, and quantum sciences. This review focuses on recent advances in optical forces, ranging from fundamentals to applications for biological exploration. First, the basics of different types of optical forces with new light-matter interaction mechanisms and near-field techniques for optical force generation beyond the diffraction limit with nanometer accuracy are described. Optical forces for biological applications from in vitro to in vivo are then reviewed. Applications from individual manipulation to multiple assembly into functional biophotonic probes and soft-matter superstructures are discussed. At the end future directions for application of optical forces for biological exploration are provided.

Biotoxoid Photonic Sensors with Temperature Insensitivity Using a Cascade of Ring Resonator and Mach-Zehnder Interferometer

The great advances in silicon photonic-sensing technology have made it an attractive platform for wide sensing applications. However, most silicon photonic-sensing platforms suffer from high susceptibility to the temperature fluctuation of an operating environment. Additional complex and costly chemical signal-enhancement strategies are usually required to improve the signal-to-noise ratio (SNR). Here, a biotoxoid photonic sensor that is resistant to temperature fluctuation has been demonstrated. This novel sensor consists of a ring resonator coupled to a Mach-Zehnder interferometer (MZI) readout unit. Instead of using costly wavelength interrogation, our photonic sensor directly measures the light intensity ratio between the two output ports of MZI. The temperature dependence (TD)-controlling section of the MZI is used to eliminate the adverse effects of ambient temperature fluctuation. The simulation and experimental results show a linear relationship between the interrogation function and the concentration of an analyte under operation conditions. The thermal drift of the proposed sensor is just 0.18%, which is a reduction of 567-fold for chemical sensing and 28-fold for immuno-biosensing compared to the conventional single-ring resonator. The SNR increases from 6.85 to 19.88 dB within a 2 °C temperature variation. The high SNR optical sensor promises great potential for amplification-free detection of nucleic acids and other biomarkers.

Optofluidic gradient refractive index resonators using liquid diffusion for tunable unidirectional emission

Resonators have been used in a wide range of fields, such as biochemical detection and microscale lasers. In recent years, optofluidic resonators have attracted a significant amount of attention owing to their unique liquid environments. Liquids containing biochemical samples can be designed to pass through the ring resonators or to directly form droplets, for sample sensing. Liquid diffusion is an important property in optofluidic applications, such as gradient refractive index lenses and waveguides. However, liquid diffusion has not been used in the study of optofluidic resonators, for both possible sensing characteristics, and unidirectional emission that is mostly acted as light sources. Here, we introduce a gradient refractive index profile formed by liquid diffusion in annular channels into a circular resonator, forming a gradient-index resonator with a tunable unidirectional emission. For both simulations and experiments, the squeezed and non-rotationally symmetrical light intensity profile was first obtained in a circular resonator. The squeezed light profile enables unidirectional emission in circular resonators, which is difficult to achieve in conventional ones. The squeezed light profile and unidirectional emission are determined by the refractive index difference of the liquids used, the dimension of the circular channels, and the working wavelengths. In experiments, different dimensions of bending radii were demonstrated and a tunable squeezed light intensity profile and unidirectional emission were exhibited. Interestingly, the squeezed coefficient of light, which was about 1.8 for a bending radius of 100 μm, enabled emission with a divergence angle as small as 14 degrees, which could be used for laser emission applications in the future. This work reveals the significant potential of the novel liquid gradient refractive index resonator, which provides a practicable approach for optofluidic resonator emission applications and also has potential for use in optofluidic sensing based on the squeezed light profile.

Extraordinary Multipole Modes and Ultra-Enhanced Optical Lateral Force by Chirality

Strong mode coupling and Fano resonances arisen from exceptional interaction between resonant modes in single nanostructures have raised much attention for their advantages in nonlinear optics, sensing, etc. Individual electromagnetic multipole modes such as quadrupoles, octupoles, and their counterparts from mode coupling (toroidal dipole and nonradiating anapole mode) have been well investigated in isolated or coupled nanostructures with access to high Q factors in bound states in the continuum. Albeit the extensive study on ordinary dielectric particles, intriguing aspects of light-matter interactions in single chiral nanostructures is lacking. Here, we unveil that extraordinary multipoles can be simultaneously superpositioned in a chiral nanocylinder, such as two toroidal dipoles with opposite moments, and electric and magnetic sextupoles. The induced optical lateral forces and their scattering cross sections can thus be either significantly enhanced in the presence of those multipoles with high-Q factors, or suppressed by the bound states in the continuum. This work for the first time reveals the complex correlation between multipolar effects, chiral coupling, and optical lateral force, providing a distinct way for advanced optical manipulation.

Optical Potential-Well Array for High-Selectivity, Massive Trapping and Sorting at Nanoscale

Optical tweezers are versatile tools capable of sorting microparticles, yet formidable challenges are present in the separation of nanoparticles smaller than 200 nm. The difficulties arise from the controversy on the requirement of a tightly focused light spot in order to create strong optical forces while a large area is kept for the sorting. To overcome this problem, we create a near-field potential well array with connected tiny hotspots in a large scale. This situation can sort nanoparticles with sizes from 100 to 500 nm, based on the differentiated energy depths of each potential well. In this way, nanoparticles of 200, 300, and 500 nm can be selectively trapped in this microchannel by appropriately tuning the laser power. Our approach provides a robust and unprecedented recipe for optical trapping and separation of nanoparticles and biomolecules, such that it presents a huge potential in the physical and biomedical sciences.

Optical Sorting at the Single-Particle Level with Single-Nanometer Precision Using Coordinated Intensity and Phase Gradient Forces

Position-controlled sorting of colloidal nanoparticles (NPs) at the single-particle level is a challenge in nanoscience. Optofluidic potential wells can partially address this challenge, but they have limited flexibility, reconfigurability, and precision. Here we introduce a strategy by feedback-controlled manipulation of NPs using reconfigurable optical traps with designed intensity and phase gradient. Spatiotemporal patterns of these optical traps coordinatively manipulate the NPs based on machine vision of their positions and differentiated scattering intensities. The NPs are always kept inside the optical field during the manipulation and stably trapped once the sorting is accomplished. To substantiate the key advantages of our approach, we present position-controlled optical sorting of single Ag and Au NPs of the same size (150 nm diameter) and ordering of monodisperse Au NPs (80 ± 9 nm diameter) according to their sub-10 nm radius variation, which can hardly be done via other approaches.

Nanovortex-Driven All-Dielectric Optical Diffusion Boosting and Sorting Concept for Lab-on-a-Chip Platforms

The ever-growing field of microfluidics requires precise and flexible control over fluid flows at reduced scales. Current constraints demand a variety of controllable components to carry out several operations inside microchambers and microreactors. In this context, brand-new nanophotonic approaches can significantly enhance existing capabilities providing unique functionalities via finely tuned light-matter interactions. A concept is proposed, featuring dual on-chip functionality: boosted optically driven diffusion and nanoparticle sorting. High-index dielectric nanoantennae is specially designed to ensure strongly enhanced spin-orbit angular momentum transfer from a laser beam to the scattered field. Hence, subwavelength optical nanovortices emerge driving spiral motion of plasmonic nanoparticles via the interplay between curl-spin optical forces and radiation pressure. The nanovortex size is an order of magnitude smaller than that provided by conventional beam-based approaches. The nanoparticles mediate nanoconfined fluid motion enabling moving-part-free nanomixing inside a microchamber. Moreover, exploiting the nontrivial size dependence of the curled optical forces makes it possible to achieve precise nanoscale sorting of gold nanoparticles, demanded for on-chip separation and filtering. Altogether, a versatile platform is introduced for further miniaturization of moving-part-free, optically driven microfluidic chips for fast chemical analysis, emulsion preparation, or chemical gradient generation with light-controlled navigation of nanoparticles, viruses or biomolecules.

A microfluidically controlled concave-convex membrane lens using an addressing operation system

Electrical control toolkits for microlens arrays are available to some extent, but for applications in environments with strong electromagnetic fields, radiation, or deep water, non-electrical actuation and control strategies are more appropriate. An integrated digital microfluidic zoom actuating unit with a logic addressing unit for a built-in membrane lens array, e.g., a flexible bionic compound eye, is developed and studied in this article. A concave-convex membrane fluidic microvalve, which is the component element of the logic gate, actuator, and microlens, is proposed to replace the traditional solenoid valve. The functions of pressure regulation and decoding can be obtained by incorporating microvalves into fluidic networks according to equivalent circuit designs. The zoom actuating unit contains a pressure regulator to adjust the focal length of lenses with three levels, and the logic addressing unit contains a decoder to choose a typical lens from a hexagonal lens array. The microfluidic chip control system is connected flexibly to the actuating part, a membrane lens array. It is shown from a simulation and experimental demonstration that the designed and fabricated system, which is composed of a whole microfluidic zoom unit, addressing technology, and a microlens array, works well. Because these components are constructed in the same fabrication process and operate with the same work media and driving source, the system can be made highly compatible and lightweight for applications such as human-machine interfaces and soft robots.

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

Lateral optical forces induced by linearly polarized laser beams have been predicted to deflect dipolar particles with opposite chiralities toward opposite transversal directions. These "chirality-dependent" forces can offer new possibilities for passive all-optical enantioselective sorting of chiral particles, which is essential to the nanoscience and drug industries. However, previous chiral sorting experiments focused on large particles with diameters in the geometrical-optics regime. Here, we demonstrate, for the first time, the robust sorting of Mie (size ~ wavelength) chiral particles with different handedness at an air-water interface using optical lateral forces induced by a single linearly polarized laser beam. The nontrivial physical interactions underlying these chirality-dependent forces distinctly differ from those predicted for dipolar or geometrical-optics particles. The lateral forces emerge from a complex interplay between the light polarization, lateral momentum enhancement, and out-of-plane light refraction at the particle-water interface. The sign of the lateral force could be reversed by changing the particle size, incident angle, and polarization of the obliquely incident light.

Generation and manipulation of oil-in-water micro-droplets by confined thermocapillary microvortices

Optofluidic manipulation of droplets is critical in droplet-based microfluidic systems for chemistry, biology, and medicine. Here, we reported a thermocapillary microvortices-based manipulation platform for controlling oil-in-water droplets through integrating a photothermal waveguide into a microfluidic chip. The sizes and shapes of the droplets can be controlled by adjusting optical power or positions of the water-oil interface. Here, teardrop-shaped droplets, which can encapsulate and accumulate mesoscopic matters easily, were generated when the water-oil interface and the channel boundaries approached the photothermal waveguide center simultaneously. The results showed that the thermocapillary microvortices have good controllability of droplet positions, droplet volumes, and encapsulated-particle distribution and thus it will be a powerful droplet manipulation strategy for microreactors and microcapsules.