Plasmonic tweezers: for nanoscale optical trapping and beyond

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.

Suppressing the radiation loss by hybrid Tamm-surface plasmon BIC modes

Tamm plasmon polaritons (TPPs), localized near the boundary of a dielectric Bragg reflector (DBR) and a thin metal film, have attracted much attention for the lower ohm loss and flexible excitation. However, the radiation loss resulting from the direct coupling to the surroundings hinders their applications. Here, we propose and experimentally demonstrate a new type of hybrid plasmonic quasi-bound state in the continuum (BIC) in a Tamm-surface plasmon polariton system to suppress the radiation loss. Leveraging the scattering of the periodic metal array, the TPP interacts with the surface plasmon polariton (SPP) mode and form a Friedrich-Wintgen type quasi-BIC state that originated from the interference of two surface waves with different natures. Through angle resolved reflectance spectrum measurement, the hybrid plasmonic quasi-BIC was observed in the experiment. Our work proposes a new method to design a high Q mode in plasmonic systems, and thus holds promise for applications in the field of light matter interactions.

Plasmonic dielectric antennas for hybrid optical nanotweezing and optothermoelectric manipulation of single nanosized extracellular vesicles

This paper showcases an experimental demonstration of near-field optical trapping and dynamic manipulation of an individual extracellular vesicle. This is accomplished through the utilization of a plasmonic dielectric nanoantenna designed to support an optical anapole state-a non-radiating optical state resulting from the destructive interference between electric and toroidal dipoles in the far-field, leading to robust near-field enhancement. To further enhance the field intensity associated with the optical anapole state, a plasmonic mirror is incorporated, thereby boosting trapping capabilities. In addition to demonstrating near-field optical trapping, the study achieves dynamic manipulation of extracellular vesicles by harnessing the thermoelectric effect. This effect is induced in the presence of an ionic surfactant, cetyltrimethylammonium chloride (CTAC), combined with plasmonic heating. Furthermore, the thermoelectric effect improves trapping stability by introducing a wide and deep trapping potential. In summary, our hybrid plasmonic-dielectric trapping platform offers a versatile approach for actively transporting, stably trapping, and dynamically manipulating individual extracellular vesicles.

Nanoscale Helical Optical Force for Determining Crystal Chirality

The deterministic control of material chirality has been a sought-after goal. As light possesses intrinsic chirality, light-matter interactions offer promising avenues for achieving non-contact, enantioselective optical induction, assembly, or sorting of chiral entities. However, experimental validations are confined to the microscale due to the limited strength of asymmetrical interactions within sub-diffraction limit ranges. In this study, a novel approach is presented to facilitate chirality modulation through chiral crystallization using a helical optical force field originating from localized nanogap surface plasmon resonance. The force field emerges near a gold trimer nanogap and is propelled by linear and angular momentum transfer from the incident light to the resonant nanogap plasmon. By employing Gaussian and Laguerre-Gaussian incident laser beams, notable enantioselectivity is achieved through low-power plasmon-induced chiral crystallization of an organic compound-ethylenediamine sulfate. The findings provide new insights into chirality transmission orchestrated by the exchange of linear and angular momentum between light and nanomaterials.

Quantification and description of photothermal heating effects in plasmon-assisted electrochemistry

A growing number of reports have demonstrated plasmon-assisted electrochemical reactions, though debate exists around the mechanisms underlying the enhanced activity. Here we address the impact of plasmonic photothermal heating with cyclic voltammetry measurements and finite-element simulations. We find that plasmonic photothermal heating causes a reduction in the hysteresis of the anodic and cathodic waves of the voltammograms along with an increase in mass-transport limiting current density due to convection induced by a temperature gradient. At slow scan rates, a temperature difference as low as 1 K between the electrode surface and bulk electrolytic solution enhances the current density greater than 100%. Direct interband excitation of Au exclusively enhances current density by photothermal heating, while plasmon excitation leads to photothermal and nonthermal enhancements. Our study reveals the role of temperature gradients in plasmon-assisted electrochemistry and details a simple control experiment to account for photothermal heating.

Whispering-Gallery Mode Optoplasmonic Microcavities: From Advanced Single-Molecule Sensors and Microlasers to Applications in Synthetic Biology

Optical microcavities, specifically, whispering-gallery mode (WGM) microcavities, with their remarkable sensitivity to environmental changes, have been extensively employed as biosensors, enabling the detection of a wide range of biomolecules and nanoparticles. To push the limits of detection down to the most sensitive single-molecule level, plasmonic nanorods are strategically introduced to enhance the evanescent fields of WGM microcavities. This advancement of optoplasmonic WGM sensors allows for the detection of single molecules of a protein, conformational changes, and even atomic ions, marking significant contributions in single-molecule sensing. This Perspective discusses the exciting research prospects in optoplasmonic WGM sensing of single molecules, including the study of enzyme thermodynamics and kinetics, the emergence of thermo-optoplasmonic sensing, the ultrasensitive single-molecule sensing on WGM microlasers, and applications in synthetic biology.

Efficient monodisperse upconversion composite prepared using high-density local field and its dual-mode temperature sensing

Enhanced upconversion via plasmonics has considerable potential in biosensors and solar cells; however, conventional plasmonic configurations such as core-shell assemblies or nanoarray platforms still suffer from the compromise between the enhancement factor and monodispersity, which has failed to meet the requirement of the materials for the in vivo all-solution-prepared solar cells and biosensors. We herein report a monodisperse metal-dielectric-metal (MDM) type upconverted hybrid material with high efficiency. The lanthanide-doped upconversion nanoparticles (UCNPs) were sandwiched by two gold nanodisk mirrors, and the highly localized excitation field around the UCNPs together with the efficient coupling enhanced the upconversion. The upconversion intensity can then be effectively regulated and improved by three to four orders of magnitude. As per the measurement of the temperature-dependent fluorescence intensity and spectra shift, a dual-mode nanothermometer based on our proposed hybrid materials was demonstrated. This MDM-type upconverted hybrid material demonstrated the merits of high efficiency and monodispersity, which demonstrated promise in in vivo biosensors and solar cell fabrication techniques such as spin-coating and roll-to-roll.

Topological state transitions of skyrmionic beams under focusing configurations

The recent emerging appearance of optical analogs of magnetic quasiparticles, i.e., optical skyrmions constructed via spin, field, and Stokes vectors, has garnered substantial interest from deep-subwavelength imaging and quantum entanglement. Here, we investigate systematically the topological state transitions of skyrmionic beams constructed by the Stokes vectors in the focusing configuration. We theoretically demonstrated that in the weak focusing, the skyrmion topological number is protected. Whereas, in the tight focusing, a unique topological transformation with skyrmion number variation is exhibited for the optical skyrmion, anti-skyrmion, and 2nd-order skyrmion structures. The significant difference between the topological state transitions of these two cases originates from the transformation from the paraxial optical system to the nonparaxial optical system, and the approximate two-dimensional polarization structure to the three-dimensional polarization structure. The results provide new insights into the topological state transitions in topological structures, which promote applications in information processing, data storage, and free-space optical communications.

Spin-orbital angular momentum degeneracy breaking in nanoplasmonic metachain

The spin and orbital angular momentum (namely SAM and OAM) mode division provides a promising solution to surmount exhausted available degrees of freedom in conventional optical communications. Nevertheless, SAM and OAM are often subjected to the degeneracy of total angular momentum (AM) because they both have integer variables of quantum eigenstates, which inevitably brings about the shortcomings specific to limited signal channels and multiplexing cross talk. Herein, we present a nanoplasmonic metachain that can discriminatively couple any input SAM and OAM components to an extrinsic orbital AM, corresponding to the chirality and topological charge of incident light. Importantly, the unambiguous measurement has a prominent advantage of detecting the arbitrary AM component rather than the total AM. The miniature metadevice offers the possibility of harnessing AM division on chip or in fiber and holds great promise to delve the spin-orbit interactions for topological photonics and quantum cryptography.

A two-fold SPR-SERS sensor utilizing gold nanoparticles and graphene thin membrane as a spacer in a 3D composite structure

Localized surface plasmonic resonance (LSPR) biosensing using optical fibers has gained popularity due to its label-free approach and high sensitivity to changes in the nanoparticle surface's local index of refraction. However, improving sensitivity remains a challenge. In this study, a two-step approach was employed to fabricate a composite structure using gold nanoparticles and monolayer graphene (Gr-AuNPs). The combination of AuNPs and graphene membrane demonstrated high potential for Surface-enhanced Raman scattering (SERS) and surface plasmonic resonance (SPR) fiber sensors. The Gr-AuNPs sensor successfully detected R6G molecules with a low detection limit of 10-12 M, indicating promising SERS activity. Numerical simulations confirmed that the graphene generated densely hot spots in the nanogap region between plasmonic layers. It's interesting that the proposed SPR-SERS Sensor can detect both glucose and thiram. This demonstrates the sensors practicality and can help with a basic environmental need to find leftover pesticides in the soil. The combination of SPR-SERS dual-mode detection provides more options for detecting and verifying data, increasing the precision and repeatability of experiments.

Mirror-Enhanced Plasmonic Nanoaperture for Ultrahigh Optical Force Generation with Minimal Heat Generation

Double Nanohole Plasmonic Tweezers (DNH) have emerged as a powerful approach for confining light to sub-wavelength volume, enabling the trapping of nanoscale particles much smaller than the wavelength of light. However, to circumvent plasmonic heating effects, DNH tweezers are typically operated off-resonance, resulting in reduced optical forces and field enhancements. In this study, we introduce a novel DNH design with a reflector layer, enabling on-resonance illumination while minimizing plasmonic heating. This design efficiently dissipates heat and redistributes the electromagnetic hotspots, making them more accessible for trapping nanoscale particles and enhancing light-matter interactions. We also demonstrate low-power trapping and release of small extracellular vesicles. Our work opens new possibilities for trapping-assisted Surface Enhanced Raman Spectroscopy (SERS), plasmon-enhanced imaging, and single photon emission applications that demand strong light-matter interactions.

Enhanced trapping properties induced by strong LSPR-exciton coupling in plasmonic tweezers

Plasmonic tweezers break the diffraction limit and enable trap the deep-subwavelength particles. However, the innate scattering properties and the photothermal effect of metal nanoparticles pose challenges to their effective trapping and the non-damaging trapping of biomolecules. In this study, we investigate the enhanced trapping properties induced by strong coupling between localized surface plasmon resonances (LSPR) and excitons in plasmonic tweezers. The LSPR-exciton strong coupling exhibits an anticrossing behavior in dispersion curves with a markable Rabi splitting of 196 meV. Plasmonic trapping forces on excitons experience a significant increase within this strong coupling system due to higher longitudinal enhancement of electric field enhancement, which enables efficient particle trapping using lower laser power and minimizes ohmic heat generation. Moreover, leveraging strong coupling effects allows the successful trapping of a 50 nm Au particle coated with J-aggregates, overcoming previous limitations associated with scattering characteristics and smaller size that hindered effective metal nanoparticle manipulation. These findings open up new possibilities for the nondestructive trapping of biomolecules and metal nanoparticles across various 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.

Efficient optical plasmonic tweezer-controlled single-molecule SERS characterization of pH-dependent amylin species in aqueous milieus

It is challenging to characterize single or a few biomolecules in physiological milieus without excluding the influences of surrounding environment. Here we utilize optical plasmonic trapping to construct a dynamic nanocavity, which reduces the diffraction-limited detection volume and provides reproducible electromagnetic field enhancements to achieve high-throughput single-molecule surface-enhanced Raman spectroscopy (SERS) characterizations in aqueous environments. Specifically, we study human Islet Amyloid Polypeptide (amylin, hIAPP) under different physiological pH conditions by combining spectroscopic experiments and molecular dynamics (MD) simulations. Based on a statistically significant amount of time-dependent SERS spectra, two types of low-populated transient species of hIAPP containing either turn or β-sheet structure among its predominant helix-coil monomers are characterized during the early-stage incubation at neutral condition, which play a crucial role in driving irreversible amyloid fibril developments even after a subsequent adjustment of pH to continue the prolonged incubation at acidic condition. Our results might provide profound mechanistic insight into the pH-regulated amyloidogenesis and introduce an alternative approach for investigating complex biological processes at the single-molecule level.

Optical Manipulation of Soft Matter

Optical manipulation has emerged as a pivotal tool in soft matter research, offering superior applicability, spatiotemporal precision, and manipulation capabilities compared to conventional methods. Here, an overview of the optical mechanisms governing the interaction between light and soft matter materials during manipulation is provided. The distinct characteristics exhibited by various soft matter materials, including liquid crystals, polymers, colloids, amphiphiles, thin liquid films, and biological soft materials are highlighted, and elucidate their fundamental response characteristics to optical manipulation techniques. This knowledge serves as a foundation for designing effective strategies for soft matter manipulation. Moreover, the diverse range of applications and future prospects that arise from the synergistic collaboration between optical manipulation and soft matter materials in emerging fields are explored.

High-efficiency plasmonic vortex generation with near-infrared bifunctional metasurfaces

Plasmonic vortices have shown a wide range of applications in on-chip photonics due to their fascinating properties of the orbital angular momenta (OAM) and phase singularity. However, conventional devices to generate them suffer from issues of low efficiencies and limited functionalities. Here, we establish a systematic scheme to construct high-efficiency bifunctional metasurfaces that can generate two plasmonic vortices exhibiting distinct topological charges, based on a series of reflective meta-atoms exhibiting tailored reflection-phases dictated by both resonant and geometric origins. As a benchmark test, we first construct a meta-coupler with meta-atoms exhibiting geometric phases only, and experimentally demonstrate that it can generate a pre-designed plasmonic vortex at the wavelength of 1064 nm with an efficiency of 27% (56% in simulation). Next, we design/fabricate two bifunctional metasurfaces with meta-atoms integrated with both resonant and geometric phases, and experimentally demonstrate that they can generate divergent (or focused) or convergent (or defocused) plasmonic vortices with district OAM as shined by circularly polarized light with opposite helicity at 1064 nm wavelength. Our work provides an efficient platform to generate plasmonic vortices as desired, which can find many applications in on-chip photonics.

Selective plasmonic trapping of nano-particles by Archimedes metalens

Optical tweezer is a non-invasive method for optical force tool applied in various fields like biology, physics, and lab on chip manipulation. The Archimedean helix shape is ideal for creating chiral nanostructures, and being able to generate plasmonic focused hotspot field for optical trapping. Here we design a metal disk with the Archimedean shape to own the ability of selective trapping nanoparticles based on the spin-orbit interactions with circularly polarized light. The plasmonic near field on the metalens can be designed by adjusting the geometric parameter flexibly. We numerically analyze the optimal size and screw pitch of the metal disk to realize the switch modulation of hotspot generation, and then demonstrate the novel switchable optical trapping ability in the view of optical force and potential well analysis under the circularly polarized light excitation by a 532 nm laser. The work shows significant potential for on-chip optical trapping in various fields.

Hypothermal opto-thermophoretic tweezers

Optical tweezers have profound importance across fields ranging from manufacturing to biotechnology. However, the requirement of refractive index contrast and high laser power results in potential photon and thermal damage to the trapped objects, such as nanoparticles and biological cells. Optothermal tweezers have been developed to trap particles and biological cells via opto-thermophoresis with much lower laser powers. However, the intense laser heating and stringent requirement of the solution environment prevent their use for general biological applications. Here, we propose hypothermal opto-thermophoretic tweezers (HOTTs) to achieve low-power trapping of diverse colloids and biological cells in their native fluids. HOTTs exploit an environmental cooling strategy to simultaneously enhance the thermophoretic trapping force at sub-ambient temperatures and suppress the thermal damage to target objects. We further apply HOTTs to demonstrate the three-dimensional manipulation of functional plasmonic vesicles for controlled cargo delivery. With their noninvasiveness and versatile capabilities, HOTTs present a promising tool for fundamental studies and practical applications in materials science and biotechnology.

Tailing Optical Pulling Force on a Metal-Dielectric Hybrid Dimer with Electromagnetic Coupling

In this work, we demonstrate that optical pulling forces (OPFs) can be induced by a hybrid dimer consisting of a Si nanoparticle (NP) and a coated nanoparticle with a gain core and Au shell under normal plane wave illumination. Analytical theory reveals that the underlying physical mechanism relies on interactions between the electric dipole (ED) modes excited in the NPs. As compared with the individual NP, it is found that the magnitude of optical force can be enlarged by almost three orders for the Si NP and one order for the coated gain NP in the coupled dimer. In addition, we find that the OPFs exerted on the NPs are heavily dependent on the gain level of the core materials, the incident polarization angle and the sizes of the NPs. More interestingly, we find that the OPF can also be exerted on a trimer system consisting of two identical Si NPs and a coated NP arranged in a line. The related results could be used to propose a versatile platform for manipulating NPs.

Optofluidic transport and assembly of nanoparticles using an all-dielectric quasi-BIC metasurface

Manipulating fluids by light at the micro/nanoscale has been a long-sought-after goal for lab-on-a-chip applications. Plasmonic heating has been demonstrated to control microfluidic dynamics due to the enhanced and confined light absorption from the intrinsic losses of metals. Dielectrics, the counterpart of metals, has been used to avoid undesired thermal effects due to its negligible light absorption. Here, we report an innovative optofluidic system that leverages a quasi-BIC-driven all-dielectric metasurface to achieve subwavelength scale control of temperature and fluid motion. Our experiments show that suspended particles down to 200 nanometers can be rapidly aggregated to the center of the illuminated metasurface with a velocity of tens of micrometers per second, and up to millimeter-scale particle transport is demonstrated. The strong electromagnetic field enhancement of the quasi-BIC resonance increases the flow velocity up to three times compared with the off-resonant situation by tuning the wavelength within several nanometers range. We also experimentally investigate the dynamics of particle aggregation with respect to laser wavelength and power. A physical model is presented and simulated to elucidate the phenomena and surfactants are added to the nanoparticle colloid to validate the model. Our study demonstrates the application of the recently emerged all-dielectric thermonanophotonics in dealing with functional liquids and opens new frontiers in harnessing non-plasmonic nanophotonics to manipulate microfluidic dynamics. Moreover, the synergistic effects of optofluidics and high-Q all-dielectric nanostructures hold enormous potential in high-sensitivity biosensing applications.

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.

Plexcitonics: plasmon-exciton coupling for enhancing spectroscopy, optical chirality, and nonlinearity

Plexcitonics is a rapidly developing interdisciplinary field that holds immense potential for the creation of innovative optical technologies and devices. This field focuses on investigating the interactions between plasmons and excitons in hybrid systems. In this review, we provide an overview of the fundamental principles of plasmonics and plexcitonics and discuss the latest advancements in plexcitonics. Specifically, we highlight the ability to manipulate plasmon-exciton interactions, the emerging field of tip-enhanced spectroscopy, and advancements in optical chirality and nonlinearity. These recent developments have spurred further research in the field of plexcitonics and offer inspiration for the design of advanced materials and devices with enhanced optical properties and functionalities.

Abruptly Autofocusing Vortex Beams for Rapid Controllable Femtosecond Two-Photon Polymerization

Micro-fabrication based on structured-beam-assisted Two-Photon Polymerization (2 PP) provides a rapid and flexible method for the manufacture of microstructures with complex morphologies. The tunable Abruptly Autofocusing Vortex (AAFV) beams were designed theoretically and generated experimentally based on a single-phase-only Spatial Light Modulator (SLM). Their specific spatial intensity distributions were further utilized to assist the fabrication of a bowl-shaped Three-Dimensional (3D) micro-trap array via 2 PP with a one-step exposure technique. Finally, the fabricated microstructures act as a novel tool for the trapping and spatial positioning of micro-particles with different diameters, which shows potential applications in fiber optics and cell study.

Enabling Self-Induced Back-Action Trapping of Gold Nanoparticles in Metamaterial Plasmonic Tweezers

The pursuit for efficient nanoparticle trapping with low powers has led to optical tweezers technology moving from the conventional free-space configuration to advanced plasmonic systems. However, trapping nanoparticles smaller than 10 nm still remains a challenge even for plasmonic tweezers. Proper nanocavity design and excitation has given rise to the self-induced back-action (SIBA) effect offering enhanced trap stiffness with decreased laser power. In this work, we investigate the SIBA effect in metamaterial tweezers and its synergy with the exhibited Fano resonance. We demonstrate stable trapping of 20 nm gold particles with trap stiffnesses as high as 4.18 ± 0.2 (fN/nm)/(mW/μm²) and very low excitation intensity. Simulations reveal the existence of two different groups of hotspots on the plasmonic array. The two hotspots exhibit tunable trap stiffnesses, a unique feature that can allow for sorting of particles and biological molecules based on their characteristics.

Exosomes trapping, manipulation and size-based separation using opto-thermo-electrohydrodynamic tweezers

Owing to the heterogeneity of exosomes in size and biomolecular composition, there is a need for new approaches for trapping, manipulating, and sorting of single exosomes in solution. Due to their small size ranging from 30 nm to 150 nm and their relatively low refractive index, their stable trapping using optical tweezers has been met with challenges. Trapping exosomes in an optical trap requires nearly 100 mW of input power, which predisposes them to photo-induced damage and membrane rupture at the laser focus. Here, we report a high stability opto-thermo-electrohydrodynamic tweezer for the stable stand-off trapping of single exosomes based on a concentric nanohole array (CNA) using laser illumination and an a.c. field. The CNA system generates two regions of electrohydrodynamic potentials several microns away from the laser focus where single exosomes are trapped. We demonstrate the rapid trapping within seconds, and selective dynamic manipulation of exosomes based on size using only 4.2 mW of input laser power. The proposed platform opens up a promising approach for stabilizing single exosomes in solution and controlling their distribution based on size without the risk of photo-induced damage.

Optical forces in heat-assisted magnetic recording head-disk interface

A main challenge in Heat-Assisted Magnetic Recording technology is the build-up of contaminants called smear on the near field transducer. In this paper, we investigate the role of optical forces originating from the electric field gradient in the formation of smear. First, based on suitable theoretical approximations, we compare this force with air drag and the thermophoretic force in the head-disk interface for two smear nanoparticle shapes. Then, we evaluate the force field's sensitivity to the relevant parameter space. We find that the smear nanoparticle's refractive index, shape, and volume significantly impact the optical force. Further, our simulations reveal that the interface conditions, such as spacing and the presence of other contaminants, also influence the magnitude of the force.

Prospects and Challenges of Synergistic Effect of Fluorescent Carbon Dots, Liposomes and Nanoliposomes for Theragnostic Applications

The future of molecular-level therapy, efficient medical diagnosis, and drug delivery relies on the effective theragnostic function which can be achieved by the synergistic effect of fluorescent carbon dots (FCDs) liposomes (L) and nanoliposomes. FCDs act as the excipient navigation agent while liposomes play the role of the problem-solving agent, thus the term "theragnostic" would describe the effect of LFCDs properly. Liposomes and FCDs share some excellent at-tributes such as being nontoxic and biodegradable and they can represent a potent delivery system for pharmaceutical compounds. They enhance the therapeutic efficacy of drugs via stabilizing the encapsulated material by circumventing barriers to cellular and tissue uptake. These agents facilitate long-term drug biodistribution to the intended locations of action while eliminating systemic side effects. This manuscript reviews recent progress with liposomes, nanoliposomes (collectively known as lipid vesicles) and fluorescent carbon dots, by exploring their key characteristics, applications, characterization, performance, and challenges. An extensive and intensive understanding of the synergistic interaction between liposomes and FCDs sets out a new research pathway to an efficient and theragnostic / theranostic drug delivery and targeting diseases such as cancer.

Optical Manipulation Heats up: Present and Future of Optothermal Manipulation

Optothermal manipulation is a versatile technique that combines optical and thermal forces to control synthetic micro-/nanoparticles and biological entities. This emerging technique overcomes the limitations of traditional optical tweezers, including high laser power, photon and thermal damage to fragile objects, and the requirement of refractive-index contrast between target objects and the surrounding solvents. In this perspective, we discuss how the rich opto-thermo-fluidic multiphysics leads to a variety of working mechanisms and modes of optothermal manipulation in both liquid and solid media, underpinning a broad range of applications in biology, nanotechnology, and robotics. Moreover, we highlight current experimental and modeling challenges in the pursuit of optothermal manipulation and propose future directions and solutions to the challenges.

Plasmon-Assisted Trapping of Single Molecules in Nanogap

The manipulation of single molecules has attracted extensive attention because of their promising applications in chemical, biological, medical, and materials sciences. Optical trapping of single molecules at room temperature, a critical approach to manipulating the single molecule, still faces great challenges due to the Brownian motions of molecules, weak optical gradient forces of laser, and limited characterization approaches. Here, we put forward localized surface plasmon (LSP)-assisted trapping of single molecules by utilizing scanning tunneling microscope break junction (STM-BJ) techniques, which could provide adjustable plasmonic nanogap and characterize the formation of molecular junction due to plasmonic trapping. We find that the plasmon-assisted trapping of single molecules in the nanogap, revealed by the conductance measurement, strongly depends on the molecular length and the experimental environments, i.e., plasmon could obviously promote the trapping of longer alkane-based molecules but is almost incapable of acting on shorter molecules in solutions. In contrast, the plasmon-assisted trapping of molecules can be ignored when the molecules are self-assembled (SAM) on a substrate independent of the molecular length.

Room-Temperature Gate Voltage Modulation of Plasmonic Nanolasers

Stable electrical modulation of plasmonic nanolasers is achieved on a hybrid graphene-insulator-metal (GIM) platform at room temperature. To support surface plasmon polariton (SPP) resonance, a zinc oxide (ZnO) nanowire is placed on the GIM platform to create a plasmonic cavity with a compact mode volume of 2.6 × 10^-2 λ³, and the graphene layer is used as a transparent electrode for electrical modulation. When a gate voltage is applied, the surface electron density of Al varied, which results in the shifting of its plasma frequency and thus affects its SPP dispersion. In particular, this variation strongly changes the internal loss of the SPP mode; thus, the lasing thresholds of the ZnO nanowire plasmonic nanolasers on the GIM platform can be modulated by the gate voltage. This study demonstrates the gate voltage modulation of ZnO nanowire plasmonic nanolasers on a GIM platform at room temperature. These nanolasers can exhibit ultrahigh modulation speed on the order of terahertz. Accordingly, plasmonic nanolasers with gate voltage modulation have high potential for plasmonic circuit applications with high operation speed and versatility.

Thermofluidic assembly of colloidal crystals

Colloidal crystals are interesting as functional structures due to their emergent photonic properties like photonic stop bands and bandgaps that can be used to redirect light. They are commonly formed by a drying process that is assisted by capillary forces at the drying fronts. In this manuscript, we demonstrate the optically induced dynamic thermofluidic assembly of 2D and 3D colloidal crystals. We quantify in experiment and simulation the structure formation and identify thermo-osmosis and temperature induced depletion interactions as the key contributors to the colloidal crystal formation. The non-equilibrium nature of the assembly of colloidal crystals and its dynamic control by laser-induced local heating promise new possibilities for a versatile formation of photonic structures inaccessible by equilibrium processes.

Near-field manipulation of Tamm plasmon polaritons

Tamm plasmon polaritons (TPPs) arise from electromagnetic resonant phenomena which appear at the interface between a metallic film and a distributed Bragg reflector. They differ from surface plasmon polaritons (SPPs), since TPPs possess both cavity mode properties and surface plasmon characteristics. In this paper, the propagation properties of TPPs are carefully investigated. With the aid of nanoantenna couplers, polarization-controlled TPP waves can propagate directionally. By combining nanoantenna couplers with Fresnel zone plates, asymmetric double focusing of TPP wave is observed. Moreover, radial unidirectional coupling of the TPP wave can be achieved when the nanoantenna couplers are arranged along a circular or a spiral shape, which shows superior focusing ability compared to a single circular or spiral groove since the electric field intensity at the focal point is 4 times larger. In comparison with SPPs, TPPs possess higher excitation efficiency and lower propagation loss. The numerical investigation shows that TPP waves have great potential in integrated photonics and on-chip devices.

A 3D Biocompatible Plasmonic Tweezer for Single Cell Manipulation

Plasmonic tweezers are an emerging research topic because of their low input power and wide operating range from homogeneous particles to complex biological objects. But it is still challenging for plasmonic tweezers to trap or manipulate objects of tens of microns, especially in biological science. This study introduces a new 3D biocompatible plasmonic tweezer for single living cell manipulation in solution. The key design is a tapered tip whose three-layer surface structure consists of nanoprobe, gold nanofilm, and thermosensitive hydrogel, thiolated poly(N-isopropylacrylamide). Incident light excites the surface plasmon polaritons on gold film and generates heat to induce thermally driven phase transition of the thermosensitive hydrogel, which enables reversible binding between functionalized surface and cell membrane and avoids both thermal and mechanical stresses in the meanwhile. The 3D biocompatible plasmonic tweezer realizes selective capture, 3D pathway free transport, and position-controlled release of target cells, and it displays excellent biocompatibility, low energy consumption, and high operational flexibility.

Collective Behaviors of Active Matter Learning from Natural Taxes Across Scales

Taxis orientation is common in microorganisms, and it provides feasible strategies to operate active colloids as small-scale robots. Collective taxes involve numerous units that collectively perform taxis motion, whereby the collective cooperation between individuals enables the group to perform efficiently, adaptively, and robustly. Hence, analyzing and designing collectives is crucial for developing and advancing microswarm toward practical or clinical applications. In this review, natural taxis behaviors are categorized and synthetic microrobotic collectives are discussed as bio-inspired realizations, aiming at closing the gap between taxis strategies of living creatures and those of functional active microswarms. As collective behaviors emerge within a group, the global taxis to external stimuli guides the group to conduct overall tasks, whereas the local taxis between individuals induces synchronization and global patterns. By encoding the local orientations and programming the global stimuli, various paradigms can be introduced for coordinating and controlling such collective microrobots, from the viewpoints of fundamental science and practical applications. Therefore, by discussing the key points and difficulties associated with collective taxes of different paradigms, this review potentially offers insights into mimicking natural collective behaviors and constructing intelligent microrobotic systems for on-demand control and preassigned tasks.

Effect of the focused gap-plasmon mode on tip-enhanced Raman excitation and scattering

As a powerful molecular detection approach, tip-enhanced Raman scattering (TERS) spectroscopy has the advantages of nanoscale spatial resolution, label-free detection and high enhancement factor, therefore has been widely used in fields of chemistry, materials and life sciences. A TERS system enhanced by the focused gap-plasmon mode composed of Surface Plasmon Polariton (SPP) focus and the metal probe has been reported, however, its underlying enhancement mechanism for Raman excitation and scattering remains to be deeply explored. Here, we focus on the different performances of optical focus and SPP focus in the TERS system, and verify that the cooperation of these two focuses can produce maximum enhancement in a local electromagnetic field. Further, the Purcell effect on sample scattering in such a system is studied for the enhancement of Raman scattering collection in the far field. Finally, the local field enhancement and the sample far-field scattering enhancement are combined to show a full view of the whole process of TERS enhancement. This research can be applied to optimize the excitation and collection of Raman signals in TERS systems, which is of great value for the research and development of TERS technology.

Ultra-compact Archimedes spiral plasmonic lens with a circular groove for low power optical trapping in the far-field region

Particle levitation is crucial in optical trapping considering contamination and alteration of the character of the particle due to physical contact with the structure. A strong field gradient along the optical axis is required in this case. To manipulate the particle at a distance from the surface, we propose an Archimedes spiral plasmonic lens with a circular groove (CG-ASPL). The optical properties and parameters influencing the trapping performance of CG-ASPL are fully analyzed and discussed. By illuminating the structure with circular polarization and structure optimization, we can reduce the required optical power down to 2.4 mW for trapping particle of 1 µm in diameter with groove width and height of 100 and 125 nm, respectively. The particle can be stably trapped with trapping potential of 4138 kBT/W in the far-field region (1.1λ) owing to constructive interference of the scattered SPP waves. Furthermore, this structure is ultra-compact with a size of about 6.7 µm in diameter. We believe the results demonstrated in this work would be very useful for lab-on-a-chip applications and many others.

Dynamically controllable plasmonic tweezers using C-shaped nano-engravings

A near-field optical trapping scheme using plasmonic C-shaped nano-engraving is presented. Utilizing the polarization sensitivity of the C-structure, a mechanism is proposed for dynamically controlling the electric field, the associated trapping force, and the plasmonic heating. Electromagnetic analysis and particle dynamics simulations are performed to verify the viability of the approach. The designed structure is fabricated and experimentally tested. Polarization control of the excitation light is achieved through the use of a half-wave plate. Experimental results are presented that show the functioning implementation of the dynamically adjustable plasmonic tweezers. The dynamic controllability can allow trapping to be maintained with lower field strengths, which reduces photo-thermal effects. Thus, the probability of thermal damage can be reduced when handling sensitive specimens.

Plasmonic phenomena in molecular junctions: principles and applications

Molecular junctions are building blocks for constructing future nanoelectronic devices that enable the investigation of a broad range of electronic transport properties within nanoscale regions. Crossing both the nanoscopic and mesoscopic length scales, plasmonics lies at the intersection of the macroscopic photonics and nanoelectronics, owing to their capability of confining light to dimensions far below the diffraction limit. Research activities on plasmonic phenomena in molecular electronics started around 2010, and feedback between plasmons and molecular junctions has increased over the past years. These efforts can provide new insights into the near-field interaction and the corresponding tunability in properties, as well as resultant plasmon-based molecular devices. This Review presents the latest advancements of plasmonic resonances in molecular junctions and details the progress in plasmon excitation and plasmon coupling. We also highlight emerging experimental approaches to unravel the mechanisms behind the various types of light-matter interactions at molecular length scales, where quantum effects come into play. Finally, we discuss the potential of these plasmonic-electronic hybrid systems across various future applications, including sensing, photocatalysis, molecular trapping and active control of molecular switches.

Electromagnetic forces in the time domain

We look beyond the standard time-average approach and investigate optical forces in the time domain. The formalism is developed for both the Abraham and Minkowski momenta, which appear to converge in the time domain. We unveil an extremely rich - and by far unexplored - physics associated with the dynamics of the optical forces, which can even attain negative values over short time intervals or produce low frequency dynamics that can excite mechanical oscillations in macroscopic objects under polychromatic illumination. The magnitude of this beating force is tightly linked to the average one. Implications of this work for transient optomechanics are discussed.

Manipulation force analysis of nanoparticles with ultra-high numerical aperture metalens

Metalens optical tweezers technology has several advantages for manipulating micro-nano particles and high integration. Here, we used particle swarm optimization (PSO) to design a novel metalens tweezer, which can get 3-dimensional trapping of particles. The numerical aperture (NA) of the metalens can reach 0.97 and the average focusing efficiency is 44%. Subsequently, we analyzed the optical force characteristics of SiO₂ particles with a radius of 350 nm at the focal point of the achromatic metalens. We found the average maximum force of SiO₂ particles in the x-direction and z-direction to be 0.88 pN and 0.72 pN, respectively. Compared with the dispersive metalens, it is beneficial in maintaining the constant of optical force, the motion state of trapped particles, and the stability of the trapping position.

Molecular Dynamics Simulation Investigation of the Binding and Interaction of the EphA6-Odin Protein Complex

Protein-protein interaction plays an important role in the development of almost all cells. Elucidating the dynamic binding and affinity of a protein-protein complex is essential for understanding the biological functions of proteins. EphA6 and Odin proteins are members of the Eph (erythropoietin-producing hepatocyte) receptor family and the Anks (ankyrin repeat and sterile α motif domain-containing) family, respectively. Odin significantly functions in regulating endocytosis, degradation, and stability of EphA receptors. In this work, the key residues of the interaction interface were determined through a hydrogen bond, root-mean-square deviation (RMSD), root-mean-square fluctuation (RMSF), and dynamic correlation analysis of the conventional molecular dynamics (MD) simulations. The calculated standard binding free energy, -7.92 kcal/mol, between EphA6 and Odin is quite consistent with the experimental measurement value, -8.73 kcal/mol. By the combination of several MD simulation techniques, our investigation of the binding process reveals the detailed representative characteristics of the entire binding pathway at the molecular level. Based on the obtained potential of the mean force (PMF) curve, the analysis of the simulation trajectories shows that the residue Arg1013 in the receptor EphA6 is responsible for capturing Asp739 and Asp740 in the ligand Odin during the initial stage of binding. In the later stage of binding, the hydrogen bonds and salt bridges between a series of residues Lys973, Leu1007, Gly1009, His1010, and Arg1012 in the receptor and residues Leu735, Asn736, Asp739, Asp740, and Asp753 in the ligand mainly contribute to the stability of the protein complex. In addition, the specific change process of the receptor-ligand-binding mode is also clarified during the binding process. Our present simulation will promote a deep understanding of the protein-protein interaction, and the identified key interresidue interaction will be theoretical guidance for the design of protein drugs.

A Critical Review on the Sensing, Control, and Manipulation of Single Molecules on Optofluidic Devices

Single-molecule techniques have shifted the paradigm of biological measurements from ensemble measurements to probing individual molecules and propelled a rapid revolution in related fields. Compared to ensemble measurements of biomolecules, single-molecule techniques provide a breadth of information with a high spatial and temporal resolution at the molecular level. Usually, optical and electrical methods are two commonly employed methods for probing single molecules, and some platforms even offer the integration of these two methods such as optofluidics. The recent spark in technological advancement and the tremendous leap in fabrication techniques, microfluidics, and integrated optofluidics are paving the way toward low cost, chip-scale, portable, and point-of-care diagnostic and single-molecule analysis tools. This review provides the fundamentals and overview of commonly employed single-molecule methods including optical methods, electrical methods, force-based methods, combinatorial integrated methods, etc. In most single-molecule experiments, the ability to manipulate and exercise precise control over individual molecules plays a vital role, which sometimes defines the capabilities and limits of the operation. This review discusses different manipulation techniques including sorting and trapping individual particles. An insight into the control of single molecules is provided that mainly discusses the recent development of electrical control over single molecules. Overall, this review is designed to provide the fundamentals and recent advancements in different single-molecule techniques and their applications, with a special focus on the detection, manipulation, and control of single molecules on chip-scale devices.

Gap Effect on Electric Field Enhancement and Photothermal Conversion in Gold Nanostructures

Plasmonic optical tweezers and thermophoresis are promising tools for nanomaterial manipulation. When a gold nanostructure is irradiated with laser light, an electric field around the nanostructure is enhanced because of the localized surface plasmon resonance, which increases the optical radiation pressure applied to the nanomaterials. In addition, a temperature gradient is also generated by the photothermal conversion, and thermophoretic force is then generated. This study numerically evaluated the electric and temperature fields induced by the localized surface plasmon resonance between two gold nanostructures. Here, we focused on the effect of the gap width between nanostructures on the optical radiation pressure and thermophoretic force. The simulation results show that the electric field is locally enhanced according to the gap width, but the effect on the temperature rise due to the photothermal heating is small. This fact suggests that the gap effect between the nanostructures is particularly dominant in nanomanipulation using optical force, whereas it has little effect in nanomanipulation using thermophoresis.

Controllable transportation of microparticles along structured waveguides by the plasmonic spin-hall effect

With the nanoscale integration advantage of near field photonics, controllable manipulation and transportation of micro-objects have possessed plentiful applications in the fields of physics, biology and material sciences. However, multifunctional optical manipulation like controllable transportation and synchronous routing by nano-devices are limited and rarely reported. Here we propose a new type of Y-shaped waveguide optical conveyor belt, which can transport and route particles along the structured waveguide based on the plasmonic spin-hall effect. The routing of micro-particles in different branches is determined by the optical force components difference at the center of the Y junction along the two branches of the waveguide. The influence of light source and structural parameters on the optical forces and transportation capability are numerically studied. The results illustrate that the proposed structured waveguide optical conveyor belt can transport the microparticles controllably in different branches of the waveguide. Due to the selective transportation ability of microparticles by the 2D waveguide, our work shows great application potential in the region of on-chip optical manipulation.

Electromagnetic forces in nanoparticles made of multilayer hyperbolic metamaterials

We theoretically study the electromagnetic forces (optical gradient force, optical torque and vacuum friction) acting on a spherical anisotropic nanoparticle, which can be characterized by multilayer hyperbolic metamaterials (mHMMs). We find three important results about these forces: (i) Firstly, we theoretically demonstrate that the optical gradient force produced on a mHMMs nanoparticle can be flexibly tuned, from pushing the particle to pulling it, just via changing incident angle of illuminating plane light wave. (ii) Secondly, we find the optical torque acting on the mHMMs nanoparticle (its filling factor is around 0.3) can be tuned between positive and negative via changing the incident angle of circularly polarized plane light. Therefore, the rotating mHMMs nanoparticle with designed filling factor can be accelerated or decelerated by the optical torque. (iii) Finally, due to the large fluctuations of dipole polarizability of mHMMs nanoparticle with appropriate filling factor, we propose a new method to obtain the large enhancement of vacuum friction torque by designing the filling factor of the rotating mHMMs nanoparticle.

Light-driven microdrones

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

Directivity-Enhanced Detection of a Single Nanoparticle Using a Plasmonic Slot Antenna

In situ refractive index sensors integrated with nanoaperture-based optical tweezers possess stable and sensitive responsivity to single nanoparticles. In most existing works, detection events are only identified using the total light intensity with directivity information ignored, leading to a low signal-to-noise ratio. Here, we propose to detect an optically trapped 20 nm silica particle by monitoring directivity of a plasmonic antenna. The main and secondary radiation lobes of the antenna reverse upon trapping because the particle-induced perturbation negates the relative phase between two antenna elements, leading to a significant change of the antenna front-to-back ratio. As a result, we obtain a signal-to-noise ratio of 20, with an order-of-magnitude improvement as compared to the intensity-only detection scheme.

Optically-assisted thermophoretic reversible assembly of colloidal particles and E. coli using graphene oxide microstructures

Optically-assisted large-scale assembly of nanoparticles have been of recent interest owing to their potential in applications to assemble and manipulate colloidal particles and biological entities. In the recent years, plasmonic heating has been the most popular mechanism to achieve temperature hotspots needed for extended assembly and aggregation. In this work, we present an alternative route to achieving strong thermal gradients that can lead to non-equilibrium transport and assembly of matter. We utilize the excellent photothermal properties of graphene oxide to form a large-scale assembly of silica beads. The formation of the assembly using this scheme is rapid and reversible. Our experiments show that it is possible to aggregate silica beads (average size 385 nm) by illuminating thin graphene oxide microplatelet by a 785 nm laser at low intensities of the order of 50-100 µW/µm2. We further extend the study to trapping and photoablation of E. coli bacteria using graphene oxide. We attribute this aggregation process to optically driven thermophoretic forces. This scheme of large-scale assembly is promising for the study of assembly of matter under non-equilibrium processes, rapid concentration tool for spectroscopic studies such as surface-enhanced Raman scattering and for biological applications.