Trapping Of Nanoparticles Research Articles

Optical trapping forces exerted by a pulsed Hermite–Gaussian beam on a dielectric nanosphere

An analytical expression for the optical forces exerted by a pulsed Hermite–Gaussian (HG) beam on a nano-dielectric sphere has been derived. This allows us to investigate how the forces are affected by different beam modes (p,l) and the duration of the light pulse (τ). Our findings demonstrate that these factors significantly influence the strength of the forces, the number of regions where the sphere can be trapped, and the size of these trapping zones. This knowledge paves the way for precise control of these tiny particles with light, potentially leading to improved techniques for trapping and sorting nanoparticles.

Precise Nanoparticle Manipulation Using Femtosecond Laser Trapping.

Optical tweezers use strongly focused light for trapping, characterizing, and manipulating objects in the microscopic and nanoscopic regimes. However, fully understanding optical trapping at the nanoscale remains a significant challenge. This holds importance because the nanoscale is the frontier for numerous promising advancements, ranging from enhancing single-molecule investigations in biology to developing hybrid devices for nanoelectronics and photonics and exploring fundamental quantum phenomena in opto-mechanics. We report an experimental and theoretical study of nanoparticles of various sizes, showing the advantages of the immense peak power of ultrashort laser pulses over conventional optical tweezers. We also demonstrate highly stable trapping of nanoparticles for extended durations at low average laser power using femtosecond lasers.

Enantioselective optical trapping of chiral nanoparticles by tightly focused fractional vector beams

Enantiomers exhibit markedly different chemical properties although they have the same chemical structure. The identification and separation of enantiomers have been significant issues in biomedicine and chemistry. In this work, we proposed an optical method that selective trapping of enantiomers by using tightly focused fractional vector beams (FVBs). In our proposed model, such a focused beam forms multiple local optical chirality densities (OCDs) with opposite signs at the focal plane. We found that focused FVBs can stably trap the enantiomers at the local positions with the minimum or maximum OCD according to the handedness of enantiomers. The positions and numbers of the trapped enantiomers have a relationship with the fractional topological charge. These results indicate that tightly focused FVBs are an all-optical method capable of dynamic modulation and achieving precise and stable trapping of multiple pairs of enantiomers. Our findings have practical applications in the multi-throughput and multi-sample manipulation of chiral materials.

Switchable optical trapping and manipulation enabled by polarization-modulated multifunctional phase-change metasurfaces

Optical trapping, a cutting-edge methodology, is pivotal for contactlessly controlling and exploring microscopic objects. However, it encounters formidable challenges such as multiparticle trapping, flexible control, and seamless integration. Here, we employ a polarization-modulated multi-foci technique for versatile nanoparticle trapping using multifunctional metasurfaces relying on geometric phase. Numerical simulations demonstrate the generation of two focused spots with orthogonal polarization distributions through our metasurfaces when illuminated with linearly polarized light, with their polarization distributions be interchanged by orthogonally switching the incident polarizations. We extend this design to an array of multi-foci metasurface tweezers modulated by polarization, highlighting the versatility and robustness of our approach. Furthermore, we demonstrate the simultaneous generation of two distinct focusing cylindrical vector beams using a monolayer metasurface, showcasing the two vector beams possess the interchange ability of their polarization distributions. By leveraging the Maxwell stress tensor, we assess the distinct contributions of the focused beams to longitudinal and transverse optical forces on SiO2 spheres, validating diverse trapping and manipulation behaviors for nanoparticles with the proposed metasurface designs. By manipulating the phase states of Sb2S3 nanopillars, binary-switchable optical trapping and manipulation are facilitated for all proposed metasurface tweezers. Our work underscores the efficacy of polarization-modulation multifunctional metasurface tweezers in consolidating multiple trapping tasks into a single device, paving the way for innovative lab-on-a-chip optical trapping applications in biophysics, nanotechnology, and photonics.

Annular optical trapping of metallic nanoparticles using the azimuthally polarized beam.

The unique physical and chemical properties make metallic nanoparticles promising for broad applications in many fields. Exploring the dynamics of metallic nanoparticles in optical traps is crucial for exploiting optical tweezers to advance the applications of metallic particles. In this paper, we present a detailed study of the annular optical trapping of gold nanoparticles with azimuthal polarization. Theoretical analysis based on the T-matrix method shows that the gold nanoparticles experience optical forces pointing to the equilibrium position along the radial direction, while there is no force along the azimuthal direction at this equilibrium position. Therefore, a tightly focused azimuthally polarized beam captures gold nanoparticles in an annular region. Experimental measurements of the motion trajectory of the confined gold nanoparticles reveal a donut profile consistent with the theoretical predictions. Our work reported in this paper is expected to deepen our understanding of the interactions between metallic nanoparticles and light and promote the application of metallic nanoparticles.

Optical trapping of nanoparticles using all-dielectric quasi-bound states in the continuum

Plasmonic nanotweezers employing metallic nanostrctures provide a powerful tool for optical trapping of nanoscale objects; however, they are limited by strong heating effects resulting from the intrinsic loss in metallic materials. Alternatively, quasi-bound states in the continuum (q-BICs), supported in all-dielectric metasurfaces, offer a higher quality-factor (Q-factor) and electric field enhancement compared to plasmonic systems, making it a promising candidate for heatless nanotweezers with high performance. Here, we demonstrate an all-dielectric nanotweezer harnessing q-BIC for effective trapping of nanoparticles with low trapping power and negligible heating effects. The quasi-BIC system provides very high electromagnetic field intensity enhancement as well as high-quality-factor resonances. The q-BIC mode is induced by symmetry breaking of periodic double ring nanostructure, which leads to an increased optical force and potential energy in comparison to prior studies, enabling attractive promise in subwavelength particle trapping applications. Moreover, the q-BIC nanotweezer creates numerous optical hotspots characterized by significant field confinement and enhancement, generating multiple trapping sites and thus facilitating high-throughput trapping of nanoscale objects. Furthermore, the trapping wavelength (1047.59 nm) in our structure exactly corresponds with the current laser choice for working with biological samples. In addition, we show that trapped particles can improve the resonance mode of the cavity in a symmetry-broken system, which in turn enhances the trapping process. Our study paves the way for applying q-BIC systems to low-power particle trapping and sensing applications, especially for parallel analyses of biological cells and protein molecules.

AC electrothermal trapping for efficient nanoparticles enrichment in a microchannel

Trapping nanoparticles in a microfluidic device holds promise for enriching low-concentration species in samples. This article explores AC electrothermal (ACET) flow as an effective mechanism for nanoparticle trapping within a microfluidic system. Employing a precisely designed microfluidic device with coplanar symmetric electrodes, we systematically analyze the trapping performance of 100-nm polystyrene nanoparticles (PsNPs) dispersed in deionized water. Through fluorescence microscopy, we observe the dynamic behavior of PsNPs under the influence of an AC electric field. The concentration factor (CF) is introduced as a metric for evaluating trapping efficiency, revealing a remarkable up to 30-fold increase in concentration within the electrode gap. The quadratic relationship between electrothermal forces and electric field strength is investigated, highlighting the direct impact on trapping enhancement. By adjusting the applied voltage to vary the electric field strength, CF is found to significantly improve, reaching a peak value of 16.3 at 3.5 MV m−1. Furthermore, we explore the influence of frequency on ACET velocity, emphasizing the importance of aligning frequency and electric field strength for effective nanoparticle trapping. This study contributes valuable insights into the optimization of ACET for precise and controlled nanoparticle manipulation, holding promise for diverse applications in fields requiring nanoparticle enrichment.

Effect of two-photon absorption on trapping of plasmonic nanoparticles

In this paper, we introduce a theoretical framework for optical trapping that integrates nonlinear polarization within the dipole approximation. This theory represents the most comprehensive analytic model to date capable of resolving the discrepancies between the observed and simulated trapping of plasmonic nanoparticles. Our theory elucidates how two-photon absorption can account for the stable trapping of gold nanoparticles, including their longitudinal stability, especially near their plasmon resonance. Furthermore, the experimentally observed split potential wells in the transverse plane, which are attributed to two-photon absorption, are in close agreement with our model’s predictions. Finally, this study provides new insights into the mechanism of optical trapping under conditions of intense light–matter interactions.

Controlled Second Harmonic Generation with Optically Trapped Lithium Niobate Nanoparticles.

We report the second harmonic generation (SHG) response from a single 34 nm diameter lithium niobate nanoparticle. The experimental setup involves a first beam devoted to the optical trapping of single nanoparticles, whereas a second arm involves a femtosecond laser source leading to the SHG emission from the trapped nanoparticles. SHG operation where one to three nanoparticles are present in the optical trap is first demonstrated, highlighting the transition between coherent and incoherent SHG, the latter known as hyper-Rayleigh scattering (HRS). With a spatial light modulator moving the optical trap in and out of the focus of the femtosecond beam, the SHG intensity is switched back and forth between a low and a high level. This controlled operation opens new avenues for nanoparticle characterization and applications in sensing or communication and information technologies and constitutes the first step in the design of active substrateless metasurfaces.

Ultracompact Lab-on-a-Chip Device for Surface-Enhanced Coherent Anti-Stokes Raman Scattering.

This paper introduces an ultracompact lab-on-a-chip device with a size of [Formula: see text] for surface-enhanced coherent anti-Stokes Raman scattering. This device comprises of a unique hybrid plasmonic-photonic vertical coupler, for light-coupling between the device and a light source, and a heptamer plasmonic nanotweezer for trapping and manipulation of nanoparticles. The coupler with its nanoscale size of [Formula: see text] offers maximum coupling efficiency and directivity of -4.2 dB and 17.8 dB with a 3 dB bandwidth across the wavelength range of [Formula: see text]. Based on a finite element method, it is theoretically shown that the tear-drop based nanotweezer can empower the device to yield a very high Raman gain of ≥ 1017 , making the detection at the single-molecule level possible. The proposed device can exhibit low damage to bioparticles through the use of near infrared wavelengths range with lower energy photons. Due to its short optical path, this device is expected to improve the Raman signal to noise ratio by reducing the level of background in-coherent Raman signals. Finally, thanks to its configuration, this device can enable the creation of parallel measurement channels, paving the way toward the development of high-throughput and mass-produced biosensors.

Plasmonic Dipole and Quadrupole Scattering Modes Determine Optical Trapping, Optical Binding, and Swarming of Gold Nanoparticles

Plasmonic Dipole and Quadrupole Scattering Modes Determine Optical Trapping, Optical Binding, and Swarming of Gold Nanoparticles

Silica seed particles improve the efficiency and throughput of nanoparticle acoustic trapping

Silica has rarely been used as a seed particle material in acoustic trapping of nanoparticles. Here we use fluorescent nanoparticles, which are frequently used as a model system, to demonstrate that throughput and nanoparticle trapping efficiency can be improved by using silica seed particles as opposed to traditionally used polystyrene seed particles. The 10 times larger dipole scattering coefficient of silica seed particles compared with polystyrene seed particles in water leads to a higher retention force against fluid flow and thus enables higher throughput. Seed particles retained at an actuation voltage of approximately 10 V p.p. can withstand flow rates up to 2100 ± 200 µl/min for silica and 200 ± 50 µl/min for polystyrene. Furthermore, silica is found to be 40%–2000% more efficient (number of trapped nanoparticles as measured by fluorescent intensity) than polystyrene seed particles in trapping 270-nm polystyrene nanoparticles from suspensions of 1010–1011 particles/ml. Moreover, after enriching nanoparticles into a silica seed particle cluster, the washing flow rate can be increased from 30 µl/min to 200 µl/min (the flow rate at which polystyrene clusters are unstable), halving the total sample processing time without losing the silica seed particle cluster or compromising the nanoparticle trapping efficiency. Thus, material properties (particularly density) of the seed particles are critical to both nanoparticle trapping efficiency and throughput. Published by the American Physical Society 2024

The Arago–Poisson Spot: New Applications for an Old Concept

Herein, we report some specific properties and applications of the so-called Arago–Poisson spot in optics. This spot results from the diffraction of a plane wave by an occulting disk that leads to a small bright spot in its shadow. We discuss some of the properties of such beams. In particular, we focus on the ultimate size that can be reached for these beams, which depends on the diameter of the disk, the wavelength, and the distance from the disk. We also highlight self-healing and faster-than-light properties. Applications are then proposed. The applications mainly deal with new traps with nanometer sizes dedicated to the trapping of nanoparticles. We also discuss beams that change frequency during propagation and their application for signal delivery in a precise and determined area.

Kinetic trapping of nanoparticles by solvent-induced interactions.

Theoretical analysis based on mean field theory indicates that solvent-induced interactions (i.e. structural forces due to the rearrangement of wetting solvent molecules) not considered in DLVO theory can induce the kinetic trapping of nanoparticles at finite nanoscale separations from a well-wetted surface, under a range of ubiquitous physicochemical conditions for inorganic nanoparticles of common materials (e.g., metal oxides) in water or simple molecular solvents. This work proposes a simple analytical model that is applicable to arbitrary materials and simple solvents to determine the conditions for direct particle-surface contact or kinetic trapping at finite separations, by using experimentally measurable properties (e.g., Hamaker constants, interfacial free energies, and nanoparticle size) as input parameters. Analytical predictions of the proposed model are verified by molecular dynamics simulations and numerical solution of the Smoluchowski diffusion equation.

Origami nanogap electrodes for reversible nanoparticle trapping.

We present a facile desktop fabrication method for origami-based nanogap indium tin oxide (ITO) electrokinetic particle traps, providing a simplified approach compared to traditional lithographic techniques and effective trapping of nanoparticles. Our approach involves bending ITO thin films on optically transparent polyethylene terephthalate (PET), creating an array of parallel nanogaps. By strategically introducing weak points through cut-sharp edges, we successfully controlled the spread of nanocracks. A single crack spanning the constriction width and splitting the conductive layers forms a nanogap that can effectively trap small nanoparticles after applying an alternating electric potential across the nanogap. We analyze the conditions for reversible trapping and optimal performance of the nanogap ITO electrodes with optical microscopy and electrokinetic impedance spectroscopy. Our findings highlight the potential of this facile fabrication method for the use of ITO at active electro-actuated traps in microfluidic systems.

Sub-wavelength focused light and optical trapping application based on two-mode interference from an optical micro-/nanofiber

The ability to focus light on a subwavelength scale is essential in modern photonics. Optical microfiber-based sub-wavelength focusing will allow a miniaturized, flexible and versatile tool for many applications such as biomedical imaging and optomechanics. For a separate mode exited from an optical micro-/nanofiber endface, the photons will experience significant diffraction into the free space. This situation can be changed by incorporating two-mode interference along with the specific spatial distributions of both <i> <b>E</b> </i>-field amplitude and phase. Herein we report a novel approach to realizing sub-wavelength focusing based on the two-mode interference exited from an optical microfiber endface. By utilizing specific distributions of <b><i>E</i></b> -field amplitude and phase of two interacting optical modes, interference field patterns with a single focus (e.g., via a two-mode set of HE<sub>11</sub> and HE<sub>12</sub>) or multiple foci (e.g., via a two-mode set of HE<sub>11</sub> and HE<sub>31</sub>) can be obtained. Then, it is proved that the constructed foci will readily facilitate and selective trapping of nanoparticles. Circular polarization of optical mode is utilized in order to bring in angular symmetry of sub-wavelength focusing patterns compared with linear polarized optical modes. Our simulation results show that the smallest focal spot produced from the EH<sub>11</sub> and HE<sub>12</sub> mode interference has a full width at half-maximum (FWHM) of ~ 348 nm (i.e. 0.65<i>λ</i>). Such a subwavelength focusing field is applied to the optical trapping of an 85 nm-diameter polystyrene nanosphere. Further calculation reveals that the stable trapping can be fulfilled with axial and transverse trap stiffness of 11.48 pN/(μm·W) and 64.98 pN/(μm·W), as well as axial and transverse potential well of 101 <i>k</i><sub>B</sub>T/W and 641 <i>k</i><sub>B</sub>T/W via two-mode interference of HE<sub>11</sub> and HE<sub>12</sub>. These values demonstrate the great improvement over conventional tapered fibers. Further investigations show that different foci, via a two-mode set of HE<sub>11</sub> and HE<sub>31</sub>, exhibit unlike trap stiffness and potential wells, justifying the potential for nanoparticle size sorting. Based on the flexible all-fiber device, this subwavelength focusing strategy by two-mode interference may find promising applications in optical manipulation, superresolution optical imaging, data storage and nanolithography.

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.

Deep learning-based size prediction for optical trapped nanoparticles and extracellular vesicles from limited bandwidth camera detection.

Due to its ability to record position, intensity, and intensity distribution information, camera-based monitoring of nanoparticles in optical traps can enable multi-parametric morpho-optical characterization at the single-particle level. However, blurring due to the relatively long (10s of microsecond) integration times and aliasing from the resulting limited temporal bandwidth affect the detected particle position when considering nanoparticles in traps with strong stiffness, leading to inaccurate size predictions. Here, we propose a ResNet-based method for accurate size characterization of trapped nanoparticles, which is trained by considering only simulated time series data of nanoparticles' constrained Brownian motion. Experiments prove the method outperforms state-of-art sizing algorithms such as adjusted Lorentzian fitting or CNN-based networks on both standard nanoparticles and extracellular vesicles (EVs), as well as maintains good accuracy even when measurement times are relatively short (<1s per particle). On samples of clinical EVs, our network demonstrates a well-generalized ability to accurately determine the EV size distribution, as confirmed by comparison with gold-standard nanoparticle tracking analysis (NTA). Furthermore, by combining the sizing network with still frame images from high-speed video, the camera-based optical tweezers have the unique capacity to quantify both the size and refractive index of bio-nanoparticles at the single-particle level. These experiments prove the proposed sizing network as an ideal path for predicting the morphological heterogeneity of bio-nanoparticles in optical potential trapping-related measurements.

Light-to-Heat Conversion of Optically Trapped Hot Brownian Particles.

Anisotropic hybrid nanostructures stand out as promising therapeutic agents in photothermal conversion-based treatments. Accordingly, understanding local heat generation mediated by light-to-heat conversion of absorbing multicomponent nanoparticles at the single-particle level has forthwith become a subject of broad and current interest. Nonetheless, evaluating reliable temperature profiles around a single trapped nanoparticle is challenging from all of the experimental, computational, and fundamental viewpoints. Committed to filling this gap, the heat generation of an anisotropic hybrid nanostructure is explored by means of two different experimental approaches from which the local temperature is measured in a direct or indirect way, all in the context of hot Brownian motion theory. The results were compared with analytical results supported by the numerical computation of the wavelength-dependent absorption efficiencies in the discrete dipole approximation for scattering calculations, which has been extended to inhomogeneous nanostructures. Overall, we provide a consistent and comprehensive view of the heat generation in optical traps of highly absorbing particles from the viewpoint of the hot Brownian motion theory.

Enhancing gradient force over scattering force for nano-trapping through compensating for aberration

One challenge of optical trapping of nanoparticles is the weak trapping force compared to the destabilizing pushing force. Here we enhance the optical gradient force (GF), which is responsible for trapping, to achieve stable nanoparticle trapping through aberration compensation. The optical forces are calculated using multipole expansion theory and the focused fields are determined using Debye focusing theory accounting for interface aberrations between oil, glass, and water. With typical oil immersion objectives, the glass-water interface aberration reduces the GF relative to the scattering force (SF), leading to unstable trapping. By optimizing the refractive index of the immersion oil, the interface aberrations can be compensated. This significantly enhances the GF while moderately improves the SF, enabling stable nanoparticle trapping. The enhancements are particularly notable for large probe depths. Further improvement can be achieved with a thicker oil layer. With optimized conditions, the GF exceeds the SF by over two-fold. And the minimum axial force and axial stiffness increased approximately three-fold. Our study provides theoretical guidance to improve nanoparticle trapping efficiency through aberration compensation and force optimization.