Optical Trapping Research Articles

Optical trapping capability of tornado circular Pearcey beams

We systemically investigate optical trapping capability of a kind of tornado waves on Rayleigh particles. Such tornado waves are named as tornado circular Pearcey beams (TCPBs) and produced by combining two circular Pearcey beams with different radii. Our theoretical exploration delves into various aspects, including the propagation dynamics, energy flux, orbital angular momentum, trapping force, and torque characteristics of TCPBs. The results reveal that the orbital angular momentum, trapping force, and torque of these beams can be finely tuned through the judicious manipulation of their topological charges (l 1 and l 2). Notably, we observe a precise control mechanism wherein the force diminishes with |l 1 + l 2| and |l 1–l 2|, while the torque exhibits enhancement by decreasing solely with |l 1 + l 2| or increasing with |l 1–l 2|. These results not only provide quantitative insights into the optical trapping performance of TCPBs but also serve as a valuable reference for the ongoing development of innovative photonic tools.

Analysis and Correction of the Additive Phase Effect Generated by Power Change in a Mach–Zehnder Interferometer Integrated to an Optical Trap

Analysis and Correction of the Additive Phase Effect Generated by Power Change in a Mach–Zehnder Interferometer Integrated to an Optical Trap

A new method to experimentally quantify dynamics of initial protein–protein interactions

Cells run on initiation of protein-protein interactions, which are dynamically tuned spatially and temporally to modulate cellular events. This tuning can be physical, such as attaching the protein to a cargo or protein complex, thereby altering its diffusive properties, or modulating the distance between protein pairs, or chemical, by altering the proteins’ conformations (e.g., nucleotide binding state of an enzyme, post-translational modification of a protein, etc.). Because a dynamic and changing subset of proteins in the cell could be in any specific state, ensemble measurements are not ideal—to untangle which of the factors are important, and how, we need single-molecule measurements. Experimentally, until now we have not had good tools to precisely measure initiation of such protein-protein interactions at the single-molecule level. Here, we develop a new method to measure dynamics of initial protein-protein interactions, allowing measurement of how properties such as the distance between proteins, and their tethered length can modulate the rate of interactions. In addition to precise measurement distance dependent motor-MT rebinding dynamics, we demonstrate the use of a dithered optical trap to measure dynamic motor-MT interactions and further discuss the possibilities of this technique being applicable to other systems.

Collimating three-axicon zoom system for interferometric Bessel beam side lobe cancellation

Optical Bessel beams are used in numerous applications like fluorescence microscopy, material processing and optical trapping. These applications require Bessel beams having a central core with defined full width at half maximum and a defined axial length. Often, the side lobes of Bessel beams, which are associated with their non-diffracting properties, can interfere with the experimental process. We theoretically describe and practically verify the performance of a new refractive optical system to generate zoomable annular ring intensities. The ability to zoom the output ring diameter allows for flexibly choosing the Bessel beam parameters. Secondly, we introduce the use of a Michelson interferometer for destructively interfering Bessel beam side lobes in one direction. If two Bessel beams of zeroth order and first kind are coherently superposed with a small shift with respect to each other, their side lobes are enhanced in one direction and cancelled in the other direction. We suggest that applications like light-sheet microscopy can exploit the axis of destructive interference to improve their contrast.

Merging toroidal dipole bound states in the continuum without up-down symmetry in Lieb lattice metasurfaces

Abstract The significance of bound states in the continuum (BICs) lies in their potential for theoretically infinite quality factors. However, their actual quality factors are limited by imperfections in fabrication, which lead to coupling with the radiation continuum. In this study, we present a novel approach to address this issue by introducing a merging BIC regime based on a Lieb lattice. By utilizing this approach, we effectively suppress the out-of-plane scattering loss, thereby enhancing the robustness of the structure against fabrication artifacts. Notably, unlike previous merging systems, our design does not rely on the up-down symmetry of metasurfaces. This characteristic grants more flexibility in applications that involve substrates and superstrates with different optical properties, such as microfluidic devices. Furthermore, we incorporate a lateral band gap mirror into the design to encapsulate the BIC structure. This mirror serves to suppress the in-plane radiation resulting from finite-size effects, leading to a remarkable ten-fold improvement in the quality factor. Consequently, our merged BIC metasurface, enclosed by the Lieb lattice photonic crystal mirror, achieves an exceptionally high-quality factor of 105 while maintaining a small footprint of 26.6 × 26.6 μm. Our findings establish an appealing platform that capitalizes on the topological nature of BICs within compact structures. This platform holds great promise for various applications, including optical trapping, optofluidics, and high-sensitivity biodetection, opening up new possibilities in these fields.

Phototaxis Flight of Microdroplets in a Laser.

Phototaxis phenomenon is fundamental and critical for optical manipulation of micro-objects. Here, we report the size-dependent negative or positive phototaxis behaviors for microdroplets containing interfacial energy absorber flying in a laser. The critical diameters for such negative-to-positive turnover are studied through both experiments and simulation with different liquids and absorbers, which establishes the mechanism and reveals the role of both the liquid and the absorber inside the microdroplets. This study offers new insight for the manipulation of the phototaxis behavior of micro-objects, showing potential applications in optical trapping and transporting systems that involve light-microdroplet interactions.

Random laser emission from dye-doped gain media with optically trapped scattering particles

In this study, an optical trapping technique is employed to manipulate the scatterer distribution within a random laser medium. By focusing the trapping beams into small regions within the three-dimensional scattering medium, the scattering particles around them are concentrated in those regions, resulting in an inhomogeneous scatterer distribution. The experimental results show that optical trapping increases the maximum spike intensity in the emission spectrum. Furthermore, the spectral spike intensity depends on the power of the trap spots. The relationship between the maximum and average spike intensities in the emission spectra exhibits a characteristic observed in other random lasers with inhomogeneous scatterer distributions.

Non-Hermitian non-equipartition theory for trapped particles

The equipartition theorem is an elegant cornerstone theory of thermal and statistical physics. However, it fails to address some contemporary problems, such as those associated with optical and acoustic trapping, due to the non-Hermitian nature of the external wave-induced force. We use stochastic calculus to solve the Langevin equation and thereby analytically generalize the equipartition theorem to a theory that we denote the non-Hermitian non-equipartition theory. We use the non-Hermitian non-equipartition theory to calculate the relevant statistics, which reveal that the averaged kinetic and potential energies are no longer equal to kBT/2 and are not equipartitioned. As examples, we apply non-Hermitian non-equipartition theory to derive the connection between the non-Hermitian trapping force and particle statistics, whereby measurement of the latter can determine the former. Furthermore, we apply a non-Hermitian force to convert a saddle potential into a stable potential, leading to a different type of stable state.

Artificial neural network assisted the design of subwavelength-grating waveguides for nanoparticles optical trapping.

In this work, we propose artificial neural networks (ANNs) to predict the optical forces on particles with a radius of 50 nm and inverse-design the subwavelength-grating (SWG) waveguides structure for trapping. The SWG waveguides are applied to particle trapping due to their superior bulk sensitivity and surface sensitivity, as well as longer working distance than conventional nanophotonic waveguides. To reduce the time consumption of the design, we train ANNs to predict the trapping forces and to inverse-design the geometric structure of SWG waveguides, and the low mean square errors (MSE) of the networks achieve 2.8 × 10-4. Based on the well-trained forward prediction and inverse-design network, an SWG waveguide with significant trapping performance is designed. The trapping forces in the y-direction achieve-40.39 pN when the center of the particle is placed 100 nm away from the side wall of the silicon segment, and the negative sign of the optical forces indicates the direction of the forces. The maximum trapping potential achieved to 838.16 kBT in the y-direction. The trapping performance in the x and z directions is also quite superior, and the neural network model has been further applied to design SWGs with a high trapping performance. The present work is of significance for further research on the application of artificial neural networks in other optical devices designed for particle trapping.

Pulsed Optical Vortex Array Generation in a Self-Q-Switched Tm:YALO3 Laser.

Optical vortex arrays are characterized by specific orbital angular momentums, and they have important applications in optical trapping and manipulation, optical communications, secure communications, and high-security information processing. Despite widespread research on optical vortex arrays, the 2 μm wavelength range remains underexplored. Pulsed lasers at 2 μm are vital in laser medicine, sensing, communications, and nonlinear optic applications. The need for 2 μm-pulsed structured optical vortices, combining the advantages of this wavelength range and optical vortex arrays, is evident. Therefore, using just three elements in the cavity, we demonstrate a compact self-Q-switched Tm:YALO3 vortex laser by utilizing the self-modulation effect of a laser crystal and a defect spot mirror. By tuning the position of the defect spot and the output coupler, the resonator delivers optical vortex arrays with phase singularities ranging from 1 to 4. The narrowest pulse widths of the TEM00 LG0,-1, two-, three-, and four-vortex arrays are 543, 1266, 1281, 2379, and 1615 ns, respectively. All the vortex arrays in our study have relatively high-power outputs, slope efficiencies, and single-pulse energies. This work paves the way for a 2 μm-pulsed structured light source that has potential applications in optical trapping and manipulation, free-space optical communications, and laser medicine.

Recent developments in biomedical applications of metasurface optics

Metasurfaces have attracted considerable attention because of their unique optical capabilities to control the fundamental properties of light, such as amplitude, phase, and polarization. The flat nature of metasurfaces can help reduce the complexities and bulk of conventional optical systems. After a decade of rapid progress, metasurfaces are close to maturity and have found their role in various optical applications. This review emphasizes the significant advancements and emerging applications of metasurfaces in biomedical optics, particularly focusing on beam shaping for laser treatments, light-sheet fluorescence microscopy, HiLo microscopy, and optical trapping. Looking forward, we discuss foreseeable challenges for integrating metasurfaces into biomedical, preclinical, and clinical systems.

Variance sum rule for entropy production.

Entropy production is the hallmark of nonequilibrium physics, quantifying irreversibility, dissipation, and the efficiency of energy transduction processes. Despite many efforts, its measurement at the nanoscale remains challenging. We introduce a variance sum rule (VSR) for displacement and force variances that permits us to measure the entropy production rate σ in nonequilibrium steady states. We first illustrate it for directly measurable forces, such as an active Brownian particle in an optical trap. We then apply the VSR to flickering experiments in human red blood cells. We find that σ is spatially heterogeneous with a finite correlation length, and its average value agrees with calorimetry measurements. The VSR paves the way to derive σ using force spectroscopy and time-resolved imaging in living and active matter.

Observation of Photoassociation Resonances in Ultracold Atom-Molecule Collisions.

We report on the observation of photoassociation resonances in ultracold collisions between ^{23}Na^{40}K molecules and ^{40}K atoms. We perform photoassociation in a long-wavelength optical dipole trap to form deeply bound triatomic molecules in electronically excited states. The atom-molecule Feshbach resonance is used to enhance the free-bound Franck-Condon overlap. The photoassociation into well-defined quantum states of excited triatomic molecules is identified by observing resonantly enhanced loss features. These loss features depend on the polarization of the photoassociation lasers, allowing us to assign rotational quantum numbers. The observation of ultracold atom-molecule photoassociation resonances paves the way toward preparing ground-state triatomic molecules, provides a new high-resolution spectroscopy technique for polyatomic molecules, and is also important to atom-molecule Feshbach resonances.

Dark matter condensates as highly nonlocal solitons: instability in the Schwarzschild metric and laboratory analog

Theories on the bosonic nature of dark matter are a promising alternative to the cold dark matter model. Here we consider a dark matter halo in the state of a Bose–Einstein condensate (BEC), subject to the gravitation of a black hole. In the low energy limit, we bring together the general relativity in the Schwarzschild metric and the quantum description of the BEC. The model is solvable in the Fermi normal coordinates with the so called highly nonlocal approximation and describes tidal deformations in the condensate wave function. The black hole deforms the localized condensate until the attraction of the compact object overcomes the self-gravitation and destabilizes the solitonic dark matter. Moreover, the model can be implemented as a gravitational analog in the laboratory; the time-dependent potential generated by the galactic black hole can be mimicked by an optical trap acting on a conventional condensate. The results open the way to new laboratory simulators for quantum gravitational effects.

Coherence-tailorable vector fiber source

Apart from the polarization and spatial mode, the degree of coherence (DoC), as a new degree of freedom (DoF), has actuated the control of structured optical fields beyond into a higher dimension. However, the control of DoC mainly focuses on the spatial coherence, and so far, there are no reports on the modulation of temporal coherence of the structured light. Here, a temporal coherence-tailorable vector fiber source is proposed and experimentally demonstrated. With a two-mode long-period fiber grating and polarization control, switchable transverse-mode outputs including azimuthally polarized mode TE01 and radially polarized mode TM01 are performed in an all-fiber compact regime. Benefitting from a tunable superfluorescent fiber source as the illumination of spatial mode control, the temporal coherence and the operating wavelength of vector beams can be simultaneously modulated. This work may not only advance a considerable protocol for creating and manipulating high-dimensional vectorial optical field, but also open up a promising avenue for potential applications in optical communication, high-resolution imaging, and particle trapping.

Dirac exciton-polariton condensates in photonic crystal gratings.

Bound states in the continuum have recently been utilized in photonic crystal gratings to achieve strong coupling and ultralow threshold condensation of exciton-polariton quasiparticles with atypical Dirac-like features in their dispersion relation. Here, we develop the single- and many-body theory of these new effective relativistic polaritonic modes and describe their mean-field condensation dynamics facilitated by the interplay between protection from the radiative continuum and negative-mass optical trapping. Our theory accounts for tunable grating parameters giving full control over the diffractive coupling properties between guided polaritons and the radiative continuum, unexplored for polariton condensates. In particular, we discover stable cyclical condensate solutions mimicking a driven-dissipative analog of the zitterbewegung effect characterized by coherent superposition of ballistic and trapped polariton waves. We clarify important distinctions between the polariton nearfield and farfield explaining recent experiments on the emission characteristics of these long lived nonlinear Dirac polaritons.

Robust autofocusing propagation in turbulence

Turbulence in complex environments such as the atmosphere and biological media has always been a great challenge to the application of beam propagation in optical communication, optical trapping and manipulation. To overcome this challenge, this study comprehensively investigates the robust propagation of traditional Gaussian and autofocusing beams in turbulent environments. In order to select stable beams that exhibit high intensity and high field gradient at the focal position in complex environments, Kolmogorov turbulence theory is used to simulate the propagation of beams in atmospheric turbulence based on the multi-phase screen method. We systematically analyze the intensity fluctuations, the variation of the coherence factor and the change in the scintillation index with propagation distance. The analysis reveals that the intensity fluctuations of autofocusing beams are significantly smaller than those of Gaussian beams, and the coherence of autofocusing beams is better than that of Gaussian beams under turbulence. Moreover, autofocusing beams exhibit less oscillation than Gaussian beams, indicating that autofocusing beams propagate in complex environments with less distortion and intensity fluctuation. Overall, this work clearly demonstrates that autofocusing beams exhibit higher stability in propagation compared with Gaussian beams, showing great promise for applications such as optical trapping and manipulation in complex environments.

Light sheet fluorescence microscopy with active optical manipulation.

We present a light sheet fluorescence microscopy (LSFM) with active optical manipulation by using linear optical tweezers (LOTs). In this method, two coaxially transmitting laser beams of different wavelengths are shaped using cylindrical lenses to form a linear optical trapping perpendicular to the optical axis and an excitation light sheet (LS) parallel to the optical axis, respectively. Multiple large-sized polystyrene fluorescent microspheres are stably captured by LOTs, and their rotation angles around specific rotation axes are precisely controlled. During a sample rotation, the stationary excitation LS scans the sample to obtain fluorescence sectioning images of the sample at different angles.

Improved detection of particle displacement in optical traps using back focal plane interferometry and image processing

Optical tweezers play a crucial role in particle manipulation and force measurement. Therefore, it is essential to calibrate the force measurement parameters, such as the optical trap stiffness and the conversion factor of the sensor deflection signal to the actual displacement of the trapped particle. This calibration is necessary to achieve accurate and efficient parameter calibration. In this study, we employed a suitable image processing method, based on a low-frequency CMOS camera, to calibrate the Displacement Conversion Factor (DCF) of the optical tweezers system. The high accuracy of this image processing method makes it ideal for analyzing the actual displacement of optically trapped microspheres. Interestingly, we discovered that the normalized deflection signal captured by the quadrant photodiode (QPD) through back focal plane interferometry is primarily correlated with the axial height of the trapped microspheres, and is relatively unaffected by laser power variations. Building on this finding, we also examined the linear detection range of the trapped microspheres at different heights. These results address certain limitations associated with the use of optical tweezer systems and provide valuable guidance for utilizing this tool effectively.

Charge transport mechanism and trap origin in methyl methacrylate copolymer with thioxanthenone side groups

The transverse charge transport mechanism in a specially synthesized novel methyl methacrylate copolymer containing 2 % electron-accepting 9H-thioxanthene-9-one side groups (poly(MMA-co-ThS)) has been studied experimentally and theoretically. These side groups are shown to be deep electron traps. The charge transport in poly(MMA-co-ThS) is governed by the model of phonon-assisted tunneling between traps. The optical ionization trap energy (Wopt = 1.8 eV), independently determined by a reduction spectroelectrochemical method, confirms the validity of the phonon-assisted tunneling model used to describe charge transport in organic dielectrics with high content of electron traps.