Optical Trapping Research Articles

Helico-conical vector beams for intensity and polarization 3D light shaping

While vector beams offer an intriguing way to structure optical beams and enhance light-based technologies across many fields, their generation remains a challenging task in practical applications. Disclosing an unprecedented manipulation of light at the subwavelength scale, metaoptics have inspired smart and efficient solutions for spatially variant polarization structuring. Concurrently, the generalization of non-separability in polarization and phase manipulation extends the vectorial nature beyond standard vector vortices. In this work, we present the design and test of dual-functional metasurfaces for the compact generation of a new type of vector beam, so-called helico-conical vector beam, providing an inhomogeneous polarization pattern over customizable one-arm or two-arm 3D spirals of light. These devices pave the way to integrated optical architectures for dynamic optical manipulation and trapping in many fields, from optofluidics to quantum computing.

Radially polarized partially coherent beams with prescribed sinh-Gauss non-uniform correlation structure

We introduce a kind of radially polarized partially coherent beam with a prescribed sinh-Gauss non-uniform correlation structure, named a radially polarized sinh-Gauss non-uniformly correlated (RPSNC) beam. Utilizing the ordinary Huygens–Fresnel principle, we derive the analytical formulas for the spectral intensity and the spectral degree of polarization (DOP) in free space and investigate the beam’s propagation properties through numerical simulations. The results demonstrate that RPSNC beams exhibit a self-focusing property during propagation, with the focal position adjustable by varying the coherence length. Additionally, the spectral DOP in the central region forms a distinctive single-ring structure as the beam propagates. These unique properties make RPSNC beams promising for applications in free-space optical communications, beam shaping, and optical trapping.

Dissecting the pH Sensitivity of Kinesin-Driven Transport.

Kinesin-1 is a crucial motor protein that drives the microtubule-based movement of organelles, vital for cellular function and health. Mostly studied at pH 6.9, it moves at approximately 800 nm/s, covers about 1 μm before detaching, and hydrolyzes one ATP per 8 nm step. Given that cellular pH is dynamic and alterations in pH have significant implications for disease, understanding how kinesin-1 functions across different pH levels is crucial. To explore this, we executed single-molecule motility assays paired with precise optical trapping techniques over a pH range of 5.5-9.8. Our results show a consistent positive relationship between increasing pH and the enhanced detachment (off rate) and speed of kinesin-1. Measurements of the nucleotide-dependent off rate show that kinesin-1 exhibits the highest rate of ATPase activity at alkaline pH, while it demonstrates the optimal number of ATP turnover and cargo translocation efficiency at the acidic pH. Physiological pH of 6.9 optimally balances the biophysical activity of kinesin-1, potentially allowing it to function effectively across a range of pH levels. These insights emphasize the crucial role of pH homeostasis in cellular function, highlighting its importance for the precise regulation of motor proteins and efficient intracellular transport.

Polarization-insensitive bifocal metalenses by combining nanoimprint lithography and atomic layer deposition in the visible spectrum

Multifocal lenses are essential components for microscopy, spectroscopic detection, and optical trapping. Benefiting from the unprecedented capability of metasurfaces in light control, metalenses are able to provide multi-foci functionality with a more compact footprint, making them attractive alternatives to traditional bulky lenses. However, current manufacturing techniques encounter some challenges, including low throughput, high cost, and limited patterning areas. Here, we demonstrate the wafer-scale, low-cost, and high-throughput production of polarization-insensitive bifocal metalenses at a wavelength of 450 nm by combining nanoimprint lithography and atomic layer deposition. The nanoimprint process is simplified by using the imprinted resin itself as meta-atoms, which exhibit high aspect ratios (∼10:1) and small critical dimensions (∼90 nm). The effective refractive index of the meta-atoms is increased through atomic layer deposition of the high-index TiO2 film, providing 0–1.5π sufficient phase coverage. Metalenses with diameters of 480 μm are fabricated on the silica substrate, exhibiting two diffraction-limited focal spots along the optical axis. Moreover, the fabricated metalenses demonstrate the polarization-insensitive feature under various polarization states. The fabrication process presented in this Letter paves the way for large-scale and low-cost production of versatile metasurfaces operating in the visible or shorter spectrum.

Hybrid Paul-optical trap with large optical access for levitated optomechanics

We present a hybrid trapping platform that allows us to levitate a charged nanoparticle in high vacuum using either optical fields, radio-frequency fields, or a combination thereof. Our hybrid approach combines an optical dipole trap with a linear Paul trap while maintaining a large numerical aperture (0.77 NA). We detail a controlled transfer procedure that allows us to use the Paul trap as a “safety net” to recover particles lost from the optical trap at high vacuum. The presented hybrid platform adds to the toolbox of levitodynamics and represents an important step towards fully controllable “dark” potentials, providing control in the absence of decoherence because of photon recoil. Published by the American Physical Society 2024

Controlling the orientation of ellipsoidal nanoparticles using fractional vector beams

In the field of nanotechnology, achieving precise manipulation of ellipsoidal nanoparticles presents a significant challenge because it requires controlling five degrees of freedom, including three spatial dimensions (position in 3D space) and two angular dimensions (polar and azimuthal angles). In this work, we investigate both the optical forces and trapping potentials on an ellipsoidal nanoparticle produced by tightly focused fractional vector beams (FVBs). Unlike the integer vector beams (IVBs), which manipulate only three spatial dimensions of ellipsoidal particles, FVB with an initial phase not only provides spatial position control but also enables precise manipulation of spatial orientation. Moreover, by adjusting the topological index and initial phase of the incident FVBs, arbitrary orientations in the 3D space of ellipsoidal nanoparticles can be achieved. Our results may find interesting applications in microfluidics, biomedical engineering, and nanotechnology.

Compensating loss via non-Hermiticity in optically trapped and bounded particles.

The non-Hermiticity in the optical trapping and binding originates from their open nature. Once the non-Hermiticity is sufficiently large such that it pushes the system across the exceptional point, the non-Hermitian force will provide an effective gain to the systems. In this scenario, the trapped particles acquire additional energies as a consequence of the non-Hermitian force field, which effectively serve as a gain. Conversely, these trapped or bound particles can also dissipate energy as a result of the damping effect experienced during oscillation within optical trapping or binding. People usually employ vacuum extraction to extend the lifetime of particles' vibrational modes. However, low-pressure environments can induce instability in the systems. Here, we propose using the "non-Hermitian gain" to compensate for damping loss and enhance the quality factor (lifetime, Q-factor) of vibrational modes. Our study also takes into account the Brownian motion in optical trapping. Nevertheless, even after taking the Brownian motion into account, the Q-factors remained high. We further unveil the physical mechanism that can enhance or diminish non-Hermitian forces, such as increasing particle radius and refractive index and utilizing propagating or standing waves.

Propagation formulas and foci transformation of autofocusing beams.

Autofocusing beams have attracted widespread attention due to their advantages in optical trapping, but their propagation behavior in complex optical systems is still unclear. Here, we obtain the analytical propagation formulas for autofocusing beams through optical systems described by ABCD matrices. Foci adjustment through a lens and oscillate behavior in a parabolic potential medium of the beams are discussed. Interestingly, besides the real focus, autofocusing beams possess a virtual focus, which can be observed with a lens or lens-like medium. Furthermore, we provide a method for predicting the focal position of autofocusing beams passing through a given optical system, which is beneficial for the design and parameter optimization of practical optical devices.

Optical twin-vortex multi-trapping by Kolakoski lenses

In this work, we design and implement a new bifocal diffractive spiral lens within an optical tweezers system. The proposed diffractive optical element coined Kolokaski Kinoform Spiral Lens (KKSL), generates twin optical vortices along the propagation direction. The axial positions, as well as the diameters of the generated vortex beams, are correlated with the Kolakoski aperiodic sequence introduced in the design of the diffractive lens. The unique properties of KKSLs make it ideal for optical trapping applications, allowing independent multiple trapping and particle displacement within each optical vortex beam. We present both the focusing properties of KKSL and the experimental results of its integration into the optical tweezer setup.

Optical trapping of lipid bodies reveals increased cytoplasm viscosity in Candida tropicalis post antimicrobial photodynamic therapy

Abstract Antimicrobial photodynamic therapy (aPDT) is a common treatment for large cell colonies, but its effectiveness is typically assessed through colony-forming unit counting, which lacks microscopic details about cell death. This study monitors the trap stiffness of optically trapped lipid bodies of C. tropicalis of approximately 1 μm of radius following aPDT treatment. Methylene blue served as the photosensitizer at 20 μM concentration, with a lethal light dose of 60 J cm−2 The results revealed a significant increase in viscosity after aPDT treatment. Additionally, image analysis confirmed substantial morphological changes indicative of cell death. These findings demonstrate the potential of optical tweezers as a non-invasive tool for assessing cellular health by providing both functional (viscosity) and morphological data on the response to aPDT.

Concurrently Probing the Mechanical and Electrical Characteristics of Living Cells via an Integrated Microdevice.

The mechanical and electrical properties of cells serve as critical indicators of their physiological and pathological state. Currently, distinct setups are required to measure the electrical and mechanical responses of cells. In addition, most existing methods such as optical trapping (OT) and atomic force microscopy (AFM) are labor-intensive, expensive, and low-throughput. Here, we developed a microdevice that integrates automated cell trapping, deformation, and electric impedance spectroscopy to overcome these limitations. Our device enables parallel aspiration of tens of trapped cells in a highly scalable manner by simply adjusting the applied pressures, allowing for rapid probing of the dynamic viscoelastic properties of cells. Furthermore, embedded microelectrodes enable concurrent investigations of the electrical impedance of the cells. Through testing on different cell types, our platform demonstrated superior capabilities in comprehensive cell characterization and phenotyping, highlighting its great potential as a versatile tool for single cell analysis, drug screening, and disease detection.

Long-Lifetime Optical Trapping of a 40Ca+ Ion

Abstract We have experimentally achieved the all-optical trapping of a 40Ca+ ion. An optical dipole trap was established using a high-power, far-detuned, tightly focused laser with a wavelength of 532 nm. The single 40Ca+ ion was trapped without any RF fields and demonstrated a long lifetime of over 3 s. In this experiment, we implemented several measures to improve the optical trapping probability, including focusing the dipole beam waist near the diffraction limit, precisely compensating for stray electric fields, and mitigating electron shelving in metastable states. The optical trapping of a 40Ca+ ion eliminates the influence of micromotion induced by RF fields, potentially paving the way for development of all-optical trapping ion optical clocks.

Measurement of circular intensity differential scattering (CIDS) from single optically trapped biological particles

The circular intensity differential scattering (CIDS), which is the normalized Mueller matrix element -S14/S11, has been measured from single biological particles as a function of scattering angle. CIDS is valuable for its potential in detecting chiral particles that may include the helical structures of DNA or RNA molecules in biological samples, and as such is a potential method for detecting biological particles. Optical trapping is employed to levitate single particles within a custom-designed elliptical reflector for CIDS measurements. The advantage of optical levitation in light-scattering measurements is that single particles can be suspended in air with sufficient working distance to prevent interference from the suspending apparatus. To measure the phase function, the reflector is used to collect the angle-dependent scattering signals. We demonstrated that we can obtain two-dimensional angular optical scattering (TAOS) patterns that cover a wide angular range from single levitated particles. These TAOS patterns are generated using 532 nm illumination of left-handed and right-handed circular polarizations and recorded from trapped single particles (silica, English Oak, Ragweed, Mulberry, Glycine, and l-Aspartic acid).

Influence of astigmatic aberration on partially coherent Gaussian-Schell vortex beam focused by lensacon

Based on the mathematical framework of the partially coherent Gaussian Schell model vortex (PCGSMV) beam and the Huygens-Fresnel integral, we conducted a study to analyze the intensity distribution and depth of focus (DOF) of the PCGSMV beam as it propagates through a classical axicon. Our numerical results indicate the influence of factors such as the beam width, spatial degree of coherence, topological charge, and axicon base angle on the intensity distribution and DOF. In addition, we investigated the relationship between the beam spot size and propagation distance, as well as the effects of the spatial degree of coherence and propagation distance on the intensity profile. Our findings highlight the importance of the intensity values along the DOF for various applications, including the optical trapping and manipulation of micro particles.

Torque-speed relationship of the flagellar motor with dual-stator systems in Pseudomonas aeruginosa.

The single polar flagellar motor in Pseudomonas aeruginosa is equipped with two stator systems, MotAB and MotCD, both driven by H+ ions. The torque-speed relationship for flagellar motors with two stator systems has not been explored previously. In this study, we developed a method that utilizes optical trapping and fluorescence labeling to measure the torque-speed relationships for the wild-type P. aeruginosa motor with dual stators and mutant strains with a single stator system, revealing surprising differences in them. Moreover, we found that the MotAB stators exhibit slip-bond behavior in load dependence, contrasting with the catch-bond behavior of the MotCD stators and Escherichia coli stators. Further examination of the solvent isotope and pH effects on the torque-speed relationships of these stator systems provided additional insights into their dynamics. Interestingly, we discovered that the torque of the wild-type motor is similar to the combined torque of motors with MotAB or MotCD stators, indicating an additive contribution from the two stator types in the wild-type motors. These findings underscore the enhanced adaptability of P. aeruginosa to a wide range of external environments with varying load conditions.IMPORTANCEWe developed a novel method to measure the flagellar motor torque-speed relationship by trapping a swimming bacterium using optical tweezers. Using the P. aeruginosa flagellar motor as a model system to investigate motor dynamics with dual stator types, we measured the torque-speed relationships for wild-type motors with dual stator types and mutants with a single type. We found drastic differences that stem from the varying load dependencies of stator stability. These variations enable bacteria to rapidly adjust their stator composition in response to external load conditions. Interestingly, we observed that the torque of the wild-type motor is akin to the cumulative torque of motors with either stator type, indicating an additive contribution from the two stator types in wild-type motors. The methodology we established here can be readily employed to study motor dynamics in other flagellated bacteria.

Cylindrical Vector Beams with an MOPA Amplifier Based on Nonlinear Polarization Rotation Mode-Locking

CVBs (cylindrical vector beams) are widely used in optical imaging, optical trapping, material processing, etc. In this study, based on mode-selective couplers and passive mode-locking fiber technology, a cylindrical vector fiber amplifier with an MOPA (master oscillator power amplifier) structure was developed. In the experiment, the pre-amp stage reached 19.87 mW output power and a CVB output with a mode purity greater than 97%. The measured beam quality factor was M2 = 2.1. The CVB output power obtained by the first-amp stage was 152.4 mW, and the mode purity was greater than 92%. The measured beam quality factor was M2 = 1.99. The internal inhomogeneity and external effects of the isotropic LMA (large-mode-area) fiber led to a decrease in beam quality and mode purity. After amplification, the gain of the fundamental mode was higher and the power was greater, resulting in a greatly reduced mode purity. This CVB fiber amplifier yielded important research value in expanding the applications of high-power fiber lasers.

Micro-integrated crossed-beam optical dipole trap system with long-term alignment stability for mobile atomic quantum technologies.

Quantum technologies extensively use laser light for state preparation, manipulation, and readout. For field applications, these systems must be robust and compact, driving the need for miniaturized and highly stable optical setups and system integration. In this work, we present a micro-integrated crossed-beam optical dipole trap setup, the µXODT, designed for trapping and cooling 87Rb. This fiber-coupled setup operates at 1064 nm wavelength with up to 2.5 W optical power and realizes a free-space crossed beam geometry. The µXODT precisely overlaps two focused beams (w0 ≈ 33µm) at their waists in a 45° crossing angle, achieving a position difference of ≤3.4µm and a 0.998 power ratio between both beams with long-term stability. We describe the design and assembly process in detail, along with optical and thermal tests with temperatures of up to 65°C. The system's volume of 25ml represents a reduction of more than two orders of magnitude compared to typically used macroscopic setups while demonstrating exceptional mechanical robustness and thermal stability. The µXODT is integrated with an 87Rb 3D MOT setup, trapping 3×105 atoms from a laser-cooled atomic cloud, and has shown no signs of degradation after two years of operation.

Propagation of spatiotemporal Airy-Laguerre complex-variable-function Gaussian wave packets in a chiral medium

We analyze the propagation of spatiotemporal Airy-Laguerre complex-variable-function Gaussian (ALCG) wave packets using phase modulation. These wave packets can split into left circularly polarized ALCG (LCP-ALCG) and right circularly polarized ALCG (RCP-ALCG) components due to circular birefringence in a chiral medium. Our discussion primarily focuses on the effects of polynomial order, angle parameters, distribution factor, cross-phase factor, first-order chirp factor, and chiral parameter on the ALCG wave packets. The distinct LCP-ALCG and RCP-ALCG wave packet characteristics are obtained, and the radiation forces on Rayleigh dielectric particles are investigated. Our results emphasize the influence of various parameters in the propagation properties of ALCG wave packets, unveiling their potential for advancements in optical communication and optical trapping domains.

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.

Rotating manipulation of femtosecond optical tweezers based on optical wedge-lens group.

Currently, research on optical tweezers technology predominantly focuses on single-trap optical tweezers, which have a limited controllable range. Multi-trap optical tweezers effectively address these limitations. This paper proposes a method for developing a dual-trap optical tweezers system utilizing basic optical elements. Two optical traps are created by reflecting a laser beam off the front and rear surfaces of a beam splitter. The transition between single-trap and dual-trap configurations is facilitated by a lens group, which allows for the adjustment of the distance between the two traps. Furthermore, by incorporating a rotatable optical wedge into the optical path, the optical trap can be rotated along an annular orbit of any radius. This study includes simulations and analyses of the effects of lens spacing, refractive index, and tilt angle on the rotational range of optical traps. An optical trapping experimental system was constructed, and its feasibility was demonstrated using polystyrene particles as the target objects.