Optical Trapping Research Articles

Optical trapping performance of circular Pearcey beams on Mie particles

Optical trapping performance of circular Pearcey beams on Mie particles

Fluidic laser beam shaper based on thermal lens effect in MoS2 and its application in optical trapping

In this work, we develop a fluidic laser beam shaper based on the thermal lens effect in MoS2 suspensions. It is demonstrated that another weak unfocused Gaussian beam (probe beam) is transformed into an annular beam due to the thermal lens effect resulting from a strong focused Gaussian beam (pump beam). The time evolution of the generated annular beam is studied, and the result shows that the annular beam exhibits asymmetric caused by heat convection with the high pump beam power. In addition, the tunability of the generated annular beam is investigated. Furthermore, the optical trapping of micro-sized particles in air using the annular beam is researched. It is found that this method exhibits stable performance capture for 3 h. The advantages of adjustable size, flexible setup and low cost of annular beam generated method are significant for lots of laser applications, such as optical trapping, optical imaging and laser machining.

Sculpting optical fields into caustic patterns based on freeform optics

Manipulating flux transportation of optical fields holds great promise across various kinds of applications, encompassing laser micro-processing, optical trapping, microscopic imaging, and illumination engineering. In recent decades, freeform optics, lauded for its capacity to efficiently control the wavefronts of optical beams with high design freedom, has garnered significant attention as a potent tool for shaping light. Nevertheless, the predominant focus in current research has centered on the development of freeform optical elements (FOEs) tailored to project a designated two-dimensional flux density onto a specific target plane. This limitation confines the potential applications of freeform light shaping optics to a rather restricted domain. Moreover, the constraints imposed by geometric optics pose a further limitation on FOEs, particularly in their ability to sculpt coherent light, such as laser beams, as these are particularly susceptible to diffraction effects. Within the context of this study, we propose a design paradigm that harnesses freeform optics to craft light trajectories in three dimensions. This approach inherently gives rise to caustics, which are singularities within the realm of flux transfer in geometric optics. Crucially, our proposed method yields the capability to generate sharply defined light patterns, empirically giving better results for mitigating diffraction effects in contrast to previous design methodologies, which is observed by experiments. This design philosophy empowers FOEs to undertake new roles and functionalities by shaping caustics. To substantiate the potential of our proposal, we present a diverse array of design examples. These encompass scenarios ranging from the illumination of parallel planes to the sculpting of light propagation into three-dimensional curves, as well as the reshaping of beam intensities with an extended depth of field. We expect that the proposed work can broaden the scope of freeform optics, particularly within non-imaging applications, and propel freeform optical elements into hitherto unexplored territories.

Propagation Properties of Generalized Schell-Model Pulse Sources in Dispersive Media

A model of a generalized pulse source, whose complex degree of temporal coherence is described by a function of the nth power difference of two instants, was constructed. As examples, we consider the generalized Gaussian and multi-Gaussian Schell-model pulse sources and study their propagation in dispersive media. It is indicated that such pulse beams present unique self-focusing, off-axis self-shifting and asymmetric self-splitting characteristics by adjusting the power exponent and phase parameters. Further, we explicitly discuss how the coherence time, summation factor as well as the dispersive coefficient significantly affect the self-focusing and self-shifting behaviors of the pulse beam. The results will benefit some applications involving pulse shaping, optical trapping and remote sensing.

Self-induced back-action for aperture trapping: Bethe-Rayleigh theory.

A dielectric (nano)particle can influence the local electromagnetic field and thereby alter its interaction with that field through the process of self-induced back-action. While this phenomenon is usually considered theoretically as a change in a cavity resonance frequency, such theoretical approaches are not as appropriate when considering systems away from resonance, such as with a subwavelength aperture in a metal film. Here we consider the interaction between an aperture, modelled with Bethe theory as a magnetic dipole, and a Rayleigh particle, modelled as an electric dipole. Using this magnetic dipole - electric dipole interaction, we quantify the self-induced back-action of the particle on the aperture transmission and the optical trapping potential. The model shows quantitative agreement with finite-difference time-domain simulations. This shows that the physics of self-induced back-action for an aperture and a nanoparticle can be understood in terms of dipole-dipole coupling.

Three-dimensional optical multiple trapping using pure amplitude octagonal almost periodic structures.

We demonstrate a novel method for three-dimensional optical multiple trapping using pure amplitude octagonal almost periodic structures (PAOAPSs). We use a Gaussian beam to diffract through these structures and create a three-dimensional array of trapping spots with the aid of an objective lens. Our device is simple, cost-effective, and easy to fabricate, and it has several advantages over conventional methods for trapping multiple particles. By adjusting the rotation of the PAOAPS and the polarization of the beam, we can simultaneously rotate the trapped particles in both axial and orbital directions. We show that our device achieves an ∼19-fold increase in trapping efficiency compared to a recently introduced method based on an amplitude radial grating. Furthermore, our device transfers about 1/70 of the transmitted beam power to each optical trap, which is much more efficient than a spatial light modulator (SLM).

Direct dielectrophoretic characterization of particles in the high-density microwell array using optical tweezers

In this work, a quantitative and intuitive measurement of dielectrophoresis (DEP) behavior in a high-density dielectrophoretic microwell array chip was accomplished in assistance with the holographic optical tweezers. A one-dimensional force balance system with high reliability was achieved between the transverse optical trapping (TOT) force and the DEP force. The value of the DEP force was inferred by accurately calculating the TOT force, and the direction of the DEP force was observed with high-magnification microscopy. In order to estimate the availability and operability of the DEP force measurement system, we systematically investigated the influences of various external factors on the DEP behaviors of microspheres and live cells. All the quantitative measurement results, which agree well with the theory, showed that this system could serve as an accurate research tool for DEP behavior characterizations to optimize and improve the DEP-based manipulations of microparticles and live cells for a broad range of applications. The system also provides an approach to quantitatively obtain important parameters such as Clausius-Mossotti factors for cells in mediums, which could potentially be used to understand the dielectric properties of different types of cells for cell differentiation and diagnosis at the single-cell level.

Particle-wall hydrodynamic effects on optical trapping viscometry

Optical tweezers are a versatile setup used to measure forces in soft materials and characterize the rheological properties of fluids and gels. An accurate measurement of these forces is complicated by the interaction between the optically trapped materials and nearby substrate walls. This work presents a comprehensive Brownian Dynamic (BD) model that accounts for the hydrodynamic interactions between an optically trapped particle and a nearby boundary. The model is based on a midpoint algorithm, which simplifies accounting for diffusion gradients that arise from hydrodynamic interactions. Experiments using an optical tweezer and a quadrant position detector (QPD) are performed to validate these experiments. We conduct experiments by varying input laser current from 100 mA to 300 mA. Our experiments show that measured viscosity at 100 mA is ∼19% higher than the value expected for room temperature water (0.89 mPa-s). The discrepancy diminishes to 4% when a 300 mA current is used. Simulations performed with the BD model show that, for a nominal spring constant of ks=4.332×10−4IlaserpN/μm, the discrepancy can be explained via hydrodynamic interactions that influence the measured effective viscosity. Wide variability in confidence interactions observed in our data can be replicated by using a range of diameters in our simulation. This is consistent with DLS measurements of the particles used in this study, which show that the mean diameter was (1.63 ± 0.28) μm. This work has implications for measuring Newtonian media viscosities up to ∼63 mPa-s, as determined by a scaling analysis of limitations for our measurement system.

An efficient Yb:YAG microchip laser for generating high-power cylindrical vector beams with four topological charges

High-power cylindrical vector beams (CVBs) in the form of Laguerre-Gaussian (LG0,l) with fixed topological charge and desirable state of polarization (SoP) have been widely needed for various applications such as quantum communication, optical trapping, manipulating microparticles, laser machining, and high-resolution imaging. Highly efficient high-order CVBs with fixed topological charge and SoP generated in solid-state lasers are desirable for various applications. Here, by adjusting annular pump beam waist and distance between focus spot and gain medium, we directly generated LG0,l (l = 2, 3, 4) CVBs with SoP in the form of E0,l(ϕ,r) = - Eϕ ×sin[(l + 1)ϕ]+ Er ×cos[(l + 1)ϕ] in a highly efficient Yb3+:Y3Al5O12 (Yb:YAG) microchip laser. The LG0,4 CVB is obtained when the input pump power is higher than 2.9 W and keeps as the input pump power further increases. The output power of the LG0,4 cylindrical vector laser is 1.3 W at the input pump power of 6.1 W. The optical conversion efficiency is as high as 21.3%. Polarization purity is over 93% for LG0,4 CVB generated in the Yb:YAG microchip laser.

Highly efficient nonlinear vortex beam generation by using a compact nonlinear fork grating

Vortex beams with an orbital angular momentum (OAM) are extremely important in optical trapping, optical micromachining, high-capacity optical communications, and quantum optics. Nonlinear generation of such a vortex beam enables vortex beams to be obtained at new wavelengths, which opens up new possibilities for all-optical switching and manipulation of vortex beams. However, previous nonlinear vortex beam generation suffers from either low efficiency or low-level integration. Here, we use the technique of ultraviolet photolithography-assisted inductively coupled plasma (ICP) etching to realize a compact nonlinear fork grating for high-efficiency nonlinear vortex beam generation. In our experiment, the depth of such a compact nonlinear fork-grating structure can be precisely controlled by etching time. The vortex beams with a topological charge of l = ±1, ± 2, ± 3 can be generated in the far field, and the normalized nonlinear conversion efficiency of such nonlinear vortex beam is 189% W−1cm−2. Our method not only provides an efficient and compact method for nonlinear vortex beam manipulation but also suits for timesaving and large-area nonlinear functional device fabrication.

Direct creation of interacting quasi-one-dimensional Bose–Einstein condensate through fast evaporative cooling

Preparation of atomic Bose–Einstein condensate (BEC) with tunable interactions in a quasi-one-dimensional (quasi-1D) optical trap is essential for both the observation of bright matter-wave solitons and the quantum simulation based on the discrete atomic momentum states. However, the quasi-1D BEC has been obtained by a complex process, which includes the creation of a three-dimensional BEC and its adiabatic transform into a quasi-1D trap. Here, we report the direct creation of a quasi-1D BEC of 133Cs atoms by the fast evaporative cooling of ultracold atoms prepared by the degenerated Raman sideband cooling. We produce the pure BEC of up to 5.5×104 atoms in a quasi-1D optical trap with an evaporative time of 6 s. We demonstrate the anisotropic expansion of the atomic cloud after the release from the quasi-1D trap and study the dependence of both the phase space density and the temperature on the number of atoms in the trap during the evaporative cooling. Our study facilitates the promising applications of quasi-1D interacting atomic gases.

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.

Thermo-osmotic slip flows around a thermophoretic microparticle characterized by optical trapping of tracers

Thermo-osmotic slip flows around a thermophoretic microparticle characterized by optical trapping of tracers

Propagation of generalized multi-Gaussian correlated Schell-model beams in electromagnetically induced transparency atomic vapor

Propagation of generalized multi-Gaussian correlated Schell-model (GMGCSM) beams through four-level electromagnetically induced transparency (EIT) atomic vapor is investigated for the first time. Analytical expression for the GMGCSM beam passing through an ABCD optical system of EIT vapor is deduced and employed to analyze its propagation properties. It is demonstrated that intensity evolution of the targeted GMGCSM beam through EIT atomic vapor is dependent on the coupling Rabi frequency of the control light, the beam index and coherence length. Moreover, analysis shows that degree of coherence of the beam can also be regulated by the coupling Rabi frequency. Additionally, it is shown that, selecting appropriate coherence length, GMGCSM beams of the first kind would form a shift from a dark-centered distribution known as “optical cage”, to a peak-centered distribution; GMGCSM beams of the second kind would shift from a flat-topped distribution to a Gaussian distribution. Therefore, EIT atomic vapor provides a method to regulate the coherence of partially coherent beams. Results obtained here could be expected applications in free-space optical communications, material thermal processing, and optical trapping.

Optimized, highly efficient silicon antennas for optical phased arrays

Silicon photonics, in conjunction with complementary metal-oxide-semiconductor (CMOS) fabrication, has greatly enhanced the development of integrated optical phased arrays. This facilitates a dynamic control of light in a compact form factor that enables the synthesis of arbitrary complex wavefronts in the infrared spectrum. We numerically demonstrate a large-scale two-dimensional silicon-based optical phased array (OPA) composed of nanoantennas with circular gratings that are balanced in power and aligned in phase, required for producing elegant radiation patterns in the far-field. For a wavelength of 1.55 μm, we optimize two antennas for the OPA exhibiting an upward radiation efficiency as high as 90%, with almost 6.8% of optical power concentrated in the field of view. Additionally, we believe that the proposed OPAs can be easily fabricated and would have the ability to generate complex holographic images, rendering them an attractive candidate for a wide range of applications like LiDAR sensors, optical trapping, optogenetic stimulation, and augmented-reality displays.

Spectroscopic Analysis of Concentration Dynamics and Crystallization of Hen Egg-White Lysozyme under Optical Trapping Conditions

Spectroscopic Analysis of Concentration Dynamics and Crystallization of Hen Egg-White Lysozyme under Optical Trapping Conditions

Optimal calibration of optical tweezers with arbitrary integration time and sampling frequencies: a general framework [Invited

Optical tweezers (OT) have become an essential technique in several fields of physics, chemistry, and biology as precise micromanipulation tools and microscopic force transducers. Quantitative measurements require the accurate calibration of the trap stiffness of the optical trap and the diffusion constant of the optically trapped particle. This is typically done by statistical estimators constructed from the position signal of the particle, which is recorded by a digital camera or a quadrant photodiode. The finite integration time and sampling frequency of the detector need to be properly taken into account. Here, we present a general approach based on the joint probability density function of the sampled trajectory that corrects exactly the biases due to the detector's finite integration time and limited sampling frequency, providing theoretical formulas for the most widely employed calibration methods: equipartition, mean squared displacement, autocorrelation, power spectral density, and force reconstruction via maximum-likelihood-estimator analysis (FORMA). Our results, tested with experiments and Monte Carlo simulations, will permit users of OT to confidently estimate the trap stiffness and diffusion constant, extending their use to a broader set of experimental conditions.

Broadband Hybridly Polarized Vector Vortex Raman Microchip Laser

AbstractHybridly polarized (HP) vector vortex Raman lasers dramatically extend their applications on optical microscopy, optical communication, and quantum information. Spatial light modulators and waveplates are widely used for generating HP vector vortex lasers, however, the performance and beam quality of HP vector vortex lasers are restricted by diffraction loss and low damage threshold of these optical elements. Here, HP vector vortex Raman microchip lasers constructed with Yb3+:Y3Al5O12 (Yb:YAG) and vanadate (YVO4) crystals is demonstrated. The states of polarization (SoP) of HP vector vortex lasers are combination of radial and anti‐radial polarizations (RP‐ARP), azimuthal and anti‐azimuthal polarizations (AP‐AAP). The SoP of HP vector vortex lasers can be controlled by adjusting the length of YVO4 crystal and applying pump power. Maximum output powers are 456 and 586 mW with optical efficiency of 7.1% and 9.2% for HP vector vortex lasers with SoP of RP‐ARP and AP‐AAP. The HP vector vortex Raman lasers with SoP of RP‐ARP and AP‐AAP oscillate ≈1076 nm with bandwidths of 11.4 and 10.8 nm. High beam quality is achieved for HP vector vortex lasers with measured M2 nearly equal to theoretical value. The broadband HP vector vortex Raman lasers with high beam quality extend applications on optical trapping, and quantum information processing.

Resonant optical trapping of Janus nanoparticles in plasmonic nanoaperture

Controlled trapping of light-absorbing nanoparticles with low-power optical tweezers is crucial for remote manipulation of small objects. This study takes advantage of the synergetic effects of tightly confined local fields of plasmonic nanoaperture, self-induced back-action (SIBA) of nanoparticles, and resonant optical trapping method to demonstrate enhanced manipulation of Janus nanoparticles in metallic nanohole aperture. We theoretically demonstrate that displacement of Au-coated Janus nanoparticles toward plasmonic nanoaperture and proper orientation of the metal coating give rise to enhanced near-field intensity and pronounced optical force. We also explore the effect of resonant optical trapping by employing a dual-laser system, where an on-resonant green laser excites the metal-coated nanoparticle, whereas an off-resonant near-infrared laser plays trapping role. It is found that, at the optimum nanoparticle configuration, the resonant optical trapping technique can result in threefold enhancement of optical force, which is attributed to the excitation of surface plasmon resonance in Janus nanoparticles. The findings of this study might pave the way for low-power optical manipulation of light-absorbing nanoparticles with possible applications in nanorobotics and drug delivery.

Concentric ring optical traps for orbital rotation of particles

Abstract Optical vortices (OVs), as eigenmodes of optical orbital angular momentum, have been widely used in particle micro-manipulation. Recently, perfect optical vortices (POVs), a subclass of OVs, are gaining increasing interest and becoming an indispensable tool in optical trapping due to their unique property of topological charge-independent vortex radius. Here, we expand the concept of POVs by proposing concentric ring optical traps (CROTs) and apply them to trapping and rotating particles. A CROT consists of a series of concentric rings, each being a vortex whose radius and topological charge can be controlled independently with respect to the other rings. Quantitative results show that the generated CROTs have weak sidelobes, good uniformity, and relatively high diffraction efficiency. In experiments, CROTs are observed to trap multiple dielectric particles simultaneously on different rings and rotate these particles with the direction and speed of rotation depending on the topological charge sign and value of each individual ring. In addition, gold particles are observed to be trapped and rotate in the dark region between two bright rings. As a novel tool, CROTs may find potential applications in fields like optical manipulation and microfluidic viscosity measurements.