Multiple Optical Trapping Research Articles

Effective multiple optical trapping of sub-micrometer particles with petal beams

Optical tweezers are noncontact and noninvasive force transducers. Techniques for multiple optical trapping are of much interest. In this paper, we investigate, numerically, optical trapping of multiple particles using so-called petal beams by trapping every single particle in a separate petal of the focused beam. We have used the generalized Lorenz–Mie theory to compute the optical trapping forces. Results demonstrate that the intensity is equally distributed over the lobes along the intensity ring for circularly polarized petal beams, but in the case of linear polarization, the intensity is not equally distributed over the petals, which means a particle trapped by every petal of a circularly polarized beam experiences the same force. It is shown that for a particle size range, the axial trapping stiffness for the l = 4 mode of a petal beam is about threefold greater than that of the l = 1 mode, and in the lateral direction, the trapping stiffness of the l = 2 mode is about twofold greater than that of the l = 4 mode. Finally, the optimum choice of topological charge for the stronger trap stiffness based on the particle size and trapping depth is identified.

Tunable optical assembly of subwavelength particles by a microfiber cavity

Optical assembly as a multiple optical trapping technique enables patterned arrangements of matter ranging from atoms to microparticles for diverse applications in biophysics, quantum physics, surface chemistry, and cell biology. Optical potential energy landscapes based on evanescent fields are conventionally employed for optical assembly of subwavelength particles, but are typically limited to predefined patterns and lacking in tunability. Here we present a microfiber photonic crystal cavity applicable for tunable optical assembly of subwavelength particles along a flexible path. This is enabled by excellent mechanical flexibility of the microfiber cavity as well as its broadband photonic crystal reflectors. By virtue of the broadband reflectors, the lattice constant of the assembled particles is precisely tunable via altering the wavelength of input light. Three-dimensional optical assembly is also realized by making use of the high-order transverse mode of the microfiber cavity. Moreover, the optical assembly process is detectable by simply monitoring the reflection/transmission spectrum of the microfiber cavity. The design of the microfiber cavity heralds a new way for tunable optical assembly of subwavelength particles, potentially applicable for development of tunable photonic crystals, metamaterials, and sensors.

Multiple optical trapping assisted bead-array based fluorescence assay of free and total prostate-specific antigen in serum

Although suspension bead-based assay technology has been widely used owing to its advantages of high-throughput and microvolume detection, its sensitivity is greatly limited because it detects the fluorescence signal emitted by microbeads for a short time in the flowing fluid. In this work, we present the approach for prostate-specific antigen (PSA) detection of both free PSA (fPSA) and total PSA (tPSA) based on bead-array based fluorescence imaging by combining multiple optical trapping and bead-based bioassays. The polystyrene beads were employed to enrich the targets using the classic sandwich immuno-binding and tagged with fluorescent quantum dots (QDs), and the QDs-tagged beads in suspension were trapped array-by-array using multiple optical tweezers constructed with a diffraction optical element and excited with a 405 nm fiber laser for wide-field fluorescence imaging. The distinctive size information from the image of the trapped beads enabled the sorting of different targets. Moreover, the limits of detection for fPSA and tPSA are 3.8 pg/mL and 2.5 pg/mL respectively with good specificity. More importantly, this strategy was successfully used to detect fPSA and tPSA simultaneously in real serum samples. The high sensitivity, good selectivity, and tiny sample volume make this strategy a promising method for life sciences and clinical applications.

Tight Focusing Properties of Phase Modulated Radially Polarized Laguerre Bessel Gaussian Beam

We propose a new approach for generating a multiple focal spot segment of subwavelength size, by tight focusing of a phase modulated radially polarized Laguerre Bessel Gaussian beam. The focusing properties are investigated theoretically by vector diffraction theory. We observe that the focal segment with multiple focal structures is separated with different axial distances and a super long dark channel can be generated by properly tuning the phase of the incident radially polarized Laguerre Bessel Gaussian beam. We presume that such multiple focal patterns and high intense beam may find applications in atom optics, optical manipulations and multiple optical trapping.

Tight focusing properties of phase modulated transversely polarized sinh Gaussian beam

We studied the tight focusing properties of an azimuthally polarized sinh-Gaussian beam phase modulated with multi belt vortex phase filter on the basis of the vector diffraction theory. The sinh-Gaussian beam can be considered as superposition of a series of eccentric Gaussian beams and it is observed that focusing such a beam with a properly designed dedicated vortex phase filter generates many novel focal patterns including transversely polarized optical needle of extended focal depth with sub wavelength scale lateral resolution and mutiple focal spots with different axial seperation distance. We expect that such a focal patterns are usefull in applications such as semiconductor inspection and multiple optical trapping.

Tight focusing properties of phase modulated azimuthally polarized doughnut Gaussian beam

The intensity distribution in the focal region of a high NA lens for the incident azimuthally polarized doughnut Gaussian beam transmitted through a multi belt spiral phase hologram (MBSPH) is studied on the basis of the vector diffraction theory. Here we report a new method that generates a needle of transversely polarized light beam with sub diffraction beam size of 0.48λ that propagates without divergence over a long distance of about 49λ in free space and designed a dedicated MBSPH to generate multiple focal spot of transversely polarized. The authors also expect such a light needle of transversely polarized beam may find its application when using optical materials or instruments responsive to the transversal field only and to use in multiple optical trapping.

Optical configurations for photophoretic trap of single particles in air.

Since Ashkin's pioneering work in the 1970's, optical trapping (OT) and manipulation have become an indispensable tool in diverse research fields. Today, there are multiple optical trapping schemes in use. In this article, we explore six different optical trapping schemes based on the photophoretic force (PPF). Within these schemes we explore 21 variants differing in such details as laser source, power, beam shape, and focusing optics. We evaluate and rate the trapping quality and performance of the six trapping schemes in terms of four key aspects: simplicity, robustness, flexibility, and efficiency. One of the schemes is novel: we introduce a simple, high quality scheme using a confocal design in which one trapping beam is effectively converted to two counter-propagating beams. The versatility of this new trapping scheme is demonstrated via application of the scheme to cavity ringdown spectroscopy. We hope this exploration of the diversity of PPF trapping schemes will extend applications of OT by providing researchers with information to assist in the selection of specific optical trapping schemes from the first-of-its-kind list of 21 configurations presented herein.

Non-coaxial superposition of vector vortex beams.

Vector vortex beams are classified into four types depending upon spatial variation in their polarization vector. We have generated all four of these types of vector vortex beams by using a modified polarization Sagnac interferometer with a vortex lens. Further, we have studied the non-coaxial superposition of two vector vortex beams. It is observed that the superposition of two vector vortex beams with same polarization singularity leads to a beam with another kind of polarization singularity in their interaction region. The results may be of importance in ultrahigh security of the polarization-encrypted data that utilizes vector vortex beams and multiple optical trapping with non-coaxial superposition of vector vortex beams. We verified our experimental results with theory.

Multiple optical trapping based on high-order axially symmetric polarized beams**Project supported by the National Natural Science Foundation of China (Grant Nos. 61108047 and 61475021), the Program for New Century Excellent Talents in University, China (Grant No. NCET-13-0667), and the Beijing Top Young Talents Support Program, China (Grant No. CIT&TCD201404113).

Multiple optical trapping with high-order axially symmetric polarized beams (ASPBs) is studied theoretically, and a scheme based on far-field optical trapping with ASPBs is first proposed. The focused fields and the corresponding gradient forces on Rayleigh dielectric particles are calculated for the scheme. The calculated results indicate that multiple ultra-small focused spots can be achieved, and multiple nanometer-sized particles with refractive index higher than the ambient can be trapped simultaneously near these focused spots, which are expected to enhance the capabilities of traditional optical trapping systems and provide a solution for massive multiple optical trapping of nanometer-sized particles.

Efficient and low cost multiple optical trap, based on interference

Interference fringes provide the means for simultaneous, multiple–micromanipulation, of particles. In biomedical investigations, multiple interference optical trapping gives the feasibility to trap a large number of biological substances. In this paper we describe a simple method for creating multiple optical tweezers from a single laser beam, using a system of ten transparent glass beam splitters. The experimental set up was tested, on polystyrene beads of intermediate size. We also measured the optical forces exerted by the fringes, in positions where the three and two dimensions optical trap was stable, and we evaluated the experimentally obtained optical efficiency. We notice that the experimentally optical efficiency value for the higher order fringes decreases.

Structured light spots projected by a Dammann grating with high power efficiency and uniformity for optical sorting

We report a method for microfluidic multiple trapping and continuous sorting of microparticles using an optical potential landscape projected by a Dammann grating, enabling a high power-efficient approach to forming a composite two-dimensional spots array with high uniformity. The Dammann grating is fabricated in a photoresist by optical lithography. It is employed to create an optical lattice for multiple optical trapping and sorting in a mixture of polymer particles (n = 1.59) and silica particles (n = 1.42) with the same diameters of 3.1 μm. In addition to the exponential selectivity by the projected optical landscapes, the proposed microfluidic sorting system has advantages in terms of high power efficiency and high uniformity due to the Dammann grating.

Surface plasmon interference formed by tightly focused higher polarization order axially symmetric polarized beams

We numerically study the surface plasmon interference formed by tightly focused higher polarization order axially symmetric polarized beams (ASPBs) based on the vectorial diffraction theory. The definition of ASPBs is stated, and the optical setup for surface plasmon polariton (SPP) excitation and mathematical expressions for interfering SPP fields are proposed. The simulation results show that the interfering SPP fields present a multi-focal spot pattern. In addition, the number of spots is related to the polarization order of the incident beams P as 2\times(P-1), indicating potential utilization in near-field multiple optical trapping and near-field imaging and sensing. The unique interfering phenomenon is also explained.

Combining multiple optical trapping with microflow manipulation for the rapid bioanalytics on microparticles in a chip

An array of four independent laser traps is combined with a polydimethylsiloxane microfluidic chip to form a very compact system allowing parallel processing of biological objects. Strong three dimensional trapping allows holding objects such as functionalized beads in flows at speeds near 1 mm/s, enabling rapid processing. By pressure control of the inlet flows, the trapped objects can be put in contact with different solutions for analysis purpose. This setup, including a fluorescence excitation-detection scheme, offers the potential to perform complex biochemical manipulations on an ensemble of microparticles.

Effects of Optical Trapping and Liquid Surface Deformation on the Laser Microdeposition of a Polymer Assembly in Solution

A polymer microassembly is formed by focusing a near-infrared (NIR) laser beam in a thin film of a polymer solution. We have investigated the mechanism of laser microdeposition of a polyfluorene assembly by measuring the surface deformation of the solution film and the morphology of the deposited assembly. It is clearly observed that a rupture is formed at the laser focus in the solution film by using laser interferometric imaging. The time necessary for the rupture formation and the volume of the deposited microassembly are analyzed as a function of laser power. Experimental results suggest that the solution surface deformation induced by local laser heating and optical trapping effects determined the volume of the laser microdeposition. By combining this method with multiple optical trapping, a polymer microassembly with a polygonal morphology is formed on the glass substrate.

Optical sculpture: controlled deformation of emulsion droplets with ultralow interfacial tensions using optical tweezers

We report a technique for deforming micron-sized emulsion droplets that have ultralow interfacial tensions, by the manipulation of multiple optical trapping sites within the droplets.

Biological samples micro-manipulation by means of optical tweezers

The goal of this work is to investigate the usefulness of the optical tweezers for biological sample micro-manipulation. We report multiple optical trapping and manipulation of Escherichia coli cells, immersed in growth medium, by means of diffractive optical elements (DOE). The DOEs are implemented on a spatial light modulator to generate movable geometries of traps. We report also an experiment that allows to mimic the mechanical environment of cells in tissues. Micro-meter sized beads are trapped in circular geometry in order to surround living cells. By dynamically varying the geometry of the configuration and the trapping forces, we can reproduce a controlled environment where only mechanical stimuli are present and biological responses can be monitored. These experiments have been implemented in two different setups. In the first, cell manipulation is performed by the use of two-dimensional optical tweezers system generated by dual axis acousto-optical deflectors. In the second setup, trapping configuration is extended to three-dimensional volumes using DOEs.

Dynamic multiple optical trapping by means of diffractive optical elements

We report multiple optical trapping of microscopic dielectric particles using diffractive optical elements (DOEs) implemented with a twisted nematic liquid crystal spatial light modulator (SLM). The particles are trapped simultaneously in an array disposed in plane or in volume and the trapping stability is tested by moving the microscope stage. When using a computer controlled SLM the particles can be moved independently by changing the configuration of the diffractive optical element with a maximum refresh rate of 60 Hz. Translation of the particles in x–y–z is demonstrated inside a volume of 8 × 8 × 8 μm3. Simultaneous rotation of multiple trapped particles along circular trajectories is demonstrated with different angular velocities. In order to evaluate the maximum speed with which a particle can be moved using our set-up, the Stokes drag force is compared with the trapping force. The performance of our multiple trapping technique is compared with other methods reported in literature.

Multiple Optical Trapping by Means of Diffractive Optical Elements

In this paper we report multiple optical trapping of microscopic dielectric particles using diffractive optical elements implemented on twisted nematic liquid crystal spatial light modulators. The particles are trapped in arrays disposed in plane or in volume and can be moved independently in x-y-z by changing the configuration of the diffractive optical element. We show also multiple trapping using Laguerre-Gaussian and Gaussian beams simultaneously. The orbital angular momentum of the Laguerre-Gaussian beam is transferred to the particle, making it to move on a circular trajectory defined by the intensity pattern specific to this beam. We use sample cells built with two microscope slides separated by 120 µm with a sticky tape. The space between the two slides is filled with 2 µm diameter silica spheres diluted in water (concentration 0.026% wt). We show that optical trapping is also possible in a small glass capillary with a diameter of 100 µm.

Dynamic multiple optical trapping by means of diffractive optical elements

We report multiple optical trapping of microscopic dielectric particles using diffractive optical elements (DOEs) implemented with a twisted nematic liquid crystal spatial light modulator (SLM). The particles are trapped simultaneously in an array disposed in plane or in volume and the trapping stability is tested by moving the microscope stage. When using a computer controlled SLM the particles can be moved independently by changing the configuration of the diffractive optical element with a maximum refresh rate of 60 Hz. Translation of the particles in x– y– z is demonstrated inside a volume of 8 × 8 × 8 μm 3. Simultaneous rotation of multiple trapped particles along circular trajectories is demonstrated with different angular velocities. In order to evaluate the maximum speed with which a particle can be moved using our set-up, the Stokes drag force is compared with the trapping force. The performance of our multiple trapping technique is compared with other methods reported in literature.

The mechanism of red cell (dis)aggregation investigated by means of direct cell manipulation using multiple optical trapping.

We used multiple optical trapping to study the mechanism of red cell (dis)aggregation. Two sets of optical 'tweezers' were used to bring two red blood cells together to form a two-cell aggregate and then to pull them apart, to study the interaction between the cells. We found that cross-bridging occurred in normal reversible aggregation as we observed binding and the occurrence of small tethers between opposite cell membranes. Furthermore, the cells could only be parted by sliding them side by side with a maximum velocity in the order of microm/s indicating accumulation of the cross-bridges.