Single-beam Gradient Force Optical Trap Research Articles

Focusing properties of Azimuthally Polarized Lorentz Gauss Vortex Beam through a Dielectric Interface

Tight focusing properties of azimuthally polarized Lorentz Gaussian vortex beam through a dielectric interface are numerically studied by vector diffraction theory. The focusing properties, such as spot size, depth of focus, and maximum intensity position, are numerically calculated by properly manipulating the Lorentz parameter with/without annular obstruction values. Thus, using annular obstruction, one can generate a highly confined focal spot of long focal depth when using an azimuthally polarized Lorentz Gaussian vortex beam. Full Text: PDF References A. Ashkin et al., "Observation of a single-beam gradient force optical trap for dielectric particles", Opt. Lett. 11, 288 (1986). CrossRef A. Ambardekar, Y.Q. Li, "Optical levitation and manipulation of stuck particles with pulsed optical tweezers", Opt. Lett. 30, 1797 (2005). CrossRef P. Zemánek, C.J. Foot, "Atomic dipole trap formed by blue detuned strong Gaussian standing wave", Opt. Commun. 146, 119(1998). CrossRef S.M. Block et al., "Bead movement by single kinesin molecules studied with optical tweezers", Nature 348, 348 (1990). CrossRef D.E. Smithet al., "The bacteriophage φ29 portal motor can package DNA against a large internal force", Nature 413, 748 (2001). CrossRef L. Oroszi et al., "Direct Measurement of Torque in an Optical Trap and Its Application to Double-Strand DNA", Phys. Rev. Lett. 97, 058301 (2006). CrossRef D.P. Biss, T.G. Brown, "Cylindrical vector beam focusing through a dielectric interface", Opt. Express 9, 490 (2001). CrossRef P. Török et al., "Electromagnetic diffraction of light focused through a planar interface between materials of mismatched refractive indices: structure of the electromagnetic field. I", J. Opt. Soc. Am. A 12, 2136 (1995). CrossRef S.H. Wiersma et al., "Comparison of different theories for focusing through a plane interface", J. Opt. Soc. Am. A 14, 1482 (1997). CrossRef L.E. Helseth, "Roles of polarization, phase and amplitude in solid immersion lens systems", Opt. Commun. 191, 161 (2001). CrossRef P. Zhou et al., "Propagation properties of a Lorentz beam array", Appl. Opt. 49, 2497 (2010). CrossRef O.E. Gawhary, S. Severini, "Lorentz beams and symmetry properties in paraxial optics", J. Opt. A: Pure Appl Opt 8, 409 (2006). CrossRef J. Yang et al., "Focusing of diode laser beams: a partially coherent Lorentz model", Proc. SPIE 6824, 68240 A (2007). CrossRef O.E. Gawhary, S. Severini, "Lorentz beams as a basis for a new class of rectangularly symmetric optical fields", Opt. Commun. 269, 274 (2007). CrossRef H. Yu, L. Xiong, B. Lü, "Nonparaxial Lorentz and Lorentz–Gauss beams", Optik 121,1455(2010). CrossRef Z. Zhang, J. Pu, X. Wang, "Tight Focusing of Radially and Azimuthally Polarized Vortex Beams through a Dielectric Interface", Chin. Phys. Lett. 25, 1664 (2008). CrossRef

Correction to “Observation of the Binary Coalescence and Equilibration of Micrometer-Sized Droplets of Aqueous Aerosol in a Single-Beam Gradient-Force Optical Trap”

Correction to “Observation of the Binary Coalescence and Equilibration of Micrometer-Sized Droplets of Aqueous Aerosol in a Single-Beam Gradient-Force Optical Trap”

Observation of the Binary Coalescence and Equilibration of Micrometer-Sized Droplets of Aqueous Aerosol in a Single-Beam Gradient-Force Optical Trap

The binary coalescence of aqueous droplets has been observed in a single-beam gradient-force optical trap. By measuring the time-dependent intensity for elastic scattering of light from the trapping laser, the dynamics of binary coalescence have been examined and the time scale for equilibration of a composite droplet to ambient conditions has been determined. These data are required for modeling the agglomeration of aqueous droplets in dense sprays and atmospheric aerosol. Elastic-light scattering from optically trapped particles has not been used previously to study the time-resolved dynamics of mixing. It is shown to offer a unique opportunity to characterize the binary coalescence of aqueous droplets with radii from 1 to 6 μm. The study of this size regime, which cannot be achieved by conventional imaging methods, is critical for understanding the interactions of droplets in the environment of dense sprays.

Optical manipulation and characterisation of aerosol particles using a single-beam gradient force optical trap

The application of optical tweezers (a single-beam gradient force optical trap) to the manipulation and characterisation of aerosol particles is discussed in this tutorial review. Optical tweezers allow not only the indefinite control over a single droplet, but control over arrays of particles. Typical particle sizes span the 1-10 microm diameter range. When coupled with spectroscopic techniques for probing evolving particle size (with nanometre accuracy), composition, phase and mixing state, detailed investigations of the thermodynamic properties of aerosol, the kinetics of particle transformation, and the nature of interparticle forces and coagulation can be undertaken.

One-dimensional jumping optical tweezers for optical stretching of bi-concave human red blood cells

We report the experimental demonstration of optical stretching of individual bio-concave human red blood cells (RBCs) with one-dimensional jumping optical tweezers. We trapped a RBC in isotonic buffer solution in a conventional stationary single-beam gradient-force optical trap and discretely scanned the trapping beam with an acousto-optic modulator such that the focal point of the trapping beam jumped back-and-forth between two fixed points. At the jumping frequency on the order of a 100 Hz and higher, and the jumping distance in the range of a few microns, the bi-concave RBC was stably trapped and stretched. The elongation of the stretched RBC was measured as a function of the beam-scanning amplitude, and the experimental results were explained qualitatively by a theoretical model.

Optical tweezers: 20 years on

In 1986, Arthur Ashkin and colleagues published a seminal paper in Optics Letters, 'Observation of a single-beam gradient force optical trap for dielectric particles' which outlined a technique for trapping micrometre-sized dielectric particles using a focused laser beam, a technology which is now termed optical tweezers. This paper will provide a background in optical manipulation technologies and an overview of the applications of optical tweezers. It contains some recent work on the optical manipulation of aerosols and concludes with a critical discussion of where the future might lead this maturing technology.

Spectroscopy of Growing and Evaporating Water Droplets: Exploring the Variation in Equilibrium Droplet Size with Relative Humidity

We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2-6 microm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from the droplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstrate that it is possible to measure the equilibrium size and composition of an aqueous droplet with variation in relative humidity. For this benchmark study we investigate the variation in equilibrium size with relative humidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueous aerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be in excellent agreement with the predictions of Köhler theory. We suggest that this approach could provide an important new strategy for characterizing the thermodynamic properties and kinetics of transformation of aerosol particles.

"Electron Tweezers" as a Tool for In-situ Manipulation and Processing of Individual Metal Nanoparticles in a Two-Phase Partially Molten Alloy

Vladimir P. Oleshko and James M. Howe University of Virginia, Department of Materials Science & Engineering, Charlottesville, VA 22904 Understanding physical principles for controlling the dynamic behavior of nanoparticles in inhomogeneous gas (liquid) – solid systems is becoming increasingly important for nanoscience and technology. Nondestructive trapping of micron- and nano-sized dielectric and metal particles in a liquid using a laser beam, which is refracted by the particle and transfers momentum to it, is known as a single-beam gradient force optical trap, or optical tweezers [1]. Recent experiments utilizing partially molten submicron-sized Al-Si alloy spheres and a focused electron beams in a medium-voltage TEM [2] and theoretical calculations of linear and angular momentum transfer from an electron beam to small particles [3] indicate that optical trapping of particles using electron beams may occur. Such thermally assisted “electron tweezers” potentially can be utilized for manipulation and processing of individual nanoobjects and fabrication of assembled nanodevices with up to atomic level sensitivity and lateral resolution provided by modern electron optical systems. In this work, we demonstrate electron-beam trapping of solid nanoparticles inside opaque submicron-sized Al-Si alloy spheres using energy filtering TEM (EFTEM) and parallel electron energy-loss spectroscopy (PEELS). Thermally assisted motion of crystalline Al nanospheres confined within the partially molten Al-Si particles was initiated by the electron beam, which was used to control and to observe the trapping in real-time by plasmon spectroscopic imaging (Fig. 1). Such approaches enable

Vibrational Probing Technique for the Nano-CMM based on Optical Radiation Pressure Control

A microprobe system with a micrometer size probe sphere is required to establish the nano-CMM that is a three dimensional coordinate measuring machine with a measuring range of mm and accuracy of nm order. In this paper, the newly developed vibrational probing technique for the microprobe sytem is presented. The principle is based on the single-beam gradient-force optical trap and the forced vibration method. An optically trapped silica particle in air is vibrated using the laser beam scanning method. Frequency response is examined to estimate the radial spring constant of the microprobe system. Fundamental measurements of a micro-step specimen fabricated using a FIB (Focused Ion Beam) process are demonstrated to investigate the position detection sensitivity.

Large capture-range of a single-beam gradient optical trap

We have observed long-range trapping with a single-beam gradient force optical trap. 6 to 10 microm polystyrene beads that are initially approximately 100 microm away from the trap-center can be pulled into the trap-center. Particle-tracking enables us to determine the trajectory of a bead as it moves towards the trap-center and map out a capture zone inside which trapping can occur.

Forces of a Single-Beam Gradient-Force Optical Trap on Dielectric Spheroidal Particles in the Geometric-Optics Regime

We developed a computational code to calculate the forces of a single-beam gradient-force optical trap exerted on dielectric ellipsoidal particles in the geometric-optics regime. Using this code, the axial and the transverse trapping stability of spheroidal particles, the semi-major axis of which is perpendicular (type-A) and parallel (type-B) to an incident beam axis, was evaluated and the effects of the nonspherical geometry of the particles were analyzed. As the fractional deformation ratio increased, the axial trapping stability improved for type-B particles, whereas it degraded for type-A particles. It can, therefore, be concluded that type-B particles can be trapped more stably than type-A particles. It was also observed that the axial trapping stability can be improved by the use of a TEM01*-mode beam instead of a TEM00-mode beam.

Multiphoton fluorescence excitation in continuous-wave infrared optical traps

The multiphoton fluorescence excitation observed in acontinuous-wave (cw) single-beam gradient force optical trap(optical tweezers) is reported for latex beads labeled with ayellow-green fluorescent dye (BODIPY). The fluorescenceemission spectra of the yellow-green beads trapped and excited by thesame 1064-nm laser light are identical to the spectra excited by the365-nm UV light. The influence of the numerical aperture of theobjective on the slope of the log-log power-dependence has beendemonstrated for BODIPY-Oil solution under cw and pulsed-laserconditions. The possibility that three-photon excitation processoccurs is discussed within the context of a dog-bone saturationmodel. Other possibilities for the observed fluorescence excitationhave been discussed.

Observation of a single-beam gradient-force optical trap for dielectric particles in air

A single-beam gradient-force optical trap for dielectric particles, which relies solely on the radiation pressure force of a TEM(00)-mode laser light, is demonstrated in air for what is believed to be the first time. It was observed that micrometer-sized glass spheres with a refractive index of n=1.45 remained trapped in the focus region for more than 30 min, and we could transfer them three dimensionally by moving the beam focus and the microscope stage. A laser power of ~40 mW was sufficient to trap a 5- microm -diameter glass sphere. The present method has several distinct advantages over the conventional optical levitation method.

Two-photon fluorescence excitation in continuous-wave infrared optical tweezers

We report the observation of two-photon fluorescence excitation in a continuous-wave (cw) single-beam gradient force optical trap and demonstrate its use as an in situ probe to study the physiological state of an optically confined sample. In particular, a cw Nd:YAG (1064-nm) laser is used simultaneously to confine, and excite visible fluorescence from submicrometer regions of, cell specimens. Two-photon fluorescence emission spectra are presented for motile human sperm cells and immotile Chinese hamster ovary cells that have been labeled with nucleic acid (Propidium Iodide) and pH-sensitive (Snarf) fluorescent probes. The resulting spectra are correlated to light-induced changes in the physiological state experienced by the trapped cells. This spectral technique should prove extremely useful for monitoring cellular activity and the effects of confinement by optical tweezers.

Autofluorescence spectroscopy of optically trapped cells.

Cellular autofluorescence spectra were monitored in a single-beam gradient force optical trap ("optical tweezers") in order to probe the physiological effects of near infrared and UVA (320-400 nm) microirradiation. Prior to trapping, Chinese hamster ovary cells exhibited weak UVA-excited autofluorescence with maxima at 455 nm characteristic of beta-nicotinamide adenine dinucleotide (phosphate) emission. No strong effect of a 1064 nm NIR microbeam on fluorescence intensity and spectral characteristics was found during trapping, even for power densities up to 70 MW/cm2 and radiant exposures of 100 GJ/cm2. In contrast to the 1064 nm trap, a 760 nm trapping beam caused a two-fold autofluorescence increase within 5 min (about 20 GJ/cm2). Exposure to 365 nm UVA (1 W/cm2) during 1064 nm trapping significantly altered cellular autofluorescence, causing, within 10 min, a five-fold increase and a 6 nm red shift versus initial levels. We conclude that 1064 nm microbeams can be applied for an extended period without producing autofluorescence changes characteristic of alterations in the cellular redox state. However, 760 nm effects may occur via a two-photon absorption mechanism, which, in a manner similar to UVA exposure, alters the redox balance and places the cell in a state of oxidative stress.

Laser induced cell fusion in combination with optical tweezers: The laser cell fusion trap

A single-beam gradient force optical trap was combined with a pulsed UV laser microbeam in order to perform laser induced cell fusion. This combination offers the possibility to selectively fuse two single cells without critical chemical or electrical treatment. The optical trap was created by directing a Nd:YAG laser, at a wavelength of 1.06 microns, into a microscope and focusing the laser beam with a high numerical aperture objective. The UV laser microbeam, produced by a nitrogen-pumped dye laser (366 nm), was collinear with the trapping beam. Once inside the trap, two cells could be fused with several pulses of the UV laser microbeam, attenuated to an energy of approximately 1 microJ/pulse in the object plane. This method of laser induced cell fusion should provide increased selectivity and efficiency in generating viable hybrid cells.

Compliance of bacterial polyhooks measured with optical tweezers

In earlier work, a single-beam gradient force optical trap ("optical tweezers") was used to measure the torsional compliance of flagella in wild-type cells of Escherichia coli that had been tethered to glass by a single flagellum. This compliance was nonlinear, exhibiting a torsionally soft phase up to 180 degrees, followed by a torsionally rigid phase for larger angles. Values for the torsional spring constant in the soft phase were substantially less than estimates based on the rigidity determined for isolated flagellar filaments. It was suggested that the soft phase might correspond to wind-up of the flagellar hook, and the rigid phase to wind-up of the stiffer filament. Here, we have measured the torsional compliance of flagella on cells of an E. coli strain that produces abnormally long hooks but no filaments. The small-angle compliance of these cells, as determined from the elastic rebound of the cell body after wind-up and release, was found to be the same as for wild-type cells. This confirms that the small-angle compliance of wild-type cells is dominated by the response of the hook. Hook flexibility is likely to play a useful role in stabilizing the flagellar bundle.

Bead movement by single kinesin molecules studied with optical tweezers

Kinesin, a mechanoenzyme that couples ATP hydrolysis to movement along microtubules, is thought to power vesicle transport and other forms of microtubule-based motility. Here, microscopic silica beads were precoated with carrier protein, exposed to low concentrations of kinesin, and individually manipulated with a single-beam gradient-force optical particle trap ('optical tweezers') directly onto microtubules. Optical tweezers greatly improved the efficiency of the bead assay, particularly at the lowest kinesin concentrations (corresponding to approximately 1 molecule per bead). Beads incubated with excess kinesin moved smoothly along a microtubule for many micrometres, but beads carrying from 0.17-3 kinesin molecules per bead, moved, on average, only about 1.4 microns and then spontaneously released from the microtubule. Application of the optical trap directly behind such moving beads often pulled them off the microtubule and back into the centre of the trap. This did not occur when a bead was bound by an AMP.PNP-induced rigor linkage, or when beads were propelled by several kinesin molecules. Our results are consistent with a model in which kinesin detaches briefly from the microtubule during a part of each mechanochemical cycle, rather than a model in which kinesin remains bound at all times.

Laser trapping in cell biology

Optical traps offer the promise of being used as noninvasive micromanipulators for biological objects. An analytical model was developed that accurately describes the forces exerted on dielectric microspheres while in a single-beam gradient force optical trap. The model can be extended to the trapping of biological objects. The model predicts the existence of a stable trapping point and effective trapping range. A minimum trapping power of approximately 5 mW and an effective trapping range of 2.4 mu m were measured for 10- mu m-diameter dielectric microspheres and are in reasonable agreement with expected results. In cell biology, the optical trap was used to alter the movement of chromosomes within mitotic cells in vitro and to hold motile sperm cells. Results for the mitotic cells indicate that chromosome movement was initiated in the direction opposite to that of the applied force.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>