Alessandro Salandrino, Shima Fardad, and Demetrios N. Christodoulides
The theory of optical forces on spherical scatterers is here generalized to arbitrary incident fields. The interaction between spherical harmonics of different order, and the degree and azimuthal parity, is studied in detail. The resulting force from all the contributing components is presented in analytical form. A further generalization of this formulation to nonspherical scatterers is also discussed.
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Basudev Roy, Sambit Bikas Pal, Arijit Haldar, Ratnesh Kumar Gupta, Nirmalya Ghosh, and Ayan Banerjee
The dynamics of an optically trapped particle are often determined by measuring intensity shifts of the back-scattered light from the particle using position sensitive detectors. We present a technique which measures the phase of the back-scattered light using balanced detection in an external Mach-Zehnder interferometer scheme where we separate out and beat the scattered light from the particle and that from the top surface of our trapping chamber. The technique has improved axial motion resolution over intensity-based detection, and can also be used to measure lateral motion of the trapped particle. In addition, we are able to track the Brownian motion of trapped 1.1 and 3 μm diameter particles from the phase jitter and show that, similar to intensity-based measurements, phase measurements can also be used to simultaneously determine displacements of the trapped particle as well as the spring constant of the trap. For lateral displacements, we have matched our experimental results with a simulation of the overall phase contour of the back-scattered light by using plane wave decomposition in conjunction with Mie scattering theory. The position resolution is limited by path drifts of the interferometer which we have presently reduced to demonstrate the capability of sub-nm displacement resolution in the axial direction for 1.1 μm diameter particles by locking the interferometer to a frequency stabilized diode laser.
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Saurabh Raj, Mónica Marro, Michal Wojdyla, and Dmitri Petrov
Two microparticles were biochemically attached to a red blood cell at diametrically opposite parts and held by optical traps allowing to impose deformations. The cell deformation was monitored from the microscopy images. Raman spectra of the cell under tunable deformations were studied. Vibrational spectra analysis at different stretching states was supported with two statistical methods. Principal Component Analysis distinguishes the most prominent changes in spectra while 2D correlation technique monitors the evolution of Raman bands during stretching. The measurements show significant changes in the cell chemical structure with stretching however the changes saturate above 20% of cell deformation. Mechanical deformation of the cell mainly affects the bands corresponding to hemoglobin but contributions from spectrin and membrane proteins can not be excluded. The saturation of bands at higher deformations suggests some structural relaxation that RBC has to undergo to bear extra load. The results confirm widely accepted belief that spectrin released from membrane proteins allows for significant shape changes of the cells. We therefore tentatively suggest that interaction between membrane and cytoskeleton during deformation can be efficiently probed by confocal Raman spectroscopy, in particular via the peak around 1035 cm−1.
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Takeshi Baba, Takahiro Sakaue, and Yoshihiro Murayama
We observed transverse fluctuations of single DNA molecules by fluorescence microscopy. The end-to-end distance of DNA molecules was varied by using dual trap optical tweezers, and the force–fluctuation relation was experimentally obtained in wide ranges of the force regime. In strong force regime, the theory of a stretched worm-like chain with fixed both ends explains the experimental results. On the other hand, in the low force regime, the fluctuations approach the value for an ideal ring polymer. We introduce an interpolate formula for the force–fluctuation relation by considering strong and low force limits, which captures the overall trend of experimental results. The proposed force–fluctuation relation will be useful for quantitative discussions in various sectors of polymer physics and biological processes which involve the conformation change of DNA and other biopolymers, where the loading and the fluctuation are relevant factors.
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Antonia E. Carruthers, Jim S. Walker, Abby Casey, Andrew Orr-Ewing and Jonathan P Reid
Bessel beams were used to create a counter-propagating optical trap for capturing and manipulating aerosol particles. Aerosol droplets were characterized through measurement of the elastic scattered light at three wavelengths; the trapping wavelength of 532 nm was used in conjunction with two probe beams at 405 nm and 633 nm to reduce the uncertainty in estimating droplet radii of 1 μm or less. Control of the aerosol size distribution sampled by the counter-propagating trap was demonstrated by varying the trapping beam core diameters and intensities. Smaller droplet sizes were preferentially selected with a 1.7 μm core diameter compared to cores of 2.7 μm and 4.5 μm. Further, an increase in core intensity was shown to broaden the range in droplet sizes that were optically trapped. The possibility of using such an approach to isolate and analyze the properties of single accumulation mode aerosol particles is discussed.
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David Engström, Michael C.M. Varney, Martin Persson, Rahul P. Trivedi, Kris A. Bertness, Mattias Goksör, and Ivan I. Smalyukh
Stable optical trapping and manipulation of high-index particles in low-index host media is often impossible due to the dominance of scattering forces over gradient forces. Here we explore optical manipulation in liquid crystalline structured hosts and show that robust optical manipulation of high-index particles, such as GaN nanowires, is enabled by laser-induced distortions in long-range molecular alignment, via coupling of translational and rotational motions due to helicoidal molecular arrangement, or due to elastic repulsive interactions with confining substrates. Anisotropy of the viscoelastic liquid crystal medium and particle shape give rise to a number of robust unconventional trapping capabilities, which we use to characterize defect structures and study rheological properties of various thermotropic liquid crystals.
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P. S. Alves and M. S. Rocha
We present a simple method to calibrate optical tweezers by using only videomicroscopy to measure the position autocorrelation function of the trapped bead in the potential well of the tweezers. To accomplish this task, we use a high-speed camera with ∼500 fps (frames per second), which provides a precise measurement of the relaxation time of the bead Brownian fluctuations. We also study the variation of the trap stiffness as a function of some parameters of interest such as the laser power and the distance from the bead center to the microscope coverslip, showing that the presented method returns precise results.
DOI
Arijit Haldar, Sambit Bikas Pal, Basudev Roy, S. Dutta Gupta, and Ayan Banerjee
Microparticle self assembly under the influence of optical forces produced by higher-order optical beams or by projection of a hologram into the trapping volume is well known. In this paper, we report the spontaneous formation of a ring of identical microspheres (each with diameter 1.1 μm) in conventional single-beam optical tweezers with a usual TEM00Gaussian beam coupled into a sample chamber having a standing wave geometry with a cover slip and glass slide. The effects of different experimental parameters on the ring formation are studied extensively. The experimental observations are backed by theoretical simulations based on a plane wave decomposition of the forward- and backward-propagating Gaussian beams. The ring patterns are shown to be caused due to geometrical aberrations produced by focusing the Gaussian beam using a high-numerical-aperture microscope objective into stratified media. It is found that the thickness of the stratified media and the standing wave geometry itself play a critical role in the formation of stable ring structures. These structures could be used in the study of optical binding, as well as of biological interactions between cells in an optical trap.
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R. S. Dutra, N. B. Viana, P. A. Maia Neto, and H. M. Nussenzveig
We extend a previous proposal for absolute calibration of optical tweezers by including optical setup aberrations into the first-principles theory, with no fitting parameters. Astigmatism, the dominant term, is determined from images of the focused laser spot. Correcting it can substantially increase stiffness. Comparison with experimental results yields agreement within error bars for a broad range of bead sizes and trap heights, as well as different polarizations. Absolute calibration is established as a reliable and practical method for applications and design of optical tweezers systems.
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Murat Muradoglu, Wesley Sung-Yee Chiu, and Tuck Wah Ng
The placement of the beam focus corresponding with the center of living matter (such as cells) in an optical tweezer can result in photodamage. We advance a scheme here that locates the focus of the beam either above or below the matter to pull and push relative to the beam axis in a predominant lateral sense based on the resultant action of scattering and gradient forces. Switching to a laser that acts oppositely serves to restore the axial position of the matter. Although an exact value could not be derived due to the statistical nature of Brownian perturbations and time frame considerations, we simulated the optical force fields to visualize the effective force envelope. The lateral optical push–pull operation was conducted experimentally on polystyrene beads in which the motion manipulation efficacy was characterized.
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