Browsing by Subject "Optical tweezers"
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Item Open Access Detection of Calcium-induced morphological changes on RBCs by digital holographic microscopy and blinking optical tweezers(IEEE, 2016) Rad, V. F.; Tavakkoli, R.; Moradi, Ali-Reza; Anand, A.; Javidi, B.Ca+2 level in the circulating red blood cells (RBCs) takes part not only in controlling biophysical properties, but also affects the membrane composition, and its morphological and rheological properties. Excessive accumulation of Ca2+ within the cells is associated with a number of important pathological diseases. In this paper, by the use of digital holographic microscopy (DHM), we quantitatively analyzed the volumetric behavior of RBC membrane under influence of excess Calcium ions. DHM in a transmission mode is an effective tool for quantitative visualization of phase objects. By deriving the associated phase changes 3D information on the morphology variation of the cells at arbitrary time scales is obtained. Individual cells are immobilized by the use of optical tweezers and are monitored live with DHM system, while the concentration of Ca2+ ions in the buffer is changed simultaneously. We utilized blinking optical tweezers, by inserting an optical chopper to modulate intensity of the trapping laser beam. Blinking optical tweezers, while keeping the cell trapped during the experiments, ensures of minimizing the photo-damage of trapping laser beam on the cell. Our experimental results are in agreement with previous biological studies and predictions, and experimental observations of living RBCs under Ca2+ influence.Item Open Access Engineering particle trajectories in microfluidic flows using speckle light fields(SPIE, 2014) Volpe, G.; Volpe, Giovanni; Gigan, S.Optical tweezers have been widely used in physics, chemistry and biology to manipulate and trap microscopic and nanoscopic objects. Current optical trapping techniques rely on carefully engineered setups to manipulate nanoscopic and microscopic objects at the focus of a laser beam. Since the quality of the trapping is strongly dependent on the focus quality, these systems have to be very carefully aligned and optimized, thus limiting their practical applicability in complex environments. One major challenge for current optical manipulation techniques is the light scattering occurring in optically complex media, such as biological tissues, turbid liquids and rough surfaces, which give rise to apparently random light fields known as speckles. Here, we discuss an experimental implementation to perform optical manipulation based on speckles. In particular, we show how to take advantage of the statistical properties of speckle patterns in order to realize a setup based on a multimode optical fiber to perform basic optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these "speckle optical tweezers" will greatly broaden the perspectives of optical manipulation for real-life applications. © 2014 SPIE.Item Open Access Experimental investigation of critical Casimir forces in binary liquid mixtures by blinking optical tweezers(OSA, 2017) Magazzu, Alessandro; Schmidt, F.; Callegari, Agnese; Gambassi, A.; Dietrich, S.; Volpe, GiovanniWe investigate, for the first time and by blinking optical tweezers, the effects of critical Casimir forces (CCFs) on the free dynamics of a pair of spherical colloidal particles, immersed in binary liquid mixtures upon approaching their critical points.Item Open Access Forces and torques on the nanoscale: from measurement to applications(SPIE, 2012) Volpe,GiovanniThe possibility of measuring microscopic forces down to the femtonewton range has opened new possibilities in fields such as biophysics and nanophotonics. I will review some of the techniques most often employed, namely the photonic force microscope (PFM) and the total internal reflection microscope (TIRM), which are able to measure tiny forces acting on optically trapped particles. I will then discuss several applications of such nanoscopic forces: from plasmonic optical manipulation, to self-propelled microswimmers, to self-organization in large ensembles of particles.Item Open Access Generation of cylindrical vector beams with few-mode fibers excited by Laguerre–Gaussian beams(Elsevier, 2004-07-01) Volpe, Giovanni; Petrov, D.We propose a novel method to efficiently produce light beams with radial, azimuthal, and hybrid polarization, through a few-mode fiber excited by a Laguerre–Gaussian beam. With different input polarization we can selectively excite different combinations of modes from the LP11 group. We propose to show how to transform the output beam into a cylindrical vector beam in free-space through various polarization transformations.Item Open Access Intracavity optical trapping with fiber laser(Bilkent University, 2019-06) Kalantarifard, FatemehAfter Ashkin's seminal works, optical trapping has been a powerful technique for capturing and manipulating sub micro particles not only in physics research fields but also in biology and photonics. Standard optical tweezers consists of a single beam with Gaussian or profile which focused by a high numerical aperture (NA) water or oil immersion microscope objective. Typically, objective with NA>1.2 is used to provide strong enough gradient forces being able to overcome Brownian uctuations and gravity and trap the particle stably. On the other hand, compare with high NA, trapping with low NA, has its own advantage and among all the advantages, low local heating of the sample has a particular interest in molecular biology and manipulating living cells. The main concern is that the interaction of trapping laser beam and biological object induces a damage on the specimen which is mainly due to light absorption of the sample. It is, therefore, recommended to use NIR (near infrared ) wavelength due to its minimal absorption by water and biological objects. Other important factors that must be considered, to secure the viability of the cell, are spot size of the focused beam and laser power at the sample plane. Thus, it deserves an effort to look for new configurations with low NA with the capability of creating 3D confinement. Standard optical tweezers rely on optical forces that arise when a focused laser beam interacts with a microscopic particle: scattering forces, which push the particle along the beam direction, and gradient forces, which attract it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement per unit laser intensity on the sample. We first present a toy model that intuitively explains how the microparticle position and the laser power become nonlinearly coupled: The loss of the laser cavity depends on the particle position due to scattering, so the laser intensity grows whenever the particle tries to escape. We describe a simple toy model to clarify how the nonlinear feedback forces emerge as a result of the interplay between the particle's motion and the laser's dynamics. It also quantifies how and to what extent this scheme reduces the average laser power to which a trapped particle is exposed. In this model, the power and hence trapping force are considered to be zero for small particle displacements. However, in reality they have small values that do operate the trap even when the particle is near the equilibrium position. Thus, we need an accurate description of the coupling between the laser and the trapped particle thermal dynamics at equilibrium to compare with experiments. In particular, accurate simulations can help to associate an effective harmonic potential to the optical trap for small displacements from the equilibrium position, and hence to define a meaningful stiffness using the standard calibration methods based on the thermal uctuations of a trapped particle. We therefore present a series of numerical simulations based on an extended theoretical model, including highly realistic descriptions of the laser dynamics, optical losses incurred by the particle, and the particle's Brownian motion in order to gain a quantitative understanding of the dynamics of intracavity optical trapping and to guide the experiments. Finally, guided by the simulation results, we have built an experimental setup to prove the operational principle of intracavity optical trapping and experimentally realize this concept by optically trapping microscopic polystyrene and silica particles inside the ring cavity of a fiber laser. One of the major advantages of the intracavity optical trapping scheme is that it can operate with very low-NA lenses, with a consequent large field-of-view, and at very low average power, resulting in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. When compared to other low-NA optical trapping schemes, positive and negative aspects can be considered, such as in terms of trap stiffness and average irradiance of the sample. These features can yield advantages when dealing with biological samples. Ultra-low intensity at our wavelength can grant a safe, temperature controlled environment, away from surfaces for micro uidics manipulation of biosamples. Accurate studies on Saccharomices cerevisiae yeast cells in near-infrared counterpropagating traps and standard optical tweezers have found no evidence for a lower power threshold for phototoxicity. We observed that we can 3D trap single yeast cells with about 0:47 mW, corresponding to an intensity of 0:036 mW m2, that is more than a tenfold less intensity than standard techniques.Item Open Access Intracavity optical trapping with ytterbium doped fiber(SPIE, 2013) Laser, R.; Sayed, R.; Kalantarifard, Fatemeh; Elahi P.; İlday, F. Ömer; Volpe, Giovanni; Marago O.M.We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution. © 2013 SPIE.Item Open Access Optical trapping of microparticles and yeast cells at ultra-low intensity by intracavity nonlinear feedback forces(SPIE, 2020) Kalantarifard, A.; Elahi, P.; Makey, Ghaith; Ünlü, B.; Marago, O. M.; İlday, Fatih Ömer; Volpe, G.; Dholakia, K.; Spalding, G. C.In standard optical tweezers optical forces arise from the interaction of a tightly focused laser beam with a microscopic particle. The particle is always outside the laser cavity and the incoming beam is not affected by the particle position. Here we describe an optical trapping scheme inside the cavity of a fiber laser where the laser operation is nonlinearly influenced by the displacement of trapped particle and there is a coupling between laser operation to the motion of the trapped particle and this can dramatically enhances optical tweezers action and gives rise to nonlinear feedback forces. This scheme operates using an aspheric lens at low numerical aperture (NA=0.125), NIR wavelength (λ = 1030 nm), and very low average power which results in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. Ultra-low intensity at our wavelength can grant a safe, temperature-controlled environment, away from surfaces for microfuidics manipulation of biosamples that are sensitive to light intensity. As the main advantage of our approach and highly relevant application, we observed that we can trap single yeast cells at a very low power, corresponding to an intensity of 0.036 mW μm-2, that is more than a tenfold less intensity than standard techniques reported in the literature.