Wednesday, 23 September 2015

SPIE OTOM XII Conference Proceedings

The proceedings of the SPIE Optical Trapping and Optical Manipulation Conference XII, held in San Diego in August 2015 have been published, including three contributions from the UCL Optical Tweezers Group:

T. J. Smart, C. J. Richards, R. Bhatnagar, C. Pavesio, R. Agrawal and P. H. Jones.  'A study of red blood cell deformability in diabetic retinopathy using optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945820, doi 10.1117/12.2191281 (2015)
From the abstract: Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus (DM) in which high blood sugar levels cause swelling, leaking and occlusions in the blood vessels of the retina, often resulting in a loss of sight. The microvascular system requires red blood cells (RBCs) to undergo significant cellular deformation in order to pass through vessels whose diameters are significantly smaller than their own. There is evidence to suggest that DM impairs the deformability of RBCs, and this loss of deformability has been associated with diabetic kidney disease (or nephropathy) - another microvascular complication of DM. However, it remains unclear whether reduced deformability of RBCs correlates with the presence of DR.

Here we present an investigation into the deformability of RBCs in patients with diabetic retinopathy using optical tweezers. To extract a value for the deformability of RBCs we use a dual-trap optical tweezers set-up to stretch individual RBCs. RBCs are trapped directly (i.e. without micro-bead handles), so rotate to assume a `side-on' orientation. Video microscopy is used to record the deformation events, and shape analysis software is used to determine parameters such as initial and maximum RBC length, allowing us to calculate the deformability for each RBC.  A small decrease in deformability of diabetes cells subject to this stretching protocol is observed when compared to control cells.

T. J. Smart, C. J. Richards, Xiang Han, S. Siwiak-Jaszek and P. H. Jones.  'Correlated fluctuations of optically trapped particles',  Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945823, doi 10.1117/12.2190820 (2015)
From the abstract: We present a study of correlated Brownian fluctuations between optically confined particles in a number of different configurations.  First we study colloidal particles held in separate optical tweezers.  In this configuration the particles are known to interact through their hydrodynamic coupling, leading to a pronounced anti-correlation in their position fluctuations at short times.  We study this system and the behavior of the correlated motion when the trapped particles are subject to an external force such as viscous drag.

The second system considered is a chain of optically bound particles in an evanescent wave surface trap.  In this configuration the particles interact both through hydrodynamic and optical coupling.  Using digital video microscopy and subsequent particle tracking analysis we study the thermal motion of the chain and map the covariance of position fluctuations between pairs of particles in the chain.  The experiments are complemented by Brownian motion simulations. 

C. J. Richards, T. J. Smart, P. H. Jones and D. Cubero.  'Low frequency dynamical stabilisation in optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945825, doi 10.1117/12.2190822 (2015)
From the abstract: It is well known that a rigid pendulum with minimal friction will occupy a stable equilibrium position vertically upwards when its suspension point is oscillated at high frequency.  The phenomenon of the inverted pendulum was explained by Kapitza by invoking a separation of timescales between the high frequency modulation and the much lower frequency pendulum motion, resulting in an effective potential with a minimum in the inverted position.

We present here a study of a microscopic optical analogue of Kapitza's pendulum that operates in different regimes of both friction and driving frequency.  The pendulum is realized using a microscopic particle held in a scanning optical tweezers and subject to a viscous drag force.  The motion of the optical pendulum is recorded and analyzed by digital video microscopy and particle tracking to extract the trajectory and stable orientation of the particle.  In these experiments we enter the regime of low driving frequency, where the period of driving is comparable to the characteristic relaxation time of the radial motion of the pendulum with finite stiffness.

In this regime we find stabilization of the pendulum at angles other than the vertical (downwards) is possible for modulation amplitudes exceeding a threshold value where, unlike the truly high frequency case studied previously, both the threshold amplitude and equilibrium position are found to be functions of friction.  Experimental results are complemented by an analytical theory for induced stability in the low frequency driving regime with friction.

Friday, 21 August 2015

Paper published in Optics Letters

Our paper on measuring the optical binding interaction between microparticles in an evanescent wave surface trap has been published as Xiang Han and P. H. Jones, Evanescent wave optical binding forces on spherical microparticles Optics Letters 40 4042-4045 (2015).
From the abstract: In this Letter, we demonstrate stable optical binding of spherical microparticles in counter-propagating evanescent optical fields formed by total reflection at a dielectric interface. The microspheres are observed to form one-dimensional chains oriented parallel to the direction of propagation of the beams. We characterize the strength of the optical binding interaction by measuring the extent of Brownian position fluctuations of the optically bound microspheres and relating this to a binding spring constant acting between adjacent particles. A stronger binding interaction is observed for particles near the middle of the chain, and the dependence of the binding strength on incident laser power and number of particles in the chain is determined.

Friday, 10 July 2015

Optical Tweezers: Principles & Applications

Our new textbook Optical Tweezers: Principles & Applications by P H Jones, O M Marago & G Volpe will be published by Cambridge University Press in October 2015.  It is availabe to pre-order from 09 July 2015 from the publisher or from Amazon.

From the back cover: Combining state-of-the-art research with a strong pedagogic approach, this text provides a detailed and complete guide to the theory, practice and applications of optical tweezers. In-depth derivation of the theory of optical trapping and numerical modelling of optical forces are supported by a complete step-by-step design and construction guide for building optical tweezers, with detailed tutorials on collecting and analysing data. Also included are comprehensive reviews of optical tweezers research in fields ranging from cell biology to quantum physics. Featuring numerous exercises and problems throughout, this is an ideal self-contained learning package for advanced lecture and laboratory courses, and an invaluable guide to practitioners wanting to enter the field of optical manipulation. The text is supplemented by, a forum for discussion and a source of additional material including free-to-download, customisable research-grade software (OTS) for calculation of optical forces, digital video microscopy, optical tweezers calibration and holographic optical tweezers.

Tuesday, 2 June 2015

UCL Physics Society Talk

Phil is giving a talk the the UCL undergraduate Physics Society, Tue 02 Jun 15 titled "Pull, push, spin, squeeze: optical forces on microparticles."

Abstract:  In this talk I will explain how the interaction of laser light with matter can give rise to a force or torque that, if the particle is small enough, can have a significant effect on its motion.  I will then go on to describe a number of experiments from the UCL Optical Tweezers Group that use optical forces in a variety of experimental geometries (optical tweezers, optical fibre traps, optical binding) and applied to a range of different objects, including nanostructures and biological material.

Monday, 1 June 2015

Paper in J Opt Soc Am A

Our paper on the optical trapping force on a particle using a radially polarised beam focused by a "devil's vortex lens" has been published as Ruili Zhang, Ziyang Chen, Jixiong Pu and P. H. Jones, 'Radiation forces on a Rayleigh particles by highly focused radially polarized beams modulated by Devil's vortex lens', Journal of the Optical Society of America A 32 797-802 (2015).

From the abstract:  The intensity and the radiation forces acting on a Rayleigh particle near the focus of completely coherent radially polarized beams whose phase are modulated by a devil’s vortex-lens (DVL) are studied. The influence of the structure of a DVL on the radiation force distribution is analyzed. It is found by numerical simulations that the modulated beams show a clear advantage over the unmodulated highly focused radially polarized beams, as the modulated beam can simultaneously trap and manipulate the multiple Rayleigh particles, while the unmodulated beam can trap only one particle under the same condition.

Wednesday, 22 April 2015

Paper published in JOSA B

Our paper describing how to constuct an advanced optical tweezers experiment has been published as G. Pesce, G. Volpe, O. M. Maragò, P. H. Jones, S. Gigan, A. Sasso & G. Volpe.  'A step-by-step guide to the realisation of advanced optical tweezers', Journal of the Optical Society of America B 32 B84-B98 (2015).  This paper forms part of the joint Special Issue of Optics Express and JOSA B on Optical Cooling and Trapping organised by the OSA Technical Group.

From the abstract: Since the pioneering work of Arthur Ashkin, optical tweezers (OT) have become an indispensable tool for contactless manipulation of micro- and nanoparticles. Nowadays OT are employed in a myriad of applications demonstrating their importance. While the basic principle of OT is the use of a strongly focused laser beam to trap and manipulate particles, more complex experimental setups are required to perform novel and challenging experiments. With this article, we provide a detailed step-by-step guide for the construction of advanced optical manipulation systems. First, we explain how to build a single-beam OT on a homemade microscope and how to calibrate it. Improving on this design, we realize a holographic OT, which can manipulate independently multiple particles and generate more sophisticated wavefronts such as Laguerre–Gaussian beams. Finally, we explain how to implement a speckle OT, which permits one to employ random speckle light fields for deterministic optical manipulation.

Thursday, 26 March 2015

Paper published in Optics Express

Our paper on optical trapping using a beam with a wavefront shaped by a fractal-generated lens structure has been published as Jixiong Pu & P. H. Jones 'Devil's lens optical tweezers' Optics Express 23 8190-8199 (2015).  This paper forms part of the joint Special Issue of Optics Express and JOSA B on Optical Cooling and Trapping organised by the OSA Technical Group.

From the abstract: We demonstrate an optical tweezers using a laser beam on which is imprinted a focusing phase profile generated by a Devil’s staircase fractal structure (Cantor set). We show that a beam shaped in this way is capable of stably trapping a variety of micron- and submicron-sized particles and calibrate the optical trap as a function of the control parameters of the fractal structure, and explain the observed variation as arising from radiation pressure exerted by unfocused parts of the beam in the region of the optical trap. Experimental results are complemented by calculation of the structure of the focus in the regime of high numerical aperture.