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 www.opticaltweezers.org, 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.

Wednesday, 18 March 2015

SPIE Conference Proceedings: Photonics West

Proceedings from the SPIE Photonics West 2015 conference have been published.  These include our paper based on Phil's invited talk: P. H. Jones, C. J. Richards, T. J. Smart & D. Cubero.  'Dynamical stabilisation in optical tweezers', Proc SPIE 9379, Complex Light & Optical Forces IX, 93790L, doi: 10.1117/12.2078961, (2015)

From the abstract: We present a study of dynamical stabilisation of an overdamped, microscopic pendulum realised using optical tweezers. We first derive an analytical expression for the equilibrium dynamically stabilised pendulum position in a regime of high damping and high modulation frequency of the pendulum pivot. This model implies a threshold behavior for stabilisation to occur, and a continuous evolution of the angular position which, unlike the underdamped case, does not reach the fully inverted position. We then test the theoretical predictions using an optically trapped microparticle subject to fluid drag force, finding reasonable agreement with the threshold and equilibrium behavior at high modulation amplitude. Analytical theory and experiments are complemented by Brownian motion simulations.