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.

Friday, 2 January 2015

MAPS Faculty Research Festival

The Faculty of Mathematical and Physical Sciences (MAPS) Research Festival will celebrate research activities across the faculty by highlighting notable achievements and ongoing projects from the eight departments.

The programme for the event is:

Prof. Ofer Lahav (MAPS Vice Dean for Research)  - "Welcome"
Prof. David Price (Vice Provost for Research)  - "UCL Research Strategy"
Prof. Nick Brook (MAPS Dean) - "MAPS Research Strategy"
Dr David Scanlon (Chemistry) - "Polymorph engineering of TiO2: Understanding the correlation between local coordination, absolute reference potentials and practical applications"
Prof. Rachel McKendry (LCN) - "Connecting for Global Health: nanosensors, mobile phones and big data"
Dr Timo Betcke (Mathematics) - "BEM++: Open-source software development in Mathematics"
Ms. Beate Franke (Statistics) - "Understanding network structure"
Dr Phil Jones (Physics and Astronomy) - "Current research in Biological Physics"
Prof. Dario Alfe (Earth Sciences) - "A window to the Earth's core"

Prof. Mat Page (MSSL) - "Discoveries with Swift"
Dr Jack Stilgoe (STS) - "Responsible Research and Innovation"

Tuesday, 14 October 2014

SPIE Conference Proceedings: OTOM XI

Proceedings from the SPIE Optics + Photonics 2014 conference have been published.  These include Chris F's paper on optical and acoustic manipulation of microbubbles in a microfluidic device: C. R. Fury, P. H. Jones and G. Memoli.  'Multi-scale manipulation of microbubbles employing simultaneous optical and acoustical trapping', Proc SPIE 9164, Optical Trapping and Optical Micromanipulation XI, 91642Z, doi: 10.1117/12.2061622 (2014).

From the abstract: We present a dual-modality microbubble trapping system that incorporates the fine spatial resolution of optical tweezers, with the long range, high force manipulation of acoustic tweezers, in a single microfluidic system. We demonstrate aggregation of polymer microbubbles in the node of an acoustic field, and subsequent selection and separation of a single microbubble using holographic optical tweezers. We further characterize the optical tweezers by measuring the transverse spring constant, and use the calibrated trap to determine the acoustic force on the bubble for varying parameters of optical trap diameter and power, and acoustic frequency and driving voltage. Further development of the system to include acoustic emission measurement is presented, with the goal of having a multi-purpose mechanical and cavitation detection set-up combined into a single system