Tuesday, 7 February 2017

Paper published in IEEE Photonics Journal

Our work on a novel optical trapping system using fractal optics and spatially inhomogneous polarisation of light has been published as Zhirong Liu and P. H. Jones, 'Fractal conical lens optical tweezers', IEEE Photonics Journal 9 6500111 (2017)

From the abstract: We propose a novel optical tweezers composed of an annular beam with alternate radially and azimuthally polarized rings modulated by a fractal conical lens (FCL) and demonstrate its optical forces on Rayleigh dielectric particles both analytically and numerically. Owing to the optical system's particular focusing properties, which could generate a dark-centered or peak-centered intensity distribution in the focal region when selecting an appropriate truncation parameter in front of the focusing lens, the proposed FCL optical tweezers could selectively trap and manipulate dielectric mesoscopic particles with low- or high-refractive indices by appropriately adjusting the radius of the pupil or the beam. Finally, the stability conditions for effective trapping and manipulation Rayleigh particles are analyzed.

Friday, 2 December 2016

Paper published in JOSA A

Our paper on the optical trapping forces of a poarization-structured beam focused by a fractal (Devil's) lens has been published as Zhirong Liu and P. H. Jones, 'Radiation forces acting upon a Rayleigh particle by highly focused alternate radially- and azimuthally-polarized beams modulated by a Devil's Lens', Journal of the Optical Society of America A 33 2501-2508 (2016).

From the abstract:  We propose and demonstrate a novel high numerical aperture (NA) focusing system composed of an annular beam with alternate radially and azimuthally polarized rings, focused by a devil’s lens (DL), and further investigate its radiation forces acting upon a Rayleigh particle both analytically and numerically. Strongly focused cylindrical vector beams produce either dark-centered or peak-centered intensity distributions depending on the state of polarization, whereas the DL produces a series of foci along the propagation direction. We exploit these focusing properties and show that by selecting an appropriate truncation parameter in front of the focusing lens, the proposed optical focusing system can selectively trap and manipulate dielectric micro-particles with low or high refractive indices by simply adjusting the radius of the pupil or the beam. Finally, the stability conditions for effectively trapping and manipulating Rayleigh particles are analyzed. The results obtained in this work are of interest in possible applications in optical confinement and manipulation, sorting micro-particles, and making use of a DL.

Wednesday, 2 November 2016

Paper published in Optics Letters

Our paper on optical binding in two-dimensions has been published as Xiang Han, Hui Luo, Guangzong Xiao and P. H. Jones.  'Optically bound colloidal lattices in evanescent optical fields', Optics Letters 41 4935 (2016).

From the abstract: In this Letter, we demonstrate the formation of a stable two-dimensional lattice of colloidal particles in the interference pattern formed by four evanescent optical fields at a dielectric interface. The microspheres are observed to form a two-dimensional square lattice with lattice vectors inclined relative to the beam propagation directions. We use digital video microscopy and particle tracking to measure the Brownian motion of particles bound in the lattice, and use this to characterize fluctuations in the local ordering of particles using the bond orientational order parameter, the probability distribution of which is shown to be a chi-squared distribution. An explanation for the form of this distribution is presented in terms of fluctuations of the modes of a ring of particles connected by springs.

Tuesday, 11 October 2016

PhD project in Advanced Characterisation of Materials CDT

The UCL-Imperial CDT in Advanced Characterisation of Materials is now accepting applications.  You can apply for a project in the UCL Optical Tweezers Group in collaboration with Valeria Garbin's Group in the Department of Chemical Engineering at Imperial on Nanomechanical Characterisation of Soft Materials.

Project abstract:

The project objective is to develop a suite of analytical techniques, including optical tweezers and microfluidics, for characterising the (nano)mechanical properties of ‘soft’ materials such as liposomes or biomembranes.  The principal aims of this project are:
(i) to study the mechanical properties of biomimetic vesicles undergoing extreme deformations as a result of an applied external stress, e.g. optical, acoustic, or fluid shear forces;
(ii) to study phase separation and rupture in artificial vesicles under external forcing;
(iii) to use the result of the above studies to  engineer membrane materials with properties optimised for applications including controlled drug release and microreactors.
During the project the student will acquire skills in microfluidics, microdevice fabrication, optics, modelling (including light scattering and transport phenomena), image analysis, and (micro)rheology.  

Contact Phil Jones or Valeria Garbin for details

Friday, 5 August 2016

Postdoc position in UCL Optical Tweezer Group

The Optical Tweezers group at UCL (www.ucl.ac.uk/~ucapphj) is looking to recruit a postdoctoral research associate to work on the British Council-funded project “Manipulation and Destruction of Cancer Cells Using Cavitation Bubbles by Optical and Acoustic Tweezers”. 

The aim of the project is to use optical and acoustic techniques to manipulate microscopic bubbles in the vicinity of cancer cells, with the goal of destroying them either by enhancement of drug uptake, or by physical disruption of the cell membrane.

The project will involve several research visits to our collaborators Dr Burcin Unlu (Bogazici University, Istanbul) and Dr Giovanni Volpe (Bilkent University, Ankara). 

This position is funded for 18 months in the first instance.

The successful candidate should have (or be about to obtain) and PhD in Physics or Medical Physics, and expertise in experimental optics and/or acoustics (including photoacoustics).

Please contact Dr Phil Jones for further detail, or see jobs.ac.uk to apply.

The closing date for applications is 26 August 2016.

Wednesday, 27 July 2016

Paper published in Nano Letters

Our paper on the dynamics of silison nanowires in optical tweezers has been published as A. Irrera et al, 'Photonic torque microscopy of the non-conservative force field for optically trapped silicon nanowires', Nano Letters 16, 4181-4188 (2016).
From the abstract: We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

Monday, 4 April 2016

Paper published in Scientific Reports

Our work on assessing the chage in deformability of red blood cells from patients with the condition diabetic retinopathy has been published as R. Agrawal et al.  'Assessment of red blood cell deformability in type 2 diabetes mellitus and diabetic retinopathy by dual optical tweezers stretching technique', Scientific Reports 6 15873, doi:10.1038/srep15873 (2016).

From the abstract: A pilot cross sectional study was conducted to investigate the role of red blood cells (RBC) deformability in type 2 diabetes mellitus (T2DM) without and with diabetic retinopathy (DR) using a dual optical tweezers stretching technique. A dual optical tweezers was made by splitting and recombining a single Nd:YAG laser beam. RBCs were trapped directly (i.e., without microbead handles) in the dual optical tweezers where they were observed to adopt a “side-on” orientation. RBC initial and final lengths after stretching were measured by digital video microscopy, and a Deformability index (DI) calculated. Blood from 8 healthy controls, 5 T2DM and 7 DR patients with respective mean age of 52.4yrs, 51.6 yrs and 52 yrs was analysed. Initial average length of RBCs for control group was 8.45 ± 0.25 μm, 8.68 ± 0.49 μm for DM RBCs and 8.82 ± 0.32 μm for DR RBCs (p < 0.001). The DI for control group was 0.0698 ± 0.0224, and that for DM RBCs was 0.0645 ± 0.03 and 0.0635 ± 0.028 (p < 0.001) for DR group. DI was inversely related to basal length of RBCs (p = 0.02). DI of RBC from DM and DR patients was significantly lower in comparison with normal healthy controls. A dual optical tweezers method can hence be reliably used to assess RBC deformability.

The movie below shows a red blood cell being stretched using our optical tweezers.