Paper published in Optics Letters

Our paper 'Trapping volume control in optical tweezers using cylindrical vector beams' has been published as S. E. Skelton et al, Optics Letters 30 28-30 (2013).

From the abstract: We present the result of an investigation into the optical trapping of spherical microparticles using laser beams with a spatially inhomogeneous polarization direction [cylindrical vector beams (CVBs)]. We perform three-dimensional tracking of the Brownian fluctuations in the position of a trapped particle and extract the trap spring constants. We characterize the trap geometry by the aspect ratio of spring constants in the directions transverse and parallel to the beam propagation direction and evaluate this figure of merit as a function of polarization angle. We show that the additional degree of freedom present in CVBs allows us to control the optical trap strength and geometry by adjusting only the polarization of the trapping beam. Experimental results are compared with a theoretical model of optical trapping using CVBs derived from electromagnetic scattering theory in the T-matrix framework.

PhD Studentships Available

Two PhD studentships are available to work on a project funded by the Leverhulme Trust titled 'Exploring stochastic thermodynamics with optical traps', starting in 2013. The aim of this project is to use optical trapping experiments as a test-bed for fluctuation relations in non-equilibrium (driven) systems.

Project abstract: The theory of equilibrium statistical mechanics is well established, and has found direct or indirect confirmation in a wide range of experiments. This is not the case for the emerging field of non-equilibrium statistical mechanics. In this case, a general theory has yet to be formulated, although a number of theorems (e.g. fluctuation and work theorems) have been introduced to link different physical quantities in systems out of equilibrium. The aim of the proposed research is to develop further, through proof-of-principle experiments with optical traps, the foundations of non-equilibrium statistical mechanics, with a view towards a unified description. 

Studentships will pay a stipend and fees at the rate applicable for UK and European Union students only.

Contact Dr Phil Jones for further details

Pietro Cicuta visit and seminar

Pietro Cicuta (Cambridge University) is visiting the lab on Wednesday 07 November, and giving the AMOPP/BioP seminar.  Pietro works on a variety of problems relevant to soft matter and biological physics, including colloidal systems, complex fluids and membranes.  His talk is titled Emergence of collective states in active colloids, and the abstract is below:

Title: Emergence of collective states in active colloids. Pietro Cicuta (Cambridge University)

Abstract:  Coupled oscillators can, under some conditions, synchronise. There are examples of this everywhere in natural phenomena, technology, and material properties.  A relatively unexplored area is the synchronisation of colloidal oscillators: these are micron-sized objects, coupled through hydrodynamic interaction.   There is reason to study these because physiologically similar structures are thought to enable fluid transport in various organs;   technologically it might be possible to construct very sensitive micro-resonators.
Over the last few years we have worked in this area by developing an experimental platform: optical tweezers are used to create phase oscillators. This talk will present various strategies for making oscillators (active colloids), and the resulting collective behaviour of two or more of these elements.

SPIE Conference Proceedings

Our contributions to the Optical Trapping and Optical Micromanipulation IX meeting at SPIE Optics + Photonics 2012 are now published in the conference proceedings as:

S. E. Skelton, M. Sergides, M. G. Donato, S. Vasi, R. Sayed, P. G. Gucciardi, R. Saija, M. A. Iatì, O. M. Maragò and P. H. Jones.  'Shaping the trapping volume in optical tweezers using cylindrical vector beams', Proc. SPIE 8458, Optical Trapping and Optical Micromanipulation IX, 84582Z, doi: 10.1117/12.929927 (2012)

S. E. Skelton, M. Sergides, G. Memoli, O. M. Maragò and P. H. Jones.  'Optical squeezing of microbubbles: Ray optics and Mie scattering calculations', Proc. SPIE 8458, Optical Trapping and Optical Micromanipulation IX, 84581F, doi: 10.1117/12.929900 (2012)

M. Sergides, S. E. Skelton, E. Karczewska, K. Thorneycroft, O. M. Maragò and P. H. Jones.  'Optically bound particle structures in evanescent wave traps', Proc. SPIE 8458, Optical Trapping and Optical Micromanipulation IX, 84583C, doi: 10.1117/12.929612 (2012)

New Group Members

This year we welcome two final year undergraduate students into the Optical Tweezers Group for their MSci projects.  Chris Richards will be working on light scattering calculations relevant to our optical trapping and optical binding experiments.  Zhi Wong will be investigating the optical and light scattering properties of metallic nanoparticles in his project on Plasmonics.

SPIE Optics + Photonics Conference

Marios and Susan are attending the Optical Trapping and Optical Micromanipulation IX conference, part of SPIE Optics + Photonics, in San Diego, CA, 12 - 16 August.  Susan will be giving a talk 'Optical squeezing of microbubbles: Ray optics and Mie scattering calculations' (Paper 8458-51) on Wed 16 Aug (Session 10: Combining optical traps with acoustics) and presenting a poster on 'Shaping of the trapping volume in optical tweezers using cylindrical vector beams' (Paper 8458-110).  Marios will be presenting a poster 'Optically bound particle structures in evanescent wave traps' (Paper 8458-124).

Paper published in Optics Letters

Our paper 'Optical trapping of single walled carbon nanotubes with cylindrical vector beams has been published as M. G. Donato et al, Opt. Lett. 37 3381-3383 (2012).  

From the abstract: We use laser beams with radial and azimuthal polarization to optically trap carbon nanotubes. We measure force constants and trap parameters as a function of power showing improved axial trapping efficiency with respect to linearly polarized beams. The analysis of the thermal fluctuations highlights a significant change in the optical trapping potential when using cylindrical vector beams. This enables the use of polarization states to shape optical traps according to the particle geometry, as well as paving the way to nanoprobe-based photonic force microscopy with increased performance compared to a standard linearly polarized configuration.

This paper has also been selected for inclusion in the Virtual Journal of Biomedical Optics 7 (2012)