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)

Royal Society Summer Exhibition

This week we have been at the Royal Society Summer Exhibition 2012 with a display titled POP! The sound of bubbles.  Opposite you can see members of the team explaining the science of bubbles to guests at the soiree on Thu 05 Jul.

The exhibition and our display also featured on the BBC news, and in BBC R4's Material World programme on 05 July. The podcast of this show will be available to download for the next few weeks. 

Trapping and deformation of microbubbles in a dual-beam fibre-optic trap

  

Our paper Trapping and deformation of microbubbles in a dual-beam fibre-optic trap has been published as S E Skelton et al, Journal of Optics 14 075706 (2012).

From the abstract:  We present results of numerical calculations to evaluate the performance of a dual-beam fibre-optic trap for low refractive index particles such as ultrasound contrast agent microbubbles. Using a geometrical optics approach, we determine the range of parameters of microbubble size and beam dimensions over which the optical trap is stable and evaluate the trapping forces and spring constants. Additionally, we calculate the optically induced stress profile over the surface of the microbubble and evaluate the resulting deformation of the microbubble using elastic membrane theory. Our results suggest that such an experiment could be a useful tool for quantifying the mechanical properties (elastic modulus) of the shell material of an ultrasound contrast agent microbubble.

This paper has been included in the IoPscience collection of featured articles "chosen for their quality and recency".

Fluctuations in Dynamical Systems Far From Equilibrium

The workshop will bring together people from applied mathematics, dynamical systems, condensed matter physics and statistical physics. The theme are fluctuations far from equilibrium, particularly in stochastic dynamics. A focus will be on so-called fluctuation relations, which generalize the second law of thermodynamics to small systems. They have been found to be generally valid very far from equilibrium. This active, recent topic of research will be discussed both from a theoretical and from an experimental point of view. Cross-links to large deviation theory and to anomalous stochastic processes will be explored.

Programme

11.30 - 12.30 Ian Ford, UCL: A rough guide to fluctuation relations

14.00 - 15.00 David Carberry, University of Bristol: Fluctuation Relations: Experimental Demonstrations

15.00 - 16.00 Adrian Baule, QMUL: Exact relations in non-equilibrium statistical mechanics

16.30 - 17.30  Nicholas Watkins, Cambridge, UK: Anomalous Diffusion, Anomalous Time Series, and the models that describe them

All talks will be held in seminar room M103 (first floor) of the Department of Mathematics at Queen Mary University of London.

For more information contact Prof Ferruccio Renzoni

Paper in Journal of Quantitative Spectroscopy and Radiative Transfer

Our paper Evanescent wave optical trapping and transport of micro- and nanoparticles on tapered optical fibers has been published online in the Journal of Quantitative Spectrocopy and Radiative Transfer doi: 10.1016/j.jqsrt.2012.06.005.

From the abstract: We investigate the manipulation of microscopic and nanoscopic particles using the evanescent optical field surrounding an optical fiber that is tapered to a micron-scale diameter, and propose that this scheme could be used to discriminate between, and thereby sort, metallic nanoparticles. First we show experimentally the concept of the transport of micron-sized spheres along a tapered fiber and measure the particle velocity. Having demonstrated the principle we then consider theoretically the application to the optical trapping and guiding of metallic nanoparticles, where the presence of a plasmon resonance is used to enhance optical forces. We show that the dynamics of the nanoparticles trapped by the evanescent field can be controlled by the state of polarization of the fiber mode, and by using more than one wavelength differently detuned from the nanoparticle plasmon resonance. Such a scheme could potentially be used for selectively trapping and transporting nano- or microscopic material from a polydisperse suspension.

Visit by Sile Nic Chormaic's Group

On Thursday 3 May we were visited by members of Sile Nic Chormaic's Group from University College Cork who were in London en route from Ireland to Japan.  Our groups collaborate on the Nanofibre Optical Interfaces for Ion Atoms and Molecules (NOIs) project.  Some pictures of the group were taken by Dr VB Tiwari (UC Cork & Raja Ramanna Centre for Advanced Technology, Indore).

New Group Members

Chris Fury has joined the UCL Optical Tweezers Group from April 2012 to study for a PhD.  Chris gained a degree in Physics from Exeter University, followed by an MSc in Fusion Energy from York University, which included a research project at the Central Laser Facility at the Rutherford Appleton Lab.

Chris joins the group to work at the National Physical Laboratory on the NPL/UCL optical and acoustic microbubble trapping project.

Physics Department Annual Review

The UCL Department of Physics & Astronomy Annual Review 2011-12 has been published.  In this year's review Phil is featured in the 'Academic showcase' section.

Royal Society Summer Exhibition 2012

The UCL/NPL microbubble trapping project will have a stand at the Royal Society Summer Exhibition 2012, called 'Pop! The sound of bubbles'.  This stand will be run by many of the scientists involved in the project including Gianluca Memoli, Eleanor Stride, Caroline Harfield & Louise Wright.  The exhibition runs from 03-08 July 2012 at the Royal Society.
Our display will feature the science of microbubble trapping as well as many other everyday applications of microbubble technology (including some that might surprise you!).
You can follow the sound of bubbles blog for the latest updates on our project, or the Summer Science 2012 twitter @summerscience for more about the exhibition.

David Carberry visit and seminar

David Carberry (Bristol University) is visiting and giving the AMOP Physics seminar on Wednesday 22 February.  David works on optical micromanipulation techniques and recently demonstrated a multi-point holographic optical tweezers controlled through an iPad.  The abstract for his seminar is below:

Title:  Non-spherical optically trapped probes: Design, control and applications
Abstract: Non-spherical probe particles have a number of advantages over microspheres: rotational motion may be monitored and controlled, the trapping points may be removed from the probe’s tip, and the tip can be more accurately positioned than a microsphere of equivalent radius.

We demonstrate a range of non-spherical probes, and discuss how they may be tailored to specific applications. We consider how probe geometry affects the region of space the tip explores – the ‘tip thermal volume’, and the relaxation times of this motion¹. By independently position-clamping translational and rotational modes in different ways, we are able to further control the shape of the tip thermal volume, and dramatically improve the position resolution of the probe, with no reduction in force sensitivity².

Using holographic optical tweezers combined with stereomicroscopy, we can both control and track the motion of our non-spherical probes in all three dimensions. Finally, we demonstrate the use of our probes to image surfaces in 3D³, and measure force and torque interactions with biological specimens. 

  

1. S.H. Simpson and S. Hanna. ‘Thermal Motion of a holographically trapped SPM-like probe’. Nanotechnology, 20 395710 (2009)
2. D.B. Phillips et al. ‘Position clamping of optically trapped microscopic non-spherical probes’. Optics Express, 19 20622 (2011)
3. D.B. Phillips et al. ‘Surface imaging using holographic optical tweezers’. Nanotechnology, 22 285503 (2011)

Visit by Onofrio Marago

Onofrio Marago from the NanoSoft Lab, IPCF-CNR (Messina) is visiting our group at UCL from 26-31 January.  Our groups have a long-running collaboration, and exchange visits are funded by the Royal Society as part of our International Joint Project on Photonic Force Microscopy.

AMOP Physics Open Day

The Atomic Molecular Optical and Positron Physics group will be holding an Open Day on Wednesday 08 February 2012 at 13:00. Come along and find out more about working towards a PhD in our group! You'll get to hear about our research programs, talk to group members and potential supervisor as well as see our labs. 

There are PhD opportunities in theoretical and experimental research in the following areas:
 

• Cold matter and optical trapping
• Positron and positronium physics
• Quantum effects in biological systems
• Quantum information and quantum computation
• Theoretical molecular atmospheric and astrophysics
• Ultracold quantum gases
• Ultrafast laser spectroscopy and strong laser interactions

For more information on studentship opportunities and the application process, please follow this link. You are strongly encouraged to email potential supervisors directly regarding potential projects and funding.

The Open Day will be held in rooms A1 and A13 located on the top floor of the Physics building on UCL's main Gower Street Campus. Please see here for further details of how to get to us.  The agenda of the day is:

13:00 - 13:30  Introductory talk by Prof Peter Baker and Prof Sougato Bose (A1)

13:00 - 13:15  Buffet Lunch (A13)

13:15 - 15:00 Poster presentations by research groups (A13)

14:00 - 16:00 Lab tours