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

New Group Members

Two new members have joined the Optical Tweezers Group at the start of the 2014-15 academic year.  Nick Tidy has joined us as a PhD student.  Nick studied Physics at Birmingham University and graduated in 2014.  He will be working with our collaborators at JAIST on optical trapping of polymer vesicles.  Stefan Siwiak-Jaszek is a fourth year undergraduate at UCL studying Natural Sciences.  Stefan spent the third year of his degree studying at the National University of Singapore, and has retured to UCL for his final year where he will be using optical tweezers to study stochastic thermodynamic processes for his MSci project.

Optics Express & JOSA B: Joint Special Issue on Optical Trapping

The OSA Technical Group on Optical Cooling and Trapping is organizing a joint special issue of the journals Optics Express and the Journal of the Optical Society of America B.  Topics for the special issues include, but are not limited to, the physics and application of laser cooling, electromagnetic trapping and other radiative manipulation of neutral atoms, ions, dielectric particles and nanostructures.  Subissions for the joint Special Issue will open on 01 November 2014 and close on 05 January 2015, with publication scheduled for early in 2015.  The Guest Editors for the Special Issue will be:

Antonio A. R. Neves (Federal University of ABC, Brazil)
Philip H. Jones (UCL, UK)
Le Luo (IUPUI, USA)
Onofrio M. Maragò (CNR-IPCF, Italy)

SPIE Optics + Photonics Conference

Last week Chris F attended the Optical Trapping and Optical Micromanipulation (OTOM) XI conference, as part of SPIE Optics + Photonics 2014 in San Diego.  Chris presented a poster on "Multi-scale manipulation of microbubbles employing simultaneous optical and acoustical trapping" based on work from the NPL-UCL-Oxford microbubble trapping project.  You can see movies from the experiments described in his poster below:

SPIE Photonics Europe Conference Proceedings

Proceedings from the SPIE Photonics Europe 2014 conference have been published.  These include our paper on the characterization of holographic optical traps for microbubbles as part of the NPL/UCL microbubble project: C. R. Fury, C. J Harfield, P. H. Jones, E. P. J. Stride & G. Memoli.  'Experimental characterisation of holographic optical traps for microbubbles', Proc SPIE 9126 Nanophotonics V, 91263L doi:10.1117/12.2055889 (2014)

From the abstract: In this study microscopic gas bubbles (7-12 μm diameter) suspended in water were optically trapped in a custom-built microfluidic slide using holographically generated Laguerre-Gaussian (‘doughnut’) beam optical tweezers. The optical potential was then characterized as a function of bubble size, trapping laser power and trapping beam diameter (Laguerre-Gaussian beam mode) using the trap spring constant in the plane transverse to the beam propagation direction, obtained from the position fluctuations of the bubble in the trap measured by video microscopy and particle tracking. It was found that microbubbles were held at the equilibrium position of buoyant and optical forces at a distance from the focus of the beam that increased with laser power, and that optical trapping in this configuration was only possible within a specific range of trap and bubble parameters. Furthermore an optimum size of the doughnut beam to microbubble diameter which maximized the transverse spring constant was found . A ray optics model of the optical forces acting on microbubbles in a focused Laguerre-Gaussian beam was used in order to calculate the trap spring constants and equilibrium trapping position as a function of the different parameters, and highlight key physical behaviours.

Paper in J Phys: Conf Ser

A paper on laser vibrometer characterization of the ultrasonic coupling into the microfluidic chips used in the NPL/UCL microbubble trapping project as presented at the 12th Anglo-French Physical Acoustics Conference (AFPAC2013) 16-18 Jan 2013, has been published as C. Fury et al, Laser vibrometry characterisation of a microfluidic lab-on-a-chip device: a preliminary investigation J Phys: Conf Ser 498 012002 (2014).

From the abstract: Since their original inception as ultrasound contrast agents, potential applications of microbubbles have evolved to encompass molecular imaging and targeted drug delivery. As these areas develop, so does the need to understand the mechanisms behind the interaction of microbubbles both with biological tissue and with other microbubbles. There is therefore a metrological requirement to develop a controlled environment in which to study these processes. Presented here is the design and characterisation of such a system, which consists of a microfluidic chip, specifically developed for manipulating microbubbles using both optical and acoustic trapping. A laser vibrometer is used to observe the coupling of acoustic energy into the chip from a piezoelectric transducer bonded to the surface. Measurement of the velocity of surface waves on the chip is investigated as a potential method for inferring the nature of the acoustic fields excited within the liquid medium of the device. Comparison of measured surface wavelengths with wave types suggests the observation of anti-symmetric Lamb or Love-Kirchhoff waves. Further visual confirmation of the acoustic fields through bubble aggregation highlights differences between the model and experimental results in predicting the position of acoustic pressure nodes in relation to excitation frequency.

Harrie Massey Lecture 2014 by Prof Steven Chu

Professor Steven Chu (1997 Nobel Laureate in Physics and former US Secretary of Energy) will give the 2014 Harrie Massey Lecture at UCL on Energy and Climate Change: Challenges and Opportunities, Wed 19 Mar 14.

Abstract: Science and technology has profoundly transformed the lives of much of humanity. The industrial and agricultural revolutions are also changing the future destiny of our planet. I will discuss the necessity, challenges, and opportunities in innovation and policy that will be needed to transition to a sustainable future.

NanoSpain 2014

Phil is giving a talk at the NanoSpain 2014 conference in Madrid, 11-14 Mar 14 on Evanescent wave optical trapping and manipulation of particles and nanostructures.

Abstract:  Optical trapping is a powerful technique for the controlled manipulation of particles with sizes in the micron, sub-micron and nanometre range¹.  Conventional optical tweezers using a single, strongly-focused laser beam to confine particles within the focal volume of ~1um³.  Optical binding describes the self-organisation of microparticles and nanostructures in an optical field that occurs over long distances and extended areas arising from the multiple scattering of light. Here we present experimental schemes for the control of optically bounds structures in evanescent optical fields.  The first relies on total internal reflection at an interface, where the evanescent field penetrates a short distance (comparable to, or less than the optical wavelength) above the interface.  We show that this geometry, shown in Figure 1(a), gives rise to one- and two-dimensional optically ordered structures of microparticles and also of nanostructures immersed in the field, shown in Figure 1(b) – (d), and quantify the binding forces and structure geometries via video microscopy^2,3.

Figure 1: Optical binding of carbon nanostructures. (a) Set-up of the optical binding experiment; (b) Optically bound chain of carbono nanotube bundles; (c) When the laser beam is turned off the chain disintegrates; (d) Laser beam on, chain re-forms

The second geometry uses optical waveguides of sub-optical wavelength dimension.  For our experiments these are optical fibres that are adiabatically tapered to 1micron in diameter. Such a waveguide supports the fundamental mode only, but a large fraction of the power propagates in an evanescent field that can penetrate a significant distance in the surroundings.  We show here how this field can be used for optical binding of particles to the nanofibre and long-range transport along the length of the tapered region⁴.

References

1. O. M. Maragò, P. H. Jones, P. G. Gucciardi, G. Volpe & A. C. Ferrari. 'Optical trapping and manipulation of nanostructures', Nature Nanotechnology 8 807-819 (2013)

2. M. Sergides, S. E. Skelton, E. Karczewska, K. Thorneycroft, O. M. Maragó & 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)

3. S. H. Simpson, P. H. Jones, O. M. Maragò, S. Hanna & M. J. Miles. 'Opticalbinding of nanowires in counter-propagating beams’, Proc SPIE 8810 Optical Trapping and Optical Micromanipulation X, 881026, doi: 10.1117/12.2024466 (2013)

4. S. E. Skelton, M. Sergides, R. Patel, E. Karczewska, O. M. Maragó & P. H. Jones. 'Evanescent wave optical trapping and transport of micro- and nanoparticleson tapered optical fibers', Journal of Quantitative Spectroscopy and Radiative Transfer 113 2512-2520 (2012)

Symposium on Optical Forces: from atoms to soft-matter

On Wed 26 Feb 14 the OSA Messina Student Chapter and EPS Young Minds Group Messina wil be holding a one-day symposium on Optical Forces at the IPCF-CNR (Messina). The programme for the day includes:

• J. J. Saenz (University of Madrid) Scattering asymmetry and non-conservative optical forces on small particles (OSA Lecture)
• P. H. Jones (University College London) Evanescent wave traps and optical binding of particles
• G. Pesce (University of Naples) Surface charge and hydrodynamic coefficient measurements of micro-particles and living micro-organisms by Optical Tweezers
• M. G. Donato (IPCF-CNR) Optical trapping of nanostructures
• C. J. Foot (University of Oxford) Laser cooling and trapping of atoms – past and present (Young Minds Lecture)
• G. Volpe (Bilkent University) Speckle optical tweezers: Tunable anomalous diffusion and selective optical manipulation
• O. M. Maragò (IPCF–CNR) Fano-Doppler laser cooling of hybrid nanostructures

Xiang Han joins the Optical Tweezers Group

Xiang Han has joined the UCL Optical Tweezers Group as a visiting student.   Xiang is a PhD student in the College of Optoelectronic Science & Engineering at the National Defense University, Changsha, Hunan, China.  His visit is funded by an award from the China Scholarship Council.  Xiang will be working with us until 2016 on a number of optical binding experiments.