Paper published in Nano Letters

Our theoretical study of optical binding effects between nonspherical particles has been published as S. H. Simpson, P. Zemánek, O. M. Maragò, P. H. Jones & S. Hanna.  'Optical binding of nanowires', Nano Letters 17 3485-3492 (2017).

 From the abstract:  Multiple scattering of light induces structured interactions, or optical binding forces, between collections of small particles. This has been extensively studied in the case of microspheres. However, binding forces are strongly shape dependent: here, we turn our attention to dielectric nanowires.  Using a novel numerical model we uncover rich behavior. The extreme geometry of the nanowires produces a sequence of stationary and dynamic states. In linearly polarized light, thermally stable ladder-like structures emerge. Lower symmetry, sagittate arrangements can also arise, whose configurational asymmetry unbalances the optical forces leading to nonconservative, translational motion. Finally, the addition of circular polarization drives a variety of coordinated rotational states whose dynamics expose fundamental properties of optical spin. These results suggest that optical binding can provide an increased level of control over the positions and motions of nanoparticles, opening new possibilities for driven self-organization and heralding a new field of self-assembling optically driven micromachines.

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.

Paper published in Optics Letters

Our paper on measuring the optical binding interaction between microparticles in an evanescent wave surface trap has been published as Xiang Han and P. H. Jones, Evanescent wave optical binding forces on spherical microparticles Optics Letters 40 4042-4045 (2015).
 

From the abstract: In this Letter, we demonstrate stable optical binding of spherical microparticles in counter-propagating evanescent optical fields formed by total reflection at a dielectric interface. The microspheres are observed to form one-dimensional chains oriented parallel to the direction of propagation of the beams. We characterize the strength of the optical binding interaction by measuring the extent of Brownian position fluctuations of the optically bound microspheres and relating this to a binding spring constant acting between adjacent particles. A stronger binding interaction is observed for particles near the middle of the chain, and the dependence of the binding strength on incident laser power and number of particles in the chain is determined.

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.

Review paper in Nature Nanotechnology

Our paper 'Optical trapping and manipulation of nanostructures' has been published in Nature Nanotechnology 8 807-819 (2013).

From the abstract:  Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light–matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate — nanoscale — range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.

SPIE Conference Proceedings

Proceedings from the SPIE Optics + Photonics 2013 conference have been published.  These include a paper containing calculations by Stephen Simpson (Bristol University) based on our optical binding experiments: S. H. Simpson, P. H. Jones, O. M. Maragò, S. Hanna & M. J. Miles. 'Optical binding of nanowires in counter-propagating beams’, Proc SPIE 8810 Optical Trapping and Optical Micromanipulation X, 881026, doi: 10.1117/12.2024466 (2013).

From the abstrast:  We present a computational model for the simulation of optically interacting nano-structures immersed in a viscous fluid. In this scheme, nanostructures are represented by aggregates of small spheres. All optical and hydrodynamic interactions, including thermal fluctuations, are included. As an example, we consider optical binding of dielectric nanowires in counterpropagating plane waves. In particular, the formation of stable, ladder like structures, is demonstrated. In these arrangements, each nanowire lies parallel to the polarization direction of the beams, with their centres of mass colinear.

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)

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 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.

New Group Members

Kelly Thorneycroft has joined the UCL Optical Tweezers Group for her MSci project.  Kelly is a fourth year undergraduate studying Natural Sciences and will be working on optical trapping and binding experiments and biophysical applications.

Electromagnetic and Light Scattering XIII

The latest conference in the series 'Electromagnetic and Light Scattering' (ELS XIII) is being held in Taormina, Sicily from 26-20 September.

Phil will be giving an invited talk about the UCL Optical Tweezers Group's research on Optical Trapping and Optical Binding Using Cylindrical Vector Beams.  The book of extended abstracts of all the conference presentations is published in a special issue of Atti della Accademia Peloritana dei Pericolanti Volume 89, Supplement No. 1 (2011).

Nuffield Bursary Update (3)

Throughout the course of her Nuffield Foundation project this summer Ewa Karczewska will be making updates of her progress on our blog.  Her final instalment is below.

Ewa writes: During the 8-week project with the UCL Optical Tweezers Group I have learnt a great deal about the laboratory work as well as the current experiments run in it, which focus on the evanescent field and the micro- and nano- structures trapping. I have also gain knowledge about the evanescent wave and the different optical fibre modes.
My research was focused on the thin fibre fabrication using the “heating and pulling”method. The light (λ = 1064nm) was coupled to the tapered fibre so that the evanescent field near the taper region was created. That field was then tested with the micro-sized spheres. When the field was strong enough and the surface of the fibre was smooth enough, the spheres were observed to move along the fibre along the direction of the light propagation.
Overall, I have managed to obtain an efficient method of obtaining thin (about 1.5μm in diameter) optical fibres using the “heating and pulling” method and observe the spheres of 2μm to move along the fibre once the laser light was coupled through it. When the laser light was coupled from both sides of the fibre I could move the spheres in two directions and also balanced the two beams so that the spheres stayed in place.
I have also investigated the behaviour of the micron-sized spheres when the polarisation of the trapping light of an optical tweezers was changed from linear to circular polarisation as well as from radial to azimuthal polarisation.
That project gave me a valuable experience and a great insight into the research work, which will be very useful in my future study for PhD.

From September Ewa will be studying for the third year of her degree as a year abroad in Toronto University.

Nuffield Bursary Update

Throughout the course of her Nuffield Foundation project this summer' Ewa Karczewska will be making updates of her progress on our blog.  Her first instalment is below.



Tapered glass optical fibre
with 2 micron polystyrene spheres 

 Ewa writes:  I have started this Nuffield Bursary project by studying the mechanical effect of evanescent optical fields on microscopic particles.  I have learned how to set up simple optical path using optical mirrors and lenses, and most importantly I have understood the basic ideas behind the use of the laser light for optical trapping.  Having been familiarised with the equipment in the laboratory and having been shown the experiment of array formation and optical binding in evanescent waves carried out by Marios Sergides, I have been using the heating and pulling method for making tapered optical fibres.  With this method I have produced several optical fibres of less than 2μm in diameter.  Using these fibres I will be trying to trap micron-sized spheres in the evanescent field, but first the efficiency of coupling and light transmission into the fibre must be improved.  To do this I have learned how to polish and connectorize optical fibres and measured the transmission efficiency.

Optical Binding Movie

We have been working on an experiment to demonstrate evanescent wave optical binding of microparticles.

The latest movie from this experiment is here and on our YouTube channel. In this movie you can see 1 micron diameter silica microparticles suspended in water above the surface of a glass prism.  The microparticles organise into 1D chains when exposed to the evanescent field of a laser beam reflected from the surface below.

Optical Binding

In a new experiment at UCL we have realised evanescent wave optical binding of microscopic spheres.  A drop of liquid containing a suspension of microspheres is placed on top of a prism and a laser beam focused onto the prism surface from below.  When the beam is incident on the glass-water interface at just greater than the critical angle for total internal reflection an evanescent field penetrates a short distance into the water.  Under these conditions we see the microparticles drawn together into short optically bound chains.

Part (a) of the diagram represents out set-up.  The beam is retro-reflected and re-focused by the concave mirror in order to balance the radiation pressure in each direction.  Part (b) shows a photograph of optically bound chains of 2 micron diameter microspheres.