SPIE OTOM XII Conference Proceedings

The proceedings of the SPIE Optical Trapping and Optical Manipulation Conference XII, held in San Diego in August 2015 have been published, including three contributions from the UCL Optical Tweezers Group:

T. J. Smart, C. J. Richards, R. Bhatnagar, C. Pavesio, R. Agrawal and P. H. Jones.  'A study of red blood cell deformability in diabetic retinopathy using optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945820, doi 10.1117/12.2191281 (2015)

From the abstract: Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus (DM) in which high blood sugar levels cause swelling, leaking and occlusions in the blood vessels of the retina, often resulting in a loss of sight. The microvascular system requires red blood cells (RBCs) to undergo significant cellular deformation in order to pass through vessels whose diameters are significantly smaller than their own. There is evidence to suggest that DM impairs the deformability of RBCs, and this loss of deformability has been associated with diabetic kidney disease (or nephropathy) - another microvascular complication of DM. However, it remains unclear whether reduced deformability of RBCs correlates with the presence of DR.

Here we present an investigation into the deformability of RBCs in patients with diabetic retinopathy using optical tweezers. To extract a value for the deformability of RBCs we use a dual-trap optical tweezers set-up to stretch individual RBCs. RBCs are trapped directly (i.e. without micro-bead handles), so rotate to assume a `side-on' orientation. Video microscopy is used to record the deformation events, and shape analysis software is used to determine parameters such as initial and maximum RBC length, allowing us to calculate the deformability for each RBC.  A small decrease in deformability of diabetes cells subject to this stretching protocol is observed when compared to control cells.

T. J. Smart, C. J. Richards, Xiang Han, S. Siwiak-Jaszek and P. H. Jones.  'Correlated fluctuations of optically trapped particles',  Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945823, doi 10.1117/12.2190820 (2015)

From the abstract: We present a study of correlated Brownian fluctuations between optically confined particles in a number of different configurations.  First we study colloidal particles held in separate optical tweezers.  In this configuration the particles are known to interact through their hydrodynamic coupling, leading to a pronounced anti-correlation in their position fluctuations at short times.  We study this system and the behavior of the correlated motion when the trapped particles are subject to an external force such as viscous drag.

The second system considered is a chain of optically bound particles in an evanescent wave surface trap.  In this configuration the particles interact both through hydrodynamic and optical coupling.  Using digital video microscopy and subsequent particle tracking analysis we study the thermal motion of the chain and map the covariance of position fluctuations between pairs of particles in the chain.  The experiments are complemented by Brownian motion simulations. 

C. J. Richards, T. J. Smart, P. H. Jones and D. Cubero.  'Low frequency dynamical stabilisation in optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945825, doi 10.1117/12.2190822 (2015)

From the abstract: It is well known that a rigid pendulum with minimal friction will occupy a stable equilibrium position vertically upwards when its suspension point is oscillated at high frequency.  The phenomenon of the inverted pendulum was explained by Kapitza by invoking a separation of timescales between the high frequency modulation and the much lower frequency pendulum motion, resulting in an effective potential with a minimum in the inverted position.

We present here a study of a microscopic optical analogue of Kapitza's pendulum that operates in different regimes of both friction and driving frequency.  The pendulum is realized using a microscopic particle held in a scanning optical tweezers and subject to a viscous drag force.  The motion of the optical pendulum is recorded and analyzed by digital video microscopy and particle tracking to extract the trajectory and stable orientation of the particle.  In these experiments we enter the regime of low driving frequency, where the period of driving is comparable to the characteristic relaxation time of the radial motion of the pendulum with finite stiffness.

In this regime we find stabilization of the pendulum at angles other than the vertical (downwards) is possible for modulation amplitudes exceeding a threshold value where, unlike the truly high frequency case studied previously, both the threshold amplitude and equilibrium position are found to be functions of friction.  Experimental results are complemented by an analytical theory for induced stability in the low frequency driving regime with friction.

SPIE Conference Proceedings: Photonics West

Proceedings from the SPIE Photonics West 2015 conference have been published.  These include our paper based on Phil's invited talk: P. H. Jones, C. J. Richards, T. J. Smart & D. Cubero.  'Dynamical stabilisation in optical tweezers', Proc SPIE 9379, Complex Light & Optical Forces IX, 93790L, doi: 10.1117/12.2078961, (2015)

From the abstract: We present a study of dynamical stabilisation of an overdamped, microscopic pendulum realised using optical tweezers. We first derive an analytical expression for the equilibrium dynamically stabilised pendulum position in a regime of high damping and high modulation frequency of the pendulum pivot. This model implies a threshold behavior for stabilisation to occur, and a continuous evolution of the angular position which, unlike the underdamped case, does not reach the fully inverted position. We then test the theoretical predictions using an optically trapped microparticle subject to fluid drag force, finding reasonable agreement with the threshold and equilibrium behavior at high modulation amplitude. Analytical theory and experiments are complemented by Brownian motion simulations.

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

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:

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.

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)

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.

PHOTOPTICS 2014

The 2nd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS2014) will be held in Lisbon, Portugal, 07-09 Jan 14.  The conference will feature three different tracks on Optics, Photonics and Lasers, covering both theoretical and practical aspects, and the Keynote Speaker is Prof David Andrews of the University of East Anglia.

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).

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).

Optical Trapping Applications

Next week Phil, Susan and Marios will all be attending the OSA Optical Trapping Applications conference in Monterey, CA.  Phil will be giving a talk about our work on trapping with novel beams, and Susan and Marios will be presenting posters on their research projects.