From the abstract: Purpose: To investigate differences in red blood cell (RBC) deformability between birdshot chorioretinopathy (BCR) subjects and matched controls, and to postulate its relationship with lack of vascular occlusion in BCR. Methods: In a single center, prospective, non-randomized mechanistic study, blood samples were collected from eight healthy controls and nine BCR patients, and subjected to biochemical and hematological tests, as well as RBC indices assessment using dual-beam optical tweezers. Results: The mean age of the controls was 52.37 ± 10.70 years and BCR patients was 53.44 ± 12.39 years. Initial cell size (Io) for the controls was 8.48 ± 0.25 μm and 8.87 ± 0.31 μm for BCR RBCs (p = 0.014). The deformability index (DI) for the controls was 0.066 ± 0.02 and that for BCR RBCs was 0.063 ± 0.03 (p = 0.441). Conclusion: There was no statistically significant difference in DI between RBCs from BCR and healthy controls. This may explain the rare occurrence of retinal vascular occlusion despite the underlying vasculitic pathophysiology of BCR.
From the abstract: A novel high numerical aperture (NA) focusing system with a fractal
conical lens (FCL) is proposed, and tight focusing of radially polarized
beams through the proposed optical system is investigated theoretically
and numerically. The influence of several relevant factors, including
the FCL’s stage , objective lens’ NA, and truncation parameter , on the targeted beam’s focusing characteristics in the focal region is discussed in detail. It is found that, when a FCL with S≥0
is employed, position of the major focal point would shift from the
geometric focal point, and the focused intensity distributions cannot
maintain symmetrical about the focus any more, although they present
different profiles for various truncation parameters . When ,
multiple focal points can be generated, i.e., a single major focus and a
series of subsidiary foci surrounding it along the optical axis, which
form a focal region. These unique focusing characteristics with a FCL
are remarkably different from that of without a FCL. The fascinating
findings here may be taken advantage of when using radially polarized
beams in exploiting new-type optical tweezers and making use of a FCL.
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.
From the abstract: This work presents an acoustofluidic device for manipulating coated microbubbles, designed for the simultaneous use of optical and acoustical tweezers. A comprehensive characterization of the acoustic pressure in the device is presented, obtained by the synergic use of different techniques in the range of acoustic frequencies where visual observations showed aggregation of microbubbles. In absence of bubbles, the combined use of laser vibrometry and finite element modelling supported a non-invasive measurement of the acoustic pressure and an enhanced understanding of the system resonances. Calibrated holographic optical tweezers were then used for a direct measurement of the acoustic forces acting on an isolated microbubble at low driving pressures and to confirm the spatial distribution of the acoustic field. This allowed quantitative pressure measurements by particle tracking using polystyrene beads and an evaluation of the related uncertainties. The extension of the tracking technique to polymer-coated microbubbles allowed acoustic force measurements at higher pressures, highlighting four peaks in the acoustic response of the device. Results and methodologies are relevant to acoustofluidic applications requiring a precise characterization of the acoustic field and, in general, to biomedical applications with microbubbles or deformable particles.
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.
From the abstract: We measure, by photonic torque microscopy, the nonconservative
rotational motion arising from the transverse components of the
radiation pressure on optically trapped, ultrathin silicon nanowires.
Unlike spherical particles, we find that nonconservative effects have a
significant influence on the nanowire dynamics in the trap. We show that
the extreme shape of the trapped nanowires yields a transverse
component of the radiation pressure that results in an orbital rotation
of the nanowire about the trap axis. We study the resulting motion as a
function of optical power and nanowire length, discussing its
size-scaling behavior. These shape-dependent nonconservative effects
have implications for optical force calibration and optomechanics with
levitated nonspherical particles.
From the abstract: A pilot cross sectional study was conducted to investigate the role of
red blood cells (RBC) deformability in type 2 diabetes mellitus (T2DM)
without and with diabetic retinopathy (DR) using a dual optical tweezers
stretching technique. A dual optical tweezers was made by splitting and
recombining a single Nd:YAG laser beam. RBCs were trapped directly
(i.e., without microbead handles) in the dual optical tweezers where
they were observed to adopt a “side-on” orientation. RBC initial and
final lengths after stretching were measured by digital video
microscopy, and a Deformability index (DI) calculated. Blood from 8
healthy controls, 5 T2DM and 7 DR patients with respective mean age of
52.4yrs, 51.6 yrs and 52 yrs was analysed. Initial average length of
RBCs for control group was 8.45 ± 0.25 μm, 8.68 ± 0.49 μm for DM RBCs
and 8.82 ± 0.32 μm for DR RBCs (p < 0.001). The DI for control group
was 0.0698 ± 0.0224, and that for DM RBCs was 0.0645 ± 0.03 and
0.0635 ± 0.028 (p < 0.001) for DR group. DI was inversely related to
basal length of RBCs (p = 0.02). DI of RBC from DM and DR patients was
significantly lower in comparison with normal healthy controls. A dual
optical tweezers method can hence be reliably used to assess RBC
deformability.
The movie below shows a red blood cell being stretched using our optical tweezers.
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.
From the abstract: The intensity and the radiation forces acting on a Rayleigh particle near the focus of completely coherent radially polarized beams whose phase are modulated by a devil’s vortex-lens (DVL) are studied. The influence of the structure of a DVL on the radiation force distribution is analyzed. It is found by numerical simulations that the modulated beams show a clear advantage over the unmodulated highly focused radially polarized beams, as the modulated beam can simultaneously trap and manipulate the multiple Rayleigh particles, while the unmodulated beam can trap only one particle under the same condition.
Our
paper describing how to constuct an advanced optical tweezers experiment has been published as G. Pesce, G. Volpe, O.
M. Maragò, P. H. Jones, S. Gigan, A. Sasso & G. Volpe. 'A
step-by-step guide to the realisation of advanced optical tweezers',
Journal of the Optical Society of America B
32 B84-B98 (2015). This paper forms part of the joint Special Issue of Optics Express and JOSA B on Optical Cooling and Trapping organised by the OSA Technical Group.
From the abstract: Since the pioneering work of Arthur Ashkin, optical tweezers (OT) have
become an indispensable tool for contactless manipulation of micro- and
nanoparticles. Nowadays OT are employed in a myriad of applications
demonstrating their importance. While the basic principle of OT is the
use of a strongly focused laser beam to trap and manipulate particles,
more complex experimental setups are required to perform novel and
challenging experiments. With this article, we provide a detailed
step-by-step guide for the construction of advanced optical manipulation
systems. First, we explain how to build a single-beam OT on a homemade
microscope and how to calibrate it. Improving on this design, we realize
a holographic OT, which can manipulate independently multiple particles
and generate more sophisticated wavefronts such as Laguerre–Gaussian
beams. Finally, we explain how to implement a speckle OT, which permits
one to employ random speckle light fields for deterministic optical
manipulation.
Our paper on optical trapping using a beam with a wavefront shaped by a fractal-generated lens structure has been published as Jixiong Pu & P. H. Jones 'Devil's lens optical tweezers' Optics Express 23 8190-8199 (2015). This paper forms part of the joint Special Issue of Optics Express and JOSA B on Optical Cooling and Trapping organised by the OSA Technical Group.
From the abstract: We demonstrate an optical tweezers using a laser beam on which is
imprinted a focusing phase profile generated by a Devil’s staircase
fractal structure (Cantor set). We show that a beam shaped in this way
is capable of stably trapping a variety of micron- and submicron-sized
particles and calibrate the optical trap as a function of the control
parameters of the fractal structure, and explain the observed variation
as arising from radiation pressure exerted by unfocused parts of the
beam in the region of the optical trap. Experimental results are
complemented by calculation of the structure of the focus in the regime
of high numerical aperture.
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.
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
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)
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.
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.
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.
The article on Optical Tweezers has now been published in the Taylor & Francis Encyclopedia of Optical Engineering.
From the abstract: Optical tweezers are devices that use a single, strongly focused laser
beam for the remote, non-contact trapping and manipulation of
microscopic objects. Since the first demonstration of optical tweezers
in 1986, they have become commonly used across physics, chemistry, and
biology for experiments as diverse as measuring the step size of motor
proteins to a demonstration of fluctuations in entropy production in
microscopic systems. In this entry, the mechanism behind optical
tweezers is introduced, followed by a discussion of practical aspects in
the use of optical tweezers, such as data collection and analysis for
quantitative measurements, and the use of holographic optical traps.
Also, a review of one of the many applications of optical tweezers is
presented, namely, the optical trapping of nanoparticles and
nanostructures and their use for imaging and force sensing.
From the abstract: Ultrasound contrast agents, in the form of coated microbubbles, are a
powerful tool in current diagnostic imaging. Given their sensitive
dynamic response they also have the potential to be used for
quantitative measurements of the properties of the surrounding tissue (e.g.
percentage perfusion or blood pressure). For this potential to be
realised, however, the theoretical descriptions of bubble behaviour, in
particular the constitutive equations for the microbubble shell, need to
be improved and a method needs to be developed for the accurate
characterisation of individual bubbles. In this paper the first steps
are taken towards deriving a complete model for the coupled radial and
translational motion of a coated bubble. It is then shown that with this
model the bubble can be characterised by a unique set of parameters
describing the bubble shell corresponding to its viscous and elastic
response. This uniqueness will enable the model to be used to interpret
experimental data and quantify these parameters for which accurate
values are currently lacking but which are critical to predicting bubble
response and hence enabling advanced diagnostic applications.
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.
