By moulding optical fields, holographic optical tweezers are able to generate structured force fields with magni- tudes and length scales of great utility for experiments in soft matter and biological physics. Optically induced force fields are determined not only by the incident optical field, but by the shape and composition of the par- ticles involved. Indeed, there are desirable but simple attributes of a force field, such as rotational control, that cannot be introduced by sculpting optical fields alone. In this work we describe techniques for the fabrication, sample preparation, optical manipulation and position and orientation measurement of non-spherical particles. We demonstrate two potential applications: we show how the motion of a non-spherical optically trapped force probe can be used to infer interactions occurring at its tip, and we also demonstrate a structure designed to be controllably rotated about an axis perpendicular to the optical axis of the beam.
In this proceedings paper we show describe how a microtool can be assembled, and tracked in three dimensions
such that its full rotational and translational coordinates, q, are recovered. This allows tracking of the motion
of any arbitrary point, d, on the microtool's surface. When the micro-tool is held using multiple optical traps
the motion of such a point investigates the inside of an ellipsoidal volume - we term this a `thermal ellipsoid. We
demonstrate how the shape of this thermal ellipsoid may be controlled by varying the relative trapping power
of the optical traps, and adjusting the angle at which the micro-tool is held relative to the focal plane. Our
experimental results follow the trends derived by Simpson and Hanna.
We present an imaging technique using an optically trapped silica microrod probe controlled
using holographic optical tweezers. The probe is raster scanned over a surface, allowing an image
to be recorded in a manner analogous to scanning probe microscopy (SPM), with closed loop
feedback control provided by high-speed CMOS camera image tracking. We demonstrate a proof of
principle of this technique by imaging the surface of an oil droplet. We estimate the normal force
exerted on the sample during imaging to be 1 pN. The resolution is limited by the diameter of
the microrod tip, thermal motion of the probe and the tracking accuracy. As our technique is not
diraction limited, there is scope for signicant improvement by reducing the tip diameter, and
position clamping to reduce unwanted thermal motion.
Holographic optical tweezers are fast becoming an established tool in micro- and nano-science. We present
developments in both the interface and the underlying technology as used in our lab, along with some of the
uses we have put them to. Real time particle tracking from stereoscopic video images enables us to determine
position to nanometre precision in 3D. This can be performed in real time at several kHz, allowing us to positionclamp
trapped objects in 3D and measure positions and forces with optically controlled tools. Our multi-touch
iPad interface gives interactive control over multiple traps at the same time, which opens new possibilities for
controlling tools, structures and other dynamic processes.
The force field experienced by a sphere, trapped in a tightly focused Gaussian beam, is approximately conservative for
small displacements. For lower symmetry systems, this is not generally the case. Even when very tightly trapped, a
particle in such a system displays the effects of the non-conservative force field to which it is exposed. It does not come to
thermal equilibrium, but reaches a steady state in which its stochastic motion is subject to a deterministic, cyclic bias. Here,
we examine the dynamics of such a system, and show that the non-conservative nature of the force field manifests itself in
both the covariance and the spectral density of the generalized coordinates of the particle. In addition, we show that the
coupling between different types of thermal motion of such particles, i.e. rotational and translational, is asymmetric, which
leads to the periodic bias to the motion. These points are illustrated through computational simulations of the Brownian
dynamics of a trapped silica disk.
A Multitouch screen is an obvious choice for a holographic optical tweezers interface, allowing multiple optical traps to be
controlled in real-time. In this paper we describe the user interface used for our original multitouch system and demonstrate
that, for the user tasks performed, the multitouch performs better than a simple point-and-click interface.
Holographic optical tweezers are used to assemble and control probes made from high aspect-ratio CdS and SiO2 nanorods
and SiO2 microspheres. Analysis of the probe position allows for the measurement of forces experienced by the tip in a manner
analogous to existing scanning probe microscopy (SPM) techniques.
Optical Tweezers have become a widespread tool in Cell Biology, microengineering and other fields requiring
delicate micromanipulation. But for those sensitive tasks, it remains difficult to handle objects without damaging
them. As the precision in position and force measurement increase, the richness of information cannot
be fully exploited with simple interfaces such as a mouse or a common joystick. For this reason, we propose
a haptic force-feedback optical tweezer command and a force-feedback system controlled by one hand. The
system combines accurate force measurement using a fast camera and the coupling of these measured forces
with a human operator. The overall transparency allows even the feeling of the Brownian motion.
CdS nanowires and silica microspheres are manipulated using optical traps into 1D and 2D structures. The bonding occurs through the use of biotin and streptavidin.
In suspensions containing microspheres and a sub-micron species, such as nanoparticles or a polymer, an attractive force
can result between the microspheres. This attraction arises due to an entropic interaction, often referred to as a depletion
force. In this work we demonstrate an application of the depletion force to the controlled assembly of crystalline templates
for the production of photonic band gap (PBG) materials. The method makes use of holographic optical tweezers to
assemble crystalline arrays of silica or polystyrene microspheres, in which depletion interactions are used to stabilise the
structures being built. In addition, we use the holographic optical tweezers to characterise the attraction between pairs of
microspheres in the system.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.