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Optical Tweezers
OPTICS IN 2008
Parallel and Real-time Trapping, Manipulating
and Characterizing Microscopic Specimens
Jesper Glückstad, Darwin Z. Palima, Jeppe S. Dam
and Ivan P.-Nielsen
(a)
Label
y axis
I
n the mesoscopic regime, very small
forces that result from light-matter
interaction are strong enough to significantly influence the motion of tiny
particles. Until just a few years ago,
virtually all laser manipulation schemes
were based on trapping particles inside a
single strongly focused beam and moving
them into a desired position by translating the laser focus. Now, two decades
later, a great deal of progress has been
achieved in optical trapping and manipulation, both in terms of applications
and technical developments.
Particularly, much more versatile and
general manipulation of particles and
cell colonies is now possible by using
specially tailored structures of light.1
Such light patterns have unprecedented
potential for manipulating mesoscopic
objects and have already been successfully
used to organize small particles, including microorganisms, in desired patterns
and to sort samples of particles according
to their size.2
Optical trapping and manipulation
of a plurality of micro-particles is now
viable using reconfigurable patterns of
optical fields.3 This opens up research
possibilities for many interdisciplinary
fields, particularly those with biomedical
relevance. With the advent of computeraddressable spatial light modulators, the
reconfigurability of light patterns that
can act as confining optical potential
landscapes is made even more feasible
with a great degree of interactive usercontrol.4
We invented the “all-optical biophotonics workstation” to trap, manipulate
and characterize microscopic specimens
in parallel. We used an optical mapping
from a beam-modulation module to
obtain reconfigurable intensity patterns,
corresponding to two independently
addressable regions relayed to the sample
volume, where the optical manipula-
x axis
The BioPhotonics Workstation and examples of real-time 3D experiments.
tion of a plurality of micro-objects takes
place. The generated array of counterpropagating trapping-beams is easily
aligned using a computer-guided alignment procedure.5
The spatial addressing of the expanded
laser source is done in real-time through
a high-speed computer-controlled spatial
light modulator that is integrated in the
beam modulation module. Through a
computer interface, the operator can
simply select, trap and move the desired objects with a mouse or joystick.
Once the object is trapped, one can also
manipulate them using arbitrary motion
patterns that can be programmed for the
micro-objects and orchestrate complicated moving patterns of many independent
samples.
The fluid-borne microscopic particles
can be ushered in through a rectangular cuvette, where they are trapped
and steered in three dimensions using
the real-time reconfigurable matrix of
counter-propagating structured laser
beams. The counter-propagating geom-
etry currently generates up to 100 powerful optical traps using well-separated
objectives; this eliminates the need for
the high-numerical-aperture oil immersion objectives that are required with
conventional optical tweezers. It also
generates a large field of view and leaves
vital space for integrating other enabling
tools for probing the trapped particles,
such as linear and nonlinear microscopy
or micro-spectroscopy. t
Jesper Glückstad (jesper.gluckstad@fotonik.dtu.dk),
Darwin Z. Palima, Jeppe S. Dam and Ivan P.-Nielsen
are with the department of photonics engineering at
DTU Fotonik, Roskilde, Denmark.
References
1. J. Glückstad. “Sorting particles with light,” Nature Materials
3, 9-10 (2004).
2. J. Glückstad et al. “Optical 3D manipulation and observation in real-time (invited paper),” J. Robotics Mechatronics
18(6), 692-7 (2006).
3. P.J. Rodrigo et al. “2D optical manipulation and assembly
of shape-complementary planar microstructures,” Opt.
Express 15, 9009-14 (2007).
4. I. Perch-Nielsen et al. “Autonomous and 3D real-time
multi-beam manipulation in a microfluidic environment,”
Opt. Express 14, 12199-205 (2006).
5. J.S. Dam et al. “Three-dimensional imaging in three-dimensional optical multi-beam micromanipulation,” Opt. Express
16, 7244-50 (2008).
OPN December 2008 | 41
Optical Tweezers
Single-Fiber Optical Tweezers for
Cellular Micro-Manipulation
Samarendra K. Mohanty, Khyati S. Mohanty
and Michael W. Berns
T
he short working distance of microscope objectives has severely restricted the application of optical tweezers
and scissors at large depths. Therefore,
researchers are paying more and more attention to the use of optical fiber for this
purpose. Recently, in-depth single fiber
optic trapping of low- and high-index
particles has been demonstrated using
micro-axicon-tip fibers.1,2
The shape of the cone angle at the
axicon’s tip enabled fiber-optic trapping in the near-field.2 Further, we have
demonstrated controlled guidance of
neuronal growth cones as well as the
trapping and stretching of neurons using
fiber-optic tweezers.3 The cells could be
stretched3 by the combined action of two
forces—an attractive gradient force due
to fiber-optic tweezers at high beam powers pulling the membrane and a scattering force on the membrane as reported in
dual-fiber trapping.
We also observed alignment of intracellular dark (high refractive index)
material along the direction of laser
beam propagation.3 By mode-locking,
the beam of the fiber-optic tweezers was
converted to fiber-optic scissors, enabling
the dissection of neuronal processes.3
This microscopic-controlled nanodissection of neurons followed by a
process of resealing and repair could
serve as a useful tool for basic and applied studies on neuronal damage, repair
and regeneration. When the femtosecond
fiber-optic microbeam was at reduced
average power, we could microinject impermeable exogenous materials into the
trapped cells. At high average powers, we
accomplished lysis of a three dimensionally trapped cell.3
In the figure, we show optical trapping as well as lysis of biological cells
using a single axicon tip fiber. The cell,
distant from the fiber tip (marked by arrow in a) is attracted toward the fiber tip
42 | OPN December 2008
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Trapping, transport and lysis of a biological cell using axicon-tip single-fiber tweezers and
scissors. All images are in the same magnification. Scale bar: 10 mm.
(b) at a power of 95 mW and was stably
trapped very close to the axicon tip (c).
The trapped cell could be transported
to a new location (d, e) by maneuvering
the fiber tip. Switching the laser beam on
and off alternatively allowed the cell to
move close (g) or away (f ) from the fiber
tip, ruling out the possibility of nonoptical attraction between the cell and
the fiber.
By mode-locking the near infrared
laser beam, we could deliver femtosecond
pulses (about 200 fs, 76 MHz), and the
same fiber probe could be used for lysis
of the trapped cells (h, i) in a timescale
of 600±200 ms. This feature is required
in many assays to terminate biochemical
reactions immediately, thus preventing
measurement artifacts.
The non-invasive micro-axicon-tipped
optical fiber can also be used in multifunctional mode for in-depth trapping,
stretching, rotation, sorting, microinjec-
tion and ablation as well as for exciting
fluorophores. The depth attainable by
optical micromanipulation is enhanced
by a single microfabricated fiber device.
Moreover, this technology could lead to
sophisticated sensing and imaging capabilities that can be applied to live cells.4 t
Samarendra K. Mohanty (smohanty@uci.edu), Khyati
S. Mohanty and Michael W. Berns are with the Beckman Laser Institute, University of California-Irvine,
Irvine, Calif., U.S.A. K.S. Mohanty and M.W. Berns
are also affiliated with the department of biomedical
engineering at the University of California-Irvine in
Irvine, Calif., U.S.A.
References
1. K.S. Mohanty et al. “In depth fiber optic trapping of lowindex microscopic objects,” Appl. Phys. Lett. 92, 151113
(2008).
2. S.K. Mohanty et al. “Organization of microscale objects
using a microfabricated optical fiber tip,” Opt. Lett. 33,
2155-7 (2008).
3. S.K. Mohanty et al. J. Biomed. Opt. 13, AIP ID code:
046805JBO (2008).
4. Y. Verma et al. “Tapered single mode fiber tip high lateral
resolution optical coherence tomography,” Las. Phys. Lett.
4, 686-9 (2007).