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Description  |
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TECHNICAL FIELD
This invention relates to trapping of particles using a single-beam
gradient force trap.
BACKGROUND OF THE INVENTION
Single-beam gradient force traps have been demonstrated for neutral atoms
and dielectric particles. Generally, the single-beam gradient force trap
consists only of a strongly focused laser beam having an approximately
Gaussian transverse intensity profile. In these traps, radiation pressure
scattering and gradient force components are combined to give a point of
stable equilibrium located close to the focus of the laser beam.
Scattering force is proportional to optical intensity and acts in the
direction of the incident laser light. Gradient force is proportional to
the optical intensity and points in the direction of the intensity
gradient.
Particles in a single-beam gradient force trap are confined transverse to
the laser beam axis by a radial component of the gradient force.
Stabilizing the particle along the axis direction of the trap is achieved
by strongly focusing the laser beam to have the axial component of
gradient force dominate the scattering force in the trap region.
In prior work using single-beam gradient force optical traps on dielectric
particles, trapping was demonstrated with a visible light laser source
(.lambda.=514.5 nm.) focused by a high numerical aperture lens objective.
See A. Ashkin et al., Optics Letters, Vol. 11, p 288-90. The dielectric
particles were closely spherical or spheroidal in shape and ranged in size
from 10 .mu.m diameter Mie glass spheres (.alpha.>>.lambda.) down to 260
Angstrom diameter Rayleigh particles (.alpha..ltoreq..ltoreq..lambda.).
Use of such regularly shaped particles in the Mie regime was desirable as
taught in this and other articles.
For Mie particles, both the magnitude and direction of the forces depend on
the particle shape. This restricts trapping to fairly simple shapes such
as spheres, ellipsoids, or particles whose optical scattering varies
slowly with orientation in the trap. In the Rayleigh regime, the particle
acts as a dipole and the direction of force is independent of particle
shape; only the magnitude of force varies with particle orientation.
It is not an insignificant result of the prior work that silica and other
dielectric particles experienced varying amounts of irreversible optical
damage from the trap. While it was suggested that the single-beam trap and
the prior results would be extensible to biological particles, the
resulting damage from exposure in the trap would destroy or significantly
incapacitate the biological particles and render them useless. Also, since
prior optical traps have been defined for quite regular-shaped, dielectric
particles, their extension to biological particles is cast in doubt
because regularity of shape is not an attribute of biological particles.
SUMMARY OF THE INVENTION
Biological particles are successfully trapped in a single-beam gradient
force optical trap incorporating an infrared light source. Reproduction of
trapped particles has been observed. After release from the trap,
particles exhibit normal motility and continued reproductivity even after
trapping for several life cycles at a high laser power of 160 mW.
In one embodiment, the higher numerical aperture lens objective in the
single-beam gradient force trap is used for simultaneous viewing of the
trapped biological particles.
Two single-beam gradient force optical traps are introduced into the same
cell to permit three- dimensional manipulation of the biological particles
.
BRIEF DESCRIPTION OF THE DRAWING
A more complete understanding of the invention may be obtained by reading
the following description of a specific illustrative embodiment of the
invention in conjunction with the appended drawing in which:
FIG. 1 is a cross-sectional schematic diagram of an embodiment of the
invention;
FIG. 2 is a cross-sectional schematic diagram of an embodiment of the
invention employing two single-beam gradient force traps in one cell; and
FIGS. 3 through 5 are schematic drawings of different modes of operation
for an optical trap on particles in a cell.
DETAILED DESCRIPTION
Single-beam gradient force optical traps are useful for confining,
isolating, translating and manipulating at least one particle in a group
of particles enclosed in a cell or hanging droplet or the like. Special
problems surface when the particles are biological. For example,
absorption of the optical energy in the trap by the confined particle may
lead to particle annihilation or a significant loss of particle motility.
Also, as the wavelength of the light beam is varied to avoid the
aforementioned problem, the intensity of the optical trap may be
sufficiently decreased so as to be rendered ineffective for the particles
of interest. While the wavelength selected may be sufficient for effective
operation of the optical trap, it may be at a wavelength which is absorbed
by the medium surrounding the particles and, therefore, which leads to
heat generation within the cell. Clearly, many factors must be considered
when selecting the operating wavelength for the optical trap.
In the prior optical trap experiments reports in the literature, particle
sensitivity has not been an issue. This is generally attributed to the
fact that dielectric particles have homogeneous compositions and uniformly
regular shapes so that it is straightforward to observe the effect of the
trap on one particle or portion of a particle and accurately predict the
effect on other particles or on other portions of the same dielectric
particle. For biological particles, sensitivity of the particles is
extremely important. Biological particles have heterogeneous compositions
and irregular shapes. Hence, the effect of the trap on one part of a
biological particle is in no way determinative of the effect in another
portion of the same particle.
FIG. 1 shows a cross-sectional schematic diagram of apparatus for creating
a single-beam gradient force optical trap in accordance with the
principles of this invention. IR laser 10 is a standard laser emitting a
coherent light beam substantially in the infrared range of wavelengths,
for example, 0.8 .mu.m to 1.8 .mu.m.
Light beam 11 from IR laser 10 impinges upon a combination of optics
elements for focusing the light beam with a sufficient degree of
convergence to form a single-beam gradient force optical trap for
confining biological particles at a desired position. The combination of
optics elements includes an adjustably mounted diverging lens 12 and a
high convergence lens 23.
Lens 12 is adjustable in any of three dimensions (x, y, z) by manipulating
adjustable mount 13. It is important that lens 12 expand that spot size of
light beam 11 to cover a substantial area on the surface of lens 23. As
shown in FIG. 1, diverging light beam 14 impinges on a large portion of
the facing surface of lens 23 so that relatively high intensity of beam 14
fills the aperture of lens 23. In order to create the forces required for
operation of the single-beam gradient force optical trap, it is desirable
that lens 23 be capable of focusing to a spot size less than .lambda.
approaching .lambda./2. In an example from experimental practice, lens 23
is a strong or high convergence water immersion microscope objective lens
having a numerical aperture of approximately 1.25 (measured in water).
wherein the numerical aperture is defined as the refractive index for the
medium multiplied by the sine of the half angle covered by the converging
light beam. Element 24 depicts the liquid (water or oil) in which lens 23
is immersed for improved optical coupling into cell 25.
The optical trap is shown within cell 25 with particle 27 captured in the
trap. Particle 27 is suspended in a liquid medium such as water, for
example, which is enclosed by cell 25. Cell 25 is a transparent enclosure
for enclosing the suspended biological particles or a transparent slide
from which particle containing droplets can be hung. In one example, cell
25 has dimensions of 1 cm..times.3 cm..times.100 .mu.m.
The position of cell 25 is adjustable in three dimensions (x, y, z) by the
use of adjustable mount 26. In practice, mount 26 is useful in locating
and manipulating the biological particles.
Viewing of biological particles in the trap is accomplished directly or
through the use of a monitor. While other types of viewing such as viewing
directly in cell 25 are possible, it is an added feature of the present
invention that the viewing is accomplished through the same lens objective
which simultaneously creates the optical trap.
Illumination for viewing is provided by visible light source 29 and is
projected through converging lens 28 onto the particles in the field of
view. High resolution viewing occurs with the aid of lens 23 through which
the visible light passes toward either the eyepiece 22 or the monitor 18.
For direct viewing, visible light shown as a dashed line is reflected from
beam splitter 19 to microscope eyepiece 21. Infrared blocking filter 22 is
placed in front of eyepiece 21 to isolate the viewing optics (viewer's
eye) from back reflections from cell 25. For monitoring, the visible light
passes through beam splitter 19 and is reflected from beam splitter 15
toward infrared blocking filter 17 and finally monitor 18. Infrared
blocking filter 17 isolates the monitor from back reflections from cell
25.
In FIG. 2, the apparatus shown in FIG. 1 is augmented by a second infrared
laser source and optics to create a second single-beam gradient force
optical trap in cell 25. Infrared laser source 30 generates light beam 31
impinging on adjustably mounted diverging lens 32. Lens 32 causes beam 31
to emerge in a diverging pattern as light beam 34. Adjustment of lens 32
is accomplished in three dimensions (x, y, z) via adjustable mount 33.
Light beam 34 is reflected by mirror 35 which coincidently permits
transmission of light beam 14. This would occur by judiciously choosing
different wavelengths of operation for the separate laser sources. On the
other hand, element 35 can be realized as a beam splitter which would
reflect approximately half of the light beam incident thereon and transmit
the remaining half. As shown in FIG. 2, light beam 34 is converged by lens
23 to form a second trap in cell 25. Particle 36 is confined in the second
trap.
While not shown, it should now be apparent to those skilled in the art that
a second trap may be created in the cell by utilizing an additional set of
optics including another high convergence microscope. The second trap may
be created from light entering the cell on the side opposite the beam for
the first trap or, for that matter, at any angle to the beam for the first
trap.
Manipulation or orientation of particles is achieved by grabbing each end
of a rod-like particle, for example, and moving it at will.
In operation, it is necessary to move the trapped biological particles into
the viewing plane. This is carried out by adjusting the position of the
diverging lens or lenses. Similarly, translation, separation or isolation
of the biological particles is easily affected by adjusting mount 26 by
the desired amount.
FIGS. 3 through 5 show several modes of operation for the same optical
trap. FIG. 3 shows the conventional mode of operation in which the focus
of the beam from lens 23 lies within cell 25 and the trapping action
relies on the backward gradient component of the optical force. Depending
on the size of the particles, it is possible to trap up to approximately
four or five particles within the trap at one time.
Both modes shown in FIGS. 4 and 5 require less intensity than for the trap
in FIG. 3. In FIG. 4, the bottom plate of cell 25 provides the backward
trapping force and the gradient provides the transverse trapping force. It
is possible to trap approximately twelve or more biological particles at
one time. In FIG. 5, the scattering force of the focused light beam
provides transverse confinement due to its inward direction; backward
trapping is supplied by the bottom plate of cell 25. In the latter mode of
operation, it is possible to trap significantly greater numbers of
particles than for the modes shown in FIGS. 3 and 4.
Various biological particles have been isolated, confined and transported
in this type of optical trap. For example, some biological particles
successful trapped are tobacco mosaic viruses (See Ashkin et al., Science,
Vol. 235, pp. 1517-20 (1987).), yeast, E. coli bacteria, blood cells
containing hemoglobbin, and complex cells or parts of cells containing
chlorophyll structures.
In general, the biological particles investigated do not have the regular
shape of the dielectric spheres studied earlier. For example, passive,
string-like organisms were trapped wherein the organism was approximately
50 .mu.m long and approximately 1 .mu.m in diameter. In the case of
tobacco mosaic virus, the particles resemble a cylinder about 200
angstroms in diameter and 3100 angstroms long.
It is a significant attribute of the present invention that particle
motility is preserved and reproductivity of the particles is maintained.
Reproduction by trapped biological particles has been observed with
offspring remaining in the trap. In other words, the optical trap permits
non-destructive manipulation of biological particles at optical powers
approaching several hundred milliwatts.
It should be noted that the use of infrared light results in a lower
intensity trap at the focal spot for the same laser power than for traps
using visible light. However, the forces in the trap are approximately
equal. Thus, the infrared trap has the added benefit over visible light
traps of inducing less local heating in the focal spot.
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