Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED APPARATUS, SYSTEM AND METHOD
FOR APPLYING OPTICAL GRADIENT FORCES
BACKGROUND OF THE INVENTION
Throughout this application various publications are referenced. The
disclosures of
these publications in their entireties are hereby incorporated by reference in
this application
in order to more fully describe the state of the art to which this invention
pertains.
1. Field of the Invention
The present invention relates generally to optical traps. In particular, the
invention
relates to an apparatus, system and method for applying optical gradient
forces to form a
plurality of optical traps to manipulate small particles.
2. Discussion of the Related Arts
An optical tweezer is an optical tool which utilizes the gradient forces of a
focused
beam of light to manipulate particles with dielectric constants higher than
the surrounding
media. To minimize its energy such particles will move to the region where the
electric
field is the highest. Stated in terms of momentum, the focused beam of light
produces
radiation pressure, creating small forces by absorption, reflection,
diffraction or refraction
of the light by a particle. The forces generated by radiation pressure are
almost negligible--
a light source, such as a diode-pumped Nd:YAG laser operating at lOmW, will
only
produce a few picoNewtons. However, a few picoNewtons of force is sufficient
to
manipulate small particles.
Other optical tools which can be used to manipulate small particles include,
but are
not limited to, optical vortices, optical bottles, optical rotators and light
cages. An optical
vortex, although similar in use to an optical tweezer, operates on an opposite
principle.
An optical vortex produces a gradient surrounding an area of zero electric f
eld which is
useful to manipulate particles with dielectric constants lower than the
surrounding media or
which are reflective, or other types of particles which are repelled by an
optical tweezer. To
minimize its energy such a particle will move to the region where the electric
field is the
lowest, namely the zero electric field area at the focal point of an
appropriately shaped laser
beam. The optical vortex provides an area of zero electric field much like the
hole in a
doughnut (toroid). The optical gradient is radial with the highest electric
field at the
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circumference of the doughnut. The optical vortex detains a small particle
within the hole
of the doughnut. The detention is accomplished by slipping the vortex over the
small
particle along the line of zero electric field.
The optical bottle differs from an optical vortex in that it has a zero
electric field
only at the focus and a non-zero electric field at an end of the vortex. An
optical bottle may
be useful in trapping atoms and nanoclusters which may be too small or too
absorptive to
trap with an optical vortex or optical tweezers. J. Arlt and M.J. Padgett.
"Generation of a
beam with a dark focus surrounded by regions of higher intensity: The optical
bottle beam,"
Opt. Lett. 25, 191-193, 2000.
The optical rotator is a recently described optical tool which provides a
pattern of
spiral arms which trap objects. Changing the pattern causes the trapped
objects to rotate. L.
Patersoh, M.P. MacDouald, J. Arlt, W. Sibbett, P.E. Brya~zt, and K. Dholakia,
"Controlled
rotation of optically trapped microscopic particles," Science 292, 912-914,
2001. This class
of tool may be useful for manipulating non-spherical particles and driving
MEMs devices or
nano-machinery.
The light cage, described by Neal in U.S. Patent No. 5,939,716, is loosely, a
macroscopic cousin of the optical vortex. A light cage forms a ring of optical
vortices to
surround a particle too large, too reflective, or with dielectric constants
lower than the
surrounding media. If the optical vortex is like a doughnut, the light cage is
like a jelly-
filled doughnut. While the doughnut hole (for the vortex) is an area of zero
electric field,
the jelly-fill is an area of lowered electric field. In a gross sense, the
gradient forces of the
plurality of optical tweezers forming the doughnut "push" a particle, with a
dielectric
constant lower than the surrounding medium, towards the jelly-fill which may
also be
thought of as the less bright region which lies between the plurality of
optical tweezers.
However, unlike a vortex, no-zero electric field area is created.
Using a single beam of laser light with a diffractive optical element to form
a
plurality of diffracted laser beams focused to form an array of optical traps
is known in the
art. U.S. Patent No. 6,055,106 issued to Grier and Dufreshe describes arrays
of optical
traps. The patent teaches the use of physical transfer lenses to direct a
diffracted laser beam
to the back aperture of a focusing lens. Multiple physical lenses are employed
to direct and
overlay the laser beams at the back aperture of a focusing lens with
sufficient overlap to
achieve an effective numerical aperture (NA) of at least about 0.8, which, as
taught in U.S.
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Patent No. 5,079,169 issued to Chu and Kronis, is considered the minimum NA
necessary
to trap and manipulate particles in three dimensions. It is a drawback of the
apparatus
described in U.S. Patent No. 6,055,106 that each lens requires a relatively
large amount of
physical space to operate within and each lens must be maintained, cleaned and
aligned.
Those familiar with transfer lens systems will recognize that the greater the
number of
lenses in the system the more opportunity for misalignment and other
maintenance
problems. Accordingly, there has existed a need to reduce the number of lenses
in a transfer
lens system used for forming an array of optical traps. The present invention
satisfies this
need.
A common modality for monitoring the activity of an optical trap described in
U.S.
Patent No. 6,055,106 is to place a beam splitter in the pathway of the laser
beam and
thereby yield an optical data-stream. One limitation of this modality is the
detrimental
effect on the optical data stream of noise. In the context of optical traps,
noise refers to the
interference with the imaging, measuring and/or viewing of the optical traps,
their contents,
or the surrounding region resulting from the presence in the system of un-
diffracted focused
beam of light or energy, light emanating from the optical traps, and light
reflected or
diffracted off a lens in a physical transfer lens system either due to
imperfections in the lens,
dust, dirt or due to misalignment. As taught in U.S. Patent No. 6,055,106, one
way to
reduce noise is to direct the laser beam at an oblique angle relative to the
diffractive element
thereby urging the un-diffracted beam away from the objective lens. While
useful for its
intended purposes, the other sources of noise remain. There exists a need for
reduction or
elimination of the noise caused by the un-diffracted laser beam, scattered and
reflected laser
light off the components of a system producing an array of optical traps. The
present
invention also satisfies this need and other needs and provides related
advantages.
SUMMARY OF THE INVENTION
The present invention provides a novel and improved method, system and
apparatus
for generating, monitoring and controlling optical trap arrays employing but a
single
physical transfer lens. The within invention also improves monitoring and
control of optical
traps by filtering out, or shuttering off, the "noise" caused by scattered, un-
diffracted and
reflected light within the system.
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Multiple transfer lenses are eliminated or reduced, in a system producing a
plurality
of optical traps, by encoding a lens function into a diffractive optical
element. The
diffractive optical element may also alter the phase of any of the beams. By
encoding the
diffractive optical element to converge the plurality of beams, the invention
creates
conditions favorable to use a single physical lens to transfer and overlay the
plurality of
beams at the back aperture of the objective lens and to form optical traps
there-through.
In a basic form, the invention (FIG. 1A) is a focused beam of light or energy,
such
as single laser beam diffracted by a diffractive optical element which has an
encoded lens
function. The laser beam is diffracted into a plurality of beams and each beam
is also
converged by the diffractive optical element then directed to a single
transfer lens which in
turn directs and overlaps the plurality of beams at the back aperture of a
focusing lens (such
as the objective lens of a microscope) thereby forming a plurality of optical
traps. To alter
the position of a given optical trap, the beam forming that trap may be
steered to a new
position, via the diffractive optical element, thereby altering the position
of the optical trap
resulting therefrom. A moving mirror may be added (FIGS. 1D, 1E, 2 and 4) to
simultaneously alter the position of all the optical traps as a unit. In some
cases, movement
of the single transfer lens may also be desirable to alter the position of the
given optical
trap.
The selective generation and control of the array of optical traps with a
single lens
transfer system may be useful in a variety of commercial applications, such
as, optical
circuit design and manufacturing, nanocomposite material construction,
fabrication of
electronic components, opto-electronics, chemical and biological sensor
arrays, assembly of
holographic data storage matrices, energy source or optical motor to drive
MEMS,
facilitation of combinatorial chemistry, promotion of colloidal self assembly,
manipulation
of biological materials, interrogating biological material, concentrating
selected biological
material, investigating the nature of biological material, and examining
biological material.
In some embodiments of the invention (FIGS. 2 and 4) real time viewing of the
activity of the optical trap array is enabled by placing a beam splitter in
the path of the
beams prior to the focusing lens and then introducing a filter to limit the
passage of un-
diffracted, scattered or reflected light along the optical data stream thus
reducing this noise
which can disrupt video or other monitoring of the optical data stream. A
moving mirror,
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useful to adjust, the position of the whole array of optical traps, may also
be combined with
the beam splitter (FIGS. 2 and 3) or added to the system (FIG. 4).
Noise reduction may also be achieved by periodically shuttering offthe laser
light
(FIG. 3) and monitoring the optical data stream, and/or by shuttering off the
optical data
stream when the laser is on.
Other features and advantages of the present invention will be set forth, in
part, in
the descriptions which follow and the accompanying drawings, wherein the
preferred
embodiments of the present invention are described and shown, and in part will
become
apparent to those skilled in the art upon examination of the following
detailed description
taken in conjunction with the accompanying drawings, or may be learned by
practice of the
present invention. The advantages of the present invention may be realized and
attained by
means of the instrumentalities and combinations particularly pointed out in
the appendent
claims.
DESCRIPTION OF THE DRAWINGS
FIG. 1A illustrates a system for manipulating an array of small particles.
FIG. 1 B illustrates a first alternate system for manipulating an array of
small
particles.
FIG. 1 C illustrates a second alternate system for manipulating an array of
small
particles with a reflective diffractive optical element.
FIG. 1D illustrates a third alternate system for manipulating an axray of
small
particles with a movable mirror.
FIG. 1 E illustrates a fourth alternate system for manipulating an array of
small
particles with a movable mirror.
FIG. 2 illustrates a fifth alternate system for manipulating an array of small
particles
adapted for real time and noise free viewing.
FIG. 3 illustrates a sixth alternate system for manipulating an array of small
particles
adapted for real time and noise free viewing.
FIG. 4 illustrates a seventh alternate system for manipulating an array of
small
particles adapted for real time viewing.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Certain terminology will be used in the following specification, for
convenience and
reference and not as a limitation, brief definitions are provided below:
A. "Beamlet" refers to a sub-beam of focused light or other source of energy
that is
generated by directing a focused beam of light or other source of energy, such
as that
produced by a laser or collimated output from a light emitting diode, through
a media which
diffracts it into two or more sub-beams. An example of a beamlet would be a
higher order
laser beam diffracted off of a grating.
B. "Phase profile " refers to the phase of light or other source of energy in
a cross-
section of a beam.
C. "Phase patterning" refers to imparting a patterned phase shift to a focused
beam
of light, other source of energy or beamlet which alters its phase profile,
including, but not
limited to, phase modulation, mode forming, splitting, converging, diverging,
shaping and
otherwise steering a focused beam of light, other source of energy or a
beamlet.
Various embodiments of the inventive apparatus for forming a plurality of
movable
optical traps, generally designated as 8, are shown in FIGS. 1A, 1B and 1C. In
the
embodiment shown in FIG. 1A, a movable array of optical traps is formed by
generating a
focused beam of energy, such as electromagnetic wave energy. In preferred
embodiments,
the electromagnetic waves are light waves, preferably having a wavelength of
from about
400 nm to about 1060 nm, and more preferably having a wavelength in the green
spectrum.
The beam is formed of a collimated light, such as the collimated output from a
light
emitting electrode or, preferably, a laser beam 10, as shown in FIGS. lA-E.
The focused beam of light is directed along optical axis 500 through a phase
patterning optical element having a variable optical surface, such as a
diffractive optical
element 12 having a variable optical surface disposed substantially in a plane
conjugate to a
planar surface 15 of a back aperture 16 of a focusing lens, such as an
objective lens 18, to
produce a plurality of beamlets 32 and 33 (two shown) having a selected phase
profile.
Varying the optical surface of the diffractive optical element alters the
beamlets.
Encoded in the diffractive optical element 12 is a virtual lens that converges
the
plurality of beamlets at a position between the encoded diffractive optical
lens and a single
transfer lens. The beamlets emanating from the encoded diffractive optical
element, after
converging, are directed thorough a transfer lens so as to overlap the
beamlets at the back
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aperture of a focusing lens, such as an objective lens objective lens 18. The
beamlets are
then converged by the focusing lens to form a plurality of optical traps 1002
and 1004 in
working focal region 2000. The working focal region 2000 is that area where a
media
containing particles 3000 or other material 3002 to be examined, measured or
manipulated
by the optical traps 1002 and 1004 is placed.
Any suitable laser can be used as the source of the laser beam 10. Useful
lasers
include solid state lasers, diode pumped lasers, gas lasers, dye lasers,
alexanderite lasers,
free electron lasers, VCSEL lasers, diode lasers, Ti- Sapphire lasers, doped
YAG lasers,
doped YLF lasers, diode pumped YAG lasers, and flash lamp-pumped YAG lasers.
Diode-
pumped Nd:YAG lasers operating between 10 mW and 5 W are preferred.
When the focused beam of light 10 is directed through the encoded diffractive
optical element 12, the encoded diffractive optical element produces a
plurality of diffracted
beamlets 32 and 33 having an altered phase profile. Depending on the type of
optical trap
desired, the alteration may include wavefront shaping, phase shifting,
steering, diverging
and converging to form different classes of optical traps including optical
tweezers, optical
vortices, optical bottles, optical rotators, light cages, and combinations of
the different
classes. For clarity, only two diffracted beamlets and two corresponding
optical tweezers
1002 and 1004 are shown, but it should be understood that an array of such
beamlets are
created by the encoded diffractive optical element.
The location of each trap is selectively controlled by the encoded diffractive
optical
element. It is a significant feature of the invention that movement of each
trap, be it rotation
in a fixed position, rotation in a non-fixed position, two-dimensional and
three dimensional,
continuous and stepped is selectively controllable. The control is achieved by
varying the
surface of the diffractive optical element through which the beam passes,
thereby altering
the position of convergence of the beamlets emanating from the encoded
diffractive optical
element.
Suitable diffractive optical elements are characterized as transmissive or
reflective
depending on how they direct the focused beam of light. Transmissive
diffractive optical
elements, as shown in FIGS. 1A and 1B, focus the beam of light, while
reflective diffractive
optical elements, as shown in FIG. 1C, reflect the beam.
Within the two general groups, a diffractive optical element can be
categorized as
being formed from either static or dynamic media. Examples of suitable static
difFractive
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optical elements include diffractive optical elements with a fixed surface,
such as gratings,
including diffraction gratings, reflective gratings, transmissive gratings,
holograms, stencils,
light shaping holographic filters, polychromatic holograms, lenses, mirrors,
prisms,
waveplates and the like.
The static difFractive optical element may have different regions, each region
configured to impart a different phase profile to the beamlets. In such
embodiments, the
surface of the static diffractive optical element can be varied by moving the
surface relative
to the laser beam 10 to select the appropriate region to change the desired
characteristics
imparted to the beamlets, i.e., to change the desired phase profile of at
least one of the
resulting beamlets. In some embodiments, the static surface contains two or
more discreet
non-homogeneous regions. In alternative embodiments, the static surface is
substantially
continuously varying.
Examples of suitable dynamic diffractive optical elements having a time
dependent
aspect to their function include variable computer generated diffractive
patterns, variable
phase shifting materials, variable liquid crystal phase shifting arrays, micro-
mirror arrays,
piston mode micro-mirror arrays, spatial light modulators, electro-optic
deflectors,
accousto-optic modulators, deformable mirrors, reflective MEMS arrays and the
like. With
a dynamic diffractive optical element, the features of the encoded surface can
be altered, for
example, by a computer, to effect a change in the number of beamlets, the
phase profile of
at least one of the beamlets, and the location of at least one of the
beamlets.
The virtual lens encoded on the diffractive optical element alters the phase
of light
incident on the optical element. A representative virtual lens is a pattern
similar to a Fresnel
lens encoded, for example, in the orientation of a reflective grating or
nematic liquid
crystals. The virtual lens is distinguishable from a physical lens, which
affects all the
beamlets 32 and 33 as a whole, as the virtual element can alter the relative
position of each
beamlet 32 and 33 independently.
The diffractive optical element is also useful to impart a particular
topological mode
to the laser light. Accordingly, one beamlet 32 may be formed in a Guass-
Laguerre mode
while another beamlet 33 is formed in a Guassian mode.
Preferred virtual lens-encoded diffractive optical elements include phase-only
spatial
light modulators such as the "PAL-SLM series X7665, manufactured by Hamanmatsu
of
Japan or the "SLM 512SA7" manufactured by Boulder Nonlinear Systems of
Layfette,
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Colorado. These encoded diffractive optical elements are computer controllable
and
multifunctional, so that they can generate the beamlets 32 and 33 by
diffracting the laser
beam 10 and selectively impart desired characteristic to the resulting
beamlets.
Each of the diffracted beamlets emanates from an area A on the front surface
13 of
the encoded diffractive optical element and each must also pass through an
area B, on the
back aperture 16, thereby the beamlets are overlapping at the back aperture 16
of the
objective lens 18. In the embodiment shown in FIG. 1, near precise overlapping
is
efficiently accomplished by combining the virtual lens encoded diffraction
optical element
with a single, movable downstream optical lens Ll.
The laser beam 10 preferably has a beam diameter w which substantially
coincides
with the diameter of the back aperture 16 and it is an advantage of the
inventive system that
there is little or no overfill of the back aperture 17 of the objective lens
18 both conserving
the intensity of the laser beam 10 and preserving the strength of the
electrical field gradient
creating effective optical traps 1002 and 1004 in the desired pattern within
the working
focal region 2000.
Stated in mathematical terms: If the effective NA to establish gradient forces
sufficient to form an optical trap is 0.8 and if the effective NA is
calculated from the
formula NA =n* Sin ~/2, wherein n represents the index of refraction for the
medium
outside of the objective lens and ~ is the angle of convergence for the
diffracted beams and
if an oil immersion objective lens having an index of refraction as high as
1.5 is used, then
to maintain an effective NA of 0.8, ~ should be maintained at least 66 degrees
during
movement of the optical trap in order to form and maintain an optical trap
effective to
manipulate particles in three dimensions.
Alternatively, it is possible to form an effective optical trap without
filling the back
aperture 16, if an oversized focusing lens having an oversized back aperture,
relative to the
laser beam 10, is used. However, such a lens would require greater physical
space and may
be more costly.
Turning to the alternative embodiment shown in FIG. 1B, the controllable array
of
optical traps is formed by passing the laser beam 10 through a diffractive
optical element 12
encoded with a virtual lens that is disposed substantially in a plane 14'
forming an acute
angle 13 relative to the optical axis 500. In this embodiment, beamlets 32 and
33 emanating
from area A on the front surface of the encoded diffractive optical element
are directed by
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the diffractive optical element so as to pass through area B on the back
aperture 16 of the
objective lens 18 and form optical traps 1002 and 1004 in the working focal
region 2000.
By altering the position of the laser beam 10 relative to the optical axis
500, a portion of the
un-diffracted light 34 is removed which in turn reduces the noise caused by
the un-
diffracted light 34 thereby improving efficiency and effectiveness of forming
optical traps
1002 and 1004. Additionally, non-movable optical traps (not shown) which may
form from
an un-diffracted portion of the laser beam when the laser beam is directed
along the optical
axis as shown in FIGS. 1A and 1C is eliminated.
FIG. 1 C illustrates an alternative embodiment where the controllable array of
optical
traps is formed by reflecting the laser beam 10 off a diffractive optical
element 12" having
an encoded virtual lens.
FIGS. 1D and 1E show alternative embodiments having a movable mirror 41 for
steering the beamlets emanating from the phase patterning optical element as a
group prior
to overlapping the beamlets at the back aperture of the focusing lens. The
movable mirror
41 is disposed upstream of the transfer lens Ll with its center of rotation at
area C. The
representative beamlet 32 passes from area A on the front surface 13 of the
encoded
diffractive optical element 12 through the transfer lens L1 and on to area C
which reflects it
to area B at the back aperture 16. Tilting the movable mirror 41 effects a
change of the
angle of incidence of the beamlet 32 relative to the mirror 41 and can be used
to translate
the array of optical traps 1002 and 1004.
This movable mirror is useful for both precisely aligning the optical trap
array
within a stationary substrate, to dynamically stiffen the optical traps
through small-
amplitude rapid oscillatory displacements, and to effectively increase the
trapping action by
precisely altering positions of the array of optical traps while pulsing the
optical traps to
form two or more alternating sets of optical traps from the same number of
beamlets.
The embodiment shown in FIG. 1D minimizes beamlet misalignments by including
a conventional telescope system 42 disposed between the movable mirror 41 and
the
objective lens 18. The telescope system is constructed of two lenses L2 and L3
placed
between conjugate planes 43 and 45, such that the beamlets pass from area A on
the front
surface 13 of the encoded diffractive optical element 12 on to the center of
rotation of beam
splitter 51 at area C in plane 43, and then through area B on the back
aperture 16 of the
objective lens 18 in plane 44. In the embodiment shown in FIG. 1E, the movable
mirror 41
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is placed in close proximity to the back aperture 16 in order to minimize
beamlet
misalignments.
Using the embodiment of the invention shown in FIGS. 2 and 3, real time
viewing
of an optical data stream of the optical traps 1002 and 1004 interacting with
small particles
3000 within the working focal region 2000 is possible. The movable array of
optical traps
1002 and 1004 are formed using a single lens transfer optic L1. Only one
beamlet 32 is
shown for clarity, but it should be understood that a plurality of such
beamlets are created
by the optical element 12.
To produce the array of optical traps, the laser beam 10 passes through the
diffractive optical element 12 to produce the beamlet 32 which emanates from
area A on the
front surface 13 of the encoded diffractive optical element 12 then passes on
to area C.
Area C is the center region on the surface of a beam splitter 51 located
before the objective
lens 18. The beam splitter 51 is constructed of a static or a movable dichroic
mirror, a static
or a movable photonic band gap mirror, a static or a movable omnidirectional
mirror, or
other similar device. The beam splitter shown in FIG. 2 is movable and,
therefore, serves
the dual function of movable mirror and beam splitter. In the alternative
embodiment
shown in FIG. 3, the beam splitter 51 is fixed.
The beam splitter 51 selectively reflects the wavelength of light used to form
the
optical traps and transmits other wavelengths to form two streams of beamlets.
Thus, as
seen in FIGS. 2 and 3, the first stream of beamlets proceeds from area C
through area B at
the back aperture 16 of the objective lens 18 thereby effectively overlapping
all the
beamlets at the back aperture and forming the optical traps 1002 and 1004. The
second
stream of beamlets is reflected by the beam splitter 51 to a monitor and used
to provide a
real time optical data stream, aided by an imaging illumination source (not
shown). The
second stream of beamlets passes through the apparatus 8 to be visually
inspected 64a by a
human monitor 65. The monitor 65 may also interface with a computer 66 and
cause the
computer to effect changes in the parameters of the system to alter the
position of the
position of one or all of the beamlets 32.
Alternatively, a spectrum 64b of the optical data stream can be obtained, then
analyzed and/or the optical data stream can be converted into a video signal
and monitored
with a video monitor 64c. In some embodiments, the optical data stream is
passed through
a spectrometer and the position of the convergence of at least one beamlet is
then altered in
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response to an analysis of the spectrum or the video monitoring to change the
location of
the corresponding optical trap(s).
Spectroscopy 64b of a sample of biological material can be accomplished with
an
imaging illumination suitable for either inelastic spectroscopy or polarized
light back
scattering, the former being useful for assessing chemical structure and the
later being
suited for measuring nucleus size. The computer 66 analyzes the data to
identify suspected
cancerous, pre-cancerous and/or non-cancerous cells, and directs the optical
array to
segregate and concentrate a sample of the selected cell types. Those skilled
in the art will
recognize that the methodology used to concentrate cells based on parameters
specific to
cancerous cells may be altered without departing from the scope of the
invention to identify
and/or concentrate other types of cells based on other parameters. The
wavelengths of the
laser beam 10 used to form optical traps useful for manipulating biological
material include
the infrared, near infrared and visible wavelengths from about 400mn to about
1060nm
In still other embodiments, the optical data stream may be received by a
computer
66 adapted to record the optical data stream, analyze the optical data stream,
and/or
precisely adjust the position of one or all of the beamlet(s) 32 via the
diffractive optical
element 13, the position of the single transfer lens L1, and/or the position
of the movable
beam splitter 51. Alternatively, the optical data stream may be processed by a
photodectector to monitor intensity, or any suitable device to convert the
optical data stream
to a digital data stream adapted for use by a computer 66.
The real time optical data stream provides more useful information if the
noise is
controlled. As seen in FIG. 2, a filter element 53, such as a polarizing
element or band pass
element, is placed within the pathway of the optical data stream to reduce the
amount of
reflected, scattered or un-diffracted laser light 10 passing along the axis of
the optical data
stream. The filter element 53 filters out one or more preselected wavelengths
and, in some
embodiments, all but a preselected wavelength of the optical data stream.
Another method of limiting the noise in the optical data stream is to shutter,
or
pulse, the optical data stream. FIG. 3 illustrates a system of controllable
shutters 62 and 63.
One advantage of shutters is to eliminate substantially all of the noise or
interference from
the optical data stream. The shutter 62 selectively blocks and unblocks the
optical data
stream from passing freely from the system by coordinating its opening with
the turning on
and off of the laser beam 10. When the laser beam is not being generated, the
optical data
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stream is blocked and when focused beam of light is being generated, the
optical data
stream is unblocked. To maintain control of the small particles (not shown)
within the
optical traps when shuttering the laser beam 10 (which in-turn will cause the
beamlet(s) 32
and the resulting optical traps) 1002 to switch "on" and "off') the pulse rate
of the shutter
is adjusted dependent on the nature of the particles being manipulated.
However, too slow a
pulse rate will allow the trapped particles to drift. For those situations in
which drift is
desired, the pulse rate may be adjusted to encourage drift.
Alternatively, the shutter 63 blocks the laser beam (not shown) or the
beamlets from
passing freely into the objective lens. By coordinating the opening and
closing of the
shutter 63 with the turning "on" and "off' of the monitoring of the optical
data stream noise
is reduced. In some instances, dual shuttering may be desirable. One advantage
of dual
shuttering is that both the laser beam 10 and the monitoring equipment can
remain "on" at
all times. In such an arrangement only the activity of the shutters 62 and 63
need be
coordinated. The computer 66 may be used to selectively control the shutters)
62 and 63.
Shown in FIG. 4 is an embodiment useful where the availability of equipment,
physical space limitations or other performance parameters will benefit from
using a
traditional telescope transfer lens system 42 placed in the system after the
single transfer
lens L1 combined with a moving mirror 41 and a beam splitter 51 to provide an
optical data
stream.
The apparatus 8 is useful as part of a system for manipulating a plurality of
small
particles. In addition to the apparatus, the system includes a light source
(not shown) fox
producing the focused beam of light, the focused beam of light 10, and a
plurality of small
particles 3000 that are manipulated by the optical traps 1002 and 1004.
The plurality of optical traps created in accordance with the invention. In
some
embodiments, the optical traps form the gradient conditions necessary to
manipulate
biological material.
Since certain changes may be made in the above apparatus without departing
from
the scope of the invention herein involved, it is intended that all matter
contained in the
above description, as shown in the accompanying drawing, the specification,
and the claims
shall be interpreted in an illustrative, and not a limiting sense.
