Extended and knotted optical traps in three dimensions

We describe a method for projecting holographic optical traps that are extended along arbitrary curves in three dimensions, and whose amplitude and phase profiles are specified independently. This approach can be used to create bright optical traps with knotted optical force fields. © 2011 Optical Society of America OCIS codes: (090.1760) Computer holography; (140.7010) Laser trapping References and links 1. Y. Roichman and D. G. Grier, “Projecting extended optical traps with shape-phase holography,” Opt. Lett. 31, 1675–1677 (2006). 2. Y. Roichman and D. G. Grier, “Three-dimensional holographic ring traps,” Proc. SPIE 6483, 64830F (2007). 3. Y. Roichman, B. Sun, Y. Roichman, J. Amato-Grill, and D. G. Grier, “Optical forces arising from phase gradients,” Phys. Rev. Lett. 100, 013602 (2008). 4. S.-H. Lee, Y. Roichman, and D. G. Grier, “Optical solenoid beams,” Opt. Express 18, 6988–6993 (2010). 5. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11, 288–290 (1986). 6. H. He, N. R. Heckenberg, and H. Rubinsztein-Dunlop, “Optical particle trapping with higher-order doughnut beams produced using high efficiency computer generated holograms,” J. Mod. Opt. 42, 217–223 (1995). 7. N. B. Simpson, L. Allen, and M. J. Padgett, “Optical tweezers and optical spanners with Laguerre-Gaussian modes,” J. Mod. Opt. 43, 2485–2491 (1996). 8. K. T. Gahagan and G. A. Swartzlander, “Optical vortex trapping of particles,” Opt. Lett. 21, 827–829 (1996). 9. K. Ladavac and D. G. Grier, “Microoptomechanical pump assembled and driven by holographic optical vortex arrays,” Opt. Express 12, 1144–1149 (2004). 10. K. Ladavac and D. G. Grier, “Colloidal hydrodynamic coupling in concentric optical vortices,” Europhys. Lett. 70, 548–554 (2005). 11. H. He, M. E. J. Friese, N. R. Heckenberg, and H. Rubinsztein-Dunlop, “Direct observation of transfer of angular momentum to absorptive particles from a laser beam with a phase singularity,” Phys. Rev. Lett. 75, 826–829 (1995). 12. J. W. Goodman, Introduction to Fourier Optics, 3rd ed. (McGraw-Hill, New York, 2005). 13. G. C. Sherman, “Application of the convolution theorem to Rayleigh’s integral formulas,” J. Opt. Soc. Am. 57, 546–547 (1967). 14. M. Reicherter, T. Haist, E. U. Wagemann, and H. J. Tiziani, “Optical particle trapping with computer-generated holograms written on a liquid-crystal display,” Opt. Lett. 24, 608–610 (1999). 15. J. Liesener, M. Reicherter, T. Haist, and H. J. Tiziani, “Multi-functional optical tweezers using computergenerated holograms,” Opt. Commun. 185, 77–82 (2000). 16. J. E. Curtis, B. A. Koss, and D. G. Grier, “Dynamic holographic optical tweezers,” Opt. Commun. 207, 169–175 (2002). 17. J. Leach, K. Wulff, G. Sinclair, P. Jordan, J. Courial, L. Thomson, G. Gibson, K. Karunwi, Z. J. Cooper, John an dLaczik, and M. Padgett, “Interactive approach to optical tweezers control,” Appl. Opt. 45, 897–903 (2006). 18. Y. Roichman, I. Cholis, and D. G. Grier, “Volumetric imaging of holographic optical traps,” Opt. Express 14, 10,907–10,912 (2006). 19. M. Polin, K. Ladavac, S.-H. Lee, Y. Roichman, and D. G. Grier, “Optimized holographic optical traps,” Opt. Express 13, 5831–5845 (2005). 20. J. C. Crocker and D. G. Grier, “Methods of digital video microscopy for colloidal studies,” J. Colloid Interface Sci. 179, 298–310 (1996). 21. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular-momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A 45, 8185–8189 (1992). 22. A. T. O’Neil, I. MacVicar, L. Allen, and M. J. Padgett, “Intrinsic and extrinsic nature of the orbital angular momentum of a light beam,” Phys. Rev. Lett. 88, 053601 (2002). 23. J. E. Curtis and D. G. Grier, “Structure of optical vortices,” Phys. Rev. Lett. 90, 133901 (2003). 24. S. Sundbeck, I. Gruzberg, and D. G. Grier, “Structure and scaling of helical modes of light,” Opt. Lett. 30, 477–479 (2005). 25. J. Leach, M. R. Dennis, J. Courtial, and M. J. Padgett, “Knotted threads of darkness,” Nature 432, 165 (2004). 26. M. R. Dennis, R. P. King, J. Barry, K. O’Holleran, and J. Padgett, Miles, “Isolated optical vortex knots,” Nature Phys. 6, 53–56 (2010). 27. W. T. M. Irvine and D. Bouwmeester, “Linked and knotted beams of light,” Nature Phys. 4, 716–720 (2008). 28. L. Faddeev and A. J. Niemi, “Stable knot-like structures in classical field theory,” Nature 387, 58–61 (1997). Extended optical traps are structured light fields whose intensity and phase gradients exert forces that confine microscopic objects to one-dimensional curves in three dimensions [1, 2, 3, 4]. Intensity-gradient forces typically are responsible for trapping [5] in the two transverse directions, while radiation pressure directed by phase gradients can move particles along the third [3, 4]. This combination of trapping and driving has been demonstrated dramatically in optical vortexes [6, 7, 8, 9, 10], ring-like optical traps that are created by focusing helical modes of light. Intensity gradients draw illuminated objects toward the ring, and phase gradients then drive them around [3, 7, 8, 11]. More recently, holographic methods have been introduced to design and project more general optical traps that are extended along lines [1], rings [2] and helices [4], with intensity and phase profiles independently specified along their lengths. Unlike optical vortexes, these traps feature nearly ideal axial intensity gradients because they are specifically designed to achieve diffraction-limited focusing [1, 2, 3, 4]. Here, we describe a method for designing and projecting optical traps whose intensity maxima trace out more general curves in three dimensions with independently specified phase and amplitude profiles. Within limitations set by Maxwell’s equations, these three-dimensional light fields can be used to trap and move microscopic objects. We demonstrate the technique by projecting diffraction-limited holographic ring traps with arbitrary orientations in three dimensions. More specifically, our goal is to project a beam of light that comes to a focus along a curve ~R0(s) = (x0(s),y0(s),z0(s)), parametrized by its arc length s, along which the amplitude a0(s) and phase φ0(s) also are specified. The three-dimensional light field u(~r,z) embodying this extended optical trap is projected by a lens of focal length f , and so passes through the lens’ focal plane where its value is u f (~r,0) = u f (~r). Associated with u f (~r) is the conjugate field uh(~r) in the back focal plane of the lens, over which we have control. A hologram that imprints this field onto the wavefronts of an otherwise featureless laser beam will project the desired trapping pattern u(~r,z) downstream of the lens, as illustrated in Fig. 1. The ideal hologram, uh(~ρ) = ah(~ρ) exp(iφh(~ρ)), is characterized by a real-valued amplitude ah(~ρ) and phase φh(~ρ) both of which vary with position ~ρ = (ξ ,η) in the back focal plane. It is related to the projected field, u f (~r), by a Fresnel transform [12]

paths yields optical traps that exert knotted force ?elds.Knot ted optical force ?elds have uses for inducing motion along knotted paths, with applications including the generation of knotted electric current loops in plasmas.Physical Society, vol. 88, No. 5, Feb. 4, 2002, 4 pages.Overfelt, Scalar optical beams with helical symmetry, The American Physical Society, Physical Review A, vol. 46, No. 6, Sep. 15, 1992, 7 pages.

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Figure 1 Figures ZA-D  This application claims priority from US. Provisional Application 61/464,445 ?ledMar. 4, 2011 which is incorpo rated herein by reference in its entirety.This application also claims priority from US. Provisional Application 61/451,705 ?ledMar. 1 1, 201 1 which is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT INTEREST
The United States Government has rights in the invention described herein pursuant to Grant Number DMR-0820341 by the MRSEC Program of the National Science Foundation.

BACKGROUND OF THE INVENTION
Light previously has been used to detect and probe objects.
For example, the use of optical traps or tweezers have been known for some time and extensively utilized in various ?elds of study.More recently, light has been used to move objects in addition to trapping them.Extended optical traps are struc tured light ?eldswhose intensity and phase gradients exert forces that con?ne microscopic objects to one-dimensional curves in three dimensions.Intensity-gradient forces typi cally are responsible for trapping in the two transverse direc tions, while radiation pressure directed by phase gradients can move particles along the third.This combination of trap ping and driving has been demonstrated dramatically in opti cal vortexes, ring-like optical traps that are created by focus ing helical modes of light.Intensity gradients draw illuminated objects toward the ring, and phase gradients then drive them around.More recently, holographic methods have been introduced to design and project more general optical traps that are extended along lines, rings and helices, with intensity and phase pro?les independently speci?ed along their lengths.Unlike optical vortexes, these traps feature nearly ideal axial intensity gradients because they are speci?cally designed to achieve diffraction-limited focusing.

SUMMARY OF THE INVENTION
One embodiment of the invention relates to a method of creating a knotted optical trap.A collimated beam of light is generated.A hologram is imprinted on the beam of light.The beam of light is projected through an objective lens.The projected beam of light is focused along a curve R0(s):(x0(s),y0(s),z0(s)), parameterized by its arc length s, along which the amplitude a0(s) and phase ¢O(s) are speci?ed.
Another embodiment relates to an apparatus for creating a knotted optical trap.The apparatus includes a collimated light source, a spatial light modulator, and an objective lens.A beam of light from the collimated light is imprinted by the spatial light modulated with a hologram projecting the beam of light along a curve R0(s):(x0(s),y0(s),z0(s)), parameter ized by its arc length s, along which the amplitude a0(s) and phase ¢O(s) are speci?ed.
Another embodiment relates to a method for controlling plasma.A knotted optical trap is projected in the plasma.A knotted current loop is initiated in the plasma such that the magnetic ?eld lines of the current loop do not pass through each other.The foregoing and other objects, aspects, features, and advantages of the disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof.In the drawings, similar symbols typically identify similar compo nents, unless context dictates otherwise.The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter pre sented here.It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the ?gures, can be arranged, substituted, com bined, and designed in a wide variety of different con?gura tions, all of which are explicitly contemplated and made part of this disclosure.
One embodiment of the present invention relates to a method for designing and projecting optical traps whose intensity maxima trace out more general curves in three dimensions with independently speci?ed phase and ampli tude pro?les.Within limitations set by Maxwell's equations, these three-dimensional light ?eldscan be used to trap and move microscopic objects.As set forth below, one embodi ment of a technique is demonstrated for projecting diffrac tion-limited holographic ring traps with arbitrary orientations in three dimensions.
More speci?cally, a beam of light is proj ected that comes to a focus along a curve §O(s):(x0(s),y0(s),z0(s)), parameter ized by its arc length s, along which the amplitude a0(s) and phase ¢O(s) also are speci?ed.The three-dimensional light ?eld36;) embodying this extended optical trap is pro jected by a lens of focal length f, and so passes through the lens' focal plane where its value is u/(?,0)q1/(?).Associated with u/(?) is the conjugate ?eld uh(?) in the back focal plane of the lens, which is controllable.A hologram that imprints this ?eldonto the wavefronts of an otherwise featureless laser beam will project the desired trapping pattern u(?, Z) down where k:2rcnm/7t is the wavenumber of light of vacuum wave length ?» in a medium of refractive index nm, and where Q is the effective aperture of the optical train.In the limit that the aperture is large, uh(F) is related to the Fourier transform, 3A3), of 11/6) by with EIkF/f.
For extended traps that lie entirely within the focal plane, the projected ?eld may be approximated by an in?nitesimally ?ne thread of light u/(?)za0(s)exp (i¢0(s))6(?-?O(s)), where ?O(s):(xo(s),yO (s)).Equation (2) then yields a holo gram associated with this idealized design.The projected ?eld comes to a focus of ?nite width because Eq. ( 2) naturally incorporates contributions from self-diffraction.
Extending this to more general three-dimensional curves poses challenges that have been incompletely addressed in previous studies.The ?eld u(?,z) at height Z above the focal plane is related to the ?eld in the focal plane by the Rayleigh Sommerfeld diffraction integral, which is expressed conve niently with the Fourier convolution theorem as Where HZ<3>IeXp<iz<qu2> 1Q) <4) is the Fourier transform of the Rayleigh-Sommerfeld propa gator.Ideally, u/(?) would be associated with a three-dimen sional ?eld u(?,z) that satis?es u(?0(s), z0(s)):a(s)exp(iq) (s)).Equation (3), however, is not simply invertible unless FIG. 3 shows a typical bright-?eld image of 5.17 pm diam eter silica spheres dispersed in water and trapped on the inclined ring.A sphere's appearance varies from bright to dark depending on its axial distance from the microscope's focal plane.This dependence can be calibrated to estimate the sphere's axial position.The image in FIG. 3 is consistent with the designed inclination of the ring trap and therefore dem onstrates the ef?cacy of Eqs. ( 2) and ( 5) for designing three dimensionally extended optical traps.
In addition to extending an optical trap's intensity along a three-dimensional curve, Eq. ( 5) also can be used to specify the extended trap's phase pro?le.Imposing a uniform phase gradient, ¢Z(s):l(s|R), redirects the light's momentum ?ux to create a uniform phase-gradient force directed along the trap.In the particular case of a ring trap, this additional tangential force may be ascribed to orbital angular momentum in the beam that is independent of the light's state of polarization and makes trapped particles circulate around the ring.This principle can be applied also to inclined ring traps.
Unlike a horizontal ring trap at [3:0, the inclined ring inevitably has phase variations along its circumference, ¢O(s):kz(s), whose azimuthal gradient 85¢(s):-k sin (is) sin [3 tends to drive trapped objects to the downstream end of the ring.Adding (1)1(s) to this intrinsic phase pro?le creates a tilted ring trap described by the hologram whose additional circumferential phase gradient that tends to drive particles around the ring.Here, JZ(.) is the Bessel func tion of the ?rst kind of order 1.A particle should circulate continuously if this tangential gradient is large enough to overcome the overall downstream gradient, which occurs for l>nm(R/7t) sin [3.Diffraction limits the maximum value of 1 that can be imposed on a ring trap of radius R. This, in turn, limits the maximum tilt angle [3 for which free circulation is possible.
The images in FIG. 3 were obtained with no imposed phase gradient, 1:0.Increasing l to 20 directs enough of the beam's radiation pressure in the tangential direction to drive the spheres around the ring at roughly 0.2 Hz.
The same formalism can be used to make more compli cated three-dimensional trapping ?elds.The image in FIG.4(a) shows the intensity in the focal plan of two tilted ring traps of radius R:l2.9 um projected simultaneously but with opposite inclination, [3:175/8.Setting these rings' separation to R/2 creates a pair of interlocking bright rings in the form of a Hopf link.These interpenetrating rings still act as three dimensional optical traps, and are shown in FIG.4(b) orga nizing 3 .01um diameter colloidal silica spheres.The trapped spheres pass freely past each other along the two rings.FIG.

4(c) offers a schematic view of the Hopf link geometry.
In one embodiment, the knotted optical trap comprises two or more tilted ring traps projected simultaneously.The two or more tilted ring traps may be projected simultaneously by a single hologram.In one embodiment, the interpenetrate to form a Hopf link.The two or more interpenetrating tilted ring traps may be created with values of 1 (Equation 10) not equal to zero such that the knotted trap creates a knotted force ?eld.
This knotted light ?elddiffers from previously reported knotted vortex ?elds in that its intensity is maximal along the knot, rather than minimal.In this respect, holographically projected Hopf links more closely resemble the Ranada-Hopf knotted ?elds that have been predicted for strongly focused circularly polarized optical pulses.Knotted force ?elds arise in this latter case from knots in each pulse's magnetic ?eld.Linked ring traps are not knotted in this sense.Nevertheless, the programmable combination of intensity-gradient and phase-gradient forces in linked ring traps can create a con stant knotted force ?eld for microscopic objects.Beyond their intrinsic interest, such knotted optical force ?elds have poten tial applications in plasma physics for inducing knotted cur rent loops and could serve as mixers for biological and soft matter systems at the micrometer scale.
In one embodiment, knotted optical force ?elds are created using intensity gradients to holdmicroscopic objects in place, and phase gradients to thread them through the knot.These optical knots are a special case of a very general class of three-dimensional optical traps that can be created with the holographic techniques.
These extended traps are useful for moving small objects such as biological cells through micro?uidic lab-on-a-chip devices.They can be used to measure vanishingly small inter actions among such objects.This is useful for medical diag nostic tests, among many other things.
Aside from being a compelling and aesthetically pleasing demonstration of the use of a static beam of light and of optical traps, such knots provide useful applications.The biggest potential application currently foreseen is for creating knotted current loops in high-temperature plasmas.This is a long sought-after goal for developing fusion energy as a prac tical power source.
In one embodiment, the holograms are projected through use of a liquid-crystal spatial light modulator.The spatial light modulator imprints a hologram that is calculated onto an ordinary laser beam.Bringing the modi?ed beam to a focus then creates the knot.The pattern that is calculated and the light ?eld that it projects are both referred to in the art as holograms.
As noted above, one application of the described knotted force ?elds is with fusion energy.Fusion reactors work by colliding light atomic nuclei into each other so hard that the nuclei fuse into heavier elements, releasing large amounts of energy in the form of hot neutrons.The best way to do this is to heat the light atoms to a high enough temperature that their kinetic energy can overcome all of the barriers to fusion during random collisions.At those temperatures, the atoms' electrons ionize and the gas becomes a plasma.The use of materials in the plasma state is also bene?cial because large electric currents can be passed through the plasma, allowing for an effective means of heating the material.Further, this allows for a mechanism of controlling the plasma, as mag netic ?elds can be utilized on the currents to contain the hot plasma, thereby preventing it from destroying its physical container.
Currently, one of the major problems with fusion reactors, and high temperature plasma in general, is the instability of the plasma.Unstable plasma can result in a loss of kinetic energy within the plasma and localized cooling.The heat and movement of the plasma can also damage the container hold ing the plasma.The energy lost to such effects cools the plasma and generally prevents the process from generating a net gain in energy.
However, it is believed that if the currents in a plasma were tied in a knot, the knot would eliminate mo st, if not all of these instabilities because the magnetic ?eld lines generated by the knotted current cannot pass through each other.Therefore, projecting a knotted optical force ?eld, in accordance with the present invention, into a plasma can provide a way to initiate a knotted current loop within the plasma.The knotted current then can be ramped up by other conventional means to heat the plasma.The result of this procedure would be a stable high-temperature plasma containing a knotted current loop for such applications as plasma fusion power generation.
A knotted force ?eld is utilized to initialize a knotted cur rent loop in the plasma.Once initiated, the current in the knotted loop can be ramped up by inductive coupling, much as in conventional tokamak design.Inductive coupling itself can not initiate a knotted current loop, thus a source is nec essary, provided by one embodiment of the invention, to initialize the current loop that inductive coupling can then ramp up.The knotted current loop then can be used to heat the plasma without the deleterious effects of plasma instabilities that are known with current plasma fusion reactor designs based upon conventional current loops that do not have knots.
One embodiment of the invention utilizes small particles to initiate a knotted current loop in a plasma.One approach is to inject small particles, dust for example, into the initial rela tively cold plasma, and to accelerate the particles around a knotted path using the knotted optical force ?eld.To do this, a wavelength of light is chosen that is absorbed weakly by the plasma, but interacts strongly with the particles in such a way that illuminated particles are driven around the knotted path by the knotted Optical force ?eld.The circulating particles will inevitably carry a charge in a plasma, as is well known from the study of so-called dusty plasmas.The circulating particles will induce a current loop along their path.If their path is knotted, then the current loop the particles trace out also will be knotted.Enhancing the current by inductive cou pling will heat the plasma, burning off the particles.Once the current is initiated, however, it will persist, even without the particles.
The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description.
It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modi?cations and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments.It is intended that the scope of the invention be de?ned by the claims appended hereto and their equivalents.
What is claimed is: 1.A method of creating a extended optical trap comprising: generating a collimated beam of light; imprinting a hologram on the beam of light; projecting the beam of light through an objective lens; and focusing the projected beam of light along a curve §O(s):(x0(s),y0(s),z0(s)), parameterized by its arc length s, along which the amplitude a0(s) and phase ¢O(s) are speci?edindependent of each other in three dimen sions.
2. The method of claim 1, further wherein a light ?eldassociated with the extended optical trap has a maximal inten sity along curve.
3. The method of claim 2, wherein the extended optical trap is associated with a light ?eldu(?,z), such that an objective lens, wherein a beam of light from the collimated light is imprinted by the spatial light modulated with a holo gram projecting the beam of light along a curve §O(s):(x0(s),y0(s),z0(s)), parameterized by its arc length s, along which the amplitude a0(s) and phase ¢O(s) are speci?ed of each other in three dimensions.12.The apparatus of claim 11, further wherein a light ?eldassociated with the extended optical trap has a maximal inten sity along the extended optical trap.
13.The apparatus of claim 11, wherein the extended opti cal trap is associated with a light ?eldu(?,z), such that cal trap comprises a ring trap of radius R, rotated by angle [3 about the y axis such that the trap's focus follows the curve: S with the arc length ranging from s:0 to s:2s'cR.
16.The apparatus of claim 11, wherein the extended opti cal trap comprises a tilted ring trap described by the hologram where J I(.) is the Bessel function of the ?rst kind of order 1. 17.The apparatus of claim 11, wherein the extended opti cal trap comprises two or more tilted ring traps projected simultaneously.

Figures
Figures 4A-C FIG. 1 Projecting three-dimensionally extended holo graphic traps.A laser beam is imprinted with a hologram in the input pupil of an objective lens.The hologram is projected through the objective's focal plane, and comes to a focus along three-dimensional curve parameterized by its arc length, s.FIG.2(a) is a volumetric reconstruction of a tilted ring trap.FIG.2(b) is a horizontal section near the midplane showing tilt around the y axis.FIG.2(c) is a vertical section along the x axis.FIG.2(d) is a vertical section along the y axis.FIG.3(a) is a bright-?eld microscope image of 5.17 um diameter colloidal silica spheres trapped on an inclined ring.FIG.3(b) is a schematic representation of the geometry in FIG.3(a).FIG.4(a) illustrates intensity in the focal plane of the microscope of two tiled ring traps projected simultaneously with opposite inclination, [3:175/ 8 FIG.4(b) illustrates colloi dal silica spheres trapped in three dimensions within the focused Hopf link.FIG.4(c) is a schematic representation of the three-dimensional intensity distribution responsible for the image in FIG.4(a).The foregoing and other objects, aspects, features, and stream of the lens, as illustrated in FIG. 1.The ideal hologram, uh(F):ah(F)exp(iq)h(F)), is charac terized by a real-valued amplitude ah(F) and phase (1)4?) both of which vary with position F:(E,n) in the back focal plane.It is related to the projected ?eld, u/(Y), by a Fresnel transform 1 my r, ( > FIG.2(a) is a volumetric reconstruction of the three-di mensional intensity distribution projected by the hologram in Eq. (7) of a ring trap of radius R:9 um tilted at [3:75/4 rad.This complex-valued hologram was approximated with a phase only hologram using the shape-phase algorithm so that it could be projected with a conventional holographic optical trapping system.The light from a diode-pumped solid-state laser (Coherent Verdi, 7t:532 nm) was imprinted with the computed hologram by a phase-only liquid-crystal spatial light modulator (Hamamatsu X8267-l 6 PPM) and relayed to the input pupil of an oil-immersion objective lens (Nikon Plan-Apo, 100x, numerical aperture 1.4) mounted in a con ventional light microscope (Nikon TE-2000U).Transverse slices of the projected intensity distribution, such as the example in FIG.2(b), were obtained by mounting a front surface mirror on the microscope's sample stage and moving the trap with respect to the focal plane.Light re?ected by the mirror was collected by the same objective lens and relayed to a video camera (NEC TI-324A II) for recording.A sequence of such slices obtained in axial steps of Az:0.l um was stacked to create a volumetric map of the trap's intensity.The surface in FIG.2(a) encompasses the brightest 70 percent of comprises a ring trap of radius R, rotated by angle [3 about the y axis such that the trap's focus follows the method of claim 1, wherein the extended optical trap comprises a tilted ring trap described by the hologram where J I(.) is the Bessel function of the ?rst kind of order 1.7.The method of claim 6, wherein the tilted ring trap consists of two or more tilted ring traps projected simulta neously.8.The method of claim 7 wherein the two or more tilted ring traps are projected simultaneously by a single hologram.9.The method of claim 8, wherein the two or more tilted ring traps interpenetrate to form a Hopf link.1 0. The method of claim 9, wherein each of the two or more interpenetrating tilted ring traps are created with values of 1 not equal to zero such that the optical ring trap creates a force ?eld.11.An apparatus for creating a extended optical trap com prising: a collimated light source; a spatial light modulator; 18.The apparatus of claim 17 wherein the two or more tilted ring traps are projected simultaneously by a single hologram.19.The apparatus of claim 18, wherein the two or more tilted ring traps interpenetrate to form a Hopf link.20.The apparatus of claim 19, wherein each of the two or more interpenetrating tilted ring traps are created with values of 1 not equal to zero such that the holographic optical ring trap creates a force ?eld.