WO2009018183A2

WO2009018183A2 - System and method for near-field optical tweezers

Info

Publication number: WO2009018183A2
Application number: PCT/US2008/071284
Authority: WO
Inventors: Ken Crozier
Original assignee: President And Fellows Of Harvard College
Priority date: 2007-07-27
Filing date: 2008-07-27
Publication date: 2009-02-05
Also published as: WO2009018183A3

Abstract

A new optical tweezer technology, based on optical antennas. Optical antennas are metallic nanostructures that, when illuminated at their resonant wavelength, generate enhanced optical fields with subwavelength spatial extent. The trapping of single virus particles using optical antennas with milliwatt laser powers, in volumes with dimensions of tens of nanometers. This represents a reduction in the necessary laser power by -2-3 orders of magnitude. The spatial resolution provided by the antennas will enable nanostructures to be manipulated in a highly precise manner. Optical antennas remove the need for costly objective lenses used in conventional tweezers to provide the gradient forces necessary for stable trapping. Traps can be produced on a chip by high-resolution lithography, enabling microsystems with thousands of traps.

Description

SYSTEM AND METHOD FOR NEAR-FIELD OPTICAL TWEEZERS INVENTOR: KEN CROZIER

CROSS-REFERENCE TO RELATED APPLICATIONS 100011 The present application claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 60/952,498 entitled "System and Method for Near-Field Optical Tweezers" filed by the present inventor on July 27, 2007. |0002| The aforementioned provisional patent application is hereby incorporated by reference in its entirety. STATEMENT REGARDING FEDERALLY

SPONSORED RESEARCH OR DEVELOPMENT jOOOcM The present invention was developed in part with funding under DARPA contract number HROOl 1-06-1-0044. BACKGROUND OF THE INVENTION

Field Of The Invention

10004] The present invention relates to optical tweezers. Brief Description Of The Related Art

100051 Optical tweezers use the forces exerted by tightly focused light beams to trap and move objects with sizes ranging from tens of nanometers to tens of micrometers. Since their introduction in 1986, optical tweezers have become an important tool in the biological sciences. A. Ashkin, J.M Dziedzic, J.E. Bjorkholm, and S. Chu, "Observation of single-beam gradient force optical trap for dielectric particles," Optics Letters vol. 11, pp. 288-290 (1986). Optical tweezers are used to study of molecule motors, the mechanical properties of polymers and biopolymers and the physics of colloids and Attn'y Docket No.: 4062.2929PCT

mesoscopic systems. A.D. Mehta, M. Rief, J.A. Spudich, D.A. Smith, R.M Simmons, "Single-molecule biomechanics with optical methods," Science, vol. 283, pp. 1689-95 (1999); J.C. Crocker and D. G. Grier, "Microscopic measurement of the pair interaction potential of a charge stabilized colloid," Physical Review Letters, vol. 73, pp. 352-355 (1994); D.G. Grier, "A Revolution in Optical Manipulation," Nature, vol. 424, pp. 810- 816 (2003). As shown in Fig. 1, conventional optical tweezers for trapping a colloidal; particle 160 may comprise a laser 110, a microscope objective 120, immersion oil 130, a covers lip 140 and water 150. >000os Conventional optical tweezers (Fig. 1) are best understood by considering the limiting cases of objects much smaller than the wavelength, and those much larger. Large objects act as lenses, refracting the rays of light and redirecting the photon momentum. This results in a recoil that draws them toward the focus. A. Ashkin, "Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime," Method in Cell Biology vol. 55, pp. 1-27 (1998). For objects much smaller than the wavelength, the incident beam creates an induced dipole, drawing the object along the electric field gradient to the point of highest intensity. s 000^" ! In both cases, gradient forces 240 compete with scattering forces 230 due to the momentum they transfer from photons in the laser beam 210, which pushes particles 260 down the optical axis 220. For trapping to be stable, the gradient force 240 must dominate over the scattering force 230, which is achieved through tight focusing of the laser beam 210, as shown in Fig. 2.

IOOOSl Antennas as NSOM probes were first experimentally demonstrated by Grober et al at microwave frequencies. R.D. Grober, R.J. Schoelkopf, and D.E. Prober, "Optical Attn'y Docket No.: 4062.2929PCT

antenna: Towards a unity efficiency near-field optical probe," Applied Physics Letters 70, pp. 1354-1356 (1997). The concept was explored at optical wavelengths by fabricating gold antennas by e-beam lithography and demonstrating antenna resonances at mid-infrared wavelengths (λ=2.5-10 μm). K.B. Crozier, A. Sundaramurthy, G. S Kino and CF. Quate, "Optical Antennas: Resonators for Local Field Enhancement," Journal of Applied Physics 94, pp. 4632-4642 (2003). Optical antennas have been demonstrated at near-infrared and visible wavelengths, with Au bowtie antennas. David P. Fromm, Arvind Sundaramurthy, P. James Schuck, Gordon Kino, and W.E. Moerner, Gap- dependent optical coupling of single "bowtie" nanoantennas resonant in the visible," Nano Letters 4, pp. 957 (2004); Arvind Sundaramurthy, K.B. Crozier, G.S. Kino, D.P. Fromm, P.J. Schuck, and W.E. Moerner, "Field enhancement and gap-dependent resonance in a system of two opposing tip-to-tip Au nanotriangles," Physical Review B 72, pp. 165409 (2005); P.J. Schuck, D.P. Fromm, A. Sundaramurthy, G.S. Kino, and W.E. Moerner, "Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas," Physical Review Letters 94, pp. 017402 (2005). The strong field enhancement in the gap of an optical antenna has been used for white-light supercontinuum generation. P. Muhlschlegel, H. -J. Eisler, O.J.F. Martin, B. Hecht, and D.W. Pohl, "Resonant Optical Antenna," Science 308, pp. 1607-1609 (2005). Optical antennas have been used to investigate quantum dots. J.F. Farahani, D.W. Pohl, H. -J. Eisler, and B. Hecht, "Single quantum dot coupled to a scanning optical antenna: a tunable superemitter," Physical Review Letters 95, pp. 017402 (2005). In addition to NSOM applications, optical antennas have been coupled to diodes and used to detect infrared and visible radiation. V. Daneu, D. Sokoloff, A. Sanchez, and A. Javan, Attn'y Docket No.: 4062.2929PCT

"Extension of laser harmonic-frequency mixing techniques into the 9μ region with an infrared metal-metal point-contact diode", Applied Physics Letters 15, pp. 398 (1969); Shyh Wang, "Antenna properties and operation of metal-barrier-metal devices in the infrared and visible regions," Applied Physics Letters 28, pp. 303-305 (1976); A. Sanchez, CF Davis, K.C Liu, and A. Javan, "The MOM tunneling diode: theoretical estimate of its performance at microwave and infrared frequencies," Journal of Applied Physics 49, pp. 5270-5277 (1978); Christophe Fumeaux, Javier Alda, and Gleen D. Boreman, "Lithographic antennas at visible frequencies," Optics Letters 24, pp. 1629 (1999). Optical antennas have been used for data storage. T. Matsumoto, Y. Anzai, T. Shintani, K. Nakamura, and T. Nishida, "Writing 40 nm marks by using a beaked metallic plate near-field optical probe," Optics Letters 31, pp. 259-261 (2006). 1 OOΘ%j The use of near- field optical nanostructures as optical tweezers has been studied by a number of groups, though thus far all of the investigations have been purely theoretical except for the work of Kwak et al. See L. Novotny, R.X. Bian, X.S. Xie, "Theory of Nanometric Optical Tweezers," Physical Review Letters, vol. 79, pp. 645-648 (1997); M. Nieto-Vesperinas, P. C. Chaumet, and A. Rahmani, "Near-field Photonic Forces," Philosophical Transactions of the Royal Society of London A, vol. 362, pp. 719- 737 (2004); P.C. Chaumet, A. Rahmani, A. Sentenac, G.W Bryant, "Efficient computation of optical forces with the coupled dipole method," Physical Review E, vol. 72, pp. 046708 (2005); P.C. Chaumet, A. Rahmani, and M. Nieto-Vesperinas, "Optical trapping and manipulation of nano-objects with an apertureless probe," Physical Review Letters, vol. 88, pp. 123601 (2002); P.C. Chaumet, A. Rahmani, M. Nieto-Vesperinas, "Selective manipulation using optical forces," Physical Review B, vol. 66, pp. 195405 Attn'y Docket No.: 4062.2929PCT

(2002); R. Quidant, D. Petrov, G. Badenes, "Radiation forces on a Rayleigh dielectric sphere in a patterned optical near-field," Optics Letters, vol. 30, pp. 1009-1011 (2005); N. Calander and M. Willander, "Optical trapping of single fluorescent molecules at the detection spots of nanoprobes," Physical Review Letters, vol. 89, pp. 143603 (2002); H. Xu and M Kail, "Surface plasmon-enhanced optical forces in silver nanoaggregates," Physical Review Letters, vol. 89, pp. 246802 (2002); K. Okamoto and S. Kawata, "Radiation force exerted on subwavelength particles near a nanoaperture," Physical Review Letters, vol. 83, pp. 4534-4537 (1999); E.-S. Kwak, T.-D. Onuta, D. Amarie, R. Potyrailo, B. Stein, S. C. Jacobson, W.L. Schaich, and B. Dragnea, "Optical Trapping with Integrated Near- field Apertures," Journal of Physical Chemistry B, vol. 108, pp. 13607-13612 (2004). Kwak et al demonstrated the trapping of 200 nm latex beads in solution with the near-fields of a 500nm-diameter aperture in a gold film. The theoretical studies carried out have been on the use of apertureless near-field probes, and nanoapertures as optical tweezers, patterned substrates for the generation of large arrays of optical traps, and trapping of fluorescent molecules in the gaps between silver nanoaggregates.

10010| The present invention, which uses optical antennas, offers several advantages over the work of Kwak. Optical antennas generate intense fields, confined to a region in the antenna gap. Optical antennas comprising pairs of triangles 410, 412 and rods 420, 422 are shown in Fig. 4(a) and 4(b), respectively. Conduction electrons in the metal move under the influence of the electric field of the incident illumination, resulting in oscillating surface charges on the metal. At any given instant, charges of opposite sign appear on either side of the antenna gap, leading to a strong electric field in the gap. The Attn'y Docket No.: 4062.2929PCT

following experiment was performed to evaluate the strength of the field produced by an optical antenna. A rigorous electromagnetic calculation of the electric field intensity <|E|²> was plotted for an optical antenna consisting of two gold rectangles separated by a 20 nm gap on an SiO₂ substrate. See A. Taflove, S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method, Artech House 2000, for the numerical method used in the calculation. Each gold antenna was 130 nm long, 40 nm wide and 50 nm thick. The optical constants of the gold included the effects of loss. See E. D. Palik (editor), Handbook of Optical Constants of Solids, Academic Press, Orlando, Florida, 1985 for the optical parameters of gold. The electric field intensity distribution plotted was that resulting on the top surface of the antenna when it was illuminated though the substrate at λ = 830 nm, with the polarization of the incident wave along the antenna axis. The intensity is normalized to that of the illumination. From that plot, it was seen that the optical antenna produces an intense field that is more than 800 times the incident intensity, and concentrated to the 20nm antenna gap. The intensity also may be expressed in absolute terms. Assuming that the antenna is illuminated by a plane wave of intensity 10 wW/μm², the peak intensity was achieved at the focus of a microscope objective (NA 1.4) with an input power of 0.97 mW. It was seen that the peak intensity was more than 8 W/μm². The spot size was far smaller than the -415 nm (λ/2) spot possible with conventional optics at λ = 830nm. 1001 1 J In contrast to the present invention, the 500nm-diameter apertures of Kwak et al do not produce the field enhancement, but rather confine fields by blocking the incident radiation around the aperture.

SUMMARY OF THE INVENTION Attn'y Docket No.: 4062.2929PCT

100121 The present invention is anew method and apparatus for optical tweezers. In a preferred embodiment, a focused laser beam illuminates an optical antenna, resulting in intense optical fields in the antenna gap (~20 nm). The field enhancement increases the trapping potential, making the trap more stable against Brownian motion. 10013 ] In a preferred embodiment, the present invention is a method for trapping nanoscale particles. The method comprises the steps of illuminating an optical antenna having an antenna gap with a focused laser beam to produce intense optical fields in the antenna gap and trapping a nanoscale particle in the antenna gap. The nanoparticle may comprise a virus. The method may further comprise the steps of positioning a cell above said antenna and turning off said laser beam to release said virus to infect said cell with said virus.

10014 J In another embodiment, the present invention is a method for trapping a nanoparticle with an optical antenna wherein the antenna comprising a section of gold fabricated at an end of an AFM tip. The method comprises the steps of illuminating said optical antenna with a laser to generate intense fields at said end of said AFM tip, trapping a nanoparticle at said end of said AFM tip near a portion of a substrate, moving said AFM tip to transport said nanoparticle to a different portion of said substrate and releasing said nanoparticle by switching off said laser. 10015] In another embodiment, the present invention is optical tweezers comprising an optical antenna having an antenna gap and a focused laser beam for illuminating said optical antenna to produce intense optical fields in said antenna gap. ] 0016 J In a preferred embodiment, the present invention is a method for trapping nanoscale particles. The method comprises illuminating an optical antenna with a focused Attn'y Docket No.: 4062.2929PCT

laser beam to produce a region of enhanced electromagnetic fields and trapping a nanoscale particle in said region of enhanced electromagnetic fields. The nanoparticle may comprise, for example, a virus. The method may further comprise the steps of positioning a cell above said antenna and turning off said laser beam to release said virus to infect said cell with said virus. The optical antenna may comprise, for example, a triangle, a disc, a rod, a pair of triangles separated by a small gap, a pair of rods separated by a small gap, or a pair of discs separated by a small gap. The optical antenna comprise a metal such as gold, silver, copper or aluminum. The optical antenna may be of a resonant length, for example, 100-300nm. 100 P I In another embodiment, the present invention is a method for trapping a nanoparticle with an optical antenna comprising a section of gold fabricated at an end of an AFM tip. The method comprises the steps of illuminating said optical antenna with a laser to generate intense fields at said end of said AFM tip, trapping a nanoparticle at said end of said AFM tip near a portion of a substrate, moving said AFM tip to transport said nanoparticle to a different portion of said substrate, and releasing said nanoparticle by switching off said laser. The optical antenna may comprise a single section of gold having first and second ends wherein said first end is sharper than said second end or a pair of triangles, rods or disks separated by a small gap. The step of trapping a nanoparticle comprises trapping the nanoparticle in said gap. jOOS ^j In another embodiment, the present invention is an apparatus for trapping a nanoparticle. The apparatus comprises an optical antenna comprising a section of metal fabricated at an end of an AFM tip, means for illuminating said optical antenna with a laser to generate intense fields at said end of said AFM tip, means for trapping a Attn'y Docket No.: 4062.2929PCT

nanoparticle at said end of said AFM tip near a portion of a substrate, means for moving said AFM tip to transport said nanoparticle to a different portion of said substrate, and means for releasing said nanoparticle by switching off said laser. The optical antenna may comprise a pair of triangles, rods, discs, or the like, separated by a small gap. The optical antenna may be comprised of a metal, for example, gold, silver, copper or aluminum. f 0019^"| In another embodiment, the present invention is a nanoparticle separator. The separator comprises a microfluidic channel, an array of optical antennas arranged on a bottom of said microfluidic channel, wherein when illuminated with a focused laser beam each optical antenna in said array produces a region of enhanced electromagnetic fields for trapping a nanoparticle.

10020 J In another embodiment, said invention is a method for separating nanoparticles. The method comprises the steps of passing a fluid containing microparticles through a microfluidic channel, wherein an array of optical antennas is arranged on the bottom of said microfluidic channel, illuminating said array of optical antennas with a focused laser beam, thereby causing said array of optical antennas to capture nanoparticles, passing a buffer fluid through said channel, and removing said focused laser beam from said optical antennas to release said captured nanoparticles into said buffer fluid. 10021 ] Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a preferable embodiments and implementations. The present invention is also capable of other and different embodiments and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the Attn'y Docket No.: 4062.2929PCT

drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. BRIEF DESCRITION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description and the accompanying drawings, in which:

JOO»cM Fig. 1 is a diagram of conventional optical tweezers in which a colloidal particle is trapped at a focus of a microscope lens. j OiO-H Fig. 2 is a diagram illustrating that light must be tightly focused for gradient force to overcome scattering force.

|00^51 Fig. 3 is a diagram of optical tweezers in accordance with a preferred embodiment of the present invention in which a laser excites surface plasmons on an optical antenna, leading to intense optical fields in gap of antenna. The resultant strong gradient force traps a nanoparticle in the gap.

JOO»oj Fig. 4(a) is a diagram illustrating an optical antenna comprising a pair of triangles.

|00^| Fig. 4(b) is a diagram illustrating an optical antenna comprising a pair of rods separated by small gaps. jOO *^j Fig. 5(a) is an illustration of a study of Seol et al, in which lOOnm-diameter gold bead is trapped by a focused laser. jθO K*! Fig. 5(b) illustrates a temperature and water viscosity at the particle's surface as a function of laser power from a study by Seol et al Attn'y Docket No.: 4062.2929PCT

100301 Figs. 6(a) and (b) are a diagrams illustrating nanosphere trapping in accordance with a preferred embodiment of the present invention.

10031 ] Figs. 7 (a) and (b) illustrate behavior of trapped nanoparticles in accordance with a preferred embodiment of the present invention. Fig. 7(a) is a graph of trapping and fluorescence laser powers. Fig. 7(b) is a graph of APD counts. See text for further details.

HI0321 Fig. 8 is a diagram of trapping probe based on optical antenna at an end of an

AFM tip in accordance with a preferred embodiment of the present invention. A nanoparticle is trapped by gradient potential, moved to another part of the substrate and then released.

10033] Figs. 9(a)-(d) are diagrams illustrating an application of near- field optical tweezers in accordance with a preferred embodiment of the present invention.

|0034] Fig. 10 is a diagram of an apparatus for measurement of field distribution on an optical antenna with scattering-type apertureless scanning near-field optical microscope. HI0351 Fig. 11 is a linescan of a measured optical near-field distribution of an antenna structure.

] 00361 Fig. 12 is an extinction cross section of an optical antenna. The black line illustrates an experiment while the green line illustrates an FDTD model.

KHLT?] Fig. 13 is a measured scattering spectrum from an optical antenna (32 nm gap) (curve with experimental noise) and numerically calculated scattering efficiency

(triangles).

]003S| Figs. 14(a)-(c) illustrate an alternate embodiment of the present invention in the form of an array of optical tweezers in a micro fluidic channel. Attn'y Docket No.: 4062.2929PCT

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A completely new method for optical tweezers is shown as Fig. 3. A focused laser beam 310 illuminates an optical antenna 320 formed on a substrate 330, resulting in intense optical fields in the antenna gap 340 (~20 nm). The field enhancement increases the trapping potential, making the trap more stable against Brownian motion. A trapped colloidal particle 350 is shown in Fig. 3 trapped in the antenna gap 340. The device of Fig. 3 will result in optical tweezers that generate gradient forces sufficiently strong to trap virus particles in nanoscale regions, using laser powers 100's of times smaller than necessary with conventional tweezers.

KHMO] The forces needed for stable trapping of nanoparticles can be generated at low to moderate laser powers by optical antennas. The radiation pressure force exerted on the nanoparticle can be described by two components. Y. Harada and T. Asakura, "Radiation forces on a dielectric sphere in the Rayleigh scattering regime, " Optics Communications, vol. 124, pp. 529-541 (1996). The first is the scattering force, resulting from scattering by the nanoparticle of the incident electromagnetic wave. This leads to momentum transfer, and the scattering force associated with these changes is exerted on the nanoparticle. The scattering force is given by:

F_scat{r) =z^C_scat I{f) (1) c

where C_scat is the scattering cross section of the nanoparticle, rib is the refractive index of the medium, c is the speed of light, I(r) is the intensity, equal to the time-averaged Poynting vector, and z is the unit vector in the beam propagation direction. K. Sovoboda Attn'y Docket No.: 4062.2929PCT

and S. M Block, "Biological Applications of Optical Forces," Annual Review of Biophyics and Biomolecular Structure, vol. 23, pp. 247-285 (1994). The second force component is the gradient force, due to the Lorentz force acting on the dipole induced by the electromagnetic field. J.P. Gordon, "Radiation Forces and Momenta in Dielectric Media," Physical Review A, vol. 8, p. 14-21 (1973). For a nanoparticle of polarizability a, the gradient force F_grad is given by:

F_grad =n_b ^E* (2)

P. W. Smith, A. Ashkin, and W.J. Tomlinson, "Four-wave mixing in an artificial Kerr medium," Optics Letters, vol. 6, pp. 284-286 (1981); A. Ashkin, J.M. Dziedzic, and P.W. Smith, "Continuous-wave self-focusing and self-trapping of light in artificial Kerr media," Optics Letters, vol. 7, pp. 276-278 (1982).

|0041 | To model the trapping of nanoparticles with optical antennas, calculations were made of the field distributions of an antenna (λ=830 nm) with illumination intensity of 10 mW/μm². The nanoparticle was a polystyrene latex sphere (n = 1.592) with a 10 nm radius. S. Kawata and T. Sugiura, "Movement of micrometer-sized particles in the evanescent field of a laser beam," Optics Letters, vol. 17, pp. 772-774 (1992). The medium above the antenna was air (rib = 1.0). Following the calculation of the electric and magnetic fields, equations (1) and (2) were used to find the scattering and gradient forces, and the total force was found.

1004« J The results clearly indicated that the fields generated by the optical antenna led to gradient forces that dominated over the scattering forces. The nanoparticle was drawn to the regions of high intensity, including the ends of the antenna as well as the antenna gap. Attn'y Docket No.: 4062.2929PCT

It can be seen that when the nanoparticle was closer than -100 nm to the substrate, the net force on the nanoparticle was toward the antenna. The assumed illumination intensity (10 mW/μm²) corresponded to the peak intensity achieved at the focus of a microscope objective (NA 1.4) with a 0.97 mW input power. Clearly, the optical antenna was able to achieve strong trapping forces for relatively low (sub mW) laser powers.

100431 For stable trapping, the requirement that the gradient force dominates is a necessary but not sufficient condition. An additional trapping condition is that the Boltzmann factor exp(-U/kT) « 1, where U is the potential of the gradient force, and given by:

U = n_b ^ E² (3)

A. Ashkin, "Trapping of Atoms by Resonance Radiation Pressure," Physical Review

Letters, vol. 40, pp. 729-732 (1978).

10044 J This is equivalent to requiring that the time to pull a particle into the trap be much less than the time for the particle to diffuse out of the trap by Brownian motion. A practical condition is:

U/kT≥lO (4)

|0045| For a Gaussian beam illuminating a particle small compared to the wavelength, the potential energy of the gradient force is given by:

where ri2 is the refractive index of the medium, a is radius of the particle, c is the speed of light, m is the ratio of the particle refractive index to that of the medium {rii/ni), P is the Attn'y Docket No.: 4062.2929PCT

laser power (proportional to the square of the electric field) and ω₀ is the waist radius of the Gaussian beam.

|004oj Consider the trapping of a 10 nm radius polystyrene latex sphere in air with a λ=830 nm laser focused by an NA 1.4 objective. Using equations (4) & (5), we found that an optical power P of 0.53 W was required for stable trapping. For a gold sphere (radius IOnm), 0.17 W was needed.

1004 ^{^} I The field enhancement provided by the optical antenna reduces the laser power needed for stable trapping. Optical antennas can provide intensity enhancements of greater than 800 times. The enhancement value in the center of the gap is -500 times. From equation (3), it can be seen that the trapping potential is proportional to the square of the peak electric field. Therefore, for an intensity enhancement of 500, the powers required for the stable trapping of latex and gold nanospheres (10 nm radii) are 1.1 mW and 0.34 mW, respectively. 10048 J As discussed, the calculations of trapping powers were carried out assuming that medium is air. For applications in the biological sciences, a water environment would be more appropriate. It can be shown from equation 5 that the laser powers necessary for the trapping of 10 nm radii latex and gold spheres in water are 0.96 W and 0.12 W, respectively. Using an optical antenna with an intensity enhancement of 500 times, laser powers of 1.90 mW and 0.24 mW would be needed. The presence of water above the antenna would modify the optimum antenna design, since plasmon resonances are red- shifted as the index of the surrounding medium increases. A.D. McFarland and R.P. Van Duyne, "Single Silver Nanoparticles as Real-Time Optical Sensors with Zeptomole Sensitivity," Nano Letters, vol. 3, pp. 1057-1062 (2003). Attn'y Docket No.: 4062.2929PCT

I OtW^ I Because the nanoparticle radius a « λ, it can be accurately modeled as a point dipole. CF. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley and Sons (1983). In addition, we assumed that due to its small size, the presence of the nanoparticle does not alter the field distribution around the optical antenna. Rigorously accurate results could be obtained by carrying out simulations of the field distributions for a system including both the antenna and nanoparticle, and then computing the Maxwell stress tensor. The Maxwell stress tensor is discussed in L. Novotny, "Force in Optical Near-fields," in S. Kawata (editor), Near-field Optics and Surface Plasmon Polaritons, Springer- Verlag (2001). However, these approximations reduce complexity, while illustrating the underlying principles.

|00?0| Also, we preliminarily discuss heating considerations. Seol et al carried out a combined theoretical/experimental study of the heating of gold nanoparticles under focused laser illumination. Y. Seol, A.E. Carpenter and T. T. Perkins, "Gold nanoparticles: enhanced optical trapping and sensitivity coupled with significant heating," Optics Letters 31, pp. 2429-2431 (2006). As illustrated in Fig. 5(a), Seol et al investigated the trapping of gold nanospheres 510 with diameters of 100 nm, with a focused laser (λ = 1064 nm) 520. In Fig. 5(b), the results of their study are given. A plot of the calculated temperature distribution for the gold nanoparticle trapped with a laser power of 205 mW showed that the peak temperature was 74.5 ⁰C, corresponding to a temperature increase of 54.5 ⁰C above room temperature (20 ⁰C). In Fig. 5(b), the temperature as a function of laser power is plotted. From their modeling, Seol et al found the nanoparticle heating to be 266°C/W. Attn'y Docket No.: 4062.2929PCT

100511 The design of the near-field optical tweezers in accordance with a preferred embodiment of the present invention was carried out using rigorous numerical electromagnetic modeling based on the finite difference time domain (FDTD) technique. A goal for this preferred embodiment was to choose the antenna geometry to maximize the field enhancement and trapping potential Electron beam lithography may be used to fabricate optical antennas on glass coverslips. Antennas with different gap sizes, optimized for trapping nanoparticles of different sizes, may be fabricated. Antennas optimized for a wavelength of 1 μm, for example, may be fabricated since it is known that this wavelength minimizes damage to biological samples, the study of which is an important application of optical tweezers.

J005- s The present invention may be sued for trapping nanospheres with a wide variety of diameters, ranging from 20 nm to 500 nm. Fluorescent nanospheres are well suited as the objects to the trapped, as the separation of excitation and emission wavelengths enables them to be observed even in the presence of a large background. For a given nanoparticle composition and size, there exists an optimum optical antenna design. Due to their stronger field enhancement, optical antennas enable stable trapping at lower input laser powers than nanoholes. An example configuration is shown in Fig. 6. ϊ005.^ In the example set-up show in Fig. 6(a), nanospheres are trapped by the strong gradient forces provided by optical antennas, and observed by confocal fluorescent microscopy. The set-up has a fluorescence excitation laser referred to here as a trapping laser (the input laser beam is shown as 610), a prism 620, water 630, immersion oil 640, a microscope objective 650, a fluorescent emission filter 660, a tube lens 670, a confocal pinhole 680, and avalanche photodiode 690, a photon counting module 692, and a Attn'y Docket No.: 4062.2929PCT

computer 694. The optical antennas may be fabricated on glass coverslips and mounted in an inverted optical microscope. They may be illuminated by trapping and fluorescence excitation lasers under total internal reflection (TIR) with a glass prism. The use of these lasers will enable the power levels of two separate functions, trapping nanospheres and exciting their fluorescence, to be optimized separately. The trapping laser wavelength in this example is 1 μm, and the optical antennas in this example are designed for this wavelength. The fluorescence excitation wavelength is 400 nm, matching the excitation wavelength of the fluorescent nanospheres. The trapping and fluorescence emission processes are shown in Fig. 6(b). The trapping laser illuminates the optical antenna, generating enhanced fields in the antenna gap, resulting in the trapping of a nanosphere in the gap. Fluorescent emission from the trapped nanosphere is generated by TIR illumination by the fluorescence laser. Note that, as shown in Fig. 6(b), fluorescent emission from untrapped nanospheres can also occur but, as discussed further below, will be distinguishable from emission from trapped nanospheres. As shown in Fig. 6(a), the emission will be collected by an oil immersion microscope objective 650 onto a confocal pinhole 680 and an avalanche photodiode 690. The set-up therefore allows us to carry out fundamental investigations of the ability of surface plasmon structures to trap nanoparticles. Study of the behavior of trapped nanoparticles in response to controlled variations in the trapping laser is illustrated in Figs. 7(a) and (b). 10054 J Figs. 7(a) and (b) illustrate the types of data that may be obtained using the example set-up of Fig. 6. Modulation of the intensities of the trapping and fluorescence lasers (Fig. 7(a)) results in nanosphere trapping, which is observed by the avalanche photodiode (APD) signal (Fig. 7(b)). In this example, the fluorescence laser is switched Attn'y Docket No.: 4062.2929PCT

on at time I_A, and the trapping laser is switched on at tβ. Therefore, over the interval I_A < t < tβ, the expected APD signal will exhibit counts from untrapped nanospheres that are within the evanescent field provided by the fluorescence laser. As shown in Fig. 7(b), because these nanoparticles are not trapped, the APD signal will fluctuate in time due the Brownian motion of the nanoparticles. At time tβ, the trapping laser is switched on. A nanoparticle in the vicinity of the antenna gap will therefore be pulled into the gap and trapped there, resulting in the steady APD signal shown in Fig. 7(b). At time t_& the trapping laser is switched off. This results in a fluctuating APD output from fluorescent emission from untrapped nanoparticles. In addition to varying the trapping laser power, the ability of different optical antenna designs for the trapping of nanospheres is characterized.

100551 By combining the near-field tweezers with atomic force microscopy (AFM), another preferred embodiment of the invention may collect images, and then use the near- field tweezers to apply forces and move nanostructures of interest. Previous investigations such as in Novotny et al, Versperinas et al and Chaumet et al have studied the use of sharp metal tips, but these previous investigations did not suggest the use of optical antennas. Martin et al calculated the intensity enhancement for a gold tip (10 nm radius) to be -16 times (at λ = 633 nm). Y. C. Martin, H. F. Hamann, and H. K. Wickramasinghe, "Strength of the electric field in apertureless near-field optical microscopy," Journal of Applied Physics, vol. 89, 5774 (2001). Despite the fact that the tip was very sharp in that simulation, with a radius of curvature of half the gap width of the above examples of the present invention, the enhancement was 1-2 orders of Attn'y Docket No.: 4062.2929PCT

magnitude smaller than the -800 enhancement of the antenna of the preferred embodiment of the present invention.

|00?C>| In a preferred embodiment of the invention, AFM tips with optical antennas fabricated at the ends of the tips are used in the trapping and manipulation of nanoparticles. The concept is shown as Fig. 8. The optical antenna 810 comprises a section of gold fabricated at the end of the tip 822 of an AFM cantilever 820. The gold section would be 100-300 nm long, depending on operating wavelength. A laser illuminates the antenna 810, generating intense fields at the end of the tip 822. The gradient potential associated with this field will allow a nanoparticle 830 to be trapped at the end of the tip 822. The trapped particle is shown at 832 in Fig. 8. The nanoparticles used will initially be polystyrene beads. By moving the AFM tip, the bead will be transported to another part of the substrate 802, and then released by switching off the laser illumination. The released nanoparticle is labeled as 834 in Fig. 8. Prior to the experiment, the Au antenna is coated with a self-assembled monolayer of mercaptosuccinic acid. This will prevent the beads from sticking to the Au film through hydrophobic or van der Waals interactions. In addition, NaCl will be added to the solution to control the electrostatic repulsion between the Au surface and bead. |005H In the embodiment shown in Fig. 8, the optical antenna may be of different structures. For example, the optical antenna may have a single section of gold, where one end is sharper than the other. Laser illumination of the antenna will produce enhanced fields at the sharp end, and it would be possible to trap nanoparticles there. The optical antenna may have other forms, such as the forms discussed elsewhere herein. Attn'y Docket No.: 4062.2929PCT

|0058i In another preferred embodiment of the present invention, fabricated optical antennas are used to trap single virus particles. Optical tweezers are used to trap single cells, for example for clinical applications such as in vitro fertilization. G. Wright, M.J. Tucker, P. C. Morton, CL. Sweitzer-Yoder, and S.E. Smith, "Micromanipulation in assisted reproduction: A review of current technology," Current Opinions in Obstetrics and Gynecology, vol. 10, pp.221-226 (1998). Optical tweezers have also been used to trap viruses and bacteria, although, very high laser powers were required. A. Ashkin and J.M. Dziedzic, "Optical trapping and manipulation of viruses and bacteria," Science vol. 235, pp. 1517-1520 (1987). A laser power of 1.5 W was required to trap virus fragments with diameters of 27 nm. Using the proposed optical antennas, trapping of virus particles with mW laser powers will be possible. For example, a plant virus known as the tomato bushy-stunt virus (TBSV), a spherical virus with a diameter of -30 nm in diameter may be trapped. 10059 J In another example of the prevent invention, optical antennas are used to trap TBSV particles, and then release these single virus particles into a tomato plant cell. It should be noted that it would not be possible to use conventional optical tweezers to trap virus particles for this application, since the high laser power required would damage the cell. The experiment is illustrated in Fig. 9. The fabricated antenna 910 is mounted in an optical microscope (not shown), and buffer solution containing TBSV particles will be introduced. The antenna 910 will be illuminated with a laser 930, generating intense fields in the gap (Fig. 9(a)). This will lead to a single TBSV particle 920 being drawn into, and trapped, in the gap (Fig. 9(b)). A cell 930 will be positioned above antenna by a conventional optical tweezers (not shown) (Fig. 9(c)). The optical antenna trapping laser Attn'y Docket No.: 4062.2929PCT

will then be turned off, releasing the virus 920, which will then infect the cell 930 (Fig. 9(d)).

KHk>Os A scanning electron micrograph (SEM) of an optical antenna fabricated on the facet of a near-IR commercial laser diode was performed. The semiconductor laser diode was manufactured by Sanyo. The laser facet was first coated with alumina and then with gold. FIB milling was then used to remove gold, resulting in two rectangular gold sections, each approximately ~120nm long and ~50nm wide, separated by a ~26nm gap. |0!H>U Experiments on measurement of the field distributions in near- field laser antennas of this type were performed. The experimental set-up (Fig. 10) is a scattering-type apertureless scanning near- field optical microscope. It allows the field distribution on the optical antenna to be measured with a spatial resolution given approximately by the radius of curvature of the AFM tip which, for sharp silicon tips, can be less than -lOnm. |00*^1 In the set-up, the near-field laser antenna device 1010 is mounted in an atomic force microscope (AFM) used as a scattering-type apertureless scanning near-field optical microscope. Apertureless SNOM has also been used to measure the field distributions in apertures fabricated on the facets of laser diodes, in a similar configuration to that employed here. Ashi Partovi, David Peale, Matthias Wuttig, Cherry A. Murray, George Zydzik, Leslie Hopkins, Kirk Baldwin, William S. Hobson, James Wynn, John Lopata, Lisa Dhar, Rob Chichester, and James H. -J. Yeh, "High Power Laser Light Source for Near-field Optics and Its Application to High-Density Optical Data Storage," Applied Physics Letters 75, pp. 1515 (1999). The laser diode 1020 is driven by a pulsed current source. A gold-coated silicon AFM tip is scanned over the optical antenna 1010 in non- contact mode, with the very end of the tip scattering light from the fields on the optical Attn'y Docket No.: 4062.2929PCT

antenna surface. Some of the light is scattered back into the laser cavity and onto the monitor photodiode 1040 that collects light from the back facet of the laser. The photodetector current is amplified and lock-in measurements are made at the oscillation frequency of the AFM cantilever 1030. This allows the signal corresponding to light scattered by the AFM tip to be extracted from the photodiode signal. Bernard Knoll and Fritz Keilmann, "Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy," Optics Communications vol. 182, pp. 321-328 (2000). ^HH>5i The measured optical near field image of the antenna structure was created using the set-up of Fig. 10. The intensity peaks were in the antenna gap, and at the ends of the antenna. In Fig. 11, a linescan of the data is plotted, showing that the antenna generates an intense field in the gap, confined to a region of -40 x 100 nm, far smaller than possible with diffraction-limited far-field approaches. Intensity enhancement also occurs at the ends of the optical antenna. We believe that this is due to the imaging method emphasizing the polarization component of the field distribution normal to the antenna surface. These results clearly indicate the ability of the near- field laser antenna to provide field enhancement. jOOo-U In addition to near- field imaging of optical antennas, we have carried out far-field measurements of the wavelength response. The simulated optical antenna was 1.56 μm long, made from gold (60 nm thick), and on a Si substrate. It was illuminated with a plane wave incident from within the Si substrate at λ = 10.375 μm. The calculated electric field intensity (<\E\²>) at the tip was -3905 times that of the illumination, and confined to a region at the tip of -280 nm (λ/37) by ~160nm (λ/66). Attn'y Docket No.: 4062.2929PCT

100651 Of importance is the result that the field enhancement was considerably stronger when the antenna was of the resonant length. For a perfect conductor, this occurs when the antenna is ~λ/2n long. CA. Balanis, Advanced Engineering Electromagnetics, John Wiley and Sons (1998). Gold antennas were fabricated on silicon substrates. To determine the antenna resonant wavelength, we measured the transmission spectra of fabricated antennas. From Fig. 12, it can be seen experimental and FDTD-calculated extinction spectra are in excellent agreement. The peak in the spectrum occurs close to the resonant length of - λ/2n, with the refractive index (3.42) of the substrate taken into account. HlOd(^ The wavelength response of optical antennas at near- infrared wavelengths has also been recently investigated. The fabricated antenna consisted of two gold triangles, each with an -88 nm side length, separated by a gap of less than 20 nm. The calculated intensity (square of electric field) on the antenna surface shown that the intensity peaked in the antenna gap, and was -1645 times the incident intensity. In Fig. 13, the experimental scattering spectra from a single optical antenna with a 32 nm gap width is shown, and compared with theory. Excellent agreement is demonstrated. 100671 An alternate embodiment of the present invention is described with reference to Figs. 14(a)-(c). An array of optical antennas 1340 is formed on the bottom of a microfluidic channel 1310. In Fig. 14(a)-(c), the optical antennas are shown as pairs of triangles, but other arrangements as discussed above may be used as well. The microfluidic channel in this exemplary embodiment is formed from a substrate 1320, such as glass or silicon, and a layer 1330 of a material such as PDMS (polydimethylsiloxane), glass, silicon or silicon nitride, into which a microfluidic channel Attn'y Docket No.: 4062.2929PCT

1310 is formed. As shown in Fig. 14(a), a fluid having particles of varied size and shape, such as virus particles and other species, flows past the optical antenna array 1340. While the laser is activated, the antenna array pulls particles of the appropriate size, i.e., whatever size particle that antennas are optimized for, onto the antenna array, which traps those particles. Other particles flow out of the microfluidic channel. A buffer fluid then flows though the channel, as shown in Fig. 14(b). The laser is then turned off, thereby releasing the virus particles into the buffer fluid flow so the virus particles may be collected. In this manner, particles such as virus particles, can be removed from a fluid and collected. |0O^S| The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.

Claims

Attn'y Docket No.: 4062.2929PCT

What is claimed is:

1, A method for trapping nanoscale particles comprising: illuminating an optical antenna with a focused laser beam to produce a region of enhanced electromagnetic fields; and trapping a nanoscale particle in said region of enhanced electromagnetic fields.

2, A method according to claim 1, wherein said nanoparticle comprises a virus.

^s A method according to claim 2, further comprising the steps of: positioning a cell above said antenna; and turning off said laser beam to release said virus to infect said cell with said virus.

4, A method according to claim 1, wherein said optical antenna comprises a triangle.

5> A method according to claim 1, wherein said optical antenna comprises one of gold, silver, copper and aluminum.

<\ A method according to claim 1 , wherein said optical antenna comprises a pair of triangles separated by a small gap and wherein said trapping step comprises trapping said nanoscale particle in said small gap.

?, A method according to claim 1, wherein said optical antenna comprises a disc.

5. A method according to claim 1 , wherein said optical antenna comprises a pair of discs separated by a small gap and wherein said trapping step comprises trapping said nanoscale particle in said small gap. Attn'y Docket No.: 4062.2929PCT

9, A method according to claim 1 , wherein said optical antenna comprises a pair of rods separated by a small gap and wherein said trapping step comprises trapping said nanoscale particle in said small gap.

I IK A method according to claim 1, wherein said optical antenna comprises an optical antenna of a resonant length.

I L A method according to claim 10, wherein said resonant length is 100- 300nm.

12.. A method for trapping a nanoparticle with an optical antenna comprising a section of gold fabricated at an end of an AFM tip, the method comprising the steps of: illuminating said optical antenna with a laser to generate intense fields at said end of said AFM tip; trapping a nanoparticle at said end of said AFM tip near a portion of a substrate; moving said AFM tip to transport said nanoparticle to a different portion of said substrate; and releasing said nanoparticle by switching off said laser.

13.. A method according to claim 12, wherein said optical antenna comprises a single section of gold having first and second ends wherein said first end is sharper than said second end.

14.. A method according to claim 12, wherein said optical antenna comprises a pair of triangles, rods or disks separated by a small gap and wherein said step of trapping a nanoparticle comprises trapping said nanoparticle in said gap.

15. An apparatus for trapping a nanoparticle, comprising: an optical antenna comprising a section of metal fabricated at an end of an AFM Attn'y Docket No.: 4062.2929PCT

tip; means for illuminating said optical antenna with a laser to generate intense fields at said end of said AFM tip; means for trapping a nanoparticle at said end of said AFM tip near a portion of a substrate; means for moving said AFM tip to transport said nanoparticle to a different portion of said substrate; and means for releasing said nanoparticle by switching off said laser. lev An apparatus according to claim 15, wherein said optical antenna comprises a pair of triangles separated by a small gap.

1 ?.. An apparatus according to claim 15, wherein said optical antenna comprises a pair of rods separated by a small gap.

1 S- An apparatus according to claim 15, wherein said optical antenna comprises a pair of disks separated by a small gap. IV, An apparatus according to claim 15, wherein said optical antenna comprises one of gold, silver and aluminum.

20, A nanoparticle separator comprising: a microfluidic channel; an array of optical antennas arranged on a bottom of said microfluidic channel, wherein when illuminated with a focused laser beam each optical antenna in said array produces a region of enhanced electromagnetic fields for trapping a nanoparticle.

2 L A method for separating nanoparticles comprising the steps of: passing a fluid containing microparticles through a microfluidic channel, wherein Attn'y Docket No.: 4062.2929PCT

an array of optical antennas is arranged on the bottom of said microfluidic channel; illuminating said array of optical antennas with a focused laser beam, thereby causing said array of optical antennas to capture nanoparticles; passing a buffer fluid through said channel; removing said focused laser beam from said optical antennas to release said captured nanoparticles into said buffer fluid.