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(nanoplasmonic optical tweezers)

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Conventional optical tweezers Consider an object of radius R much smaller than the wavelength of the incident trapping light (known as the Rayleigh regime). Reducing the size of the object causes two main effects that both work against stable trapping. First, the magnitude of the restoring force decreases abruptly (following an R 3 law), which results in a shallower trapping well. Second, the damping of the trapped specimen decreases because of a reduction in viscous drag. Mathieu L. Juan, Maurizio Righini and Romain Quidant, Plasmon nano-optical tweezers, Nature Photonics, Vol. 5, pp. 349, 2011

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Evanescent wave An evanescent wave is a near-field standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. They are formed at the boundary between two media with different wave motion properties, and are most intense within one third of a wavelength from the surface of formation. http://www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfintro.html

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Silicon photonic crystal resonators Photonic crystal resonator for enhanced optical trapping. The stable trapping of particles ranging in size from 50 to 500 nm polystyrene nanoparticle (refractive index n = 1.59). (a) 3D schematic of the one- dimensional photonic crystal resonator optical trapping architecture (b) 3D FEM simulation illustrating the strong field confinement and amplification within the one- dimensional resonator cavity. The black arrows indicate the direction and magnitude of the local optical forces. Sudeep Mandal, Xavier Serey, and David Erickson, Nanomanipulation Using Silicon Photonic Crystal Resonatorsm, Nano Letters, Vol. 10, pp. 99, 2010

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Whispering gallery mode The sensor belongs to a class of devices called whispering-gallery-mode resonators. One famous whispering gallery is St. Paul's Cathedral in London. If you stand under the dome close to the wall and speak softly to the wall, someone on the opposite side of the gallery is able to hear what you say. The reason is the sound bounces along the wall of the gallery with very little loss of energy and so can be heard at a great distance. However, if you speak at normal volume, what you say can no longer be understood. The sound travels around the dome more than once, and the recirculating signal gets mixed up and garbled. In a miniature version of a whispering gallery, laser light is coupled into a circular waveguide, such as a glass ring. When the light strikes the boundary of the ring at a grazing angle it is reflected back into the ring. The light wave can make many trips around the ring before it is absorbed, but only frequencies of light that fit perfectly into the circumference of the ring can do so. If the circumference is a whole number of wavelengths, the light waves superimpose perfectly each trip around. This perfect match between the frequency and the circumference is called a resonance, or whispering-gallery mode. http://www.science20.com/news_articles/whisperinggallerymode_resonators_why_those_antiodor_socks_you_buy_will_be_safer http://www.sonicwonders.org/?p=57 http://en.wikipedia.org/wiki/File:St_Paul%27s_Cathedral_Whispering_Gallery.jpg#filelinks

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Silicon microring resonators We observe trapping of 1.1 m diameter polystyrene particles that revolve around the microring at a few hertz and are stable for several minutes. (a) Schematic diagram of planar microring cavity trapping system; (b) cross section of the instantaneous field distribution (Hx: x-component of magnetic field) of the whispering-gallery mode of the microring by 3D-FDTD simulation with a 1W input power in the waveguide. The outer edge of the microring is at x 5 m. Shiyun Lin, Ethan Schonbrun, and Kenneth Crozier, Optical Manipulation with Planar Silicon Microring Resonators, Nano Letters, Vol. 10. pp. 2408, 2010

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Ring resonator switch Schematic of optofluidic ring resonator switch. 3 m diameter fluorescent polystyrene beads were trapped. (a) Rendered picture of device with PDMS microfluidics. (b and c) Illustration of switching mechanism due to optical gradient forces when the ring is strongly coupled at the resonant wavelength. Allen H. J. Yanga and David Erickson, Optofluidic ring resonator switch for optical particle transport, Lab on a Chip, Vol. 10, pp. 769, 2010

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Surface plasmons polaritons In this experiment, the patterned surface was illuminated through a glass prism (known as the Kretschmann configuration) by an unfocused (~100 m waist), linearly p polarized near- infrared laser beam. The gold pattern was covered by a static fluidic chamber containing a diluted aqueous suspension of micrometre-sized polystyrene beads. When the direction of the incident light matched the SPP (surface plasmon polaritons) resonance angle, the light efficiently coupled to the SPP mode supported by the top gold/water interface. Owing to the combined effects of the boundaries and asymmetrical illumination, the plasmonic field was concentrated in the forward portion of the disk. No solution flow was used; it was the incident evanescent field that pushed the polystyrene beads at the glasswater interface along the incident in-plane k vector, thus guiding them towards the trapping region of the surface Trapping times of up to several hours were achieved for an incident laser intensity of 107 W m 2 around two orders of magnitude smaller than that required to trap a bead of the same size through conventional optical tweezers. Good for 1 m particles Maurizio Righini, Giovanni Volpe, Christian Girard, Dmitri Petrov, and Romain Quidant1, Surface plasmon optical tweezers: tunable optical manipulation in the femtonewton range, Physics Reiew Letters, Vol. 100, pp. 186804, 2008 MAURIZIO RIGHINI, ANNA S. ZELENINA, CHRISTIAN GIRARD AND ROMAIN QUIDANT, Parallel and selective trapping in a patterned plasmonic landscape, Nature Physics, Vol. 3, pp. 477, 2007

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Field enhancement at sharply point metal tip under laser illumination Near field of a gold tip in water illuminated by two different monochromatic waves at = 810 nm. Direction and Polarization of the incident wave are indicated by the k and E vectors. The figures show contours of E 2 (factor of 2 between successive lines). The scaling is given by the numbers in the figures (multiples of the exciting field). No enhancement at the tip in (a); enhancement of 3000 in (b). The field in (b) is almost rotationally symmetric in the vicinity of the tip. Lukas Novotny, Randy X. Bian, and X. Sunney Xie, Theory of Nanometric Optical Tweezers, Physics Review Letters, Vol. 79, No. 4, pp. 645, 1997

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SPP and LSP Although the simple disk geometry enables dielectric beads as small as 1 m to be trapped by down-scaling the pad diameter accordingly, it fails for smaller sizes. At the heart of this failure is the change in nature of SPs when the disk becomes commensurable with or smaller than the SP wavelength. Owing to boundary effects, subwavelength gold structures support localized surface plasmon (LSP) resonances, which differ in several ways from SPPs in an extended film. Unlike SPPs on flat and extended metal interfaces, LSPs are associated with bound electron plasmas in nanovoids or particles with dimensions much smaller than the incident wavelength. SPPs have a continuous dispersion relation and therefore exist over a wide range of frequencies, but LSP resonances exist only over a finite frequency range owing to the additional constraints imposed by their finite dimensions. The spectral position of this resonance is governed by the particles size and shape, as well as by the dielectric functions of both the metal and the surrounding media. LSPs can be directly coupled with propagating light, whereas SPPs cannot. Mathieu L. Juan, Maurizio Righini and Romain Quidant, Plasmon nano-optical tweezers, Nature Photonics, Vol. 5, pp. 349, 2011

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Gap antennas Much higher control of plasmonic fields can be achieved through an alternative technique that exploits the strong electromagnetic coupling between several adjacent plasmonic nanostructures. Among the most interesting geometries, plasmonic antennas (also known as plasmonic dimers) have received much attention for their ability to concentrate propagating light well beyond the diffraction limit. Gap antennas usually consist of two identical metallic particles separated by a nanoscale dielectric gap. When the incident field is linearly polarized along the vector connecting the particles, capacitive effects lead to a confined and intense light spot within the gap region. To demonstrate the action of nanometric optical tweezers, solid polystyrene beads (6 mm, 1 mm and 200 nm in diameter, refractive index of 1.6) were trapped and nanomanipulated by a focused laser beam A. N. Grigorenko, N. W. Roberts, M. R. Dickinson and Y. Zhang, Nanometric optical tweezers based on nanostructured substrates, Nature Photonics, Vol. 2, pp. 365, 2011

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Waveguide gap tweezer The geometry of the coupled waveguide system with a dielectric cylinder waveguide above the substrate with a nanoscale gap in between. An attractive optical force will be exerted on the waveguide due to the coupling between the waveguide mode and the substrate mode. The optical trapping force will be applied on a single nanoparticle toward the center of the nanoscale gap. Wave guide diamter 220 nm, gap 10~30nm. Polystyrene of diamter 5nm can be trapped (COMSOL simulation) Xiaodong Yang, Yongmin Liu, Rupert F. Oulton, Xiaobo Yin, and Xiang Zhang, Optical Forces in Hybrid Plasmonic Waveguides, Nano Letters, Vol. 11, pp. 321, 2011 metal

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Self-induced back-action trapping Plasmonic structures can also be devised such that the trapped object induces a constructive effect that favours trapping. For this, one first aims to optimize the trapping efficiency by properly engineering the plasmonic mode such that the local intensity within the trap is maximized when the object is present. As a consequence, the momentum of the (plasmonic) photons interacting with the object experiences significant changes as the object moves in and out of the trap. Owing to momentum conservation, these changes create an additional dynamical force field that is by definition automatically synchronized with the objects dynamics. This self-induced back-action (SIBA) was recently demonstrated using a nanoaperture in a metallic film. 50 nm polystyrene sphere Mathieu L. Juan, Reuven Gordon, Yuanjie Pang, Fatima Eftekhari and Romain Quidant, Self-induced back-action optical trapping of dielectric nanoparticles, Nature Physics, Vol. 5, pp. 915, 2009

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Double hole We use a double- nanohole (100nm diameters, 15 nm tip separation)in a gold film to experimentally trap individual nanospheres, including 20 nm polystyrene spheres and 12 nm silica spheres, and lately of 3.4nm single bovine serum albumin Yuanjie Pang and Reuven Gordon, Optical Trapping of 12 nm Dielectric Spheres Using Double-Nanoholes in a Gold Film, Nano Letters, Vol. 11, pp. 3763, 2011 Yuanjie Pang and Reuven Gordon, Optical Trapping of of a single protein, Nano Letters, Vol. 12, pp. 402, 2012

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Diabolo nanoantenna 300-nm-diameter fluorescent polystyrene particles Ju-Hyung Kang, Kipom Kim, Ho-Seok Ee, Yong-Hee Lee, Tae-Young Yoon, Min-Kyo Seo, Hong-Gyu Park, Low-power nano-optical vortex trapping via plasmonic diabolo nanoantennas Nature Communications Vol. 2, pp. 582, 2011

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