The very long range nature of capillary interactions in liquid films

Author(s): Di Leonardo R, Saglimbeni F, Ruocco G.

Abstract:

“Micron-sized objects confined in thin liquid films interact through forces mediated by the deformed liquid-air interface. These capillary interactions provide a powerful driving mechanism for the self-assembly of ordered structures such as photonic materials or protein crystals. We demonstrate how optical micro-manipulation allows the direct measurement of capillary interactions between mesoscopic objects. The force falls off as an inverse power law in particles separation. We derive and validate an explicit expression for this exponent whose magnitude is mainly governed by particle size. For micron-sized objects we found an exponent close to, but smaller than 1, making capillary interactions a unique example of strong and very long ranged forces in the mesoscopic world.”

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Publication: Physical Review Letters
Issue/Year: Phys. Rev. Lett., Volume 100, Issue 10, 106103 (2008)
DOI: 10.1103/PhysRevLett.100.106103

Non-scanning motionless fluorescence three-dimensional holographic microscopy

Author(s): Joseph Rosen and Gary Brooker

Abstract:

“Holography is an attractive imaging technique as it offers the ability to view a complete three-dimensional volume from one image. However, holography is not widely applied to the field of three-dimensional fluorescence microscopic imaging, because fluorescence is incoherent and creating holograms requires a coherent interferometer system. Although scanning one beam of an interferometer pattern across the rear aperture of an objective to excite fluorescence in a specimen overcomes the coherence limitation, the mechanical scanning is complicated, which makes the image capturing slow, and the process is limited to low-numerical-aperture objectives. Here we present the first demonstration of a motionless microscopy system (FINCHSCOPE) based on Fresnel incoherent correlation holography, and its use in recording high-resolution three-dimensional fluorescent images of biological specimens. By using high-numerical-aperture objectives, a spatial light modulator, a CCD camera and some simple filters, FINCHSCOPE enables the acquisition of three-dimensional microscopic images without the need for scanning.”

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Publication: Nature Photonics
Issue/Year: Nature Photonics 2, 190 – 195 (2008)
DOI: 10.1038/nphoton.2007.300

Holographic optical tweezers with real-time hologram calculation using a phase-only modulating LCOS-based SLM at 1064 nm

Author(s): Andreas Hermerschmidt, Sven Krüger, Tobias Haist, Susanne Zwick, Michael Warber, Wolfgang Osten

Abstract:

“We present a method that enables the generation of arbitrary positioned dual-beam traps without additional hardware in a single-beam holographic optical tweezers setup. By this approach stable trapping at low numerical aperture and long working distance is realized with an inverse standard research microscope. Simulations and first experimental results are presented. Additionally we present first steps towards using the method to realize a holographic 4pi-microscope. We will also give a detailed analysis of the phase-modulating properties and especially the spatial-frequency dependent diffraction efficiency of holograms reconstructed with the phase-only LCOS spatial light modulator used in our system. Finally, accelerated hologram optimization based on the iterative Fourier transform algorithm is done using the graphics processing unit of a consumer graphics board.”

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Publication: SPIE Digital Library
Issue/Year: SPIE Proceedings, Vol. 6905, 690508 (2008),
DOI: 10.1117/12.764649

Stimulated emission depletion microscopy with a supercontinuum source and fluorescence lifetime imaging

Author(s): Egidijus Auksorius, Bosanta R. Boruah, Christopher Dunsby, Peter M. P. Lanigan, Gordon Kennedy, Mark A. A. Neil, and Paul M. W. French

Abstract:

“We demonstrate stimulated emission depletion (STED) microscopy implemented in a laser scanning confocal microscope using excitation light derived from supercontinuum generation in a microstructured optical fiber. Images with resolution improvement beyond the far-field diffraction limit in both the lateral and axial directions were acquired by scanning overlapped excitation and depletion beams in two dimensions using the flying spot scanner of a commercially available laser scanning confocal microscope. The spatial properties of the depletion beam were controlled holographically using a programmable spatial light modulator, which can rapidly change between different STED imaging modes and also compensate for aberrations in the optical path. STED fluorescence lifetime imaging microscopy is demonstrated through the use of time-correlated single photon counting.”

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Publication: Optics Letters
Issue/Year: Optics Letters, Vol. 33, Issue 2, pp. 113-115, 2008
DOI: 10.1364/OL.33.000113

Eigenmodes of a hydrodynamically coupled micron-size multiple-particle ring

Author(s): R. Di Leonardo, S. Keen, J. Leach, C. D. Saunter, G. D. Love, G. Ruocco and M. J. Padgett

Abstract:

“We use a continuous acquisition, high-speed camera with integrated centroid tracking to simultaneously measure the positions of a ring of micron-sized particles held in holographic optical tweezers. Hydrodynamic coupling between the particles gives a set of eigenmodes, each one independently relaxing with a characteristic decay rate (eigenvalue) that can be measured using our positional data. Despite the finite particle size, we find an excellent agreement between the measured eigenvalues and those numerically predicted by Oseen theory applied to the two-dimensional (2D) ring geometry. Particle motions are also analyzed in terms of the alternative eigenmode set obtained by wrapping onto the ring the eigenmodes of a 1D periodic chain. We identify the modes for which the periodic chain is a good approximation to the ring and those for which it is not.”

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Publication: Physical Review E
Issue/Year: Phys. Rev. E 76, Issue 6, 061402 (2007)
DOI: 10.1103/PhysRevE.76.061402

Electrostatic Interactions of Colloidal Particles in Nonpolar Solvents: Role of Surface Chemistry and Charge Control Agents

Author(s): Sunil K. Sainis, Vincent Germain, Cecile O. Mejean, and Eric R. Dufresne

Abstract:

“We study the electrostatic and hydrodynamic interactions of colloidal particles in nonpolar solvents. Using blinking optical tweezers, we can extract the screening length, K-1, the effective surface potential, |eζ*|, and the hydrodynamic radius, ah, in a single measurement. We apply this technique to suspensions of polystyrene and poly(methyl methacrylate) particles in hexadecane with soluble charge control agents, aerosol sodium di-2-ethylhexylsulfosuccinate (AOT) and polyisobutylene succinimide (OLOA-1200). We find that the electrostatic interactions of these particles depend sensitively on surface composition as well as on the concentration and chemistry of the charge control agent.”

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Publication: Langmuir
Issue/Year: Langmuir, 2008, 24 (4), pp 1160–1164
DOI: 10.1021/la702432u

Parallel photopolymerisation with complex light patterns generated by diffractive optical elements

Author(s): Lóránd Kelemen, Sándor Valkai, and Pál Ormos

Abstract:

“Photopolymerisation by scanning a focused laser beam is a powerful method to build structures of arbitrary complexity with submicrometer resolution. We introduce parallel photopolymerisation to enhance the efficiency. Instead of multidimensional scanning of a single focus, the structure is generated simultaneously with diffractive patterns. We used fixed diffractive optical elements (DOEs), kinoforms, and Spatial Light Modulators (SLMs). The possibilities of photopolymerisation using SLM were investigated: the added flexibility using the programmable device is demonstrated. By using these DOEs, straight and helical cross shaped columns were produced with a single scan at a rate about an order of magnitude faster than by simple scanning. The produced helical structures could be rotated by optical tweezers.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 15, Issue 22, pp. 14488-14497 (2007)
DOI: 10.1364/OE.15.014488

Parallel photopolymerisation with complex light patterns generated by diffractive optical elements

Author(s): Lóránd Kelemen, Sándor Valkai, and Pál Ormos

Abstract:

“Photopolymerisation by scanning a focused laser beam is a powerful method to build structures of arbitrary complexity with submicrometer resolution. We introduce parallel photopolymerisation to enhance the efficiency. Instead of multidimensional scanning of a single focus, the structure is generated simultaneously with diffractive patterns. We used fixed diffractive optical elements (DOEs), kinoforms, and Spatial Light Modulators (SLMs). The possibilities of photopolymerisation using SLM were investigated: the added flexibility using the programmable device is demonstrated. By using these DOEs, straight and helical cross shaped columns were produced with a single scan at a rate about an order of magnitude faster than by simple scanning. The produced helical structures could be rotated by optical tweezers.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 15, Issue 22, pp. 14488-14497 (2007)
DOI: 10.1364/OE.15.014488

The dark spots of Arago

Author(s): Pascal Fischer, Susan E. Skelton, Christopher G. Leburn, Casey T. Streuber, Ewan M. Wright, and Kishan Dholakia

Abstract:

“We explore the diffraction and propagation of Laguerre- Gaussian beams of varying azimuthal index past a circular obstacle both experimentally and numerically. When the beam and obstacle centers are aligned the famous spot of Arago, which arises for zero azimuthal index, is replaced for non-zero azimuthal indices by a dark spot of Arago, a simple consequence of the conserved phase singularity at the beam center. We explore how the dark spot of Arago behaves as the beam and obstacle centers are progressively misaligned, and find that the central dark spot may break into several dark spots of Arago for higher incident azimuthal index beams. ”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 15, Issue 19, pp. 11860-11873
DOI: 10.1364/OE.15.011860

Optical ferris wheel for ultracold atoms

Author(s): S. Franke-Arnold, J. Leach, M. J. Padgett, V. E. Lembessis, D. Ellinas, A. J. Wright, J. M. Girkin, P. Ohberg, and A. S. Arnold

Abstract:

“We propose a versatile optical ring lattice suitable for trapping cold and quantum degenerate atomic samples. We demonstrate the realisation of intensity patterns from pairs of Laguerre-Gauss (exp(iℓө) modes with different ℓ indices. These patterns can be rotated by introducing a frequency shift between the modes. We can generate bright ring lattices for trapping atoms in red-detuned light, and dark ring lattices suitable for trapping atoms with minimal heating in the optical vortices of blue-detuned light. The lattice sites can be joined to form a uniform ring trap, making it ideal for studying persistent currents and the Mott insulator transition in a ring geometry. ”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 15, Issue 14, pp. 8619-8625 (2007)
DOI: 10.1364/OE.15.008619
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