Mixing via thermocapillary generation of flow patterns inside a microfluidic drop

Author(s): María Luisa Cordero, Hans Olav Rolfsnes, Daniel R Burnham, Paul A Campbell, David McGloin, Charles N Baroud

Abstract:

“The heating produced by a focused laser has been shown to provide a range of manipulation tools on droplets in microfluidic situations, through the generation of thermocapillary flows whose net result is to produce a force on the drop. In particular, droplets of water in oil that are produced in microchannels can be blocked in a special test section. Here, the manipulation of the flow within the droplet is explored through spatial and temporal modulation of the laser pattern used to block the drop. When a stationary pattern of two laser spots is used, the flow preserves the mirror symmetry inside the drop, as happens in the case of two alternating spots if the frequency of the switching is higher than the response rate of the fluid. Lower frequency switching produces a time periodic flow that breaks the mirror symmetry and which leads to efficient mixing inside the droplet. The mixing that is produced by this alternating flow is studied both experimentally and using numerical simulations of particle trajectories from measured velocity fields. This mixing can be optimized for certain parameter ranges, namely by varying the distance between the spots and the forcing frequency.”

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Publication: New Journal of Physics
Issue/Year: New Journal of Physics, Volume 11, July 2009
DOI: 10.1088/1367-2630/11/7/075033

Color translucent head-up display based on a segmented Fourier hologram

Author(s):

Jaroslaw Suszek, Maciej Sypek, Andrzej Siemion, Agnieszka Fajst, Michal Makowski and Andrzej Kolodziejczyk

Abstract:

“The paper presents the numerical and experimental results of reconstructing images from a spatially segmented Fourier hologram with additional carrier frequencies, acting as a translucent head-up display (HUD). Successful reconstructions of colorful images visible at the infinity were achieved. The design procedure is described and some limitations of the method are discussed, like the appearance of ghost images. Exemplary results from numerical and real experiments are given. In order to obtain color images three beams of primary colors were successfully utilized to create a color mixture.”

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Publication: Photonics Letters of Poland
Issue/Year: Vol 1, No 3 (2009)
DOI: 10.4302/plp.2009.3.02

Simplified approach to diffraction tomography in optical microscopy

Author(s): Reto Fiolka, Kai Wicker, Rainer Heintzmann, and Andreas Stemmer

Abstract:

“We present a novel microscopy technique to measure the scattered wavefront emitted from an optically transparent microscopic object. The complex amplitude is decoded via phase stepping in a common-path interferometer, enabling high mechanical stability. We demonstrate theoretically and practically that the incoherent summation of multiple illumination directions into a single image increases the resolving power and facilitates image reconstruction in diffraction tomography. We propose a slice-by-slice object-scatter extraction algorithm entirely based in real space in combination with ordinary z-stepping. Thereby the computational complexity affiliated with tomographic methods is significantly reduced. Using the first order Born approximation for weakly scattering objects it is possible to obtain estimates of the scattering density from the exitwaves.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 17, Issue 15, pp. 12407-12417 (2009)
DOI: 10.1364/OE.17.012407

Phase and amplitude control of a multimode LMA fiber beam by use of digital holography

Author(s): M. Paurisse, M. Hanna, F. Druon, P. Georges, C. Bellanger, A. Brignon, and J. P. Huignard

Abstract:

“Amplitude and phase control of the output beam of a multimode LMA fiber supporting 4 modes is demonstrated by digital holography in both continuous and ns pulsed regimes at 1064 nm. Our system allows dynamic compensation of beam pointing instabilities, external perturbations introducing low order aberrations and fluctuations of the relative phase of the modes supported by the fiber.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 17, Issue 15, pp. 13000-13008 (2009)
DOI: 10.1364/OE.17.013000

Spiral phase filtering and orientation-selective edge detection/enhancement

Author(s): Guohai Situ, Giancarlo Pedrini, Wolfgang Osten

Abstract:

“A spiral phase plate with an azimuthal structure exp[iϕ](0⩽ϕ<2π) has been used as a filter in a 4f system to achieve edge enhancement. Generally such edge-enhanced effect is isotropic, i.e., each edge of an input pattern is enhanced to the same degree regardless of its orientation. We found that one can achieve anisotropic edge enhancement by breaking down the symmetry of the filtering process. This can be done in two ways: first, by use of a fractional spiral phase filter (SPF) with a fractional topological charge and a controllable orientation of the edge discontinuity, and second, by the lateral shifting of the SPF. We interpret this process as a vortex formation due to the diffraction of the Fourier spectrum of the input pattern by a SPF with an integer and fractional topological charge. Optical experiments using a spatial light modulator were carried out to verify our proposal.”

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Publication: J. Opt. Soc. Am. A
Issue/Year: JOSA A, Vol. 26, Issue 8, pp. 1788-1797 (2009)
DOI: 10.1364/JOSAA.26.001788

Characterization of a spatial light modulator and its application in phase retrieval

Author(s): C. Kohler, F. Zhang, and Wolfgang Osten

Abstract:

“Recently a phase retrieval method using a movable phase plate as modulator has been proposed [Phys. Rev. A75, 043805 (2007)PLRAAN1050-294710.1103/PhysRevA.75.043805]. This method is applicable to general complex-valued fields and exhibits rapid convergence and high robustness to noise. In this paper, we demonstrate how to use this technique to characterize the phase shifting properties of a liquid-crystal modulator, and in turn we use the characterized modulator as the modulation device in the presented phase retrieval method. The adoption of a dynamic modulator gives a much more robust and flexible setup.”

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Publication: Applied Optics
Issue/Year: Applied Optics, Vol. 48, Issue 20, pp. 4003-4008 (2009)
DOI: 10.1364/AO.48.004003

Structured illumination microscopy of a living cell

Author(s): Liisa M. Hirvonen, Kai Wicker, Ondrej Mandula und Rainer Heintzmann

Abstract:

“Due to diffraction, the resolution of imaging emitted light in a fluorescence microscope is limited to about 200 nm in the lateral direction. Resolution improvement by a factor of two can be achieved using structured illumination, where a fine grating is projected onto the sample, and the final image is reconstructed from a set of images taken at different grating positions. Here we demonstrate that with the help of a spatial light modulator, this technique can be used for imaging slowly moving structures in living cells.”

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Publication: European Biophysics Journal
Issue/Year: European Biophysics Journal, Volume 38, Number 6, 807-812 (2009)
DOI: 10.1007/s00249-009-0501-6