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

Poincaré-sphere representation of phase-mostly twisted nematic liquid crystal spatial light modulators

Author(s): V Durán, P Clemente, Ll Martínez-León, V Climent, J Lancis

Abstract:

“We establish necessary conditions in order to build a phase-only wavefront modulation system from a liquid crystal display. These conditions determine the dependence of the polarization state of the light emerging from the display on the addressing gray level. The analysis, which is carried out by means of the coherence-matrix formalism, includes the depolarization properties of the device. Two different types of polarization distributions at the output of the liquid crystal cells are found. This approach is applied to a twisted nematic liquid crystal display. In this case, an optimization algorithm must be designed in order to select the input polarization state that leads to the required distributions. We show that the Poincaré-sphere representation provides a convenient framework to design the optimization algorithm as it allows for a reduced number of degrees of freedom. This feature significantly decreases the computation time. Laboratory results are presented for a liquid crystal on silicon display showing a phase modulation depth greater than 2π rad with an intensity variation lower than 6%. In addition, a hybrid ternary modulation (HTM), an operation regime employed in holographic data storage, is achieved.”

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Publication: Journal of Optics A
Issue/Year: J. Opt. A: Pure Appl. Opt. (2009), Opt. 11 085403
DOI: 10.1088/1464-4258/11/8/085403

Influence of the incident angle in the performance of Liquid Crystal on Silicon displays

Author(s): A. Lizana, N. Martín, M. Estapé, E. Fernández, I. Moreno, A. Márquez, C. Iemmi, J. Campos, and M. J. Yzuel

Abstract:

“In this paper we experimentally analyze the performance of a twisted nematic liquid crystal on silicon (LCoS) display as a function of the angle of incidence of the incoming beam. These are reflective displays that can be configured to produce amplitude or phase modulation by properly aligning external polarization elements. But we demonstrate that the incident angle plays an important role in the selection of the polarization configuration. We performed a Mueller matrix polarimetric analysis of the display that demonstrates that the recently reported depolarization effect observed in this type of displays is also dependant on the incident angle. ”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 17, Issue 10, pp. 8491-8505 (2009)
DOI: 10.1364/OE.17.008491

Programmable two-dimensional optical fractional Fourier processor

Author(s): Jose Augusto Rodrigo, Tatiana Alieva, Maria L. Calvo

Abstract:

“A flexible optical system able to perform the fractional Fourier transform (FRFT) almost in real time is presented. In contrast to other FRFT setups the resulting transformation has no additional scaling and phase factors depending on the fractional orders. The feasibility of the proposed setup is demonstrated experimentally for a wide range of fractional orders. The fast modification of the fractional orders, offered by this optical system, allows to implement various proposed algorithms for beam characterization, phase retrieval, information processing, etc.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 17, Issue 7, pp. 4976-4983 (2009)
DOI: 10.1364/OE.17.004976

A novel three-dimensional holographic display system based on LC-R2500 spatial light modulatora

Author(s): Huadong Zheng, Yingjie Yu, Cuixia Dai

Abstract:

“A novel holographic display system is proposed in this paper. The system takes LC-R2500, a kind of reflective liquid crystal spatial light modulator, (LC-SLM) as the core display unit, which can meet the requirement of real-time reconstruction of three-dimensional (3D) objects from holograms in free space. The relationship between hologram recording and image reconstruction is discussed, and the parameters associated with the magnification of reconstructed image over original object are determined. Experimental results of holographic display using the system are also given in the end.”

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Publication: Optik – International Journal for Light and Electron Optics
Issue/Year: Optik – International Journal for Light and Electron Optics, Volume 120, Issue 9, May 2009, Pages 431-436
DOI: 10.1016/j.ijleo.2007.11.002

Fast parallel diffractive multi-beam femtosecond laser surface micro-structuring

Author(s): Zheng Kuang, Dun Liu, Walter Perrie, Stuart Edwardson, Martin Sharp, Eamonn Fearon, Geoff Dearden, Ken Watkins

Abstract:

“Fast parallel femtosecond laser surface micro-structuring is demonstrated using a spatial light modulator (SLM). The Gratings and Lenses algorithm, which is simple and computationally fast, is used to calculate computer generated holograms (CGHs) producing diffractive multiple beams for the parallel processing. The results show that the finite laser bandwidth can significantly alter the intensity distribution of diffracted beams at higher angles resulting in elongated hole shapes. In addition, by synchronisation of applied CGHs and the scanning system, true 3D micro-structures are created on Ti6Al4V.”

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Publication: Applied Surface Science
Issue/Year: Applied Surface Science, Volume 255, Issues 13-14, 15 April 2009, Pages 6582-6588
DOI: 10.1016/j.apsusc.2009.02.043

Correction of aberration in holographic optical tweezers using a Shack-Hartmann sensor

Author(s): Carol López-Quesada, Jordi Andilla, Estela Martín-Badosa

Abstract:

“Optical aberration due to the nonflatness of spatial light modulators used in holographic optical tweezers significantly deteriorates the quality of the trap and may easily prevent stable trapping of particles. We use a Shack-Hartmann sensor to measure the distorted wavefront at the modulator plane; the conjugate of this wavefront is then added to the holograms written into the display to counteract its own curvature and thus compensate the optical aberration of the system. For a Holoeye LC-R 2500 reflective device, flatness is improved from 0.8λ to λ/16 (λ=532 nm), leading to a diffraction-limited spot at the focal plane of the microscope objective, which makes stable trapping possible. This process could be fully automated in a closed-loop configuration and would eventually allow other sources of aberration in the optical setup to be corrected for.”

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Publication: Applied Optics
Issue/Year: Applied Optics, Vol. 48, Issue 6, pp. 1084-1090 (2009)
DOI: 10.1364/AO.48.001084

Generation of laser-induced cavitation bubbles with a digital hologram

Author(s): P. A. Quinto-Su, V. Venugopalan, and C.-D. Ohl

Abstract:

“We demonstrate a method using a spatial light modulator (SLM) to generate arbitrary 2-D spatial configurations of laser induced cavitation bubbles. The SLM acts as a phase hologram that controls the light distribution in the focal plane of a microscope objective. We generate cavitation bubbles over an area of 380×380 µm² with a 20x microscope objective through absorption of the pulsed laser light in a liquid ink solution. We demonstrate the ability to accurately position up to 34 micrometer sized bubbles using laser energies of 56 µJ.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 16, Issue 23, pp. 18964-18969
DOI: 10.1364/OE.16.018964

Time fluctuations of the phase modulation in a liquid crystal on silicon display: characterization and effects in diffractive optics

Author(s): I. Moreno, A. Lizana, A. Márquez, C. Iemmi, E. Fernández, J. Campos, and M. J. Yzuel

Abstract:

“In this paper we provide evidence of the temporal fluctuations of the phase modulation property of a liquid crystal on silicon (LCoS) display, and we analyze its effect when the device is used for displaying a diffractive optical element. We use a commercial twisted nematic LCoS display configured to produce a phase-only modulation, and we provide time resolved measurements of the diffraction efficiency that show rapid fluctuations of the phase modulation, in the millisecond order. We analyze how these fluctuations have to be considered in two typical methods for the characterization of the phase modulation: two beam interference and diffraction from a binary grating. We finally provide experimental results on the use of this device for displaying a computer generated hologram. A reduction of the modulation diffraction efficiency results from the phase modulation fluctuation.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 16, Issue 21, pp. 16711-16722 (2008)
DOI: 10.1364/OE.16.016711

Three-dimensional parallel holographic micropatterning using a spatial light modulator

Author(s): Nathan J. Jenness, Kurt D. Wulff, Matthew S. Johannes, Miles J. Padgett, Daniel G. Cole, and Robert L. Clark

Abstract:

“We present a micropatterning method for the automatic transfer and arbitrary positioning of computer-generated three-dimensional structures within a substrate. The Gerchberg-Saxton algorithm and an electrically addressed spatial light modulator (SLM) are used to create and display phase holograms, respectively. A holographic approach to light manipulation enables arbitrary and efficient parallel photo-patterning. Multiple pyramidal microstructures were created simultaneously in a photosensitive adhesive. A scanning electron microscope was used to confirm successful replication of the desired microscale structures. ”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 16, Issue 20, pp. 15942-15948 (2008)
DOI: 10.1364/OE.16.015942
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