Computer generation of optimal holograms for optical trap arrays

Author(s): Roberto Di Leonardo, Francesca Ianni, and Giancarlo Ruocco

Abstract:

“We propose a new iterative algorithm for obtaining optimal holograms targeted to the generation of arbitrary three dimensional structures of optical traps. The algorithm basic idea and performance are discussed in conjunction to other available algorithms. We show that all algorithms lead to a phase distribution maximizing a specific performance quantifier, expressed as a function of the trap intensities. In this scheme we go a step further by introducing a new quantifier and the associated algorithm leading to unprecedented efficiency and uniformity in trap light distributions. The algorithms performances are investigated both numerically and experimentally.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 15, Issue 4, pp. 1913-1922
DOI: 10.1364/OE.15.001913

Optically controlled grippers for manipulating micron-sized particles

Author(s): Graham Gibson, Louise Barron, Fiona Beck, Graeme Whyte and Miles Padgett

Abstract:

“We report the development of a joystick controlled gripper for the real-time manipulation of micron-sized objects, driven using holographic optical tweezers (HOTs). The gripper consists of an arrangement of four silica beads, located in optical traps, which can be positioned and scaled in order to trap an object indirectly. The joystick can be used to grasp, move (lateral or axial), and change the orientation of the target object. The ability to trap objects indirectly allows us to demonstrate the manipulation of a strongly scattering micron-sized metallic particle.”

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Publication:New Journal of Physics
Issue/Year: New J. Phys. 9 14, Jan. 2007
DOI: 10.1088/1367-2630/9/1/014

Diffractive optical elements designed for highly precise far-field generation in the presence of artifacts typical for pixelated spatial light modulators

Author(s): Gabriel Milewski, David Engström, and Jörgen Bengtsson

Abstract:

“Diffractive optical elements (DOEs) realized by spatial light modulators (SLMs) often have features that distinguish them from most conventional, static DOEs: strong coupling between phase and amplitude modulation, a modulation versus steering parameter characteristic that may not be precisely known (and may vary with, e.g., temperature), and deadspace effects and interpixel cross talk. For an optimal function of the DOE, e.g. as a multiple-beam splitter, the DOE design must account for these artifacts. We present an iterative design method in which the optimal setting of each SLM pixel is carefully chosen by considering the SLM artifacts and the design targets. For instance, the deadspace-interpixel effects are modeled by dividing the pixel to be optimized, and its nearest neighbors, into a number of subareas, each with its unique response and far-field contribution. Besides the customary intensity control, the design targets can also include phase control of the optical field in one or more of the beams in the beam splitter. We show how this can be used to cancel a strong unwanted zeroth-order beam, which results from using a slightly incorrect modulation characteristic for the SLM, by purposely sending a beam in the same direction but with the opposite phase. All the designs have been implemented on the 256 × 256 central pixels of a reflective liquid crystal on silicon SLM with a selected input polarization state and a direction of transmission axis of the output polarizer such that for the available different pixel settings a phase modulation of ~2Pi rad could be obtained, accompanied by an intensity modulation depth as high as >95%.”

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Publication: Applied Optics
Issue/Year: Applied Optics, Vol. 46, Issue 1, pp. 95-105
DOI: 10.1364/AO.46.000095

Automated trapping, assembly, and sorting with holographic optical tweezers

Author(s): Stephen C. Chapin, Vincent Germain, and Eric R. Dufresne

Abstract:

“We combine real-time feature recognition with holographic optical tweezers to automatically trap, assemble, and sort micron-sized colloidal particles. Closed loop control will enable new applications of optical micromanipulation in biology, medicine, materials science, and possibly quantum computation.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 14, Issue 26, pp. 13095-13100 , 2006
DOI: 10.1364/OE.14.013095

Wavelet-modified fringe-adjusted joint transform correlator

Author(s): Alpana Bhagatji, Naveen K. Nishchal, and Arun K. Gupta, B. P. Tyagi

Abstract:

“In this paper, we implement a wavelet-modified fringe-adjusted joint transform correlator (JTC) for real-time target recognition applications. In real-time situation the input scene is captured using a CCD/thermal camera. The obtained joint power spectrum is multiplied with a pre-synthesized fringe-adjusted filter and the resultant function is processed with an appropriately scaled wavelet filter. The wavelet-modified fringe-adjusted JTC has been found to yield better results in comparison to the conventional fringe-adjusted JTC. To suppress the undesired strong dc, the resultant function is differentiated. Differential processing the wavelet-modified fringe-adjusted joint power spectrum removes the zero-order spectra and hence improves the detection efficiency. To focus the correlation terms in different planes in order to capture one of the desired autocorrelation peaks and discard the strong dc and another autocorrelation peak, chirp-encoding technique has also been applied. Computer simulation and experimental results are presented. ”

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Publication: SPIE Proceedings
Issue/Year: Proc. SPIE, Vol. 6405, 640513 (2006);
DOI: 10.1117/12.697891

A liquid crystal atmospheric turbulence simulator

Author(s): Lifa Hu, Li Xuan, Zhaoliang Cao, Quanquan Mu, Dayu Li, and Yonggang Liu

Abstract:

“In the paper, a method to calculate the time evolution turbulence wavefronts based on the covariance method is theoretically presented in detail. According to it, the time-evolution wavefronts disturbed by atmospheric turbulence were experimentally generated by our LC atmospheric turbulence simulator (ATS) based on liquid crystal on silicon (LCOS) with high pixel density, and measured with a wavefront sensor. The advantage of such a LC ATS over a conventional one is that it is flexible with considering the weather parameters of wind speed and Cn², and is relatively easy to control.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 14, Issue 25, pp. 11911-11918 (2006)
DOI: 10.1364/OE.14.011911

Direct measurement of the skew angle of the Poynting vector in a helically phased beam

Author(s): Jonathan Leach, Stephen Keen, Miles J. Padgett, Christopher Saunter, and Gordon D. Love

Abstract:

“We measure the local skew angle of the Poynting vector within a helically-phased, exp (il φ), beam using a Shack Hartmann wavefront sensor. It is the skew angle of the Poynting vector with respect to the beam axis that gives rise to the orbital angular momentum of a light beam. We confirm that this skew angle is l/kr, corresponding to an orbital angular momentum of lћ per photon. Measurement of orbital angular momentum in this way is an alternative to interferometric techniques giving a non-ambiguous result to both the magnitude and sign of l from a single measurement, without any restriction on the optical bandwidth.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 14, Issue 25, pp. 11919-11924 (2006)
DOI: 10.1364/OE.14.011919

Optimization of Liquid-Crystal Spatial Light Modulator for Precise Phase Generation

Author(s): Seow-Hwang Eng; Dong Mei Cai; Zhenglin Wang; Alameh, K.; Wenhan Jiang;

Abstract:

“Spatial light modulators (SLMs) are recently emerging as wavefront generation or reconstruction devices. In this paper, we use a liquid-crystal SLM (LC-SLM) as a wavefront generation device through the modulation of the spatial phase distribution of an incident light beam. We characterize the phase modulation performance of the LC-SLM, and identified a polarization-configuration to minimize its amplitude modulation effects in order to maximize the contrast of reconstructed phase holograms. We also investigated the feasibility of determining the optical surface uniformity of the LC-SLM through computations of the interference fringes, and identified the disadvantages of this method. This new approach may be used to determine the optical surface quality of other optical devices.”

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Publication: Conference on Optoelectronic and Microelectronic Materials and Devices
Issue/Year: Conference on Optoelectronic and Microelectronic Materials and Devices (2006)
DOI: 10.1109/COMMAD.2006.4429891

Digital speckle shearing interferometer using a liquid-crystal spatial light modulator

Author(s): Shuai Zhao and Po Sheun Chung

Abstract:

“A digital speckle shearing interferometer is developed using a dynamic binary phase grating realized by an electrically addressed, twisted nematic liquid-crystal spatial light modulator as the shearing element. The standard phase-shifting algorithm can be easily achieved by changing the grating pattern displayed on the spatial light modulator. The method has a number of advantages including variable sensitivity, no moving part, and no requirement for calibration of the value of the phase shift. The error due to the unexpected diffraction order is also discussed. ”

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Publication: SPIE – Optical Engineering
Issue/Year: SPIE – Optical Engineering, Vol. 45, 105606 (2006);
DOI: 10.1117/1.2360940

Reverse orbiting of microparticles in optical vortices

Author(s): A. Jesacher, S. Fürhapter, C. Maurer, S. Bernet, and M. Ritsch-Marte

Abstract:

“We report the observation of particles trapped at an air-water surface orbiting in a reverse direction with respect to the orbital angular momentum of the light field. The effect is explained by a combination of asymmetric particle shape and confinement of the particle on the 2D air-water interface. The experiment highlights the strong influence of the particle shape on the momentum transfer, an effect that is often not considered in optical trapping experiments.”

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Publication: Optics Letters
Issue/Year: Optics Letters, Vol. 31, Issue 19, pp. 2824-2826
DOI: 10.1364/OL.31.002824
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