Collinear superposition of multiple helical beams generated by a single azimuthally modulated phase-only element

Author(s): J. Lin, X.-C. Yuan, S. H. Tao, and R. E. Burge

Abstract:

“We propose a highly efficient approach to generating multihelix beams that contain more than one helical mode, and the power distribution over helical modes is adjustable. A multihelix beam embedded with three collinear helical modes is demonstrated by use of a spatial light modulator.”

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Publication: Optics Letters
Issue/Year: Optics Letters, Vol. 30, Issue 24, pp. 3266-3268, 2006
DOI: 10.1364/OL.30.003266

Femtosecond laser ablation elemental mass spectrometry

Author(s): Roland Hergenröder, Ota Samek, Vanja Hommes

Abstract:

“Laser ablation mass spectrometry (LA-MS) has always been an interesting method for the elemental analysis of solid samples. Chemical analysis with a laser requires small amounts of material. Depending on the analytical detection system, subpicogram quantities may be sufficient. In addition, a focused laser beam permits the spatial characterization of heterogeneity in solid samples typically with micrometer resolution in terms of lateral and depth dimensions. With the advent of high-energy, ultra-short pulse lasers, new possibilities arise. The task of this review is to discuss the principle differences between the ablation process of short (>1 ps) and ultra-short (<1 ps) pulses. Based on the timescales and the energy balance of the process that underlies an ablation event, it will be shown that ultra-short pulses are less thermal and cause less collateral damages than longer pulses. The confinement of the pulse energy to the focal region guarantees a better spatial resolution in all dimensions and improves the analytical figures of merit (e.g., fractionation). Applications that demonstrate these features and that will be presented are in-depth profiling of multi-layer samples and the elemental analysis of biological materials.”

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Publication: Mass Spectrometry Reviews
Issue/Year: Mass Spectrometry Reviews, Volume 25, Issue 4 , Pages 551 – 572 (2005)
DOI: 10.1002/mas.20077

Accurate encoding of arbitrary complex fields with amplitude-only liquid crystal spatial light modulators

Author(s): Victor Arrizón, Guadalupe Méndez, and David Sánchez-de-La-Llave

Abstract:

“We show that computer generated holograms, implemented with amplitude-only liquid crystal spatial light modulators, allow the synthesis of fully complex fields with high accuracy. Our main discussion considers modified amplitude holograms whose transmittance is obtained by adding an appropriate bias function to the real cosine computer hologram of the encoded signal. We first propose a bias function, given by a soft envelope of the signal modulus, which is appropriate for perfect amplitude modulators. We also consider a second bias term, given by a constant function, which results appropriate for modulators whose amplitude transmittance is coupled with a linear phase modulation. The influence of the finite pixel size of the spatial light modulator is compensated by digital pre-filtering of the encoded complex signal. The performance of the discussed amplitude CGHs is illustrated by means of numerical simulations and the experimental synthesis of high order Bessel beams.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 13, Issue 20, pp. 7913-7927, 2005
DOI: 10.1364/OPEX.13.007913

Fractional optical vortex beam induced rotation of particles

Author(s): S. Tao, X-C. Yuan, J. Lin, X. Peng, and H. Niu

Abstract:

“We experimentally demonstrate optical rotation and manipulation of microscopic particles by use of optical vortex beams with fractional topological charges, namely fractional optical vortex beams, which are coupled in an optical tweezers system. Like the vortex beams with integer topological charges, the fractional optical vortex beams are also capable of rotating particles induced by the transfer of orbital angular momentum. However, the unique radial opening (low-intensity gap) in the intensity ring encompassing the dark core, due to the fractional nature of the beam, hinders the rotation significantly. The fractional vortex beams orbital angular momentum and radial opening are exploited to guide and transport microscopic particles.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 13, Issue 20, pp. 7726-7731, 2005
DOI: 10.1364/OPEX.13.007726

Creating permanent 3D arrangements of isolated cells using holographic optical tweezers

Author(s): Pamela Jordan, Jonathan Leach, Miles Padgett, Paul Blackburn, Neil Isaacs, Mattias Goksör, Dag Hanstorp, Amanda Wright, John Girkin and Jonathan Cooper

Abstract:

“We report the creation of permanent 3D configurations of cells, at predefined positions, within a gelatin matrix. The technique used holographic optical tweezers to manipulate individual E. coli within a solution comprising monomer precursors. The matrix was then set and after the laser beam was removed, we were able to demonstrate that the structures remained intact for many days. We were also able to demonstrate that, in the presence of appropriate nutrients, the E. coli survived within the gelatin matrix for several days. The technique could have a number of potential future applications, including the arrangement of a variety of different cell types in complex architectures, as motifs for promoting tissue differentiation and growth within the field of cell engineering.”

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Publication: Lab on a Chip (free download)

Issue/Year/DOI: Lab Chip, 2005, 5, 1224-1228
DOI: 10.1039/b509218c

Raman imaging of floating cells

Author(s): Caitriona Creely, Giovanni Volpe, Gajendra Singh, Marta Soler, and Dmitri Petrov

Abstract:

“Raman imaging can yield spatially resolved biochemical information from living cells. To date there have been no Raman images published of cells in suspension because of the problem of immobilizing them suitably to acquire space-resolved spectra. In this paper in order to overcome this problem the use of holographic optical tweezers is proposed and implemented, and data is shown for spatially resolved Raman spectroscopy of a live cell in suspension.”

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Publication: Optics Express
Issue/Year: Optics Express, Vol. 13, Issue 16, pp. 6105-6110, 2005
DOI: 10.1364/OPEX.13.006105

Femtosecond pulse shaping using a liquid–crystal display: Applications to depth profiling analysis

Author(s): Ota Samek, Vanja Hommes, Roland Hergenröder, Sergei V. Kukhlevsky

Abstract:

“We report on a beam shaping technique for femtosecond laser pulses based on a liquid-crystal display. The system is capable of modifying femtosecond Gaussian beams to a flattop beam. A pattern projected onto liquid-crystal display modifies the incoming Gaussian beam intensity so that flattop intensity profile is obtained. The process is monitored online using a charge-coupled device camera so that the intensity distribution of each pulse is known. An experimental example of the depth profile of a Cr layer on a Si substrate obtained using such a modified beam is presented.”

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Publication: Review of Scientific Instruments
Issue/Year: Rev. Sci. Instrum. 76, 086104 (2005)
DOI: 10.1063/1.1994897

Spiral interferometry

Author(s): Severin Fürhapter, Alexander Jesacher, Stefan Bernet, and Monika Ritsch-Marte

Abstract:

“We present a surprising modification of optical interferometry. A so-called spiral phase element in the beam path of a standard microscope results in an interferogram of phase samples, for which the interference fringes have the shape of spirals instead of closed contour lines as in traditional interferograms. This configuration overrides the basic problem of interferometry, i.e., that elevations and depressions cannot be distinguished. Therefore a complete sample profile can be reconstructed from a single exposure, promising, e.g., high-speed metrology with a single laser pulse. The method is easy to implement, it does not require a spatially separated reference beam, and it is optimally stable against environmental noise.”

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Publication: Optics Letters
Issue/Year: Optics Letters, Vol. 30, Issue 15, pp. 1953-1955 (2005)
DOI: 10.1364/OL.30.001953

Encoding fully-complex transmittance with coupled amplitude-phase liquid-crystal modulator

Author(s): Victor M. Arrizón, Luis A. Gonzalez Lopez

Abstract:

“We propose a holographic code for synthesis of fully-complex transmittance, which can be implemented employing a twisted-nematic liquid-crystal display, two linear polarizers, and a He-Ne laser. This simple setup provides a reduced phase range and amplitude modulation with significant variance. Our holographic code efficiently exploits this constrained modulation for the accurate encoding of arbitrary complex transmittance. Two experimental examples illustrate the good performance of the holographic code.”

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Publication: SPIE – Optical Engineering
Issue/Year: Opt. Eng., Vol. 44, 070502 (2005)
DOI: 10.1117/1.1978427

Shadow Effects in Spiral Phase Contrast Microscopy

Author(s): Alexander Jesacher, Severin Fürhapter, Stefan Bernet, and Monika Ritsch-Marte

Abstract:

“Recently it has been demonstrated that spatial filtering of images in microscopy with a spiral phase element in a Fourier plane of the optical path results in a strong edge enhancement of object structures. In principle the operation is isotropic, i.e., all phase edges of a sample object are highlighted simultaneously, independent of their local direction. However, here we demonstrate that the symmetry can be broken intentionally by controlling the phase of the central area of a spiral phase hologram, which is displayed at a computer controlled spatial light modulator. This produces an apparent shadow effect which can be rotated at video rate. The resulting relieflike impression of the sample topography with a longitudinal resolution in the subwavelength regime is demonstrated by imaging a standard low contrast test sample consisting of a human cheek cell.”

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Publication: Physical Review Letters
Issue/Year: Phys. Rev. Lett. 94, 233902 (2005)
DOI: 10.1103/PhysRevLett.94.233902
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