Vertical differential interference contrast

Author(s): Michael Warber, Tobias Haist, Malte Hasler, and Wolfgang Osten

Abstract:

“We propose a new phase contrast filtering technique based on a combination of a focused and a defocused point-spread-function. This way, an axial shear is introduced in the imaging system. Compared to conventional differential interference contrast, an isotropic behavior is achieved. The lateral resolution is improved compared to conventional defocusing. Furthermore, the digital combination of multiple images leads to strongly enhanced visualization of small structures. We show simulated results as well as experimental results using a spatial-light modulator-based microscope.”

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Publication: Optical Engineering, (subscription required)

Issue/Year/DOI: Opt. Eng., Volume 51, Issue 1, 013204, (2012)
doi:10.1117/1.OE.51.1.013204

Scattered light fluorescence microscopy in three dimensions

Author(s): Giulia Ghielmetti and Christof M. Aegerter

Abstract:

“Recently, we have proposed a method to image fluorescent structures behind turbid layers at diffraction limited resolution using wave-front shaping and the memory effect. However, this was limited to a raster scanning of the wave-front shaped focus to a two dimensional plane. In applications, it can however be of great importance to be able to scan a three dimensional volume. Here we show that this can be implemented in the same setup. This is achieved by the addition of a parabolic phase shift to the shaped wave-front. Via the memory effect, this phase shift leads to a shift of the interference based focus in the z-direction, thus opening the possibility of three dimensional imaging using scattered light fluorescence microscopy. Here, we show an example of such a three dimensional image of fluorescent nano-beads taken behind a turbid layer more than 10 mean free paths thick. Finally, we discuss the differences of the scanning in the z-direction with that in the x–y plane and the corresponding possibilities and limitations of the technique.”

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Publication: Optics Express, (free download)

Issue/Year/DOI: Optics Express, Vol. 20, Issue 4, pp. 3744-3752 (2012)
doi:10.1364/OE.20.003744

Positional stability of holographic optical traps

Author(s): Arnau Farré, Marjan Shayegan, Carol López-Quesada, Gerhard A. Blab, Mario Montes-Usategui, Nancy R. Forde, and Estela Martín-Badosa

Abstract:

“The potential of digital holography for complex manipulation of micron-sized particles with optical tweezers has been clearly demonstrated. By contrast, its use in quantitative experiments has been rather limited, partly due to fluctuations introduced by the spatial light modulator (SLM) that displays the kinoforms. This is an important issue when high temporal or spatial stability is a concern. We have investigated the performance of both an analog-addressed and a digitally-addressed SLM, measuring the phase fluctuations of the modulated beam and evaluating the resulting positional stability of a holographic trap. We show that, despite imparting a more unstable modulation to the wavefront, our digitally-addressed SLM generates optical traps in the sample plane stable enough for most applications. We further show that traps produced by the analog-addressed SLM exhibit a superior pointing stability, better than 1 nm, which is comparable to that of non-holographic tweezers. These results suggest a means to implement precision force measurement experiments with holographic optical tweezers (HOTs).”

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Publication: Optics Express, (free download)

Issue/Year/DOI: Optics Express, Vol. 19, Issue 22, pp. 21370-21384 (2011)
doi:10.1364/OE.19.021370

Controlling ghost traps in holographic optical tweezers

Author(s): Christina Hesseling, Mike Woerdemann, Andreas Hermerschmidt, Cornelia Denz.

Abstract:

“Computer-generated holograms displayed by phase-modulating spatial light modulators have become a well- established tool for beam shaping purposes in holographic optical tweezers. Still, the generation of light intensity patterns with high spatial symmetry and simultaneously without interfering ghost traps is a challenge. We have implemented an iterative Fourier transform algorithm that is capable of controlling these ghost traps and demonstrate the benefit of this approach in the experiment.”

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Publication: Optics Letters, (subscription required)

Issue/Year/DOI: Optics Letters, Vol. 36, Issue 18, pp. 3657-3659 (2011)
doi:10.1364/OL.36.003657

Intracavity vortex beam generation

Author(s): Darryl Naidoo, Andrew Forbes, Kamel Aït-Ameur

Abstract:

“In this paper we explore vortex beams and in particular the generation of single LG0l modes and superpositions thereof. Vortex beams carry orbital angular momentum (OAM) and this intrinsic property makes them prevalent in transferring this OAM to matter and to be used in quantum information processing. We explore an extra-cavity and intra-cavity approach in LG0l mode generation respectively. The outputs of a Porro-prism resonator are represented by “petals” and we show that through a full modal decomposition, the “petal” fields are a superposition of two LG0l modes.”

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Publication: SPIE Proceedings, (subscription required)

Issue/Year/DOI: Proc. SPIE, Volume 8130, 813009 (2011)
doi:10.1117/12.902330

Poynting vector and orbital angular momentum density of superpositions of Bessel beams

Author(s): Igor A. Litvin, Angela Dudley, and Andrew Forbes

Abstract:

“We study theoretically the orbital angular momentum (OAM) density in arbitrary scalar optical fields, and outline a simple approach using only a spatial light modulator to measure this density. We demonstrate the theory in the laboratory by creating superpositions of non-diffracting Bessel beams with digital holograms, and find that the OAM distribution in the superposition field matches the predicted values. Knowledge of the OAM distribution has relevance in optical trapping and tweezing, and quantum information processing.”

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Publication: Optics Express, (free download)

Issue/Year/DOI: Optics Express, Vol. 19, Issue 18, pp. 16760-16771 (2011)
doi:10.1364/OE.19.016760

Generation of an axially super-resolved quasi-spherical focal spot using an amplitude-modulated radially polarized beam

Author(s): Han Lin, Baohua Jia, and Min Gu

Abstract:

“An axially super-resolved quasi-spherical focal spot can be generated by focusing an amplitude-modulated radially polarized beam through a high numerical aperture objective. A method based on the unique depolarization pro perties of a circular focus is proposed to design the amplitude modulation. The generated focal spot shows a ratio of x∶y∶z=1∶1∶1.48 for the normalized FWHM in three dimensions, compared to that of x∶y∶z=1∶0.74∶1.72 under linear polarization (in the x direction) illumination. Moreover, the focusable light efficiency of the designed amplitude-modulated beam is 65%, which is more than 3 times higher than the optimized case under linear polarization and thus make the amplitude-modulated radial polarization beam more suitable for a wide range of applications.”

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Publication: Optics Letters, (subscription required)

Issue/Year/DOI: Optics Letters, Vol. 36, Issue 13, pp. 2471-2473 (2011)
doi:10.1364/OL.36.002471

Phase extraction in microscopy using tunable defocusing by means of a SLM

Author(s): Luis Camacho, Vicente Micó, Javier García, Zeev Zalevsky

Abstract:

“In many practical microscopy applications the use of phase information is crucial. In this contribution we propose a method for phase extraction in a microscopy system based on analysis of images with varying defocusing. The system has no mobile parts owing to the defocusing by means of a spatial light modulator. The base of the method is the captre of images in a microscope with varying tube lens focal lengths. This produce a set of intensity images, all of them related, because the can be generated by free space propagation of a complex distribution which is unknown.”

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Publication: SPIE Proceedings, (subscription required)

Issue/Year/DOI: Proc. SPIE 8082, 80820O (2011);
doi:10.1117/12.889591

Optical Trapping of Thermo‐responsive Microgel Particles by Holographic Optical Tweezers

Author(s): M. R. Rajesh Kannan, B. V. R. Tata, R. Dasgupta, S. Ahlawat, and P. K. Gupta

Abstract:

“Holographic Optical Tweezers (HOT) is a technique in which the phase of trapping laser is modulated for generating steerable, multiple optical traps in a sample chamber. An indigenously developed HOT set‐up at Raja Ramanna Centre for Advanced Technology, Indore has been used to trap thermo‐responsive poly(N‐isopropylacrylamide‐co‐acrylic acid) (pNIPAM‐co‐AAc) spherical particles of 1.6 μm diameter suspended in aqueous medium. The videos of the trapped particles were digitally processed to track the particle positions as a function of time. From these measurements lateral trap stiffness for pNIPAM‐co‐AAc particles was determined as a function of trap power and temperature using Equipartition and Boltzmann Statistics methods. Both the methods gave similar results and the value for the trap stiffness at 25 °C with trapping laser power of 33 mW was estimated to be 0.14±0.01 μN/m. Since the optical trap stiffness depends on particle size and refractive index which vary as a function of temperature the variation of the measured optical trap stiffness as a function of temperature could be used to determine the volume phase transition of the thermo‐responsive microgel particles. The results should also be useful in investigating the interaction between pNIPAM‐co‐AAc particles trapped in different lattice configurations that can be generated using HOT.”

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Publication: AIP Conference Proceedings, (subscription required)

Issue/Year/DOI: AIP Conf. Proc., Volume 1391, pp. 359-362, (2011)
doi:10.1063/1.3643549

Optical macro-tweezers: trapping of highly motile micro-organisms

Author(s): G Thalhammer, R Steiger, S Bernet and M Ritsch-Marte

Abstract:

“Optical micromanipulation stands for contact-free handling of microscopic particles by light. Optical forces can manipulate non-absorbing objects in a large range of sizes, e.g., from biological cells down to cold atoms. Recently much progress has been made going from the micro- down to the nanoscale. Less attention has been paid to going the other way, trapping increasingly large particles. Optical tweezers typically employ a single laser beam tightly focused by a microscope objective of high numerical aperture to stably trap a particle in three dimensions (3D). As the particle size increases, stable 3D trapping in a single-beam trap requires scaling up the optical power, which eventually induces adverse biological effects. Moreover, the restricted field of view of standard optical tweezers, dictated by the use of high NA objectives, is particularly unfavorable for catching actively moving specimens. Both problems can be overcome by traps with counter-propagating beams. Our ‘macro-tweezers’ are especially designed to trap highly motile organisms, as they enable three-dimensional all-optical trapping and guiding in a volume of 2 × 1 × 2 mm³. Here we report for the first time the optical trapping of large actively swimming organisms, such as for instance Euglena protists and dinoflagellates of up to 70 µm length. Adverse bio-effects are kept low since trapping occurs outside high intensity regions, e.g., focal spots. We expect our approach to open various possibilities in the contact-free handling of 50–100 µm sized objects that could hitherto not be envisaged, for instance all-optical holding of individual micro-organisms for taxonomic identification, selective collecting or tagging.”

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Publication: Journal of Optics, (subscription required)

Issue/Year/DOI: J. Opt. 13 (2011) 044024 (6pp),
doi:10.1088/2040-8978/13/4/044024