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

Reference wave adaptation in digital lensless Fourier holography by means of a spatial light modulator

Author(s): Thomas Meeser, Claas Falldorf, Christoph von Kopylow, and Ralf B. Bergmann.

Abstract:

“In this publication an experimental configuration for Digital Holography is presented which allows for a modification of the reference wave. Using a reflective liquid crystal Spatial Light Modulator (SLM) placed in the reference arm. The benefit of this approach is demonstrated by applying it to Digital Lensless Fourier Holography. As the optimal configuration of the reference wave depends on the position of the object under investigation, we use the approach to electronically adapt the reference wave to varying positions of the object along and perpendicular to the optical axis and without the requirement of mechanically moving parts.”

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Publication: SPIE Proceedings
Issue/Year: Proc. SPIE, Volume 8082, 808206 (2011)
DOI: 10.1117/12.889472

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

Far field subwavelength focusing using optical eigenmodes

Author(s): Jörg Baumgartl, Sebastian Kosmeier, Michael Mazilu, Edward T. F. Rogers, Nikolay I. Zheludev, and Kishan Dholakia.

Abstract:

“We report the focusing of light to generate a subdiffractive, subwavelength focal spot of full width half maximum 222 nm at an operating wavelength of 633 nm using an optical eigenmode approach. Crucially, the spot is created in the focal plane of a microscope objective thus yielding a practical working distance for applications. The optical eigenmode approach is implemented using an optimal superposition of Bessel beams on a spatial light modulator. The effects of partial coherence are also discussed. This far field method is a key advance toward the generation of subdiffractive optical features for imaging and lithographic purposes.”

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Publication: Applied Physics Letters
Issue/Year: Appl. Phys. Lett. 98, 181109 (2011)
DOI: 10.1063/1.3587636