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.”

Link to Publications Page

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.”

Link to Publications Page

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.”

Link to Publications Page

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.”

Link to Publications Page

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.”

Link to Publications Page

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.”

Link to Publications Page

Publication: Journal of Optics, (subscription required)

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

Optimal resolution in Fresnel incoherent correlation holographic fluorescence microscopy

Author(s): Gary Brooker, Nisan Siegel, Victor Wang, and Joseph Rosen

Abstract:

“Fresnel Incoherent Correlation Holography (FINCH) enables holograms and 3D images to be created from incoherent light with just a camera and spatial light modulator (SLM). We previously described its application to microscopic incoherent fluorescence wherein one complex hologram contains all the 3D information in the microscope field, obviating the need for scanning or serial sectioning. We now report experiments which have led to the optimal optical, electro-optic, and computational conditions necessary to produce holograms which yield high quality 3D images from fluorescent microscopic specimens. An important improvement from our previous FINCH configurations capitalizes on the polarization sensitivity of the SLM so that the same SLM pixels which create the spherical wave simulating the microscope tube lens, also pass the plane waves from the infinity corrected microscope objective, so that interference between the two wave types at the camera creates a hologram. This advance dramatically improves the resolution of the FINCH system. Results from imaging a fluorescent USAF pattern and a pollen grain slide reveal resolution which approaches the Rayleigh limit by this simple method for 3D fluorescent microscopic imaging.”

Link to Publications Page

Publication: Optics Express, (free download)

Issue/Year/DOI: Optics Express, Vol. 19, Issue 6, pp. 5047-5062 (2011)
doi:10.1364/OE.19.005047

Dynamic generation of Debye diffraction-limited multifocal arrays for direct laser printing nanofabrication

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

Abstract:

“We propose a Debye-theory-based iterative method to produce accurate phase patterns for generating highly uniform diffraction-limited multifocal arrays with a high-NA objective. It is shown that by using the Debye method, the uniformity of the diffraction-limited focal arrays can reach 99%, owing to the critical consideration of the depolarization effect associated with high-NA objectives. The generated phase patterns are implemented in fast dynamic laser printing nanofabrication for the generation of individually controlled high-quality microvoid arrays in a solid polymer material by a single exposure of a femtosecond laser beam. As a result of the high-quality multifocal arrays, functional three-dimensional photonic crystals possessing multiple stopgaps with suppression up to 80% in transmission spectra are demonstrated.”

Link to Publications Page

Publication: Optics Letters, (subscription required)

Issue/Year/DOI: Optics Letters, Vol. 36, Issue 3, pp. 406-408 (2011)
doi:10.1364/OL.36.000406

Flexible contrast for low-coherence interference microscopy by Fourier-plane filtering with a spatial light modulator

Author(s): Stefan E. Schausberger, Bettina Heise, Christian Maurer, Stefan Bernet, Monika Ritsch-Marte, and David Stifter

Abstract:

“We propose a full-field low-coherence interference (LCI) microscope that can provide different contrast modes using Fourier-plane filtering by means of a spatial light modulator. By altering the phase and spatial frequencies of the backreflected wavefront from the sample arm of the interferometer, we are able to change the contrast in the depth-resolved LCI images. We demonstrate that different types of contrast modes, such as, e.g., spiral phase contrast, can successfully be emulated to provide specific enhancement of internal structures and edges and to reveal complementary details within the samples under investigation.”

Link to Publications Page

Publication: Optics Letters, (subscription required)

Issue/Year/DOI: Optics Letters, Vol. 35, Issue 24, pp. 4154-4156 (2010)
doi:10.1364/OL.35.004154

Isotropic Diffraction-Limited Focusing Using a Single Objective Lens

Author(s): Emeric Mudry, Eric Le Moal, Patrick Ferrand, Patrick C. Chaumet, and Anne Sentenac

Abstract:

“Focusing a light beam through a lens produces an anisotropic spot elongated along the optical axis, because the light comes from only one side of the focal point. Using the time-reversal concept, we show that isotropic focusing can be realized by placing a mirror after the focal point and shaping the incident beam. This idea is applied to confocal microscopy and brings about a dramatic improvement of the axial resolution.”

Link to Publications Page

Publication: Physical Review Letters, (subscription required)

Issue/Year/DOI: Phys. Rev. Lett., Volume 105, 203903 (2010)
doi:10.1103/PhysRevLett.105.203903