Jochen Arlt

Optical micromanipulation using a Bessel light beam

Abstract We demonstrate a technique for optical manipulation of micron-sized particles, including biological samples, using a zeroth-order Bessel light beam. The central maximum of such a beam offers a “non-diffracting” focal line of light. This line focus is well suited to rotationally align rod-like particles along the beam direction and to build stacks of particles. We have stacked up to nine 5 μ m spheres above one another and manipulated this particle chain as a whole. Furthermore, we have observed laser guiding (transport) of 1 μ m particles along the Bessel beam axis over 1 mm, which is over 10 times the Rayleigh range for a comparable Gaussian beam.

Generation of a beam with a dark focus surrounded by regions of higher intensity: the optical bottle beam

A computer-generated hologram is used to form an optical beam with a localized intensity null at its focus. The beam is a superposition of two Laguerre-Gaussian modes that are phased so that they interfere destructively to give a beam focus that is surrounded in all directions by regions of higher intensity. Beams of this kind will have applications in the optical trapping of macroscopic objects or atoms; hence the term optical bottle beam.

Orbital angular momentum of a high-order Bessel light beam

The orbital angular momentum density of Bessel beams is calculated explicitly within a rigorous vectorial treatment. This allows us to investigate some aspects that have not been analysed previously, such as the angular momentum content of azimuthally and radially polarized beams. Furthermore, we demonstrate experimentally the mechanical transfer of orbital angular momentum to trapped particles in optical tweezers using a high-order Bessel beam. We set transparent particles of known dimensions into rotation, where the sense of rotation can be reversed by changing the sign of the singularity. Quantitative results are obtained for rotation rates. This papers animations are available from the Multimedia Enhancements page.

The production of multiringed Laguerre–Gaussian modes by computer-generated holograms

Abstract A computer-generated hologram is used for the production of high-order multiringed Laguerre–Gaussian modes. These holograms differ from those previously reported in that they have an additional circular discontinuity. The holograms are used in transmission and are designed to convert the fundamental Hermite–Gaussian laser mode into a Laguerre–Gaussian mode with specific azimuthal and radial indices. The optical efficiency exceeds 40% with a radial mode purity of approximately 80%.

An experiment to observe the intensity and phase structure of Laguerre–Gaussian laser modes

We outline an easily reproduced experiment that allows the student to investigate the intensity and phase structure of transverse laser modes. In addition to discussing the usual Hermite–Gaussian laser modes we detail how Laguerre–Gaussian laser modes can be obtained by the direct conversion of the Hermite–Gaussian output. A Mach–Zehnder interferometer allows the phase structure of the Laguerre–Gaussian modes to be compared with the phase structure of a plane wave with the same frequency. The resulting interference patterns clearly illustrate the azimuthal phase dependence of the Laguerre–Gaussian modes, which is the origin of the orbital angular momentum associated with each of them.

Cell Biology of Conidial Anastomosis Tubes in Neurospora crassa

ABSTRACT Although hyphal fusion has been well documented in mature colonies of filamentous fungi, it has been little studied during colony establishment. Here we show that specialized hyphae, called conidial anastomosis tubes (CATs), are produced by all types of conidia and by conidial germ tubes of Neurospora crassa. The CAT is shown to be a cellular element that is morphologically and physiologically distinct from a germ tube and under separate genetic control. In contrast to germ tubes, CATs are thinner, shorter, lack branches, exhibit determinate growth, and home toward each other. Evidence for an extracellular CAT inducer derived from conidia was obtained because CAT formation was reduced at low conidial concentrations. A cr-1 mutant lacking cyclic AMP (cAMP) produced CATs, indicating that the inducer is not cAMP. Evidence that the transduction of the CAT inducer signal involves a putative transmembrane protein (HAM-2) and the MAK-2 and NRC-1 proteins of a mitogen-activated protein kinase signaling pathway was obtained because ham-2, mak-2, and nrc-1 mutants lacked CATs. Optical tweezers were used in a novel experimental assay to micromanipulate whole conidia and germlings to analyze chemoattraction between CATs during homing. Strains of the same and opposite mating type were shown to home toward each other. The cr-1 mutant also underwent normal homing, indicating that cAMP is not the chemoattractant. ham-2, mak-2, and nrc-1 macroconidia did not attract CATs of the wild type. Fusion between CATs of opposite mating types was partially inhibited, providing evidence of non-self-recognition prior to fusion. Microtubules and nuclei passed through fused CATs.

Optical dipole traps and atomic waveguides based on Bessel light beams

We theoretically investigate the use of Bessel light beams generated using axicons for creating optical dipole traps for cold atoms and atomic waveguiding. Zeroth-order Bessel beams can be used to produce highly elongated dipole traps, allowing for the study of one-dimensional trapped gases and realization of a Tonks gas of impenetrable bosons. First-order Bessel beams are shown to be able to produce tight confined atomic waveguides over centimeter distances.

High-order Laguerre-Gaussian laser modes for studies of cold atoms

Abstract The generation of high-order Laguerre–Gaussian (LG) laser modes at 780 nm from a simple external-cavity diode laser is demonstrated. The beams are derived from computer-generated holograms and a high conversion efficiency of 40% is achieved for LG beams with mode indices from l =1 to l =6. The line width of the external cavity diode laser is below 1 MHz. This system is suitable for studies of trapped and cooled atoms using LG beams.

The generation of Bessel beams at millimetre-wave frequencies by use of an axicon

An axicon is used to generate a Bessel beam at 90 GHz in the millimetre-wave region of the spectrum. The Bessel beam has a central intensity maximum of approximately 4 mm in diameter that is maintained over a propagation distance greater than 60 mm

Revolving interference patterns for the rotation of optically trapped particles

Abstract Optically trapped objects are rotated controllably in the interference pattern between a Laguerre–Gaussian (LG) beam and a Gaussian beam. In this work the interference pattern is analysed and its properties as it propagates are modelled, showing the important role played by the Guoy-phase of the two interfering beams. An analysis of producing controlled rotation of the interference pattern using a glass plate is presented demonstrating the ease with which the rotation can be controlled.