Kevin O'Holleran

Light beams with fractional orbital angular momentum and their vortex structure

Light emerging from a spiral phase plate with a non-integer phase step has a complicated vortex structure and is unstable on propagation. We generate light carrying fractional orbital angular momentum (OAM) not with a phase step but by a synthesis of Laguerre-Gaussian modes. By limiting the number of different Gouy phases in the superposition we produce a light beam which is well characterised in terms of its propagation. We believe that their structural stability makes these beams ideal for quantum information processes utilising fractional OAM states.

Topology of optical vortex lines formed by the interference of three, four, and five plane waves

When three or more plane waves overlap in space, complete destructive interference occurs on nodal lines, also called phase singularities or optical vortices. For super positions of three plane waves, the vortices are straight, parallel lines. For four plane waves the vortices form an array of closed or open loops. For five or more plane waves the loops are irregular. We illustrate these patterns numerically and experimentally and explain the three-, four- and five-wave topologies with a phasor argument.

Investigation of the confocal wavefront sensor and its application to biological microscopy

Wavefront sensing in the presence of background light sources is complicated by the need to restrict the effective depth of field of the wavefront sensor. This problem is particularly significant in direct wavefront sensing adaptive optic (AO) schemes for correcting imaging aberrations in biological microscopy. In this paper we investigate how a confocal pinhole can be used to reject out of focus light whilst still allowing effective wavefront sensing. Using a scaled set of phase screens with statistical properties derived from measurements of wavefront aberrations induced by C. elegans specimens, we investigate and quantify how the size of the pinhole and the aberration amplitude affect the transmitted wavefront. We suggest a lower bound for the pinhole size for a given aberration strength and quantify the optical sectioning provided by the system. For our measured aberration data we find that a pinhole of size approximately 3 Airy units represents a good compromise, allowing effective transmission of the wavefront and thin optical sections. Finally, we discuss some of the practical implications of confocal wavefront sensing for AO systems in microscopy.

Polarization effects on contrast in structured illumination microscopy

In this Letter, we present an analysis of the effects of polarization state on the pattern contrast in a structured illumination microscope. Using vectorial ray tracing methods, we show that the contrast varies nonmonotonically with both the numerical aperture of the microscope objective lens and the orientation of the electric field with respect to the meridional plane. By careful selection of these two parameters, high pattern contrast can be obtained without polarization rotation, reducing the cost and complexity of structured illumination imaging systems and increasing light throughput and imaging speed. We present experimental results that show good agreement with theoretical predictions and discuss the implications for super-resolution imaging.

Knotted and tangled threads of darkness in light beams

When multiple light beams overlap in three-dimensional space, their interference produces tangled lines of complete darkness. These lines are called optical vortices and may be infinitely long, or form closed loops which can be linked or knotted. The vortex lines can be obtained from combining random waves (such as optical speckle). Alternatively, specific configurations of looped, linked or knotted vortex lines may be produced using holographic techniques to implement mathematically derived constructions. In the random superpositions, numerical experiments indicate that the tangle of vortex lines has a fractal nature, whereas the holographic approach allows the construction of isolated optical vortex knots. The presence of such a knot has implications for the topology of the whole field. Whether such topological features produced by interfering waves are merely curiosities or correspond to subtle physical phenomena remains an open question.

Observation of Gouy-phase-induced transversal intensity changes in focused beams

We created superpositions of two different TEM modes in focused beams. Such modes show relative dephasing along the beam axis due to Gouys phase. This leads to interference effects and significant modifications of a beams transverse intensity distribution near the beam focus. We investigate the features of the resulting field profiles.

The Hunt for Vortex Knots in 3-D Speckle Fields

Random optical speckle patterns in three dimensions contain complex tangles of optical vortices. We investigate the knotting and linking of these lines in computer simulations of optical fields using tools from knot theory.