Since (at least) a quarter of a century researchers are fascinated by light that carries orbital angular momentum. At Glasgow we routinely use a variety of techniques to imprint structure in the spatial amplitude, phase and polarization of a light beam, aiming to study the interaction with atomic vapours. In this presentation however, I will concentrate on the light fields themselves, and in particular on the strong correlations between the polarisation and spatial degree of freedom. We quantify these correlations via concurrence measurements, investigate their use for enhanced focussing, and employ them in a single-shot broad band polarimeter.
The mechanism by which light is slowed through ruby has been the subject of great debate. To distinguish between the two main proposed mechanisms, we investigate the problem in the time domain by modulating a laser beam with a chopper to create a clean square wave. By exploring the trailing edge of the pulsed laser beam propagating through ruby, we can determine whether energy is delayed beyond the input pulse. The effects of a time-varying absorber alone cannot delay energy into the trailing edge of the pulse, as a time-varying absorber can only attenuate a coherent pulse. Therefore, our observation of an increase in intensity at the trailing edge of the pulse provides evidence for a complicated model of slow light in ruby that requires more than just pulse reshaping. In addition, investigating the Fourier components of the modulated square wave shows that harmonic components with different frequencies are delayed by different amounts, regardless of the intensity of the component itself. Understanding the difference in delays of the individual Fourier components of the modulated beam reveals the cause of the distortion the pulse undergoes as it propagates through the ruby.
Light beams with unusual forms of wavefront offer a host of useful features to extend the repertoire of those developing
new optical techniques. Complex, non-uniform wavefront structures offer a wide range of optomechanical applications,
from microparticle rotation, traction and sorting, through to contactless microfluidic motors. Beams combining
transverse nodal structures with orbital angular momentum, or vector beams with novel polarization profiles, also present
new opportunities for imaging and the optical transmission of information, including quantum entanglement effects.
Whilst there are numerous well-proven methods for generating light with complex wave-forms, most current methods
work on the basis of modifying a conventional Hermite-Gaussian beam, by passage through suitably tailored optical
elements. It has generally been considered impossible to directly generate wave-front structured beams either by
spontaneous or stimulated emission from individual atoms, ions or molecules. However, newly emerged principles have
shown that emitter arrays, cast in an appropriately specified geometry, can overcome the obstacles: one possibility is a
construct based on the electronic excitation of nanofabricated circular arrays. Recent experimental work has extended
this concept to a phase-imprinted ring of apertures holographically encoded in a diffractive mask, generated by a
programmed spatial light modulator. These latest advances are potentially paving the way for creating new sources of
A high-intensity laser pulse can lead to a change of the group index of a material, so that the pulse within that
material is slowed to only hundreds of meters per second. This kind of slow-light phenomenon scales with the
optical intensity of the pulse. While previous experiments have produced this effect with an elliptical beam
passing through a spinning ruby window, a question remains as to whether the effect would be present in a
circular beam. Here we use two different methods of producing slow light in a round beam, showing that, while
less pronounced than the effect with an elliptical beam, a slow-light effect can be seen in a round beam.
The orbital angular momentum of light, and also of waves beyond the electromagnetic spectrum, is a powerful
concept in all systems with cylindrical or rotational symmetry. Expressing quantum images in terms of orbital
angular momentum modes allows one to describe image rotations in terms of OAM dependent phase shifts.
We discuss image rotations, and in particular Faraday rotations in optical systems, and predict a Faraday
rotation for electron vortices. Our considerations highlight connections between orbital angular momentum
features in different systems, in particular between image rotations in optical and electron systems, and also
between parametric processes in parametric down-conversion and atomic cascades. We compare the phasematching
conditions of the two latter systems and demonstrate the efficient transfer of OAM modes and their
superpositions from near-infrared pump light to blue light in a four-wave mixing process in rubidium vapour.
The bandwidth of any communication system, classical or quantum, is limited by the number of orthogonal states in which the information can be encoded. Quantum key distribution systems available commercially rely on the two-dimensional polarisation state of photons. Quantum computation has also been largely designed on the basis of qubits. However, a photon is endowed with other degrees of freedom, such as orbital angular momentum (OAM). OAM is an attractive basis to be used for quantum information because it is discrete and theoretically infinite-dimensional. This promises a higher information capacity per photon which can lead to more complex quantum computation protocols and more security and robustness for quantum cryptography. Entanglement of OAM naturally arises from spontaneous parametric down-conversion (SPDC). However, any practical experiment utilising the innately high-dimensional entanglement of the orbital angular momentum (OAM) state space of photons is subject to the modal capacity of the detection system. Only a finite subset of this space is accessible experimentally. Given such a constraint, we show that the number of measured, entangled OAM modes in photon pairs generated by SPDC can be increased by tuning the phase-matching conditions in the SPDC process. We achieve this by tuning the orientation angle of the nonlinear crystal generating the entangled photons.
A spinning medium is predicted to induce a slight rotation in a transmitted image.
We amplify this effect by use of ruby as a slow light medium, giving image rotations of
several degrees. In terms of the orbital angular momentum such rotations are analogous to the
mechanical Faraday effect.
We report a violation of the CHSH inequality for ghost-images. This is achieved by using two spatially separated
phase modulators within the context of a two-photon parametric down-conversion experiment. We obtain edge
enhanced images as a direct consequence of the quantum correlations in the orbital angular momentum (OAM)
of the down-converted photon pairs. For phase objects, with differently orientated edges, we show a violation of
the CHSH Bell-type inequality for an OAM subspace, thereby unambiguously revealing the quantum nature of
our ghost-imaging arrangement.
The angular profile and the orbital angular momentum of a light mode are related by Fourier transform. Any
modification of the angular distribution, e. g. via diffraction off a suitably programmed spatial light modulator,
influences the orbital angular momentum spectrum of the light. This holds true even at the single photon level.
We observe the influence of various angular masks on the orbital angular momentum spectrum, both in the near
and the far field, and describe the resulting patterns in terms of angular diffraction. If photons are entangled in
their orbital angular momentum, diffraction of one photon affects the orbital angular momentum spectrum of
its partner photon, and angular ghost diffraction can be measured in the coincidence counts. We highlight the
role of the angular Fourier relationship for these effects.
In the 1970s, Jones demonstrated a photon drag by showing that the translation of a window caused a slight displacement
of a transmitted light beam. Similarly he showed that a spinning medium slightly rotated the polarization state. Rather
than translating the medium, the speed of which is limited by mechanical considerations, we translate the image and
measure its lateral delay with respect to a similar image that has not passed through the window. The equivalence, or
lack of it, of the two frames is subtle and great care needs to be taken in determining whether or not similar results are to
Laguerre-Gaussian (LG) light beams possess discrete values of orbital angular momentum (OAM) of <i>l&barh;</i> per photon, where <i>l </i>is the azimuthal index of the mode. In principle <i>l </i>can take on any integer number, resulting in an unlimited amount of information that can be carried by any part of the beam - even a single photon. We have developed a technology demonstrator that uses OAM to encode information onto a light beam for free-space optical communications. In our demonstrator units both the encoding and decoding of the orbital angular momentum states is achieved using diffractive optical components (holograms). We use 9 different OAM values; one value is used for alignment purposes, the others carry data.