The November 2008 research newsletter

Contents

• Small stuff
• Vectorial wave billiards: Some insight in VCSELs
• It has a certain ring to it
• Postgrads meet at Ross Priory
• Ultrafast physicists & chemists meet at Strathclyde (briefly)
• People with HiDEaS
• Iman Roqan in new University
• Rob Martin's holiday in France
• Force field
• A matter of gravitas

Editorial

Welcome to the November 2008 research newsletter. Apologies for the long delay since the last newsletter but we do have a rather full issue to make up for that.

Vectorial wave billiards: Some insight in VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) are a special kind of semiconductor lasers which combine a high potential for applications with intricate dynamical behaviour of fundamental interest in physics. In particular, due to their large width to length ratio, the coherence of broad-area VCSELs is limited by the appearance of high order transverse modes. Particularly interesting is that VCSEL structures open a quite simple access for studying the relation between wave and ray optics, quantum and classically trajectories respectively, in billiard problems.

Fig. 1 illustrates the last statement by comparing the intensity distribution in the active zone of the VCSEL (“near field”, 40×40 μm, upper row) and the modulus squared of the spatial Fourier spectrum for different operating temperatures. The emission direction is towards you (fairly low power, no safety issue). For high temperature (left columns), the emission is close to the optical axis in far field and concentrates along the perimeter of the laser in near field. This preference can be explained by current crowding at the aperture. For intermediate temperatures (centre column) the whole near field is lasing though in a modulated state. For even higher temperature (right columns) something weird happens. The lasing emission localizes along a rectangle, i.e. the laser decides not to use the whole available inversion. The fine modulation (barely visible in the resolution of the figure) is due to the interference of the transverse wave vectors of the counterprograting travelling visible in the Fourier spectrum, i.e. a transverse standing wave (“diamond” pattern). Obviously, this corresponds to a wave function localized along the closed orbit of a classical ray bouncing around the device aperture (defined by a refractive index jump). It is still unclear why the laser chooses to go along this classical orbit apart from the general argument that quantum systems at high quantum numbers (short wavelength, large wave vector) become more classical.

A consortium of German, Belarus and Taiwanese scientists coordinated by T. Ackemann at Strathclyde looked now at the polarization properties of these intriguing structures. Polarization is a hot topic in VCSELs because the ideal VCSEL is polarization isotropic and thus has an additional continuous phase variable leading to enhanced complexity. Experiments were done at the University of Muenster, the devices fabricated at the National Chiao Tung University, and theoretical support came from the Max-Born Institut and the Belorussian academy of Sciences.

It turns out that polarization selection is best understood in Fourier space (Fig. 2, centre and lower row). The findings yield a random polarization determined by unavoidable parasitic anisotropies for on-axis radiation (Fig. 2, left column). The device filling modulated patterns (centre column) have a polarization in tendency parallel to one of the boundaries (the one best compatible to be orthogonal to the wavevector at the same time) but actually each pair has a slightly different polarization direction. The wave vectors at the diagonal forming the diamond pattern (right column) turn out to have essentially arbitrary polarization indicating a polarization degeneracy.

It turns out that the polarization properties are governed by a competition between the transverse boundary conditions (the Billiard problem) and the longitudinal boundary conditions (reflection properties of Bragg reflectors closing the cavity). The polarization properties of the eigenmodes of the transverse waveguide are NOT preserved at the Bragg and vice visa. As a consequence, the problem is intrinsically vectorial and it appears that the billiard problem is chaotic though the scalar version with the same billiard shape would be integrable. This opens up a new field of wave/quantum chaos in vectorial fields.

The results are published in Physical Review Letters, Vol. 100, 213901 (2008).

[TA]

People with HiDEaS

Steve Barnett, Gian-Luca Oppo and Miles Padgett (at Glasgow University) have been awarded €480k from the European Commission to participate to the FP7 Project on High Dimensional Entangled Systems (HiDEaS) from 1 November 2008 to 31 October 2011. The kick-off meeting took place in Varenna in Italy, 5-7 November 2008. The picture shows the three PIs with a lot of HiDEaS in Varenna on Lake Como.

HiDEaS aims at a breakthrough in the information capacity of Quantum Communication (QC), well beyond the standard single-mode approach, by exploiting the intrinsic multi-modal and multivariate character of the radiation field. Our long-term vision is that of a broadband Quantum Communication, where all the physical properties of the photons are used to store information. Working at the quantum level requires: i) to produce quantum entanglement of light in high dimensional and multivariate spaces and ii) to create multimode quantum interfaces between light and matter in order to store high-D quantum states of light in long-lived matter degrees of freedom. Beneficial impacts will be also on Quantum Metrology. The HiDEaS collaboration is formed by partners in Como (Italy), Paris (France), Leiden (Netherlands), Lille (France), Copenhagen (Denmark), Glasgow (UK), Saint Petersburg (Russia), and – outside Europe – Canberra (Australia).

The Strathclyde-Glasgow team will specifically work at bringing to the quantum realm the spectacular progress achieved by the introduction of frequency combs in the classical domain and at realising a very high-D entanglement between twin photons produced by parametric down-conversion with conjugate variables angle and optical angular momentum.

For more information see the Cordis site and search for 'hideas'.

[GLO, SB]

Force field

Bob Bingham, a Professor in the Plasmas Division of the Physics Department, is part of a team which has created a 'force field' with the potential to shield humans from the 'space weather' which has so far made deep space travel, and the cherished dream of a visit to the Red Planet, impossible. Radiation from the sun and cosmic rays pose a deadly threat to astronauts in space but the new research shows how knowledge gained from the study of nuclear fusion may reduce the threat to tolerable levels, making manned missions beyond Earth's orbit a much greater possibility.

Professor Bingham conducted experiments along with colleagues from the University of York, Instituto Superior Técnico in Lisbon and the Science & Technology Facilities Council’s Rutherford Appleton Laboratory in Oxford, where he is also an STFC Fellow. They used theory from 50 years of research into nuclear fusion to show that it is possible for astronauts to shield their spacecrafts with a portable magnetosphere, scattering the highly charged, ionised particles of the solar wind and flares away from their spacecraft.

Solar energetic particles, although only one part of the cosmic rays spectrum, are a matter of great concern because they are the most likely to cause deadly radiation damage to astronauts. Large numbers of these energetic particles occur intermittently as 'storms,' with little warning, and are already known to pose a great threat to mankind but a giant magnetic bubble, the magnetosphere, around Earth helps to protect the planet.

Professor Bingham said: "The Apollo astronauts of the 1960s and 1970s who walked on the Moon are the only humans to have travelled beyond the Earth’s natural force field, the magnetosphere. With typical journeys lasting only about eight days, it was possible for them to miss an encounter with such a storm- a journey to Mars, however, would take about 18 months, during which time it is almost certain that astronauts would be enveloped by a solar storm."

Spacecraft visiting the Moon or Mars could maintain some of this protection by taking their own portable mini-magnetosphere. The idea has been around since the 1960s but it was thought impractical because it was believed that only a very large magnetic bubble- more than 100km wide- could work. Professor Bingham said: "By recreating in miniature a tiny piece of the Solar Wind into a bottle, we were able to confirm that a small 'hole' in the Solar Wind is all that would be needed to keep the astronauts safe on their journey to our nearest neighbouring planet."

This breakthrough, published on November 4, 2008, in the refereed journal Plasma Physics and Controlled Fusion, has been publicised internationally this month in news items by the BBC, the Telegraph and Der Spiegel.