Vol. 47 No.1 - Highlights

Bringing the chaos in light sources under control (Vol. 47 No. 1)

Study investigates how best to stabilise the output of quantum dot LEDs Noise is an issue in optical telecommunications. And findings means of controlling noise is key to physicists investigating light-emitting diodes or lasers. The authors have worked on a particular type of light source, called the quantum dot light-emitting diode (QDLED). They demonstrate that modulating bias current of the QDLED could lead to countering the noise. This, in turn, leads to stabilising such light sources, making them better suited for optical telecommunications.

Most light sources exhibit fluctuations due to the quantum nature of the process underlying the emission of light. However, experiments show that these fluctuations—often described as quantum noise—are inherently chaotic and subject to oscillations, dubbed mixed mode oscillations. The authors have developed a theoretical model, which is able to reproduce the chaotic and oscillating phenomena observed experimentally. This can help them understand the nature of such phenomena.

They found that spiking competition of quantum dots in the part of the diode that emits lights enhances the way in which the diode receives its own self-feedback in terms of the light being emitted and it also has an effect on the impact of noise perturbation. They also show that the dynamics of these fluctuations are completely determined by the variation of the injecting bias current feeding into the QDLED.

As a result, that fluctuations can be brought under control by changing the bias current. The next step in their research will involve focusing on synchronisation phenomena in QDLED arrays for using this source in optical telecommunications. Other potential applications could include quantum dot-enhanced LED-backlighting of LCD televisions.

K. Al Naimee, H. Al Husseini, S.F. Abdalah, A. Al Khursan, A. H. Khedir, R. Meucci and F.T. Arecchi, Complex dynamics in Quantum Dot Light Emitting Diodes, Eur. Phys. J. D 69, 257 (2015)

Laser-based accelerators: yes, we CAN! (Vol. 47 No. 1)

Cover picture of the EPJ Special Topic issue on the Science and the applications of the coherent amplifying network (CAN) laser
© Fraunhofer IOF
Jena, Germany, Bernd Müller

Future ultra-fast high power lasers, dubbed Coherent Amplification Network (CAN) lasers, will deliver unprecedented accelerating power and efficiency

Few technologies have the power that particle accelerator technology has to touch upon such a broad range of applications at the many frontiers of modern science. Today, thanks to improvements in laser technology, a new generation of accelerators could soon emerge to replace accelerators relying on radio frequencies. This special issue explores the requirements necessary to make such laser accelerators a reality, by presenting the work of the International Coherent Amplification Network (ICAN) research collaboration. The articles study: average/peak power and efficiency limits of coherently combined ultrafast laser systems; synchronization, spatial and temporal recombination of a large number of fibre amplifiers; temporal and spatial beam quality; combining efficiency of coherent addition amplitude and phase stability as a function of the number of fibres and their individual performance; and reduction of pulse duration and manipulation of pulse shape. Potential applications include future colliders, solutions for vacuum physics, design of Higgs-particle factories, and creation of sources of high-flux protons and of neutrons, among others. Further, such accelerators open the door to solutions in nuclear pharmacology and proton therapy as well as orbital debris remediation.

Science and applications of the coherent amplifying network (CAN) laser, Eur. Phys. J. Special Topics, 224, Number 13 (2015)

Initial state entanglement in inflationary cosmology (Vol. 47 No. 1)

Entangled vacuum state of quantum fields in the inflationary universe.

Recent observational data indicate that inflationary cosmology gives an excellent description of the very early universe. Inflationary cosmology assumes that quantum fluctuations seed the observed large scale structure in the universe. So we may be able to test the initial quantum state of the universe observationally in the future. Especially, if primordial vacuum state is entangled, the effect of entanglement could then be observed.

We give a new interpretation of the effect of initial state entanglement on the spectrum of vacuum fluctuations. We consider an initially entangled state between two free massive scalar fields in de Sitter space. We construct the initial state by making use of a Bogoliubov transformation between the Bunch-Davies vacuum and a four-mode squeezed state, and then derive the exact power spectrum for one of the scalar fields. We demonstrate that an oscillatory spectrum hardly appears for the initially entangled state unless an ad hoc absolute value of the Bogoliubov coefficients is chosen. We stress that, on the contrary, an initially non-entangled state may naturally produce an oscillatory spectrum due to quantum interference if the initial state deviates from the Bunch-Davies vacuum.

S. Kanno, A note on initial state entanglement in inflationary cosmology, EPL 111, 60007 (2015)

May the 5th force be with you (Vol. 47 No. 1)

The author revisits the wealth of research emerging from the quest for the fifth force, which he hypothesised in the 1980s as being a new fundamental force in nature.

Discovering possible new forces in nature is no mean task. The discovery of gravity linked to Newton’s arguably apocryphal apple experiment has remained anchored in popular culture. In January 1986, the author had his own chance to leave his mark on collective memory. His work made the front page of the New York Times after he and his co-authors published a study uncovering the tantalising possibility of the existence of a fifth force in the universe. In an article published recently, he gives a personal account of how the existence of the gravity-style fifth force has stimulated an unprecedented amount of research in gravitational physics - even though its existence, as initially formulated, has not been confirmed by experiment.

E. Fischbach, The fifth force: A personal history, Eur. Phys. J. H 40, 385 (2015)

Refraction index of shock compressed water at megabar pressure (Vol. 47 No. 1)

Phase plane of water showing the experimental results for normal samples and for samples precompressed at 10 kbar.

Compressing water at megabar pressure through laser-driven shocks induces phase changes and may finally produce a metallic fluid. Such phase is particularly relevant for planetology since water is one of the main constituents of the mantles of giant planets like Uranus and Neptune, and its metallization has been recognized as a possible source of the magnetic field of such planets. In this work we study the transition of water by looking at its optical properties. Increasing pressure water changes from transparent to opaque (absorbing) and finally reflecting. We provide the first quantitative measurement of water refractive index in the megabar range, a measurement, which can give information on how the material is approaching gap closure (metallization). We also performed measurements on water precompressed at 10 kbar, allowing getting off-Hugoniot states at high pressure but low temperature. Refraction index for transparent and opaque water was measured using a VISAR system. At high compression a sharp increase of the real and imaginary part of the refraction index was observed. Experiments were performed at the LULI and RAL laboratories.

D. Batani, K. Jakubowska, A. Benuzzi-Mounaix, C. Cavazzoni, C. Danson, T. Hall, M. Kimpel, D. Neely, J. Pasley, M. Rabec Le Gloahec and B.Telaro, Refraction index of shock compressed water in the megabar pressure range, EPL 112, 36001 (2015)

Minutest absolute magnetic field measurement (Vol. 47 No. 1)

Geometry of the experiment.

High-precision and high-accuracy magnetic field measurement to support quest for missing antimatter in the universe.

Every measurement is potentially prone to systematic error. The more sensitive the measurement method, the more important it is to make sure it is also accurate. This is key for example in measuring magnetic fields in state-of-the-art fundamental physics experiments. Now, an international team of physicists has developed an extremely high-precision method for the determination of magnetic fields. The resulting device, they found, has an intrinsic sensitivity that makes it ideal for fundamental physics and cosmology experiments attempting to explain the missing antimatter of the universe. The findings of the authors have been published recently. They calculated the sensitivity of the magnetometer in the envisioned application in an experiment searching for the electric dipole moment of neutrons (nEDM), which are basic constituents of ordinary matter. Observing an nEDM would imply a broken symmetry of the laws of physics, called CP-violation. Such a finding could help to account for the primordial matter-antimatter imbalance at Big Bang stage, leading to the current abundance of matter.

H.-C. Koch, G. Bison, Z. D. Grujić, W. Heil, M. Kasprzak, P. Knowles, A. Kraft, A. Pazgalev, A. Schnabel, J. Voigt and A. Weis, Design and performance of an absolute 3He/Cs magnetometer, Eur. Phys. J. D 69, 202 (2015)

Slowing dynamics of a supersonic beam (Vol. 47 No. 1)

2D imaging of the slowed Ar* supersonic beam for a final velocity vF = 61 m/s.

This investigation of the slowing dynamics of a supersonic atom beam by a counter-propagating resonant laser light is characterized by two special features: (i) a close coupling between simulations and experiments using a nozzle beam of metastable argon atoms, (ii) the use of a Monte-Carlo (MC) scheme aimed at analysing step by step the slowing process and describing in a realistic way atom random walks due to the spontaneous emission. It allows to calculate 2D images and radial profiles of the slowed beam, in good agreement with experiment. Other important characteristics as angular aperture, velocity spreads, coherence radius (not easy to be measured experimentally), etc. also result from the simulation. Since the 3D atomic motion within the laser field is considered, border effects, not directly accessible in a simple radiative force model, can be studied. The calculations, assuming a point-like source, reproduce the experimental characteristics of the slowed beam. In general a laser beam is an efficient tool to manipulate the atomic motion. Its interaction with atoms can be accurately characterized by the present MC-code. Actually any configuration combining resonant light and atoms is relevant (if the semi-classical approximation is valid). A “pushing” laser to generate a slow atomic beam from a magneto-optical trap has been successfully tested with metastable argon atoms. The MC-code predicts accurately the characteristics of the generated beam.

M. Hamamda, T. Taillandier-Loize, J. Baudon, G. Dutier, F. Perales and M. Ducloy, Slowing dynamics of a supersonic beam, simulation and experiments, Eur. Phys. J. Appl. Phys. 71, 30502 (2015)

Zooplankton: not-so-passive motion in turbulence (Vol. 47 No. 1)

Probability of speed increments for living copepods in still water (black) and for living (blue) and inert (red) copepods in turbulence, for different separation times

Physicists show that despite their limited swimming abilities, zooplankton called calanoid copepods display active, energetic behaviour in turbulent flows.

Imagine a species that is only one millimetre long and has only a limited swimming ability. Yet, its mobility is sufficient for moving, feeding and reproducing in freshwater and seawater. That’s exactly what a type of zooplankton of the crustaceans family—namely the calanoid copepods—does. In a study published recently, the authors shed new light on how these zooplankton steer large-scale collective motion under strong turbulence. To do so, they study the zooplankton’s small-scale motion mechanisms when subjected to background flow motion. They found that at short time scales, due to the copepods’ frequent relocation jumps, the intermittent nature of their self-induced motion amplifies the intermittent properties of the underlying flow. Ecological applications in the field of zooplankton behaviour ecology include, for example, modelling the feeding efficiency of their predator, fish larvae.

F.-G. Michalec, F.G. Schmitt, S. Souissi and M. Holzner, Characterization of intermittency in zooplankton behaviour in turbulence, Eur. Phys. J. E 38, 108 (2015)

Trapping climate pollutant methane gas in porous carbon (Vol. 47 No. 1)

Schematic diagram of the confined lattice gas model used in some of the study’s calculations

New adsorption of gas into porous carbon simulations are of interest to energy research and climate change mitigation.

As talks of global warming are once again making headlines, scientists have renewed their efforts to understand how to best limit its effects. For example, sequestrating short-lived climate pollutants, such as methane and black carbon, yields much faster reductions in global warming compared to reductions in CO2. To do so, it is essential to have a better grasp of the nature of physico-chemical properties of gases interacting with porous carbon. Now, a team of chemical engineering researchers has established ways of accurately simulating methane adsorption and desorption in carbon with nanopores. These findings have been published recently. Alternative applications for such findings are relevant for future energy research, such as energy storage and the development of natural gas extraction methods.

M. Lasich and D. Ramjugernath, Influence of unlike dispersive interactions on methane adsorption in graphite: a grand canonical Monte Carlo simulation and classical density functional theory study, Eur. Phys. J. B 88, 313 (2015)

Lasing with topological defects (Vol. 47 No. 1)

Scanning-electron microscope (SEM) images of the topological defect laser where a photonic crystal surrounds the optical cavity.

A new laser based on a swirling vortex of light has been created by the authors. The ‘topological-defect laser’ could be a useful addition to lab-on-a-chip devices, where it could manipulate fluids and tiny particles. The design could also be modified to create beams of light with orbital angular momentum.

Conventional lasers confine light by bouncing it back and forth in an optical cavity made of two opposing mirrors. The authors have taken a new twist on this design by making an optical cavity that confines light by having it swirl around in a vortex. They made their optical cavity within a photonic crystal, which is a material containing a regular array of elements which are separated by distances on par with the wavelength of light. Light at certain wavelengths and travelling in certain directions will pass freely through a photonic crystal, whereas light not meeting these criteria will be diffracted into a new trajectory.

S. Knitter, S. F. Liew, W. Xiong, M. I. Guy, G. S. Solomon and H. Cao, Topological defect lasers, J. Opt. 18, 014005 (2016)

Cancer risk myth debunked (Vol. 47 No. 1)

Cancer risk is not just bad luck
© tilialucida / Fotolia

Cancer risk debate laid to rest by novel calculations distinguishing population-wide risks for each organ and individual risks linked to environmental and genetic factors.

A recent study suggests that variations in terms of cancer risk among tissues from various organs in the body merely amount to pure bad luck. In other words, cancer risk is linked to random mutations arising in the normal course of DNA replication of healthy cells. It is also claimed that environmental and genetic factors play a lesser role. The scientific community has primarily reacted negatively to this interpretation and promptly refuted it with qualitative arguments and empirical evidence. Joining these voices are the authors, who uncovered the statistical fallacy at the source of the recent study's conclusion. The key is to distinguish between individual organ risks and population risks, they wrote in this work. They also contend that the role of genetic and environmental factors must not be underplayed, even if these factors cannot explain differences in cancer rates between different organs.

D. Sornette and M. Favre, Debunking mathematically the logical fallacy that cancer risk is just “bad luck”, EPJ Nonlinear Biomedical Physics 3, 10 (2015)

Anti-clumping strategy for nanoparticles (Vol. 47 No. 1)

Schematic representation for a functionalized nanoparticle (NP) in brine

Scientists identify the factors involved in preventing nanoparticles used in industrial applications from aggregating.

Nanoparticles are ubiquitous in industrial applications ranging from drug delivery and biomedical diagnostics to developing hydrophobic surfaces, lubricant additives and enhanced oil recovery solutions in petroleum fields. For such nanoparticles to be effective, they need to remain well dispersed into the fluid surrounding them. In a study published recently, the authors identified the conditions that lead to instability of nanoparticles and producing aggregates. This happens when the electric force on their surface no longer balances by the sum of the attractive or repulsive forces between nanoparticles.

L. S. de Lara, V. A. Rigo and C. R. Miranda, The stability and interfacial properties of functionalized silica nanoparticles dispersed in brine studied by molecular dynamics, Eur. Phys. J. B 88, 261 (2015)