Vol. 47 No.5-6 - Highlights

Polychromatic cylindrically polarized beams (Vol. 47 No. 5-6)

Various polarization patterns (arrows) and intensity distributions (underlying doughnut) of a co-rotating radially polarized X-wave

Cylindrically polarized beams represent a class of solutions, where the polarization can be radially or azimuthally distributed across the intensity profile. These beams have very intriguing properties, both from a fundamental and an applied perspective. Despite their great success, they have been almost exclusively studied and realized within the monochromatic regime.

An open question is if non-monochromatic cylindrically polarized solutions of Maxwell equations exist. New research answers to this question by employing X waves with orbital angular momentum (the polychromatic counterpart of Bessel beams) as building blocks to generate optical pulses with radial and azimuthal polarization. This approach is different from the monochromatic case where Hermite-Gaussian beams are typically used. Solutions are investigated in the paraxial and the nonparaxial regime and the role of the pulse’s spectrum in the polarization properties of the pulse itself is pointed out. Analysis shows that the generalization of the concept of non-uniform polarization to the domain of optical pulses leads to new intriguing applications, such as spatially resolved Raman spectroscopy. Cylindrically polarized X-waves with orbital angular momentum could also open new intriguing scenarios for fundamental research and quantum optics.

M. Ornigotti, C. Conti and A. Szameit, Cylindrically polarized nondiffracting optical pulses, J. Opt. 18, 075605 (2016)

Asymmetrical magnetic microbeads transform into micro-robots (Vol. 47 No. 5-6)

Transformation of particle clusters while exposed to an oscillating external magnetic field

Thanks to the ordering effects of two-faced magnetic beads, they can be turned into useful tools controlled by a changing external magnetic field

Janus was a Roman god with two distinct faces. Thousands of years later, he inspired material scientists working on asymmetrical microscopic spheres—with both a magnetic and a non-magnetic half—called Janus particles. Instead of behaving like normal magnetic beads, with opposite poles attracting, Janus particle assemblies look as if poles of the same type attract each other. A new study reveals that the dynamics of such assemblies can be predicted by modelling the interaction of only two particles and simply taking into account their magnetic asymmetry. These findings were recently published by the authors. It is part of a topical issue entitled "Nonequilibrium Collective Dynamics in Condensed and Biological Matter." The observed effects were exploited in a lab-on-a-chip application in which microscopic systems perform tasks in response to a changing external magnetic field, such as, for instance, to create a zipper-style micro-muscle on a chip.

G. Steinbach, S. Gemming and A. Erbe, Non-equilibrium dynamics of magnetically anisotropic particles under oscillating fields, Eur. Phys. J. E, 39 69 (2016)

The effect of spatiality on multiplex networks (Vol. 47 No. 5-6)

The multiplex structure arising from beginning with nodes on a lattice and connecting them through two layers of links (gray and black) with the length of each link following an exponential distribution

When a node can only form a link to its nearest neighbour, the topology is entirely determined by the spatial locations of the nodes. But when near and far links can form, the influence of the spatial embedding of the topology is much less. In this paper, we use this to modulate the strength of spatial effects on network topology. This allows us to consider the question: Does increasing the allowed geometric length of links in a network improve its robustness? In single-layer networks, the answer is generally that it does. However, in multiplex networks, we find that increasing the link lengths actually makes the network vulnerable to more severe cascade behaviours. This is because in multiplex networks, longer links allow for a discontinuous percolation transition which is characterized by a nucleation process. Our model and results demonstrate the surprising effects of spatial embedding and provide a simple new framework for assessing spatial networks of one or more layers.

M. M. Danziger, L. M. Shekhtman, Y. Berezin and S. Havlin, The effect of spatiality on multiplex networks, EPL 115, 36002 (2016)

New method helps stabilise materials with elusive magnetism (Vol. 47 No. 5-6)

Visualisation of itinerant ferromagnetic domains

Stabilising materials with transient magnetic characteristics makes it easier to study them

Magnetic materials displaying what is referred to as itinerant ferromagnetism are in an elusive physical state that is not yet fully understood. They behave like a magnets under very specific conditions, such as at ultracold temperatures near absolute zero. Realising the itinerant ferromagnetic state experimentally using ultracold gas is a challenging undertaking. This is because when three atoms - one with the opposite spin of the other two - come close to each other two atoms with opposite spin will form molecules and the other one carries the binding energy away; a phenomenon called rapid three-body recombination. Now, the authors, have introduced two new theoretical approaches to stabilise the ferromagnetic state in quantum gases to help study the characteristics of itinerant ferromagnetic materials. The first approach involves imposing a moderate optical lattice. There, the three-body recombination is small enough to permit experimental detection of the phase. In a second approach, they suggest to prepare two initially separated clouds and study their time evolution. The ferromagnetic domains has longer life time because of the reduced overlap region between the two spins. These results were recently published.

I. Zintchenko, L. Wang and M. Troyer,, Ferromagnetism of the repulsive atomic Fermi gas: three-body recombination and domain formation, Eur. Phys. J. B 89, 180 (2016)

Versatile method yields synthetic biology building blocks (Vol. 47 No. 5-6)

Fluorescence microscopy image of polymersomes, taken 3 days after production

New high-throughput method to produce both liposomes and polymersomes on the same microfluidic chip Synthetic biology involves creating artificial replica that mimic the building blocks of living systems. It aims at recreating biological phenomena in the laboratory following a bottom-up approach. Today scientists routinely create micro-compartments, so called vesicles, such as liposomes and polymersomes. Their membranes can host biochemical processes and are made of self-assembled lipids or a particular type of polymers, called block copolymers, respectively. In a new study, researchers have developed a high-throughput method--based on an approach known as microfluidics--for creating stable vesicles of controlled size. The method is novel in that it works for both liposomes and polymersomes, without having to change the design of the microfluidic device or the combination of liquids. The authors recently published these findings. Typical applications in synthetic biology include the encapsulation of biological agents and creation of artificial cell membranes with a specific biochemical function. They anticipate that their method might also be applicable for the controlled fabrication of hybrid vesicles used in bio-targeting and drug-delivery.

J. Petit, I. Polenz, J.- C. Baret, S. Herminghaus and O. Bäumchen, Vesicles-on-a-chip: A universal microfluidic platform for the assembly of liposomes and polymersomes, Eur. Phys. J. E 39, 59 (2016)