Metamaterials — artificially engineered structures with building blocks smaller than the wavelength of light — have delivered a new way to design and make materials with exotic electromagnetic properties. The current challenge is to make these metamaterials into meta-devices that convert the promising research into practical applications. Nanotechnology has made it possible to fabricate ultrathin metamaterials – less than a 15^th of the wavelength – shrinking conventional optical devices into planar form. In the coming years, research in metamaterials, plasmonics and nanofabrication will revolutionize device form and function throughout the electromagnetic spectrum.

This paper reports experimental demonstration of a planar ultrathin (50 nm) gold diffraction grating, mimicking the function of a bulk dielectric grating but tens of times thinner. It uses the resonant properties of individual sub-wavelength meta-atoms to change the phase of the light passing through it, in the same way as a blazed diffraction grating. It functions throughout the visible region of the spectrum (400 – 900 nm), with peak efficiency at 736 nm, and exhibits asymmetric diffraction, sending 25 times more light to the left than the right.

Imaging spectrometers acquire three-dimensional spectral data cubes (x, y, λ) to enable chemical imaging in fields ranging from microscopy and biomedicine to remote sensing. Traditional systems, employing time-sequential recording of the complete data cube, cannot record time-varying phenomena and are optically highly inefficient. Our technique enables spectra to be recorded in real time from a programmable sparse array of spots within a microscope sample. Real-time computer-controlled manipulation of the spot array enables tracking and video-rate spectroscopy of these points with the very high optical efficiency. This is achieved with a digital micro-mirror device (DMD) that deflects light from the sparse multiple points in the sample into a slitless spectrometer consisting of a dispersive prism and CCD camera. We demonstrate real-time spectra of multiple fluorescent microbeads in aqueous solution as they diffuse in the sample.

Investigation of charge state distributions (CSD) in astrophysical as well as laboratory plasmas, such as those provided by electron-beam ion traps, electron– cyclotron-resonance ion source (ECRIS) and tokamaks, is very important for the understanding of plasma processes. This knowledge is crucial for the optimization of a given ion source so that higher yields of higher charge states can be obtained. Furthermore, characterization of the CSD enables precise diagnostics of injected elements and impurities, which are important for the performance of fusion devices.

In this work, we have determined the CSD of an Ar plasma through the analysis of x-ray spectra obtained with a double crystal spectrometer. It is the first time that such a spectrometer is used coupled to an ECRIS for measuring inner-shell transitions in highly charged ions. The very high resolution of this apparatus enables us to correctly obtain the CSD of the plasma from x-ray spectra, even in highly populated energy regions.

Comparison to extracted ion currents show that the CSD in the center of the plasma can be quite different from the ion beam yields, due to the fact that the ions are extracted from the plasma edges.

Semiconductor nanowires have attracted huge attention recently due to their unique and often superior properties compared to bulk or planar counterparts. Complex heterostructures can be made and several nanowire-based devices (e.g. solar cells) have been realized. GaAsSb is an interesting ternary compound semiconductor because of its tunable bandgap and the possibility for both type I and type II band alignment with GaAs. In the present study 20-80 nm long zinc blende GaAsSb segments in wurtzite GaAs bare-core and GaAs/AlGaAs core-shell nanowires were studied. The work established the presence of both axial and radial compositional variations in the GaAsSb segments and their effect on the optical properties of these nanowires. The Sb concentration profiles within the inserts were determined using energy dispersive X-ray spectroscopy and quantitative scanning transmission electron microscopy and related directly to micro-photoluminescence measurements for the same single nanowires. The results of the article are relevant for further growth optimization and tailoring of the optical properties of GaAs/GaAsSb heterostructured nanowires.

New research gives a theoretical explanation as to how transport of single atoms that may be applied to optical lattices is made possible through a chain of quantum dots.

The authors have pushed back the boundaries of atom-based transport, creating a current by characterising the many-body effects in the transport of the atoms along a periodic lattice. This work has adopted a new analytical approach before comparing it to approximate numerical simulations, and is reported in the present paper.

Ultra-cold atoms trapped in optical potentials offer solutions for the transport of particles capable of producing a current. In this study, the authors extended previous single-atoms transport approaches to a model reflecting the many-body setting of bosonic atoms transport. Their challenge was to develop an analytical approach that allows particles to jump in and out and therefore produce a controlled current through the sample under study. They used a chain of quantum dots coupled to two bosonic reservoirs that keep the system far from equilibrium.

Self-avoiding walks are walks on a lattice, which are not allowed to self-intersect. Despite the apparent simplicity of the self-avoiding walk model, it is an important model of polymers, and over the past 60 years it has resisted all attempts to find an exact solution.

One of the basic features of self-avoiding walks is the number of walks for a given number of steps. The number of walks grows exponentially with length, and the rate of exponential growth, called the connective constant, is a quantity of fundamental interest.

Using a novel divide and conquer Monte Carlo algorithm, the number of self-avoiding walks on the simple cubic lattice for selected lengths of up to 38 million steps were estimated to high precision. For instance, the number of walks with 606 207 steps is 7.7 × 10^406 535! Using these estimates the connective constant was found to be 4.684 039 931 ± 0.000 000 027, which is significantly more accurate than estimates obtained via alternative methods.

A key open question is whether similarly powerful enumeration methods can be found for other models in statistical mechanics.

This work shows that a successful interpretation of quantum mechanics can be seen to emerge by taking the actual, or internal, states of a sub-system to correspond to one the eigenvectors of its reduced density matrix. Previous work has highlighted serious objections to such a modal type interpretation because it apparently leads to macroscopic superpositions and physically unacceptable instabilities near degeneracies. We show that both these problems are solved if the sub-system consists of a large number of coarse-grained degrees of freedom which is natural as real measuring devices have both finite spatial and temporal resolution. What results is an interpretation in which both decoherence and coarse graining play key roles and from which the rules of the Copenhagen Interpretation are seen to emerge in realistic situations. In this interpretation a measurement process is smooth but results in internal states that corresponds to the distinct outcomes ones expects. Previous work has suggested that the internal states of a device measuring the position of a particle would be spread out macroscopically.

The electromagnetic polarisabilities of the nucleons characterise their responses to external fields. The simplest are the electric and magnetic polarisabilities that describe the induced dipole moments. For spin-1/2 particles there are also four spin polarisabilities, analogous to rotations of the polarisation of light by optically active media. The best experimental window on them is Compton scattering of photons, which has provided good determinations of the electric and magnetic polarisabilities of the proton. Future experiments with polarised protons will give access to its spin polarisabilities. In contrast, much less is known of about the neutron since it does not exist as a stable target. Nonetheless, its properties can be obtained from Compton scattering on light nuclei, most notably the deuteron -- a weakly bound proton and neutron. A new generation of experiments is planned to provide beams of polarised photons on targets of polarised deuterons. If the spins of the final particles are not observed, there are 18 independent observables. This work provides, for the first time, the complete set of these, which will be needed for the experimental analyses. More importantly, it also examines their sensitivities to the various polarisabilities, which will be crucial for the design of the experiments.

The article reports results of investigations of generation of coherent terahertz (THz) radiation from 29 nm thick GaAs/Al0.37Ga0.63As quantum wells (QWs) with transverse electric bias under interband femtoseconds laser photoexcitation at room temperature. The detected THz radiation is attributed to the excitation of time-varying dipole moment induced by polarization of non-equilibrium electron–hole pairs in QWs. Noticeable sub-linearity in the dependence of THz amplitude on excitation density is observed. A theoretical model, which accounts for the dynamic screening of the electric field in wide GaAs QWs by nonequilibrium carriers, has been developed. The model describes well the properties of the observed THz signal.

The dynamics of electron ionization are an important topic in attosecond science. Applying a strong laser pulse, electrons can quantum mechanically tunnel through the potential barrier created by the combined Coulomb field of the atom and the laser field. At the tunnel exit, it is commonly assumed that the electron velocity parallel to the electric field is zero, contrary to the well-described distribution of transverse momenta.

After ionization, electrons propagate in the remainder of the laser pulse, where they acquire a momentum spread due to the different phases of the field at their individual exit time. However, the longitudinal momentum spread measured in experiments on helium is considerably larger than that.

Monte Carlo simulations with zero initial longitudinal momentum agree with the theoretical predictions of acquired spread, while simulations that include a longitudinal momentum spread at the tunnel exit are compatible with experimental data. The authors introduced a new method to investigate electron velocity spreads after ionization. Applying this method to experimental data lead to a more accurate reconstruction of the electron wave packet immediately after tunnelling.

A new manipulation tool exploits the fact that when light interacts with matter, it creates a force that produces material properties in macromolecules and biological cells.

Romanian scientists have discovered a novel approach for the optical manipulation of macromolecules. The authors had the idea to use green photon beams. With them, it is possible to perform optical manipulation of macrostructures, such as biological proteins, with greater precision than with optical tweezers.

The authors used high-density green photon beams (HDGP) capable of inducing a polarisation effect within complex macrostructures. They found that the effect of the beam leads to ‘biological optical matter’. This includes newly-organised material structures, such as molecular aggregates and micro-particles, and can feature new characteristics such as antioxidant properties. The authors realised that this approach covers a larger area than focused tweezers and is capable of organising mesoscopic matter into a new 3D molecular architecture.

Stress generated by nano-motors within animal cells can lead to the creation of a condensed layer of filaments beneath the outer cell membrane.

The authors have found that a well-defined layer beneath animal cells’ outer membrane forms beyond a certain critical level of stress generated by motor proteins within the cellular system. They have created hydrodynamic models of active gels to model the cell cortex. They first derived the equations providing a coarse-grained description of cortical dynamics, then calculated the configuration in which their model was in a steady state.

They found that for sufficiently high levels of contractile stress it consisted of a dense layer near the membrane, which abruptly cut off beyond a certain thickness. The key advance in their model is the inclusion of gel disassembly throughout the system, and the contractility due to molecular motors. 

Many features of the Fractional Quantum Hall effect (FQHE) can be understood by considering electrons on the surface of semiconductors as a very peculiar, charged, two-dimensional fluid in the presence of a strong magnetic field. In this paper a classical hydrodynamic model of such a fluid is presented.

The model incorporates a relation between the vorticity and density of the fluid specific for FQHE and exhibits the Hall viscosity and Hall conductivity found in FQHE liquids. The relation of the model to previous effective models such as the Chern–Simons–Ginzburg– Landau theory of FQHE is explained. It is also shown how the Laughlin’s wavefunction is annihilated by the quantum velocity operator.

Magnetic hyperthermia is the process by which cycling magnetic nanoparticles in an alternating magnetic field leads to heat dissipation. It is a very attractive approach for the treatment of cancer because it generates no side effects unlike more conventional therapies such as radiotherapy or chemotherapy. The development of this therapy has been hampered by the lack of a clear understanding of the physical mechanisms leading heat generation. At the present time it is not possible for clinicians to be given details of the dosage and field conditions required for a given therapeutic outcome.

There are three mechanisms by which exposing magnetic nanoparticles to a cycling field can generate heat: susceptibility loss, hysteresis loss and viscous heating. We have found that these mechanisms are highly particle size dependent as shown schematically in the figure and will also depend upon the degree of aggregation of the particles. In experiments of magnetic nanoparticles of different sizes dispersed in solvents of varying viscosities, hysteresis heating has been shown to be the dominant mechanism. Although the contribution arising from viscous heating is significant its effects are uncontrollable and will not occur in vivo due to the high viscosity of tumour tissue.

Memristive devices are reshaping computing paradigms as one of the leading candidates for the future of computer memory. In conventional memristors, metallic filaments form and extend stochastically under applied electrical bias, producing a highly non-uniform conduction front. This produces devices with large variations in electrical properties that are difficult to tune.

Here, we introduce and simulate a specialized multi-layered device structure with alternating ionic conductor layers that enables the development of a quasi-uniform conduction front. This reduces catastrophic switching (Fig. 1b) in which devices rapidly exit the linear-tuning region and switch state. In our simulations of a single layer memristor, the majority of the resistance change during a switching event occurs catastrophically, while in the multilayer device the majority of switching occurs in the linear tuning region. This ability to fine-tune switching events in devices is an important property for multi-bit memory and neuromorphic computing applications.