Vol. 43 No.3 - Highlights

Sb-free quantum cascade lasers (QCLs) in the 3-4 µm spectral range (Vol. 43 No. 3)

image Spectral tuning behaviour with corresponding peak optical output power of the external-cavity QCL and schematic sketch of the setup.

The mid-IR spectral range is a region of great interest for numerous scientific and industrial applications such as environmental sensing, metrology and clinical diagnosis. In particular the first atmospheric window between 3-5µm is crucial where a large number of gases e.g. methane, nitric oxide, carbon mono-/dioxide or formaldehyde can be detected. The presence of very strong fundamental stretching modes of O-H, C-H and N-H bonds that can be orders of magnitude stronger than the overtones in the near-IR, brings the detection limits down to sub-ppb concentrations.

The unique feature of QCLs to tailor the emission wavelength makes them appealing sources for this kind of applications. Unfortunately the realization of QCLs in the first atmospheric window is especially challenging because a large conduction band discontinuity is needed to obtain high-energy photons. This is solved by using antimony in the lasing material. However, the growth of Sb-containing devices can be difficult and the fabrication techniques used for high performance QCLs lack compatibility.

Therefore the ETH team has focused on developing a Sb-free system by using strain-compensated InGaAs/InAlAs-AlAs on InP. In pulsed-operation watt-level emission at 3.3µm was obtained at room temperature, and lasing above 350 K could be observed. The laser performance is comparable to Sb-containing QCLs. Tunable single-mode emission between 3.15-3.4µm (Figure 1) was observed in a Littrow external-cavity configuration. The team has begun to develop buried heterostructure QCLs to obtain continuous wave operation. By incorporating first-order distributed feedback gratings, for the first time in this spectral range single-mode emitting buried heterostructure QCLs could be realized.

Sb-free quantum cascade lasers (QCLs) in the 3-4 µm spectral range
A. Bismuto, S. Riedi, B. Hinkov, M. Beck and J. Faist, Semicond. Sci. Technol. 27, 045013 (2012)

Micro-pattern formation of extracellular-matrix (ECM) layers (Vol. 43 No. 3)

image Micrograph of HEK 293 cell arrangement over patterned ECM strips on a Si substrate. The ECM shown here is Poly-L-Lysine and was patterned by application of low-temperature APPs through thin slits of a metal stencil mask placed firmly on the substrate. After plasma application and removal of the mask, HEK 293 cells were cultured on the substrate. The cells adhered to and proliferated on the remaining ECM strips. The white dashed lines here delineate the mask slit patterns.

Cell-based biochips/biosensors may advance various scientific and technological fields. For example, a neuronal network chip that simulates how our brains function, may be used to detect the cause of brain diseases such as Alzheimer's. Since living cells typically do not survive in direct contact with semiconductor surfaces, one of the major challenges for the development of cell-based biochips/biosensors is the formation of bio-interfaces that maintain living cells in an environment of microelectronics. Especially desirable is inexpensive technologies for attachment and arrangement of living cells on large areas of substrate surfaces according to one's design. In this study, we have developed a new and simple micro-patterning technique for extracellular matrices (ECMs) deposited on Si substrates by low-temperature atmospheric-pressure plasmas (APPs) and a metal stencil mask. Low-temperature APPs are suitable for such patterning because of their ability to produce highly reactive species efficiently without causing thermal damages to ECMs. In this study, it is shown that, with a short-period application of APP, ECM layers of 1 cm2 area deposited on Si substrates can be patterned for lines and intervals whose typical dimensions are in the range of 100 µm.

Micro-pattern formation of extracellular-matrix (ECM) layers by atmospheric-pressure plasmas and cell culture on the patterned ECMs
A. Ando, T. Asano, T. Urisu and S. Hamaguchi, J. Phys. D: Appl. Phys. 44, 482002 (2011)

New model for epidemic contagion (Vol. 43 No. 3)

image Star-shaped network representing human mobility

Improved estimates on the geographical spread of infectious diseases are achieved by studying human mobility networks. Given that humans are considered as the hosts spreading the epidemics, the speed at which an epidemic spreads is now better understood thanks to a new model accounting for the provincial nature of human mobility.

The authors modelled human mobility by accounting for the recurrent bi-directional travels of individuals around a central node representing their home location and forming a star-shaped network. Previous models are based on diffusion and would imply that people travel randomly in space, not necessarily returning to their home location again and again.

The researchers found that older diffusion-based models overestimated the speed at which epidemics spread. The speed of epidemics spreading through bi-directional travel, which is dependent on the travel rate, is significantly lower than the speed of epidemics spreading by diffusion. In addition, the authors discovered that it is the time an individual spend outside their home location and not, as diffusion models suggests, the rate of travel between locations, which influences the speed of epidemics spreading and whether an outbreak goes global. This model has yet to be tested against real data on human mobility before it can be used as a risk analysis and decision making tool for epidemics or in population dynamics and evolutionary biology.

Recurrent hostmobility in spatial epidemics: beyond reaction-diffusion
V. Belik, T. Geisel and D. Brockmann, Eur. Phys. J. B 84, 579 (2011)

Electronic rescattering dynamics in intense few-cycle laser fields (Vol. 43 No. 3)

image (a) Time-frequency analysis of the harmonic spectrum in a Coulomb potential; (b) Cut along the position of the nucleus of the Wigner quantum phase space distribution of the rescattering electron, visualizing the kinetic energy distribution of the electron as a function of time associated with the region near the parent ion.

The interaction of intense laser fields with atoms gives rise to characteristic strong-field effects, most notably above-threshold ionization and high-harmonic generation. After the electron tunnels out of the atomic potential, it is accelerated by the laser field before being driven back to the parent ion by the same field. The returning electron energy ranges from a few to a few hundred eV depending on the field parameters. Upon rescattering, parts of the electron wave-packet may recombine with the atom under emission of energetic radiation of harmonics of the laser frequency. The harmonic radiation carries characteristics of the atomic potential, as well as of the rescattering electron. This sequence of processes can be directly visualized by a quantum phase space analysis. Using a Wigner phase-space distribution, one can relate the coordinate-momentum space distribution of the returning electron to the temporal emission pattern of the harmonic spectrum (see Figure).

Such relation between the time-frequency vs. the coordinate-momentum space distribution has been calculated for different atomic model potentials, the Coulomb potential supporting a Rydberg series and short-range potentials, with a finite number of bound states. The important role of dynamic bound-state polarization effects in the harmonic emission can be identified. The long-range part of the Coulomb spectrum modifies the local momentum distribution at the moment of rescattering. This might become important for an improved description of experiments aiming towards a tomographic reconstruction of the electronic structure of a system using the harmonic spectrum.

Quantum phase-space analysis of electronic rescattering dynamics in intense few-cycle laser fields
S. Gräfe, J. Doose, J. Burgdörfer, J. Phys. B: At. Mol. Opt. Phys. 45, 055002 (2012)

Laser-atom interacts: the electron at the mercy of the laser (Vol. 43 No. 3)

image Double-differential electron momentum distributions

The interaction between atoms and intense electromagnetic pulses with durations in the (sub) femtosecond time domain has proved to be a powerful tool to understand the dynamics of electrons inside matter. Among the theoretical methods developed to describe the electronic transitions produced by such ultrashort pulses we can mention the Coulomb-Volkov (CV) approximation, in which the combined action of both the atomic and laser potentials is taken into account only in the final channel. The CV approach has become a widespread method to investigate the physics behind photoinduced ionization processes. Nevertheless, it fails when the perturbative conditions are not fulfilled, for instance, when the ionization produced by an intense laser field takes place in an effective time much shorter than the pulse duration, or when the ionization requires multiphoton transitions.

Here we propose a doubly distorted CV (DDCV) model that goes beyond the standard CV theory by incorporating the effect of the laser field on both the initial and final electronic states on an equal footing. This improvement allows the method to account for dynamic Stark effects, which play an important role for long electron excursions originated by the laser electric field.

We found that the DDCV approach provides reliable predictions of photoinduced electron emission distributions from H(1s) for different field intensities and wavelengths, including a range of laser parameters where the CV approach is inadequate. In addition, the extension of the DDCV approximation to complex atoms and molecules appears to be perfectly viable.

Doubly-distorted-wave method for atomic ionization by ultrashort laser pulses
M. S.Gravielle, D. G.Arbó, J. E. Miraglia and M. F. Ciappina, J. Phys. B: At. Mol. Opt. Phys. 45, 015601 (2012)

Neutron-induced capture and fission reactions together (Vol. 43 No. 3)

image Minor actinide part of the table of isotopes indicating for which isotopes n_TOF has carried out measurements (TOF-Ph1) and for which measurements are intended (n TOF-Ph2), differentiating between fissile and non-fissile isotopes.

The accurate knowledge of (n,gamma) neutron-capture cross-sections for fissile isotopes is highly relevant for next-generation applications of nuclear technology. However, accurate measurements are difficult due to the gamma-ray background generated in competing (n,f) fission reactions. Scientists at the CERN n_TOF facility have developed a new experimental setup that is capable of simultaneously measuring and identifying the capture and fission reactions.

The setup combines the existing 4pi BaF2 Total Absorption Calorimeter (TAC) with a set of three MicroMegas detectors (MGAS). A successful test experiment was performed using moderated spallation neutrons in an energy region of 6-22 eV with a 235U sample.

Neutron captures were measured in the TAC while fission reactions were simultaneously detected in the TAC and MGAS. Coincident events recorded in the TAC and MGAS were selected and the specific TAC response to capture and fission events was exploited to select the two components by imposing conditions on sum energy and event multiplicity to disentangle them.

It was important to precisely determine the different detector efficiencies for both reaction types, which in turn allowed to derive capture and fission cross-sections, as well as their ratios. The comparison of the experimentally determined values with evaluated cross-sections showed good agreement between measurement and evaluation for both types of reaction.

Thus the experimental method was validated for an unambiguous extraction of cross-sections from a simultaneous measurement of the capture and fission reaction.

With the successful commissioning of the combined experimental setup, the n_TOF collaboration has now developed a tool that will enable them to target the accurate measurement of neutron-induced capture cross-sections of fissile isotopes.

This new method brings interesting perspective for the necessary improvement of nuclear data for next-generation nuclear technology applications. The figure shows the neutron capture measurements of actinides performed at n_TOF (233,234U, 237Np, 240Pu and 243Am) and those intended for future experiments (236,238U and 241Am)

Simultaneous measurement of neutron-induced capture and fission reactions at CERN
C. Guerrero et al. (85 co-authors), Eur. Phys. J. A, 48, 29 (2012)

Tiling pattern governs quasicrystals' magnetism (Vol. 43 No. 3)

image The five-fold symmetry of the Penrose tiling

Few material classes have generated as much interest as quasicrystals. The present paper outlines the progress to date in the theory explaining the magnetic properties of these quasicrystals.

In 2011, the fascination with quasicrystals was rekindled by the awarding of the Chemistry Nobel Prize to Daniel Schechtman for their discovery in 1984. Their structure differs from standard crystals in that they are packed in patterns that repeat in space, in an aperiodic fashion. The resulting structures exhibit a highly complex ordering with 5, 8, 10 or 12-fold symmetries, which are not manifested in periodic, classic crystals.

This review shows that by focusing on two-dimensional tiling, it is possible to study the magnetic properties of these quasi-periodic materials at the atomic scale. This article provides an overview of the specificities of 2D quasicrystal structures, including those known as Penrose (see illustration) and Ammann tilings. The author considers a model called the Heisenberg model to explain their magnetic properties compared with other types of perfectly ordered as well as disordered crystals and quasicrystals.

The author discusses the use of several simulation methods, both quantitative and numerical, to show that the magnetic state in quasicrystals is characterised by a relatively high degree of magnetic order, in which an internal characteristic of electrons, called spin, alternates between the up and down direction, described as the antiferromagnetic or Néel state. In addition, highly complex spatial distribution of local staggered magnetisation also occurs within such materials.

The Heisenberg antiferromagnetic model in two dimensional quasicrystals
A. Jagannathan, Eur. Phys. J. B, 85, 68 (2012)

Single-particle interference versus two-particle collisions (Vol. 43 No. 3)

image Electronic interferometer fed by two independent single-particle sources. The magnetic field dependent interference, as a function of the phase difference Φ, in the transmitted charge Q2 is found to be suppressed with the occurrence of particle collisions at time difference Δtu=0.

The recent experimental realization of an on-demand coherent single-electron source [G. Fève et al., Science (2007)] allows exploiting the individual particles' quantum nature in controlled single- and multi-particle effects in solid-state devices. In particular, it inspires to perform interference studies in electronic interferometers into which well-separated single particles - electrons or holes - are injected.

Stimulated by these possibilities, this article proposes a setup where the single-particle interference in an electronic interferometer is influenced by the presence of a second particle-emitting source placed in one of the interferometer arms. The two sources can be synchronized with respect to each other creating tunable and coherent modulation, and even suppression, of interference. Importantly, this study envisions and theoretically analyzes an experimental setup, in which both aspects of the quantum nature of electrons can be observed simultaneously: its wave-like and its particle-like behaviour. The time-dependent current shows an interference pattern, to which the second source adds a peculiar time-dependent phase determined by its working mode: this intriguing interference effect is convincingly explained by the particles' wave-like behaviour. Yet at the same time, a tunable coherent suppression of the interference is expected in the total transmitted charge, which is shown to be a feature of the particles' ability to collide. The coexistence of the two effects leading to interference suppression in the absence of dephasing is a fascinating challenge for our understanding of quantum mechanics.

Single-particle interference versus two-particle collisions
S. Juergens, J. Splettstoesser and M. Moskalets, EPL, 96, 37011 (2011)

Spin-charge-density wave in a rounded-square Fermi surface for ultra-cold atoms (Vol. 43 No. 3)

image Band structure: the Fermi energy (concentric squircles) is depicted by white contours (see also projection into the momentum plane).

Ultra-cold atoms in optical lattices can nearly ideally realize the simple Hamiltonians that model the behaviour of real condensed-matter systems, but with full control of parameters. Here, we show that Raman coupled ultra-cold fermions in a two-dimensional square optical lattice can have a different behaviour depending on their position in the lattice: at a certain site, the system gains energy if the fermion flips its spin, whereas in the sites around this one, the same process costs energy. This physical system is described by a tight-binding model with a Zeeman coupling that is different for neighbouring sites, thus dividing the lattice into AB sublattices. The single-particle spectrum has four bands; the third of which is shaped like a squarish Mexican hat (Figure). By filling up the energy levels up to the third band, the Fermi surface is squircle-shaped. This squarish-circle favours nesting and the system develops a coupled modulation in the density and spin, analogous to a spin-charge-density wave in solid- state systems.

Using field-theoretical methods, we develop a generalized formalism, which allows us to account for coupled charge and spin degrees of freedom simultaneously. We then determine the critical value of the parameters for the occurrence of the phase transition to this inhomogeneous density and spin state, which occurs at an incommensurate wave vector. Our results could be observed with state-of-the-art spectroscopic techniques. The investigation of spin-dependent optical lattices is an important direction of research in the field of spintronics with ultra-cold atoms, which will further strengthen the bonds between condensed matter and atomic physics.

Spin-charge-density wave in a rounded-square Fermi surface for ultra-cold atoms
D. Makogon, I. B. Spielman and C. Morais Smith, EPL, 97, 33002 (2012)

High carrier injection for all-silicon laser

Presently, laser diodes are mainly used to convert electronic signals into optical signals in fast communication systems. They are manufactured with a wide variety of semiconductors different from silicon and the incorporation of these lasers in silicon layers leads to distortion of the signals, degradation of sensitivity, and limits the reliability. Silicon, with more than 95% of the international market share, dominates other semiconductors. A silicon laser source would provide the ideal improvement for future high-speed electronics. The structure of silicon limits its light-emitting efficiency and, despite the very fast development of silicon based electronics, optical applications of silicon devices have not been conducted thoroughly.

The question “which silicon device will be useful for transforming electronic signals into optical signals?” is still relevant today. This paper presents an advance in optoelectronic silicon devices since it introduces and formulates a process for the creation of an optical active layer inside silicon devices. A degradation of the structure is induced by hot carriers injection. The process has been controlled by the analysis of the junction characteristic. The authors suggest that the created defects disturb the lattice periodicity by creating energy states in the band gap of the silicon. The model is based on a population inversion associated with a defect layer for carrier confinement and an electrical stimulation of light is demonstrated. The emitted light is localized is an area close the emitter-base junction. The authors measure the amplification of emitted light, and they showed that defect layer appears as an optical cavity. This groundwork introduces practical ways for improving the optical properties of silicon devices for optoelectronic applications.

High carrier injection for all-silicon laser
H. Toufik, W. Tazibt, N. Toufik, M. El Tahchi, F. Pélanchon and P. Mialhe, Eur. Phys. J. AP 58, 10103 (2012)