Table of contents

There is an error in appendix A of the published paper, please see the PDF for details.

The discussion on page 9 of the published article, including figures 3 and 4, needs to be corrected. Please see the PDF for details.

We study the effect of defects in the Mott insulator phase of ultracold atoms in an optical lattice on the dynamics of resonant electronic excitations. These excitations can be described analogous to Frenkel excitons in solids and form polaritons, when coupled to an optical resonator. Defects, which are empty sites in a singly occupied Mott insulator state or singly occupied sites for a filling factor two, change the exciton dynamics. We show that vacancies behave like hard sphere scatterers, while singly occupied sites in a doubly filled region generate either attractive or repulsive interaction potentials. We suggest cavity polaritons as tools which detect such defects and show how the scattering can be controlled by changing the exciton–photon detuning. In the case of aspherical individual lattice sites, we calculate the effective scattering potential as a function of the cavity photon polarization direction which exhibits a crossover from a repulsive into an attractive potential and should give a clearly observable signal.

Fundamental processes leading to the erosion of hydrocarbon films due to energetic argon ions and hydrogen atoms have been investigated using molecular dynamics simulations. A generic mechanism has been identified for carbon erosion due to energetic (150 eV) argon ions in the presence of sub-eV hydrogen atoms. This surface erosion process, which we call hydrogen enhanced physical sputtering (HEPS), is primarily a physical sputtering mechanism, enhanced due to the screening effect of hydrogen atoms. The energetic argon ions create open bonds within their penetration range. The hydrogen atoms passivate the open bonds created within the first few atomic layers. Subsequent ion bombardment causes the breaking of C–C bonds within and beyond the H penetration range. The steric effect of H atoms bound to the top layer of carbon atoms prevents the re-attachment of the broken bonds, and this leads to unsaturated molecule emission from the surface. The kinetic energy of the emitted molecules is above thermal energy and the emission takes place within 5 ps after the ion impact.

Using angle-resolved photoemission, we have mapped the dispersion relations and Fermi contours for surface-localized electron states onto clean and hydrogen-covered Cr(110) surfaces. In particular, we have probed the relationship between hydrogen adsorption and the evolution of the spin density wave (SDW) periodicity in chromium thin films observed previously. We find qualitatively similar surface band dispersion relations to those on W(110) and Mo(110), although with a narrower bandwidth, broader spectral features, and a smaller impact from the spin–orbit interaction. We compare our results to existing first-principles calculations and find a significant disagreement for a surface band that produces a prominent surface Fermi contour. Upon hydrogen adsorption, the Fermi contour for a particular surface band becomes well nested at a wave vector that stabilizes a commensurate SDW. We suggest that a competition between commensurate two-dimensional (2D) and incommensurate 3D Fermi surface nesting plays an important role in the SDW energetics in thin Cr(110) films.

We show that specially designed two-dimensional arrangements of full elastic cylinders embedded in a nonviscous fluid or gas define (in the homogenization limit) a new class of acoustic metamaterials characterized by a dynamical effective mass density that is anisotropic. Here, analytic expressions for the dynamical mass density and the effective sound velocity tensors are derived in the long wavelength limit. Both show an explicit dependence on the lattice filling fraction, the elastic properties of cylinders relative to the background, their positions in the unit cell, and their multiple scattering interactions. Several examples of these metamaterials are reported and discussed.

We investigate how to create entangled states of ultracold atoms trapped in optical lattices by dynamically manipulating the shape of the lattice potential. We consider an additional potential (the superlattice) that allows both the splitting of each site into a double well potential, and control of the height of the potential barrier between sites. We use superlattice manipulations to perform entangling operations between neighbouring qubits encoded on the Zeeman levels of the atoms without having to perform transfers between the different vibrational states of the atoms. We show how to use superlattices to engineer many-body entangled states resilient to collective dephasing noise. Also, we present a method to realize a two-dimensional (2D) resource for measurement-based quantum computing via Bell-pair measurements. We analyse measurement networks that allow the execution of quantum algorithms while maintaining the resilience properties of the system throughout the computation.

Competitive exclusion, a key principle of ecology, can be generalized to understand many other complex systems. Individuals under surviving pressure tend to be different from others, and correlations among them change correspondingly to the updating of their states. We show with numerical simulation that these aptitudes can contribute to group formation or speciation in social fields. Moreover, they can lead to power-law topological correlations of complex networks. By coupling updating states of nodes with variation of connections in a network, structural properties with power-laws and functions like multifractality, spontaneous ranking and evolutionary branching of node states can emerge simultaneously from the present self-organized model of coevolutionary processes.

The nonlinear properties of two-dimensional cylindrical quantum dust-ion-acoustic (QDIA) and quantum dust-acoustic (QDA) waves are studied in a collisionless, unmagnetized and dense (quantum) dusty plasma. For this purpose, the reductive perturbation technique is employed to the quantum hydrodynamical equations and the Poisson equation, obtaining the cylindrical Kadomtsev–Petviashvili (CKP) equations. The effects of quantum diffraction, as well as quantum statistical and geometric effects on the profiles of QDIA and QDA solitary waves are examined. It is found that the amplitudes and widths of the nonplanar QDIA and QDA waves are significantly affected by the quantum electron tunneling effect. The addition of a dust component to a quantum plasma is seen to affect the propagation characteristics of localized QDIA excitations. In the case of low-frequency QDA waves, this effect is even stronger, since the actual form of the potential solitary waves, in fact, depends on the dust charge polarity (positive/negative) itself (allowing for positive/negative potential forms, respectively). The relevance of the present investigation to metallic nanostructures is highlighted.

The properties of the phononic distribution of a mechanical oscillator coupled to a single-electron transistor are investigated in the sequential tunnelling regime. It is shown that for not too strong electron–phonon interaction the electrical current may induce a distribution of phonons with sub-Poissonian statistics, which is characterized by a selective population of few phonon states. Depending on the choice of parameters, such a sub-Poissonian phonon distribution can be accompanied either by a super- or a sub-Poissonian electronic Fano factor.

We introduce a prime number generator in the form of a stochastic algorithm. The character of this algorithm gives rise to a continuous phase transition which distinguishes a phase where the algorithm is able to reduce the whole system of numbers into primes and a phase where the system reaches a frozen state with low prime density. In this paper, we firstly present a broader characterization of this phase transition, both in analytical and numerical terms. Critical exponents are calculated, and data collapse is provided. Further on, we redefine the model as a search problem, fitting it in the hallmark of computational complexity theory. We suggest that the system belongs to the class NP. The computational cost is maximal around the threshold, as is common in many algorithmic phase transitions, revealing the presence of an easy-hard-easy pattern. We finally relate the nature of the phase transition to an average-case classification of the problem.

We study how heralded qubit losses during the preparation of a two-dimensional cluster state, a universal resource state for one-way quantum computation, affect its computational power. Above the percolation threshold, we present a polynomial-time algorithm that concentrates a universal cluster state, using resources that scale optimally in the size of the original lattice. On the other hand, below the percolation threshold, we show that single qubit measurements on the faulty lattice can be efficiently simulated classically. We observe a phase transition at the threshold when the amount of entanglement in the faulty lattice directly relevant to the computational power changes exponentially.

In this paper, we provide necessary and sufficient conditions for a completely positive trace-preserving (CPT) map to be decomposable into a convex combination of unitary maps. Additionally, we set out to define a proper distance measure between a given CPT map and the set of random unitary maps, and methods for calculating it. In this way one could determine whether non-classical error mechanisms such as spontaneous decay or photon loss dominate over classical uncertainties, for example, in a phase parameter. The present paper is a step towards achieving this goal.

The great advantage of measurement-based quantum computation is that one would simply need the ability to prepare a particular state, known as the cluster state, and subsequently to perform single-qubit measurements on it. Nevertheless, a scalable implementation is yet to be realized. Here, we propose a hybrid light–matter system consisting of coupled cavities interacting with two level systems. Utilizing the stable, individually addressable, qubits resulting from the localized long-lived atom–photon excitations, we demonstrate how to use the natural system dynamics to 'weave' these qubits into a cluster state and propose the implementation of quantum algorithms employing just two rows of qubits. Finally, we briefly discuss the prospects for experimental implementation using atoms, quantum dots or Cooper pair boxes.

Cluster ion beams create considerably more damage in silicon and other substrates and eject more material than single ions that deposit at the same kinetic energy on the substrate. The mechanisms that causes the non-linear growth of damage and sputtering are interesting from the point of view of both basic materials research and industrial applications. Using classical molecular dynamics, we analyse the dynamics of collision cascades that are induced in amorphous silicon by small noble gas nanoclusters. We show that the sputtering and other non-linear effects emerge due to the high-energy density induced in a relatively small region in the substrate during the cluster stopping phase and because of the timing of consequent events that dissipate the energy over a larger volume of the substrate.

We determine the symmetry of Cooper pairs, on the basis of the perturbation theory in terms of the Coulomb interaction U, for the two-dimensional Hubbard model on the square lattice. The phase diagram is investigated in detail. The Hubbard model for small U is mapped on to an effective Hamiltonian with the attractive interaction using the canonical transformation: H_eff = e^SHe^−S. The gap equation of the weak coupling formulation is solved without numerical ambiguity to determine the symmetry of Cooper pairs. The superconducting gap crucially depends on the position of the van Hove singularity. We show the phase diagram in the plane of the electron filling n_e and the next nearest-neighbor transfer t'. The d-wave pairing is dominant for the square lattice in a wide range of n_e and t'. The d-wave pairing is also stable for the square lattice with anisotropic t'. The three-band d–p model is also investigated, for which the d-wave pairing is stable in a wide range of n_e and t_pp (the transfer between neighboring oxygen atoms). In the weak coupling analysis, the second-neighbor transfer parameter -t' could not be so large so that the optimum doping rate is in the range of 0.8 < n_e < 0.85.

Optical tweezers manipulate microscopic particles using foci of light beams. Their performance is therefore limited by diffraction. Using computer simulations of a model system, we investigate the application of superresolution holography for two-dimensional (2D) light shaping in optical tweezers, which can beat the diffraction limit. We use the direct-search and Gerchberg algorithms to shape the center of a light beam into one or two bright spots; we do not constrain the remainder of the beam. We demonstrate that superresolution algorithms can significantly improve the normalized stiffness of an optical trap and the minimum separation at which neighboring traps can be resolved. We also test if such algorithms can be used interactively, as is desirable in optical tweezers.

The electrically induced bending of nonionic polyvinyl alcohol gels, bending over 90° within 100 ms, is the fastest motion in the field of electroactuation of polymers. This rapid bending produces initial mechanical vibrations followed by a durable displacement that contrasts highly with the relaxation observed with elastomer- and polyelectrolyte-based actuators. Here, we characterize the bending process using video imaging and laser detecting technology and establish a physical model for the electromechanical conversion, based on our observation of an induced solvent migration. Our results show excellent agreement between the measurements and calculations. This study provides general rules for understanding the electrically induced bending of isotropic dielectrics and may also shed light on nonmuscular biological engines.

We present a detailed numerical study of the interaction between fast particles and large-scale magnetic perturbations and toroidal field ripple. In particular we focus our study on the losses of fast ions created by neutral beam injection (NBI) for an ASDEX Upgrade discharge with neoclassical tearing mode (NTM) activity. For these investigations, we use as input an equilibrium carefully reconstructed from experimental data. The magnetic field ripple is self-consistently included by a three-dimensional, free-boundary equilibrium computation. The magnetic islands caused by a (2,1)-NTM are introduced by a field perturbation superimposed on the equilibrium magnetic field. The experimental data are used to reproduce size and location of those islands numerically. Starting from a realistic seed distribution, the guiding centres of about 100 000 fast ions are traced up to a given time limit, or until they hit plasma-facing structures. A detailed analysis of the particle trajectories provides important information on the underlying loss mechanisms such as: (i) losses of passing particles caused by drift island formation, and (ii) losses of trapped particles due to stochastic diffusion.

We present a study of the magnetic susceptibility of La_{2- x}Sr_xCoO₄ single crystals in a doping range 0.3 ⩽ x ⩽ 0.8. Our data show a pronounced magnetic anisotropy for all compounds. This anisotropy is in agreement with a low-spin ground state (S = 0) of Co³⁺ for x ⩾ 0.4 and a high-spin ground state (S = 3/2) of Co²⁺. We compare our data with a crystal-field model calculation assuming local moments and find a good description of the magnetic behavior for x ⩾ 0.5. This includes the pronounced kinks observed in the inverse magnetic susceptibility, which result from the anisotropy and low-energy excited states of Co²⁺ and are not related to magnetic ordering or temperature-dependent spin-state transitions.

Binding to cell surface receptors is thought to be essential for the transport of certain morphogens in developing tissues. The finite number of receptors per cell turns the tissue into a subdiffusive medium for the morphogens. We study a simple microscopic model of receptor-mediated transport and find superdiffusive spreading of morphogens. We propose that the superdiffusive spreading in a subdiffusive medium is due to a ratchet effect. A phenomenological model within the framework of the fractional Fokker–Planck equation allows us to analytically study the formation of morphogen gradients. Within this model, we show furthermore that the same features leading to the anomalous transport behavior also result in gradients that are robust against changes in the morphogen secretion rate. Together these findings show that anomalous transport in biological systems can be intimately linked to essential biological features.

We study in this paper the properties of a two-body random matrix ensemble for distinguishable spins. We require the ensemble to be invariant under the group of local transformations and analyze a parametrization in terms of the group parameters and the remaining parameters associated with the 'entangling' part of the interaction. We then specialize to a spin chain with nearest-neighbour interactions and numerically find a new type of quantum-phase transition related to the strength of a random external field, i.e. the time-reversal-breaking one-body interaction term.

We show analytically that the profile of the Bose–Einstein condensate can evolve exact self-similarly when the time-dependent interatomic interaction and harmonic potential satisfy a certain condition for one-, two- and three-dimensional geometries, respectively. Based on the exact scaling laws for the amplitude and width of the condensate profile, we propose experimentally feasible ways to improve the accuracy in measuring the stationary-state profile of the condensate when its spatial size is so small that the direct measurement is not available, and to squeeze the condensate to high local particle density to check the validity of the Gross–Pitaevskii equation via the controllable exact self-similar evolution of the condensate profile.

We combine electron irradiation experiments in a transmission electron microscope with kinetic Monte Carlo simulations to determine the mobility of interstitial carbon atoms in single-walled carbon nanotubes. We measure the irradiation dose necessary to cut nanotubes repeatedly with a focused electron beam as a function of the separation between the cuts and at different temperatures. As the cutting speed is related to the migration of displaced carbon atoms trapped inside the tube and to their recombination with vacancies, we obtain information about the mobility of the trapped atoms and estimate their migration barrier to be about 0.25 eV. This is an experimental confirmation of the remarkably high mobility of interstitial atoms inside carbon nanotubes, which shows that nanotubes have potential applications as pipelines for the transport of carbon atoms.

We have studied laser interference patterns, which consist of line defects on the surface of a GaAs substrate, generated by four-beam interference lithography. The orientation and periodicity of the defects are shown to depend on the configuration of the incident laser beams, while the widths of the defects are modified by varying the beam intensity. Influences of the phase and polarization on the simulated patterns are discussed.

Rare earth (RE) nanoparticles with single-crystal structure and controllable size, and RE nanocrystalline bulks with ultrafine grain sizes (3–8 nm), were prepared with a home-configured `oxygen-free' in situ synthesis system. Systematic investigations revealed the characteristic structures and properties of the nano REs. As compared with the conventional polycrystalline REs, the prepared ultrafine nanocrystalline REs show remarkably different physical, thermal and mechanical properties, which are of broad scientific and engineering interest. The series of structure and property characterizations are significant to further develop the fundamental database for the nano REs. As a particular contribution to the characterization of the RE family on the nanoscale, this work implements a clear guideline for the composition design and the scale control in the development of advanced nano RE functional and structural materials.

We studied the information traffic in Barabási–Albert scale-free networks wherein each node has a finite queue length to store the packets. It is found that in the case of the shortest path routing strategy, the networks undergo a first-order phase transition, i.e. from a free flow state to a full congestion state, with increasing packet generation rate. We also incorporate the random effect (namely random selection of a neighbor to deliver packets) as well as a control method (namely the packet-dropping strategy of the congested nodes after some delay time T) into the routing protocol to test the traffic capacity of the heterogeneous networks. It is shown that there exists an optimal value of T for the networks to achieve the best handling ability, and the presence of an appropriate random effect also contributes to the performance of the networks.

To generate white light using semiconductor nanocrystal (NC) quantum dots integrated on light emitting diodes (LEDs), multiple hybrid device parameters (emission wavelengths of the NCs and the excitation platform, order of the NCs with different sizes, amount of the different types of NCs, etc) need to be carefully designed and properly implemented. In this study, we introduce and demonstrate white LEDs based on simple device hybridization using only a single type of white emitting CdS quantum dot nanoluminophores on near-ultraviolet LEDs. Here we present their design, synthesis-growth, fabrication and characterization. With these hybrid devices, we achieve high color rendering index (>70), despite using only a single NC type. Furthermore, we conveniently tune their photometric properties including the chromaticity coordinates, correlated color temperature, and color rendering index with the number of hybridized nanoluminophores in a controlled manner.

This paper reports a novel single mode source of narrow-band entangled photon pairs at telecom wavelengths under continuous wave (CW) excitation, based on parametric down conversion. For only 7 mW of pump power it has a created spectral radiance of 0.08 pairs per coherence length and a bandwidth of 10 pm (1.2 GHz). The effectively emitted spectral brightness reaches 3.9×10⁵ pairs s⁻¹ pm⁻¹. Furthermore, when combined with low jitter single photon detectors, such sources allow for the implementation of quantum communication protocols without any active synchronization or path length stabilization. A Hong–Ou–Mandel (HOM)-dip with photons from two autonomous CW sources has been realized demonstrating the setup's stability and performance.

A sheet consisting of an array of small, aligned Dove prisms can locally (on the scale of the width of the prisms) invert one component of the ray direction. A sandwich of two such Dove-prism sheets that inverts both transverse components of the ray direction is a ray-optical approximation to the interface between two media with refractive indices +n and –n. We demonstrate the simulated imaging properties of such a Dove-prism-sheet sandwich, including a demonstration of pseudoscopic imaging.

We demonstrate that dynamic speckle patterns can be utilized to improve the optical sectioning power of wide-field coherent anti-Stokes Raman scattering (CARS) microscopy. The time-dependent speckle patterns are generated by randomly moving a multimode fiber delivering one of the excitation laser pulses. The standard deviation of various CARS images with changing speckle illumination yields an enhanced axial resolution as compared with a simply averaged CARS image. The procedure makes use of the intrinsically high speckle contrast even in scattering materials.

Electrical characterization of DNA molecules using the mechanically controlled break-junction technique is presented. The main advantage of the technique is the control over the electrode distance during the measurement. This can be used to stretch the DNA and search for the influence of the conformation on the conduction process. The DNA is characterized in liquid and dry environments. From our data, we conclude that only a small number of molecules are contacted in each measurement.

Reactive network forming polymer systems like epoxies are of huge technological interest because of their adhesive properties based on specific interactions with a large variety of materials. These specific interactions alter the morphology of the epoxy within areas determined by the correlation length of these interactions. The changed morphology leads to interphases with altered (mechanical) properties. Besides these surface-induced interphases, bulk interphases do occur due to segregation, crystallization, diffusion, etc. A new experimental technique to characterize such mechanical interphases is μ-Brillouin spectroscopy (μ-BS). With μ-BS, we studied interphases and their formation in epoxies due to segregation of the constituent components and due to selective diffusion of one component. In the latter case, we will demonstrate the influence of changing the boundary conditions of the diffusion process on the shape of the interphase.

Linear media are predicted to exist whose relative permeability is an operator in the space of quantum states of light. Such media are characterized by a photon statistics-dependent refractive index. This indicates a new type of optical dispersion: the photon-statistics dispersion. Interaction of quantum light with such media modifies the photon number distribution and, in particular, the degree of coherence of light. An excitonic composite—a collection of non-interacting quantum dots—is considered as a realization of the medium with the photon-statistics dispersion. Expressions are derived for generalized plane waves in an excitonic composite and input–output relations for a planar layer of the material. Transformation rules for different photon initial states are analyzed. Utilization of the photon-statistics dispersion in potential quantum-optical devices is discussed.

The magnetic Fe nanostructures formed on a Cu(111) surface were simulated by the kinetic Monte Carlo (kMC) method, in which the substrate-mediated long-range interactions between Fe adatoms are involved. The dependence of coverage and temperature on the formation of local nanostructures was investigated to reveal the microprocess of Fe nanostructures formed on a Cu(111) substrate. The simulation results show that the long-range interactions between Fe adatoms lead first to the formation of short linear chains and then to locally ordered nanostructures when they become steady at low coverages between 10 and 18 K. Based on the analysis of the formation mechanism of the Fe superlattice, it is found that the size of the repulsive ring, the distance to the first minimum potential and the diffusion barrier are the important parameters for Fe adatoms to form a superlattice on the Cu(111) surface.

Epitaxial films of graphene on SiC(0001) are interesting from a basic physics as well as an applications-oriented point of view. Here, we study the emerging morphology of in vacuo prepared graphene films using low-energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES). We obtain an identification of single-layer and bilayer graphene films by comparing the characteristic features in electron reflectivity spectra in LEEM to the π-band structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to accurately determine the local extent of graphene layers as well as the layer thickness.

The use of interference microscopy has enabled the direct observation of transient concentration profiles generated by intracrystalline transport diffusion in nanoporous materials. The thus accessible intracrystalline concentration profiles contain a wealth of information which cannot be deduced by any macroscopic method. In this paper, we illustrate five different ways for determining the concentration-dependent diffusivity in one-dimensional systems and two for the surface permeability. These methods are discussed by application to concentration profiles evolving during the uptake of methanol by the zeolite ferrierite and of methanol by the metal organic framework (MOF) manganese(II) formate. We show that the diffusivity can be calculated most precisely by means of Fick's 1st law. As the circumstances permit, Boltzmann's integration method also yields very precise results. Furthermore, we present a simple procedure that enables the estimation of the influence of the surface barrier on the overall uptake process by plotting the boundary concentration versus the overall uptake.

Biologically inspired models of self-propelled interacting agents display a wide variety of collective motion such as swarm migration and vortex formation. In these models, active interactions among agents are typically included such as velocity alignment and cohesive and repulsive forces that represent agents' short- and long-range 'sensing' capabilities of their environment. Here, we show that similar collective behaviors can emerge in a minimal model of isotropic agents solely due to a passive mechanism—inelastic collisions among agents. The model dynamics shows a gradual velocity correlation build-up into the collective motion state. The model displays a discontinuous transition of collective motion with respect to noise and exhibits several collective motion types such as vortex formation, swarm migration and also complex spatio-temporal group motion. This model can be regarded as a hybrid model, connecting granular materials and agent-based models.

We use high-resolution surface stress measurements to monitor the surface stress during the growth of pentacene (C₂₂H₁₄) on the (7×7) reconstructed silicon (111) surface. No significant change in the surface stress is observed during the pentacene growth. Compared to the changes in the surface stress observed for Si and Ge deposition on the Si(111)-(7×7) surface, the insignificant change in the surface stress observed for the pentacene growth suggests that the pentacene molecules of the first adsorbate layer, although forming strong covalent bonds with the Si adatoms, do not alter the structure of the (7×7) reconstruction. The (7×7) reconstruction remains intact and, with subsequent deposition of pentacene, eventually becomes buried under the growing film. This failure of the pentacene to affect the structure of the reconstruction may represent a fundamental difference between the growth of organic thin films and that of inorganic thin films on semiconductor surfaces.

In order to investigate the binding mechanism of weakly bound states of positronic alkali atoms, we calculate the energies and wavefunctions using the Gaussian expansion method (GEM) where a positronium (Ps)–alkali ion channel and a positron–alkali atom channel are explicitly introduced. The energies of the bound states are updated using a model potential that reproduces well the observed energy levels of alkali atoms. The binding mechanism of the positronic alkali atom is analyzed by the wavefunctions obtained. The structure of the positronic alkali atom has been regarded as a Ps cluster orbiting the alkali ion, which is described by the Ps–alkali ion channel. We point out that the fraction having the positron–alkali atom configuration is small but plays an indispensable role for the weakly bound system.

In the present work, a detailed theoretical analysis of a surface plasmon resonance (SPR)-based fiber optic sensor with an alternating dielectric multilayer system is carried out. The dielectric system consists of silica and titanium oxide layers. The effect of critical design parameters on the sensor's sensitivity and detection accuracy is studied. The results are explained in terms of appropriate physical phenomena, wherever required. Based on the analysis, a new design of a fiber optic SPR sensor for gas detection is proposed. The analysis of such a gas sensor is carried out for four metals separately for a clear understanding. The proposed gas sensor is able to provide reasonably high values of all the performance parameters simultaneously, as required for an efficient detection of gaseous media.

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni–Mn–Ga films. Ni–Mn–Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45° relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing microstructural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.

We propose a feasible scheme for generation of strongly non-Gaussian states using the cross-Kerr nonlinearity. The resultant states are highly non-classical states of an electromagnetic field and exhibit negativity of their Wigner function, sub-Poissonian photon statistics and amplitude squeezing. Furthermore, the Wigner function has a distinctly pronounced 'banana' or 'crescent' shape specific for the Kerr-type interactions, which so far has not been demonstrated experimentally. We show that creating and detecting such states should be possible with the present technology using electromagnetically induced transparency in a four-level atomic system in N-configuration.

Recently a technique has been introduced to Ω-modify a Hamiltonian so that the Ω-modifiedHamiltonian thereby produced is isochronous: all its solutions are periodic in all degrees of freedom with the same period . In this paper—after briefly reviewing this approach—we focus in particular on the Ω-modified version of the most general realistic many-body problem whose behavior, over time intervals much shorter than the isochrony period , differs only marginally from the thermodynamically irreversible evolution of the corresponding, unmodified and realistic many-body system. We discuss the (apparently paradoxical) periodic recurrence of the irreversible processes occurring in this Ω-modified model, implying a periodic reversal of its irreversible behavior. We then discuss the equilibrium statistical mechanics of this Ω-modified model, including the compatibility of standard thermodynamic notions such as entropy with the peculiar phenomenology featured by its time evolution. The theoretical discussion is complemented by numerically simulated examples of the molecular dynamics yielded by the (standard and classical) Hamiltonian describing (many) particles interacting pairwise via potentials of Lennard–Jones type and via harmonic potentials in two-dimensional space, and by its Ω-modified version. In the latter case, the simulation displays (approximate) returns to configurations away from thermodynamic equilibrium after relaxation to equilibrium had occurred.

We report a comparative Raman spectroscopic study of the quasi-one-dimensional charge-density-wave (CDW) systems A_0.3MoO₃ (A = K, Rb). Temperature- and polarization-dependent experiments reveal charge-coupled vibrational Raman features. The strongly temperature-dependent collective amplitudon modes in the two materials differ by about 3 cm⁻¹, thus revealing the role of the alkali atom. We discuss the observed vibrational features in terms of the CDW ground state accompanied by a change in the crystal symmetry. A frequency-kink in some modes seen in K_0.3MoO₃ between T = 80 and 100 K supports the first-order lock-in transition, unlike the case of Rb_0.3MoO₃. The unusually sharp Raman lines (limited by the instrumental response) at very low temperatures and their temperature evolution suggests that the decay of the low-energy phonons is strongly influenced by the presence of the temperature-dependent CDW gap.

The directed polymerization of a branched actin network against a functionalized surface drives cell protrusions and organelle propulsion in living cells. Solid microspheres or giant unilamellar vesicles, functionalized with neural Wiskott–Aldrich syndrome protein (N-WASP), initiate the formation of a branched actin array using actin-related protein 2/3 (Arp2/3) complex, when placed in a motility assay reconstituted with pure proteins. These systems are useful biomimetic models of actin-based propulsion that allow to address how the interplay between the physical properties of the functionalized surface and the dynamics of the actin cytoskeleton determines motile behavior. Both solid beads and deformable vesicles display either continuous or saltatory propulsive motions, which are analyzed comparatively; we show that the deformability of liposomes and the mobility of N-WASP at the lipid surface affect the dynamic and structural parameters of the actin meshwork. Our results indicate that beads and vesicles use different mechanisms to translate insertional polymerization of actin at their surface into directed movement: stress relaxation within the actin gel prevents the accumulation of filaments at the front of moving beads, while segregation of nucleators reduces actin polymerization at the front of moving vesicles.

Stress fibers are contractile cytoskeletal structures, tensile actomyosin bundles which allow sensing and production of force, provide cells with adjustable rigidity and participate in various processes such as wound healing. The stress fiber is possibly the best characterized and most accessible multiprotein cellular contractile machine. Here we develop a quantitative model of the structure and relaxation kinetics of stress fibers. The principal experimentally known features are incorporated. The fiber has a periodic sarcomeric structure similar to muscle fibers with myosin motor proteins exerting contractile force by pulling on actin filaments. In addition the fiber contains the giant spring-like protein titin. Actin is continuously renewed by exchange with the cytosol leading to a turnover time of several minutes. In order that steady state be possible, turnover must be regulated. Our model invokes simple turnover and regulation mechanisms: actin association and dissociation occur at filament ends, while actin filament overlap above a certain threshold in the myosin-containing regions augments depolymerization rates. We use the model to study stress fiber relaxation kinetics after stimulation, as observed in a recent experimental study where some fiber regions were contractile and others expansive. We find that two distinct episodes ensue after stimulation: the turnover–overlap system relaxes rapidly in seconds, followed by the slow relaxation of sarcomere lengths in minutes. For parameter values as they have been characterized experimentally, we find the long time relaxation of sarcomere length is set by the rate at which actin filaments can grow or shrink in response to the forces exerted by the elastic and contractile elements. Consequently, the stress fiber relaxation time scales inversely with both titin spring constant and the intrinsic actin turnover rate. The model's predicted sarcomere velocities and contraction–expansion kinetics are in good quantitative agreement with experiment.

In this paper, our current understanding of calorimetry is discussed, in view of the challenges offered by future applications in experiments at the LHC and the ILC/CLIC. Calorimetry is likely to become an even more crucial component of the detector complex than in the present generation of experiments. And the demands on performance will increase, in particular concerning the detection of fragmenting quarks and gluons. The (underlying reasons for the) obstacles one faces in trying to meet these demands are discussed in some detail, emphasizing the difficulties encountered in calibrating a longitudinally segmented calorimeter system. Generic R&D efforts that are being carried out in this context are described, and recent results of these projects are presented.

Investigations of light–matter interactions and motion in the microcosm have entered a new temporal regime, the regime of attosecond physics. It is a main 'spin-off' of strong field (i.e., intense laser) physics, in which nonperturbative effects are fundamental. Attosecond pulses open up new avenues for time-domain studies of multi-electron dynamics in atoms, molecules, plasmas, and solids on their natural, quantum mechanical time scale and at dimensions shorter than molecular and even atomic scales. These capabilities promise a revolution in our microscopic knowledge and understanding of matter.

The recent development of intense, phase-stabilized femtosecond (10⁻¹⁵ s) lasers has allowed unparalleled temporal control of electrons from ionizing atoms, permitting for the first time the generation and measurement of isolated light pulses as well as trains of pulses on the attosecond (1 as = 10⁻¹⁸ s) time scale, the natural time scale of the electron itself (e.g., the orbital period of an electron in the ground state of the H atom is 152 as). This development is facilitating (and even catalyzing) a new class of ultrashort time domain studies in photobiology, photochemistry, and photophysics.

These new coherent, sub-fs pulses carried at frequencies in the extreme ultraviolet and soft-x-ray spectral regions, along with their intense, synchronized near-infrared driver waveforms and novel metrology based on sub-fs control of electron–light interactions, are spawning the new science of attosecond physics, whose aims are to monitor, to visualize, and, ultimately, to control electrons on their own time and spatial scales, i.e., the attosecond time scale and the sub-nanometre (Ångstrom) spatial scale typical of atoms and molecules. Additional goals for experiment are to advance the enabling technologies for producing attosecond pulses at higher intensities and shorter durations. According to theoretical predictions, novel methods for intense attosecond pulse generation may in future involve using overdense plasmas.

Electronic processes on sub-atomic spatio-temporal scales are the basis of chemical physics, atomic, molecular, and optical physics, materials science, and even some life science processes. Research in these areas using the new attosecond tools will advance together with the ability to control electrons themselves. Indeed, we expect that developments will advance in a way that is similar to advances that have occurred on the femtosecond time scale, in which much previous experimental and theoretical work on the interaction of coherent light sources has led to the development of means for 'coherent control' of nuclear motion in molecules.

This focus issue of New Journal of Physics is centered on experimental and theoretical advances in the development of new methodologies and tools for electron control on the attosecond time scale. Topics such as the efficient generation of harmonics; the generation of attosecond pulses, including those having only a few cycles and those produced from overdense plasmas; the description of various nonlinear, nonperturbative laser–matter interactions, including many-electron effects and few-cycle pulse effects; the analysis of ultrashort propagation effects in atomic and molecular media; and the development of inversion methods for electron tomography, as well as many other topics, are addressed in the current focus issue dedicated to the new field of 'Attosecond Physics'.

Focus on Attosecond Physics Contents

Observing the attosecond dynamics of nuclear wavepackets in molecules by using high harmonic generation in mixed gases Tsuneto Kanai, Eiji J Takahashi, Yasuo Nabekawa and Katsumi Midorikawa

Core-polarization effects in molecular high harmonic generation G Jordan and A Scrinzi

Interferometric autocorrelation of an attosecond pulse train calculated using feasible formulae Y Nabekawa and K Midorikawa

Attosecond pulse generation from aligned molecules—dynamics and propagation in H₂⁺E Lorin, S Chelkowski and A D Bandrauk

Broadband generation in a Raman crystal driven by a pair of time-delayed linearly chirped pulses Miaochan Zhi and Alexei V Sokolov

Ultrafast nanoplasmonics under coherent control Mark I Stockman

Attosecond pulse carrier-envelope phase effects on ionized electron momentum and energy distributions: roles of frequency, intensity and an additional IR pulse Liang-You Peng, Evgeny A Pronin and Anthony F Starace

Angular encoding in attosecond recollision Markus Kitzler, Xinhua Xie, Stefan Roither, Armin Scrinzi and Andrius Baltuska

Polarization-resolved pump–probe spectroscopy with high harmonics Y Mairesse, S Haessler, B Fabre, J Higuet, W Boutu, P Breger, E Constant, D Descamps, E Mével, S Petit and P Salières

Macroscopic effects in attosecond pulse generation T Ruchon, C P Hauri, K Varjú, E Mansten, M Swoboda, R López-Martens and A L'Huillier

Monitoring long-term evolution of molecular vibrational wave packet using high-order harmonic generation M Yu Emelin, M Yu Ryabikin and A M Sergeev

Intense single attosecond pulses from surface harmonics using the polarization gating technique S G Rykovanov, M Geissler, J Meyer-ter-Vehn and G D Tsakiris

Imaging of carrier-envelope phase effects in above-threshold ionization with intense few-cycle laser fields M F Kling, J Rauschenberger, A J Verhoef, E Hasović, T Uphues, D B Milošević, H G Muller and M J J Vrakking

Self-compression of optical laser pulses by filamentation A Mysyrowicz, A Couairon and U Keller

Towards efficient generation of attosecond pulses from overdense plasma targets N M Naumova, C P Hauri, J A Nees, I V Sokolov, R Lopez-Martens and G A Mourou

Quantum-path control in high-order harmonic generation at high photon energies Xiaoshi Zhang, Amy L Lytle, Oren Cohen, Margaret M Murnane and Henry C Kapteyn

Time-resolved mapping of correlated electron emission from helium atom in an intense laser pulse C Ruiz and A Becker

Pump and probe ultrafast electron dynamics in LiH: a computational study M Nest, F Remacle and R D Levine

Exploring intense attosecond pulses D Charalambidis, P Tzallas, E P Benis, E Skantzakis, G Maravelias, L A A Nikolopoulos, A Peralta Conde and G D Tsakiris

Attosecond timescale analysis of the dynamics of two-photon double ionization of helium Emmanuel Foumouo, Philippe Antoine, Henri Bachau and Bernard Piraux

Generation of tunable isolated attosecond pulses in multi-jet systems V Tosa, V S Yakovlev and F Krausz

Electron wavepacket control with elliptically polarized laser light in high harmonic generation from aligned molecules Y Mairesse, N Dudovich, J Levesque, M Yu Ivanov, P B Corkum and D M Villeneuve

Tracing non-equilibrium plasma dynamics on the attosecond timescale in small clusters Ulf Saalmann, Ionut Georgescu and Jan M Rost

Ionization in attosecond pulses: creating atoms without nuclei? John S Briggs and Darko Dimitrovski

Angular distributions in double ionization of helium under XUV sub-femtosecond radiation P Lambropoulos and L A A Nikolopoulos

Potential for ultrafast dynamic chemical imaging with few-cycle infrared lasers Toru Morishita, Anh-Thu Le, Zhangjin Chen and C D Lin

Attosecond electron thermalization in laser-induced nonsequential multiple ionization: hard versus glancing collisions X Liu, C Figueira de Morisson Faria and W Becker

Ion-charge-state chronoscopy of cascaded atomic Auger decay Th Uphues, M Schultze, M F Kling, M Uiberacker, S Hendel, U Heinzmann, N M Kabachnik and M Drescher

Measurement of electronic structure from high harmonic generation in non-adiabatically aligned polyatomic molecules N Kajumba, R Torres, Jonathan G Underwood, J S Robinson, S Baker, J W G Tisch, R de Nalda, W A Bryan, R Velotta, C Altucci, I Procino, I C E Turcu and J P Marangos

Wavelength dependence of sub-laser-cycle few-electron dynamics in strong-field multiple ionization O Herrwerth, A Rudenko, M Kremer, V L B de Jesus, B Fischer, G Gademann, K Simeonidis, A Achtelik, Th Ergler, B Feuerstein, C D Schröter, R Moshammer and J Ullrich

Attosecond metrology in the few-optical-cycle regime G Sansone, E Benedetti, C Vozzi, S Stagira and M Nisoli

Attosecond x-ray pulses produced by ultra short transverse slicing via laser electron beam interaction A A Zholents and M S Zolotorev

We propose a method of generation of ∼115 attosecond x-ray pulses in a free electron laser (FEL) by means of producing ultra-fast angular modulation of the electron trajectories prior to entering the FEL. For this modulation, we employ a few-cycle laser pulse in a higher-order Gaussian mode and with carrier-envelope phase stabilization.

The polarization gating method in combination with few-optical-cycle driving pulses with controlled waveform is a powerful technique for the generation of isolated few-cycle attosecond pulses. We show that such a technique allows one to generate attosecond pulses tunable in a broad spectral region, corresponding to more than 26 eV. Complete temporal characterization of the attosecond pulses has been obtained by using the frequency resolved optical gating for complete reconstruction of attosecond bursts technique. The physical processes which determine the temporal confinement of the extreme ultraviolet radiation and the effects of various experimental parameters on the electric field of the attosecond pulses have been investigated using numerical simulations based on the nonadiabatic saddle-point method.

Recoil-ion momentum distributions for double and triple ionization of Ne and Ar, as well as for double ionization of N₂ molecule by intense (0.3–0.5 PW cm^-2), short (∼35–40 fs) laser pulses have been recorded in a so far unexplored long laser-wavelength regime at 1300 nm. Compared to earlier results at 800 nm, the direct (e, ne) ionization pathway during recollision is strongly enhanced manifesting itself in a pronounced double-hump structure in the longitudinal ion momentum spectra not only for Ne, but also surprisingly distinct for Ar and, found for the first time, for molecules. Observed wavelength dependence of the sub-laser-cycle correlated few-electron dynamics might be of paramount importance for possible future applications in attosecond science, in particular, for imaging of ultrafast molecular processes via recollision-induced fragmentation.

We have explored the use of laser driven high-order harmonic generation to probe the electronic structure and symmetry of conjugated polyatomic molecular systems. We have investigated non-adiabatically aligned samples of linear symmetric top, nonlinear symmetric top and asymmetric top molecules, and we have observed signatures of their highest occupied molecular orbitals in the dependence of harmonic yields on the angle between the molecular axis and the polarization of the driving field. A good quantitative agreement between the measured orientation dependence of high harmonic generation and calculations employing the strong field approximation has been found. These measurements support the extension of molecular imaging techniques to larger systems.

It has recently been demonstrated that apart from the electron detection realized in the attosecond streak camera, also ion detection can be used for establishing extreme-ultraviolet pump/visible probe experiments, temporally resolving the dynamics of atomic inner-shell relaxation processes. We utilize this method for studying the Auger decay of krypton atoms following the creation of vacancy states in the 3d shell. It is shown that the electronic relaxation occurs through different pathways, each involving cascades of sequential steps which are followed in their native temporal succession.

A recollision-based largely classical statistical model of laser-induced nonsequential multiple (N-fold) ionization of atoms is further explored. Upon its return to the ionic core, the first-ionized electron interacts with the other N- 1 bound electrons either through a contact or a Coulomb interaction. The returning electron may leave either immediately after this interaction or join the other electrons to form a thermalized complex which leaves the ion after the delay Δt, which is the sum of a thermalization time and a possible additional dwell time. Good agreement with the available triple and quadruple ionization data in neon and argon is obtained with the contact scenario and delays of Δt=0.17 T and 0.265 T, respectively, with T the laser period.

We studied the photoelectron spectra generated by an intense few-cycle infrared laser pulse. By focusing on the angular distributions of the back rescattered high energy photoelectrons, we show that accurate differential elastic scattering cross-sections of the target ion by free electrons can be extracted. Since the incident direction and the energy of the free electrons can be easily changed by manipulating the laser's polarization, intensity and wavelength, these extracted elastic scattering cross-sections, in combination with more advanced inversion algorithms, may be used to reconstruct the effective single-scattering potential of the molecule, thus opening up the possibility of using few-cycle infrared lasers as powerful table-top tools for imaging chemical and biological transformations, with the desired unprecedented temporal and spatial resolutions.

We present photoelectron angular distributions resulting from the two-photon direct double ionization of helium, under XUV radiation, by solving the time-dependent (TD) Schrödinger equation. The helium TD wavefunction is expanded in terms of fully correlated multichannel states normalized with incoming-wave boundary conditions. The present study focuses on fields of pulse durations within the subfemtosecond regime and at photon energy of 45 eV where the direct double ionization channel dominates the sequential channel. In addition, at this photon energy, the ejected electrons, resulting from the direct and the sequential path, acquire non-overlapping kinetic energy spectra. Our study reveals a trend for back-to-back ejection asymmetry independently of the kinetic energies of the electrons, thus implying that angular correlations are taking place at the time of the ionization. In addition, for given kinetic-energy sharing, it appears that this asymmetry is developed within an interatomic time interval of subfemtosecond scale, intimately connected with the electron–electron interaction strength.

It is shown that with one full cycle of a suitable attosecond laser pulse it is possible to detach the ground-state electrons from an atom and deposit them, with their wavefunction largely unchanged, as a wavepacket removed spatially from the nucleus and whose centre of charge is stationary with respect to the nucleus.

It is shown by microscopic calculations that the energy absorption of a rare-gas cluster from a vacuum-ultraviolet (VUV) pulse can be traced with time-delayed extreme-ultraviolet (XUV) attosecond probe pulses by measuring the kinetic energy of the electrons detached by the probe pulse. By means of this scheme we demonstrate that, for pump pulses as short as one femtosecond, the charging of the cluster proceeds during the formation of an electronic nano-plasma inside the cluster. Using moderate harmonics for the VUV and high harmonics for the XUV pulse from the same near-infrared laser source, this scheme with well defined time delays between pump and probe pulses should be experimentally realizable. Going to even shorter pulse durations we predict that pump and probe pulses of about 250 attoseconds can induce and monitor non-equilibrium dynamics of the nano-plasma.

We study experimentally and theoretically the high harmonic emission from aligned samples of nitrogen and carbon dioxide, in an elliptically polarized laser field. The ellipticity induces a lateral shift of the recombining electron wavepacket in the generation process. We show that this effect, which is well known from high harmonic generation (HHG) in atoms, can be useful to maintain the plane wave approximation in the case of HHG from molecules whose orbitals contain nodal planes. The study of the harmonic signal as a function of molecular alignment also reveals the role of the ellipticity on the recollision angle of the electron wavepacket, which can be used to accurately track the position of resonances in harmonic spectra.

We theoretically investigate how the generation of attosecond pulses from high-order harmonics can be controlled by using a specially designed sequence of gas jets. We demonstrate that quasi-phase-matching provided by such a multi-jet system can be limited to a sub-femtosecond time window, while adjusting the multi-jet structure allows tuning of the central frequency of the generated isolated attosecond pulse.

We consider the two-photon double ionization (DI) of helium and analyze electron dynamics on the attosecond timescale. We first re-examine the interaction of helium with an ultrashort XUV pulse and study how the electronic correlations affect the electron angular and energy distributions in the direct, sequential and transient regimes of frequency and time duration. We then consider pump–probe processes with the aim of extracting indirect information on the pump pulse. In addition, our calculations show clear evidence for the existence under certain conditions of direct two-color DI processes.

After introducing the importance of non-linear processes in the extreme-ultra-violet (XUV) spectral regime to the attosecond (asec) pulse metrology and time domain applications, we present two successfully implemented techniques with excellent prospects in generating intense asec pulse trains and isolated asec pulses, respectively. For the generation of pulse trains two-color harmonic generation is exploited. The interferometric polarization gating technique appropriate for the generation of intense isolated asec pulses is discussed and compared to other relevant approaches.

A time-dependent multiconfiguration method with a large electronic basis set is used to compute the response of all the electrons of LiH to a few-cycle intense pump field followed by a probe pulse. The ultrashort pump pulse excites a coherent superposition of stationary electronic states and, by changing the pump parameters such as intensity, duration, polarization and phase of carrier frequency, one can steer the motion of the electrons. Particular attention is given to the control provided by the polarization and by the phase. For example, a change in polarization is used to select an electronic wave packet that is rotating in a plane perpendicular to the bond or rotation in a plane containing the bond. The electronic wave packet can be probed by a delayed second pulse. This delayed probe pulse is also included in the Hamiltonian with the result that the frequency dispersed probe spectrum can be computed and displayed as a two-dimensional plot.

We apply and analyze the concept of mapping ionization time on to the final momentum distribution to the correlated electron dynamics in the nonsequential double ionization of helium in a strong laser pulse (λ=800 nm) and show how the mapping provides insight into the double ionization dynamics. To this end, we study, by means of numerical integration of the time-dependent Schrödinger equation of a fully correlated model atom, the temporal evolution of the center-of-mass momentum in a short laser pulse. Our results show that in the high intensity regime (I₀=1.15×10¹⁵ W cm^-2), the mapping is in good agreement with a classical model including binary and recoil rescattering mechanisms. In the medium intensity regime (I₀=5×10¹⁴ W cm^-2), we identify additional contributions from the recollision-induced excitation of the ion followed by subsequent field ionization (RESI).

We show through experiment and calculations how all-optical quasi-phase-matching of high-order harmonic generation can be used to selectively enhance emission from distinct quantum trajectories at high photon energies. Electrons rescattered in a strong field can traverse short and long quantum trajectories that exhibit differing coherence lengths as a result of variations in intensity of the driving laser along the direction of propagation. By varying the separation of the pulses in a counterpropagating pulse train, we selectively enhance either the long or the short quantum trajectory, and observe distinct spectral signatures in each case. This demonstrates a new type of coupling between the coherence of high-order harmonic beams and the attosecond time-scale quantum dynamics inherent in the process.

Theoretical studies and computer simulations predict efficient generation of attosecond electromagnetic pulses from overdense plasma targets, driven by relativistically strong laser pulses. These predictions need to be validated in time resolved experiments in order to provide a route for applications. The first available femtosecond sources for these experiments are likely to be 10 fs pulses of a few millijoules, which could provide focal intensities at about the relativistic threshold. With particle-in-cell simulations, we demonstrate that the radiation resulting from interaction of such pulses with solid targets is expected to be attosecond trains with very high conversion efficiency as relativistic effects start to act.

During the propagation of intense femtosecond laser pulses in a transparent medium, pulse shortening can occur without external guiding. Experimental evidence for this effect and a description of its physical origin are presented. Nearly single cycle pulses at 800 nm with an energy of 0.120 mJ can be obtained with excellent beam quality. Carrier envelope offset phase (CEP) stability is conserved or even improved after the nonlinear propagation stage. Prospects for further improvement are discussed.

Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed.

Harmonics generated at solid surfaces interacting with relativistically strong laser pulses are a promising route towards intense attosecond pulses. In order to obtain single attosecond pulses one can use few-cycle laser pulses with carrier-envelope phase stabilization. However, it appears feasible to use longer pulses using polarization gating—the technique known for a long time from gas harmonics. In this paper, we investigate in detail a specific approach to this technique on the basis of one-dimensional-particle-in-cell (1D PIC) simulations, applied to surface harmonics. We show that under realistic conditions polarization gating results in significant temporal confinement of the harmonics emission allowing thus the generation of intense single attosecond pulses. We study the parameters needed for gating only one attosecond pulse and show that this technique is applicable to both normal and oblique incidence geometry.

We present a method for probing ultrafast nuclear dynamics in molecules using transient enhancement of high-order harmonic generation (HHG). The method exploits strong dependence of the overall harmonic yield in a fixed spectral range on the nuclear separation in a molecule, which is shown to take place for both aligned and randomly oriented molecules. Our numerical simulations show that this method is capable of monitoring long-term evolution of the nuclear vibrational wave packets in molecules, even in light-weight ones, with very high time resolution. By the example of D₂⁺ vibrational wave packet launched via tunnelling ionization of D₂ and probed by a time-delayed 8 fs laser pulse with λ = 800 nm and the peak intensity of 10¹⁴ W cm^-2, we show that the time-delay dependence of the high harmonic signal exhibits pronounced features, which are due to the collapses and revivals of the nuclear wave packet. The time-frequency analysis of the pump–probe signal reveals structures with a periodicity down to 6 fs, which correspond to the fractional revivals of orders 1/5 and 1/10. With the laser parameters used, the deuteron motion during the probe pulse is shown to have almost negligible effect on the resulting signal.

We examine how the generation and propagation of high-order harmonics in a partly ionized gas medium affect their strength and synchronization. The temporal properties of the resulting attosecond pulses generated in long gas targets can be significantly influenced by macroscopic effects, in particular by the intensity in the medium and the degree of ionization which control the dispersion. Under some conditions, the use of gas targets longer than the absorption length can lead to the generation of compressed attosecond pulses. We show these macroscopic effects experimentally, using a 6 mm-long argon-filled gas cell as the generating medium.

High harmonic generation in gases can be used as a probe of the electronic structure of the emitting medium, with attosecond temporal resolution and angström spatial resolution. The prospect of measuring molecular dynamics by pump–probe spectroscopy with such precision is attracting a lot of interest. An important issue in pump–probe spectroscopy lies in the ability to detect small signals: the detected signal can be easily dominated by the contributions from non-excited molecules or from a carrier gas. In this paper, we demonstrate that polarization-resolved pump–probe spectroscopy can be used to overcome this issue. We study high harmonic generation from rotationally excited molecules. We show that by measuring the harmonic field that is generated orthogonally to the driving laser field, the contrast in the detection of alignment revivals in nitrogen can be increased by a factor 4. We use this configuration to measure alignment revivals in an argon–nitrogen mixture, in which the total harmonic signal is dominated by the contributions from argon.

We describe a general concept of using the spatial information encoded in the time-dependent polarization of high harmonic radiation generated by orthogonally polarized two-color laser fields. The main properties of recolliding electron wave packets driven by such fields are reviewed. It is shown that in addition to the recollision energy the angle of recollision of such wave packets, which is directly mapped onto the polarization direction of the emitted high harmonic radiation, varies on a sub-laser-cycle time-scale. Thus, a mapping between the polarization angle and the frequency of the emitted radiation is established on an attosecond time scale. While the polarization angle encodes the spatial properties of the recollision process, the frequency is linked to time via the well-known dispersion relations of high harmonic generation. Based on these principles, we show that in combination with polarization selective detection the use of orthogonally polarized drive pulses for high harmonic generation permit one to construct spatially resolved attosecond measurements. Here, we present two examples of possible applications: (i) a method for isolating a single attosecond pulse from an attosecond pulse train which is more efficient than the cut-off selection method, and (ii) a technique for orbital tomography of molecules with attosecond resolution.

The effects of the carrier-envelope phase (CEP) of a few-cycle attosecond pulse on ionized electron momentum and energy spectra are analyzed, both with and without an additional few-cycle IR pulse. In the absence of an IR pulse, the CEP-induced asymmetries in the ionized electron momentum distributions are shown to vary as the 3/2 power of the attosecond pulse intensity. These asymmetries are also found to satisfy an approximate scaling law involving the frequency and intensity of the attosecond pulse. In the presence of even a very weak IR pulse (having an intensity of the order of 10¹¹–10¹² W cm⁻²), the attosecond pulse CEP-induced asymmetries in the ionized electron momentum distributions are found to be significantly augmented. In addition, for higher IR laser intensities, we observe for low electron energies peaks separated by the IR photon energy in one electron momentum direction along the laser polarization axis; in the opposite direction, we find structured peaks that are spaced by twice the IR photon energy. Possible physical mechanisms for such asymmetric, low-energy structures in the ionized electron momentum distribution are proposed. Our results are based on single-active-electron solutions of the three-dimensional, time-dependent Schrödinger equation including atomic potentials appropriate for the H and He atoms.

Recently, there has been an increased attention and rapid development in the field of nanoscale collective electronic dynamics on surfaces of metal nanostructures, which is due to excitations called surface plasmons (SPs). This field, known as nanoplasmonics, is very promising for such applications as the next generation optoelectronics, computations and information storage on the nanoscale, ultrasensitive detection of threats and spectroscopy of physical, chemical and biological nano-objects. Due to their broad spectral bandwidth, SPs possess ultrafast dynamics, with times as short as hundreds of attoseconds. In this paper, we discuss progress in the excitation, control and applications of the ultrafast nanoplasmonic fields. Special attention is devoted to attosecond visualization and coherent control of the nanoscale optical fields. Prospective applications of ultrafast nanoplasmonics are discussed.

A pair of time-delayed linearly chirped pulses with sub-picosecond duration is used to selectively excite Raman transitions in a lead tungstate crystal. Significant molecular coherence leads to generation of up to 40 anti-Stokes and 5 Stokes sidebands. High conversion efficiency (from the two pump beams to the sidebands) is measured. The broadband generation with chirped pulses whose duration is comparable to the Raman coherence lifetime is considerably more efficient, when compared to the case of excitation by two-color femtosecond pulses. In the future, mutual coherence among the generated sidebands may allow ultrashort pulse synthesis.

The dynamics and propagation effects in attosecond (asec) pulse generation from high-order harmonic generation (HHG) of aligned one-dimensional (1D) H₂⁺ molecules are investigated from numerical solutions of fully coupled Maxwell and time-dependent Schrödinger equations (Maxwell-TDSEs), in the highly nonlinear nonperturbative regime of laser–molecule interaction. Density, laser-phase and propagation length effects are studied on the total electric field and nonlinear polarization from the Maxwell-TDSE for intense few cycle (800 nm) laser pulses interacting with a 1D H₂⁺ gas. We show how single and double asec pulses can be generated and propagated as a function of the phase of individual harmonics created by ultrashort intense laser pulses in aligned H₂⁺ molecules. We find furthermore extension of maximum HHG plateaux with increasing gas pressure.

The autocorrelation trace of an attosecond pulse train (APT) directly revealed the pulse envelope in our recent experiment on measuring the two-photon Coulomb explosion of a nitrogen molecule as a correlation signal. Although the spatial overlap of the two replicas of the APT in the correlation measurement was only achieved near the focal region owing to the spatial split of the measured APT field, which is a situation quite different from that of the correlation measurement using a Michelson interferometer, the interference fringes clearly appeared on the correlation envelope and provedthe odd symmetry of the electric field to the time translation with a half-period of the driving laser field. In this paper, we show a simple and practical analysis for the propagation and the nonlinear interaction of an APT to simulate the experimental result of the interferometric autocorrelation of the spatially split APT. The spatial convolution of the focused electric field is essential for obtaining the fringes. We also discuss how the autocorrelation should be described in the context of the second-order perturbation theory within a dipole approximation.

By comparing three-dimensional multi-electron calculations with several simplifying models, we show that polarization of the molecular ion can crucially contribute to the high harmonic spectra of molecules. We solve the time-dependent Schrödinger equation for diatomic model molecules with up to 4 active electrons using the multi-configuration time-dependent Hartree–Fock method. Single electron models fail to reproduce the harmonic spectra as they do not account for polarization of the multi-electron ionic core by the laser field. We find the dominant mechanism in the modification of the recombination dipole matrix element, while ionization is only mildly affected.

We probe the attosecond dynamics of nuclear wavepackets in H₂ and D₂ molecules by measuring the relative phase of high harmonics generated in each molecule by using a novel method with a mixed gas of H₂ and D₂. We find that not only the single molecule responses but also the propagation effects of harmonics differ between the two isotopes and we conclude that in order to discuss the dynamics of molecules in the single molecule responses, the propagation effects need to be excluded from the raw harmonic signals. The measured relative phase as well as the intensity ratio are found to be monotonic functions of the harmonic order and are successfully reproduced by applying Feynman's path integral method fully to the dynamics of the nuclei and electrons in the molecules.