Table of contents

It is customary to denote as 'nano-systems' entities whose size varies between 1 and 100 nm. Such systems span an immense variety of structures, ranging from large molecules, quantum dots, and nanowires, to viruses. Their existence is based on, and accompanied by, an amazing abundance of phenomena, depending on a huge number of parameters, all of which can in principle be used for nanoscale control. One of the most potent means of controlling nano-systems is light: it can serve as a tool for the addressing, preparation, stabilization and excitation of nano-systems. Design of light control scenarios is based on the laser techniques of coherent and optimal control at wavelengths and time sequencing most appropriate to the nano-systems, or on more traditional, incoherent forms of light–matter interactions.

This special issue of Journal of Physics B: Atomic, Molecular and Optical Physics (J. Phys. B) devoted to the subject of light control at the nanoscale contains a collection of articles that cover several key areas in this fast-growing field. These include: preparation and manipulation of nano-systems by light, nanoscale plasmonics and spintronics, Bose–Einstein condensation and excitation of novel molecular and crystalline systems. We have grouped the papers according to these general areas.

The guest editors gratefully acknowledge the help of the staff of J. Phys. B, especially that of Alice Malhador from the editorial team, and Adrian Corrigan from production, in preparing this issue, and Professor Jan-Michel Rost, the Editor-in-Chief, for his support of this project.

The tight focus of Gaussian beams is commonly used to trap dielectric particles in optical tweezers. The corresponding field distribution generates a well-defined trapping potential that is only marginally controllable on a nanometre scale. Here we investigate the influence of a metal nanostructure that is located in the vicinity of the trapping focus on the trapping potential by calculating the corresponding field and force distributions. Even for an excitation wavelength that is tuned far from the plasmonic resonance of the nanostructure, the presence of the latter alters significantly the trap potential. For the given nanostructure, a ring of spheres that is illuminated in the axial direction, a smaller focus volume is observed in comparison to free focus. The superposition of this non-resonant Gaussian field with a planar wave illumination that is tuned to the plasmonic resonance gives a handle to modify the trapping potential. Polarization and intensity of the resonant illumination allows modifying the equilibrium position of the trapping potential, thus providing means to steer dielectric particles with nanometre precision.

Localized and propagating surface plasmons excited with 10 fs, 400 nm laser pulses in silver gratings are imaged with a sub-wavelength spatial resolution. Microscopic images of two-photon photoemission from the nanostructured silver surface representing nonlinear maps of surface plasmon fields are recorded with a photoemission electron microscope (PEEM). Tuning the laser wavelength into the resonance of a silver grating enhances the emission from the propagating mode and attenuates that from the localized modes. Time-resolved interferometric PEEM movies taken at 330 as/frame intervals reveal the dynamics of the oscillation and dephasing of individual localized surface plasmons.

Intense ultrashort light pulses interacting inside dielectrics can create nanoplasmas due to localized inhomogeneous nonlinear ionization. These nanoplasmas are bound inside the dielectric and are transient as their density changes during the light pulse—from underdense to quasi-metallic plasma densities. Interaction of light at the transient plasma–dielectric interface can lead to local field enhancements, similar to that observed in the metal-dielectric interface, which control the growth of nanoplasmas. We discuss the differences in the interaction of light at these two interfaces and demonstrate that transient nanoplasmonics can imprint periodic nanostructures inside the dielectric.

We illustrate the possibility of manipulating light in the nanoscale using the combination of plasmonics physics with concepts and tools developed for coherent control of molecular dynamics. Phase and polarization control are applied to guide electromagnetic energy through metal nanoparticle junctions and control its branching ratios at array intersections. Optimal control theory is applied as a design tool, to develop constructs with desired functionality. We suggest also that nanoplasmonics could be used to make spatially localized light sources with predesigned coherence and polarization properties, which could serve to coherently control individual nano-systems.

We study the collective two-channel dissociation dynamics of a molecular Bose–Einstein condensate into bosonic fragments under tight harmonic confinement. Bose-stimulated dissociation in either channel can only take place provided that the respective trap size l_i for the fragments is large with respect to the healing length ζ_i of the atom–molecule resonance. Thus, even when both channels are equally coupled, differences in mass or in dynamical polarizability enable the control of the reaction outcome by variation of the trap frequency. In particular, if l₁ > ζ₁ and l₂ < ζ₂, only the first channel will be amplified. This behaviour demonstrates a unique feature of 'superchemistry' wherein a chemical reaction may be controlled by the manipulation of the reaction vessel.

We study phonon-assisted dephasing in optically excited semiconductor quantum dots within the frameworks of the independent-boson model and optimal control. Using a realistic description for the quantum-dot states and the phonon coupling, we demonstrate that such dephasing has a drastic impact on the coherent optical response. We employ optimal-control theory to search for control strategies that allow us to fight decoherence, and show that appropriate tailoring of laser pulses allows a complete control of the optical excitation despite the phonon dephasing.

We investigate the dynamics of two interacting electrons confined in a symmetric double quantum dot structure under the influence of bichromatic electric fields. The theoretical analysis is based on an effective two-level system approach and the conditions for two-electron localization in the same quantum dot are analytically derived. The analytical results are compared to numerical results obtained from the solution of the time-dependent Schrödinger equation.

We report on rapid adiabatic passage (RAP) in a Pr³⁺:Y₂SiO₅ crystal, cooled to cryogenic temperatures. The medium is prepared by optical pumping and spectral hole burning, creating a spectrally isolated two-level system within the inhomogeneous bandwidth of the ³H₄ → ¹D₂ transition of the Pr³⁺ ions. A chirped laser pulse drives a RAP process in the medium, i.e. inverts the initial population distribution. We study the properties and dynamics of RAP by means of fluorescence detection, absorption spectroscopy and amplified spontaneous emission. Time-resolved absorption measurements serve to monitor the adiabatic population dynamics during the excitation process. In addition, we compare the results with coherent excitation at fixed laser frequency detuned from resonance, i.e. coherent population return (CPR).

A novel technique is proposed to control the dissociation mechanism of small diatomic molecules. This technique, relying upon the creation of a coherent nuclear wavepacket, uses intense (>10¹⁴ W cm⁻²), ultrashort (∼10 fs) infrared laser pulses in a pump and probe scheme. In applying this technique to D⁺₂ good agreement has been observed between a quantum simulation and experiment. This represents a major step towards quantum state control in molecules, using optical fields.

High resolution ultraviolet-to-visible image conversion using photo-luminescence emitted by self-assembled CdSe/ZnCdMgSe quantum dots was investigated. The visible images centred at 585 nm with a resolution of 2 µm and 5 µm were obtained using a cw-He-Cd laser at the 325 nm excitation wavelength and with 150 fs laser pulses tuned to the second harmonic generation at 400 nm, respectively. The resolution limitations of visible images are discussed, and a high resolution optical system for the ultraviolet-to-visible image conversion is proposed.

The magnetic-octupole (M3) and electric-quadrupole (E2) transitions between the ground state 3d¹⁰¹S₀ and the lowest excited 3d⁹4s(5/2, 1/2)J = 3 and J = 2 states in the Ni-like tungsten are shown to exhibit a strong dependence on electron density N_e in the range of values typical for tokamak plasmas. Remarkably, the total intensity of these overlapping lines remains almost constant, which may explain the strong emission in the 7.93 Å line observed in tokamak experiments (Neu R et al1997 J. Phys. B: At. Mol. Opt. Phys.30 5057). Utilization of the M3 and E2 line ratios for density diagnostics in high-spectral-resolution experiments is discussed as well.

The multiphoton ionization of hydrogen by a strong bichromatic microwave field is a complex process prototypical for atomic control research. Periodic orbit analysis captures this complexity: through the stability of periodic orbits we can match qualitatively the variation of experimental ionization rates with a control parameter, the relative phase between the two modes of the field. Moreover, an empirical formula reproduces quantum simulations to a high degree of accuracy. This quantitative agreement shows how short periodic orbits organize the dynamics in multiphoton ionization.

We investigate the energy eigenvalues and eigenstates of a resonant field in a Kerr-like medium by an efficient method. All the energy eigenvalues and eigenstates can be obtained for an arbitrary total particle number N and other parameters involved in the system. We show that the eigenstates can be explicitly expressed analytically in terms of a single parameter γ whose values are determined by the roots of a polynomial of the order of at most 1 + int(N/2) with int(x) being x's integer part. By studying the spectra, the erratic level crossings appear apparent when N ⩾ 40 and γ over critical values.

We consider a single ion confined in a trap under the radiation of two travelling waves of lasers. In the strong-excitation regime and without the restriction of the Lamb–Dicke limit, the Hamiltonian of the system is similar to a driving Jaynes–Cummings model without the rotating wave approximation (RWA). The approach we developed enables us to present complete eigensolutions, which makes it possible to compare with the solutions under the RWA. We find that the ground state in our non-RWA solution is energetically lower than the counterpart under the RWA. If we have the ion in the ground state, it is equivalent to a spin dependent force on the trapped ion. Discussion is given for the difference between the solutions with and without the RWA, and for the relevant experimental test, as well as for the possible application to quantum information processing.

A simple model to describe the low-temperature behaviour of some atoms and molecules is proposed and applied to the hydrogen atom. To this end, the low-temperature dynamics of the approach to equilibrium of the hydrogen atom is analysed by means of a standard Monte Carlo simulation. It is shown that, before approaching ionization, the atom may live for a long time in a quasi-equilibrium state whose duration increases exponentially for decreasing temperatures. Essentially, this effect is directly related to the low probability associated with the transition between the ground- and first-excited states, which demands an enormous amount of energy (75% of the whole energy spectrum). Therefore, for low temperatures, the atom may take a long time to overcome such an energy barrier. It is argued that the dynamical behaviour associated with the approach to equilibrium of some composite particles, characterized by an energy spectrum presenting an upper bound, preceded by the accumulation of an infinite number of levels—for which the hydrogen atom represents a prototype—can be described, at low temperatures, by a special class of q-oscillators. By suitably adjusting the deformation parameter q, characteristic of these q-oscillator systems, one obtains a dynamical behaviour at low temperatures which resembles that associated with the composite particle of interest. In order to reproduce the results of the hydrogen atom, the central idea is that this parameter may be set to a value, in such a way that the energy gap between the ground- and first-excited states coincide in the two systems. The method is illustrated by choosing q = 1/4, in which case one gets a remarkable agreement, from both qualitative and quantitative points of view, with the dynamical behaviour of the hydrogen atom. The conditions of applicability of the method are discussed.

The multiconfiguration Hartree–Fock (MCHF) and the plane-wave Born approximations (PWBA) based on Dirac–Fock multiconfiguration (MCDF) computations are used to calculate the photoionization cross section and electron-impact ionization cross section of Yb from the 6s6p ³P₁ state, respectively. The wavefunctions of open-shell systems were modelled by Hartree–Fock Slater and Dirac–Fock Slater nonorthogonal orbitals with allowance for the relaxation effects. The one-electron wavefunction of the continuous spectrum for the photoelectron and for the ejected electrons was obtained using single-configuration Hartree–Fock and Dirac–Fock methods, respectively. The orthogonalization of the ejected and photoelectron wavefunctions to all occupied orbitals of the target atom is performed. The calculations of the cross sections for electron-impact ionization based on MCHF and MCDF approximations are compared with each other. The ratio of the reduced dipole matrix elements and the phaseshift difference for transition 6pεs and εd are first compared with available polynomial fitting of the experimental photoelectron angular distribution data from laser-excited aligned Yb* atoms ionized by vacuum ultraviolet radiation.

We provide arguments which demonstrate that the Berry phase effects involving particle interference in the presence of external electric and magnetic fields—namely the Aharonov–Bohm (AB), Aharonov–Casher (AC) and the Röntgen (R) effects—cannot be unambiguously explained (as has been controversially argued) through the use of any classical mechanical force. In fact, in the three scenarios which we consider, the only means to reclaim the standard quantum mechanical results is to use the expressions for the force that fail to satisfy the necessary conditions of gauge invariance. Our arguments explain the foundation of the controversy and confirm the standard idea that the Berry phase effects in question are non-local phenomena which only arise in the application of quantum theory. We lay special emphasis on the use of the concept of a canonical force, this being the quantity which ensures the agreement between the classical and quantum formulations. It is shown that if the Aharonov–Bohm effect is interpreted as being due to a force, through the use of the Bohmian formulation of quantum mechanics, then this force is equal in magnitude to the canonical force.

The photoelectron spectrum of selenophene has been recorded using synchrotron radiation in the photon energy range 20–80 eV and the inner valence region has been studied in detail for the first time. Green's function methods have been employed to evaluate the energies and spectral intensities of all valence shell ionization transitions and the results have facilitated an interpretation of the experimental spectra. Strong configuration interaction results in a redistribution of the intensity associated with the low lying π₁(1b₁) orbital amongst several satellite states located in the outer valence region. The continuum multiple scattering approach has been used to calculate photoelectron asymmetry parameters for selenophene, thiophene and hydrogen sulphide, and these theoretical predictions have been compared with the corresponding experimental data to assess the influence of Cooper minima and shape resonances. The comparison indicates that the Se 4p and the S 3p Cooper minima have little effect on the valence shell photoionization dynamics of selenophene and thiophene, respectively. This outcome is discussed in connection with the closely related hydrogen selenide and hydrogen sulphide molecules where strong resonant phenomena are observed.

The classical region of a Bose gas consists of all single particle modes that have a high average occupation and are well described by a classical field. Highly occupied modes only occur in massive Bose gases at ultra-cold temperatures, in contrast to the photon case where there are highly occupied modes at all temperatures. For the Bose gas the number of these modes is dependent on the temperature, the total number of particles and their interaction strength. In this paper, we characterize the classical region of a harmonically trapped Bose gas over a wide parameter regime. We use a Hartree–Fock approach to account for the effects of interactions, which we observe to significantly change the classical region as compared to the idealized case. We compare our results to full classical field calculations and show that the Hartree–Fock approach provides a qualitatively accurate description of a classical region for the interacting gas.

Simulations were performed describing the motion and breakup of energetic C₆₀ ions interacting with crystalline targets. A hybrid algorithm was used that employs a binary collision model for the scattering of the carbon ions by the atoms of the solid, and molecular dynamics for the Coulomb interactions of the 60 carbon ions with one another. For the case of yttrium iron garnet (YIG), directions such as [1 1 0], [1 0 0], [0 1 0] and [0 0 1] demonstrate channelling for a large fraction of the C ions. For directions such as [1 1 1], [2 1 1] and [7 5 3] the trajectories show no more channelling than for random directions. The effects of tilt, shielding and wake-field interactions were investigated for YIG and α-quartz.

We consider a two-dimensional Bose gas formed in a planar atomic trap in conditions where the two-dimensional scattering length exceeds all other microscopic length scales in the system and, accordingly, the gas parameter assumes relatively high values. We show that, unlike in the three-dimensional case, for sufficiently low areal densities the two-dimensional gas remains stable against collapse even in the resonant regime. Furthermore, we evaluate the three-body recombination rate that sets the upper limit for the lifetime of the two-dimensional resonant atomic condensate.

We propose a new numerically optimized discrete variable representation using eigenstates of diabatic Hamiltonians. This procedure provides an efficient method to solve non-adiabatic coupling problems since the generated basis sets take into account information on the diabatic potentials. The method is applied to the B¹Σ⁺ − D'¹Σ⁺ Rydberg-valence predissociation interaction in the CO molecule. Here we give an account of the discrete variable representation and present the procedure for the calculation of its optimized version, which we apply to obtain the total photodissociation cross sections of the CO molecule.

Angle-resolved metastable fragment yields following photo-excitation have been measured in the photon energy region 405.7–430 eV in N₂. Structures corresponding to multiple excitations associated with parallel and perpendicular transitions are observed in the metastable yield curves recorded using detectors orientated at 0° and 90° to the electric field vector of the linearly polarized incident radiation. The double-excitation structures around 415 eV are interpreted in terms of the overlap of three broad peaks and vibrational progressions. Multiple excitation structures corresponding to high Rydberg states converging to N⁺₂ shake-up satellites are seen above 418 eV. The present work demonstrates that the angle-resolved detection of metastable fragments is an effective spectroscopic tool for probing core-valence multiply excited states in molecules.

A new kind of bipartite coherent–entangled state (CES) is introduced which exhibits both coherent state and entangled state properties. A protocol of generating such a CES is proposed using an asymmetric beamsplitter. It is shown that the bipartite EPR entangled state can be a superposition of the CESs. Applications of CESs in quantum optics are also presented.

We investigate many characteristic features of revival and fractional revival phenomena via derived analytic expressions for an autocorrelation function of a one-dimensional Rydberg atom with weighting probabilities modelled by a Gaussian or a Lorentzian distribution. The fractional revival phenomenon in the ionization probabilities of a one-dimensional Rydberg atom irradiated by two short half-cycle pulses is also studied. When many states are involved in the formation of the wave packet, the revival is lower and broader than the initial wave packet and the fractional revivals overlap and disappear with time.

A problem which motivated a great deal of work about 20 years ago, namely, satellite lines occurring for atomic emitters undergoing a harmonic perturbation, is revisited. On a theoretical point of view, two photon mechanisms or equivalent are involved to explain those satellites due to high-frequency electric fields. Although today the activity on these problems is rather low, interest in observing such effects in the domain of x-ray spectroscopy exists, namely for hot and dense plasmas. More generally, satellites can be also seen as connected to turbulence diagnostics. This mainly motivates the design of plasmas and improvements of x-ray spectroscopy techniques. However, up to now, attempts to extend the methods of nonlinear spectroscopy to this domain have been rather disappointing. As a promotion for a resurgence of the field, an improved theory, founded on formalisms of nonlinear optics, is developed to suggest a new interpretation of the experiments. Previous publications are modified and an old problem is closed. Hopefully, this will help us to stimulate new applications of two-photon techniques in plasmas.

Calculations on electron collisions with CO⁺ molecular ion are presented as a function of electron energy and CO⁺ geometry. Resonance positions and widths are obtained for the low-lying Feshbach resonances in the system. Plots of resonance curves suggest that low-energy dissociative recombination can occur via resonances of ¹Π, ¹Δ, ³Σ⁺ and ³Π symmetries. Electron impact electronic excitation is considered both for excitation to the first excited A ²Π state and to yield a dissociation cross section. The latter calculations suggest that the lower of two experimental measurements of this process is likely to be correct. Finally results are presented for bound states in the continuum for both ¹Σ⁻ and ³Σ⁻ symmetries.

The dimensional analyses of the position and momentum variance based quantum mechanical Heisenberg uncertainty measure and the other useful net entropic information measures for the bound states of two constrained Coulomb potentials are reported for the first time. The potentials describe an electron moving in the central field due to a nucleus of charge Z with radius R defining the constraints as (a) the truncated potential given by , and (b) the radius of the impenetrable spherical wall. The net information measures for the two potentials are explicitly shown to be independent of the scaling of the set [Z, R] at a fixed value of ZR. Analytic proof is presented, for the first time, showing the presence of a characteristic extremum in the variation of the net information entropy as a function of the radius R with its location scaling as Z⁻¹. Numerical results are presented which support the validity of the scaling properties.

A theoretical description of atomic photoionization by attosecond pulses in the presence of an intense laser pulse is presented. It is based on the numerical solving of the non-stationary Schrödinger equation which includes on an equal footing the realistic atomic potential and the electric fields of both pulses. The calculated energy spectra and angular distributions of photoelectrons are compared with those obtained using a simple approximate model based on the strong-field approximation. The agreement is excellent for large energy of photoelectrons. When the energy is small, the rescattering of electrons by the ionic core affects the cross section considerably making the strong-field approximation inadequate. Influence of the electron orbital polarization on the ionization cross section is investigated.

The origin of the intense emission band at about 5 nm, dominating the emission spectra of tungsten ions in the ASDEX Upgrade tokamak and EBIT, is discussed. It is shown that the emission spectra of various ions calculated taking into account only the excitations from the ground level agree fairly well with the results obtained in the collisional-radiative model; thus, the contribution of the excitations from the other levels is small. Though the excitation spectrum for all sequence of ions W²⁹⁺–W³⁷⁺ corresponds to the same transitions 4p⁶4d^N→ 4p⁵4d^N+1+ 4p⁶4d^N−1 4f, its energetic width essentially changes going on from the charge of ion q = 34 to q = 35. It is caused by the appearance of the excitations 4p_1/2–4d_3/2 to the open 4d^N_3/2 subshell, which are not quenched by configuration mixing. The satellite line at about 4.5 nm is explained by the transitions of the same type, although between configurations with one spectator 5s electron. The existence of one more group of intense lines in the region of 2 nm, corresponding to 5s–4p transitions, is predicted.

K shell fluorescence parameters such as fluorescence yield, fluorescence cross section and absorption jump factor for pure elemental targets of Mo, Ag, Cd, In and Sn have been determined by adopting a 2π geometrical configuration. The K x-ray photons are excited in the target using 123.6 keV gamma rays from a weak ⁵⁷Co source, and detected with a GMX 10P HPGe detector coupled to an 8K multichannel analyser. Measured values of these parameters have been compared with theoretical and others' experimental values.

Absolute cross sections for electron impact dissociative excitation and ionization of CD⁺ leading to the formation of ionic products (D⁺, C⁺, C²⁺ and C³⁺) are reported in the energy range from their respective thresholds to 2.5 keV. Around the maximum, cross sections are found to be (10.5 ± 1.0) × 10⁻¹⁷ cm², (20.6 ± 3.5) × 10⁻¹⁷ cm², (1.20 ± 0.11) × 10⁻¹⁷ cm² and (8.2 ± 1.5) × 10⁻²⁰ cm² for D⁺, C⁺, C²⁺ and C³⁺, respectively. In the very low-energy region, dissociative excitation leading to the C⁺ formation dominates over the D⁺ one. The cross section for dissociative ionization (C⁺ + D⁺ formation) is found to be (6.9 ± 1.3) × 10⁻¹⁷ cm² around 105 eV and the corresponding threshold energy is (22.1 ± 0.5) eV. The animated crossed-beams method is used and the analysis of ionic product velocity distributions allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 15 eV both for C⁺ and for D⁺, and up to 40 eV both for C²⁺ and for C³⁺. For singly charged products, the comparison of the present energy thresholds and kinetic energy release with published data allows the identification of states contributing to the observed processes. In particular, contributions from primary ions formed in the a³Π metastable state are perceptible. At fixed electron energy, the cross sections for the various ionization channels are seen to reduce exponentially with the potential energy of each dissociated ion pair. Anisotropies are estimated to be in the range 8 ± 2% for both C⁺ and D⁺. The total CD⁺ single ionization cross section calculated by application of the Deutsch–Märk formalism is found to be in good agreement with experimental results.

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