We investigate the fragmentation and ionization of naphthalene by protons at intermediate velocities (between 1.41 and 2.68 a.u.). Relative cross sections for electron capture (EC), electron emission (EE), and capture ionization are measured. The EC cross sections decrease rapidly over the energy range under consideration (50–150 keV) and are lower than EE cross sections. The EE cross sections, on the other hand, change very slowly in this energy range. The energetics of interactions is quantified by comparing the mass spectra with the photodissociation breakdown curves from literature. In the case of single capture, resonant electron transfer to $n$ = 1 state in H${}^{+}$ is seen to dominate the interaction but is shown to be accompanied by a small amount of electronic energy loss. In the EE mode, two mechanisms are shown to be active in the collision process: large impact parameter plasmon excitation mode, and closer encounters with higher amounts of electronic energy loss.

The high-order harmonic-generation (HHG) spectrum of Be is investigated in the multiphoton regime by solving the full-dimensional, two-active-electron, time-dependent Schrödinger equation in an intense ($\approx {10}^{13}$ W/cm${}^2$), 30-cycle laser field. As the laser frequency ${\omega }_L$ varies from $1.7$ to $1.8$ eV (which is in the tunable range of a Ti:sapphire laser), the seventh harmonic becomes resonant sequentially with the transition between the ground state and two doubly excited autoionizing states, $2p4s$(${}^{1}P$) (at ${\omega }_L=1.734$ eV) and $2p5s$(${}^{1}P$) (at ${\omega }_L=1.785$ eV), while the third harmonic becomes resonant with the $2s2p$(${}^{1}P$) singly excited state (at ${\omega }_L=1.766$ eV). At each of these resonant frequencies, the HHG power spectrum is found to increase by an order of magnitude over a range of harmonics that form a plateau, extending from the resonant harmonic up to a cutoff at the 25th harmonic. In contrast to the well-known rescattering plateau cutoff law appropriate in the tunneling regime (which predicts a cutoff at the fifth or seventh harmonic), the multiphoton regime plateau we find for Be originates from atomic resonance effects. Off resonance, the Be HHG spectrum decreases monotonically with harmonic order. By taking the ratio of the integrated harmonic power of the seventh harmonic to that of the fifth harmonic, one can isolate the resonant effects of the two doubly excited states in the HHG spectrum from those of singly excited resonance states. These ratios exhibit resonance profiles for driving laser-pulse durations much longer than the lifetimes of these autoionizing states. The energy widths of these resonance features are comparable to the widths of the laser pulse and are much smaller than the autoionizing state widths. These results demonstrate an important role for electron correlations in enhancing harmonic-generation rates in the multiphoton regime.

The intermediate-energy $R$-matrix approach (IERM) has been developed as a first step towards representing a double-electron continua within a general multielectron $R$-matrix code. Using this method, we study photo-double-ionization of the ground states of the He and Be isoelectronic sequences and show excellent agreement with other theoretical and experimental results. We confirm scaling relations for the photo-double-ionization cross section along the He isoelectronic sequence and suggest a scaling relation, which takes into account the screening effect of the $1s$ electrons, for the photo-double-ionization cross section for Be-like ions. We also examine trends in the ratio of double to single ionization.

The solution of the Stark problem is obtained in terms of the Stokes multipliers for the triconfluent Heun equation (the quartic oscillator equation). The Stokes multipliers are found in an analytical form at positive energies. For negative energies, the Stokes parameters are calculated in frames of a consistent asymptotic approach. The scattering phase, positions, and widths of the Stark resonances are determined as solutions of an implicit equation.

We present an ab initio approach for the study of macroscopic high-order-harmonic generation (HHG) of rare-gas atoms (Ar) in intense ultrashort laser fields. The single-atom response is calculated by means of the self-interaction-free time-dependent density-functional theoretical (TDDFT) approach with proper long-range potential, and the macroscopic propagation effects are taken into account by solving Maxwell's equations using the induced dipole moments of the single-atom multiple electron shell. We perform all-electron nonperturbative investigation of macroscopic HHG of rare-gas atoms with the emphasis on the role of multiple valence electron subshells. In particular, we found that the valence electron subshell features a destructive interference distribution in multiphoton resonant HHG. In addition, our results show the Cooper minimum in macroscopic HHG from Ar atoms is located at 53 eV, in good agreement with the experimentally measured minimum position.

We discuss how a recent pump-probe study [Kelkensberg et al., Phys. Rev. Lett. 103, 123005 (2009)] of the dissociative ionization of ${\text{H}}_2$, under the combined effect of a single extreme ultraviolet attosecond pulse and an intense near-infrared pulse, actually represents a transition-state spectroscopy of the strong-field dissociation step, i.e., of the (probe-pulse-)dressed H${}_2$${}^{+}$ molecular ion. The way the dissociation dynamics is influenced by the duration of the near-infrared probe pulse, and by the time delay between the two pulses, is discussed in terms of adiabatic versus nonadiabatic preparation and transport of time-parametrized Floquet resonances associated with the dissociating molecular ion. Under a long probe pulse, the field-free vibrational states of the initial wave packet are transported, in a one-to-one manner, onto the Floquet resonances defined by the field intensity of the probe pulse and propagated adiabatically under the pulse. As the probe pulse duration shortens, nonadiabatic transitions between the Floquet resonances become important and manifest themselves in two respects: first, as a vibrational shake-up effect occurring near the peak of the short pulse, and second, through strong interference patterns in the fragment's kinetic energy spectrum, viewed as a function of the time delay between the pump and the probe pulses.

Here we analyze cavity-mediated laser cooling for an experimental setup with an external trap which strongly confines the motion of a particle in the direction of the cavity axis. It is shown that the stationary-state phonon number exhibits three sharp minima as a function of the atom-cavity detuning due to a direct atom-phonon-photon coupling term in the system Hamiltonian. These resonances have the same origin as the Mollow triplet in the resonance fluorescence of a laser-driven atomic system. It is shown that a laser-Rabi-frequency-dependent atom-cavity detuning yields the lowest stationary-state phonon number for a wide range of experimental parameters.

Adiabatic techniques offer some of the most promising tools for achieving high-fidelity control of the center-of-mass degree of freedom of single atoms. Because the main requirement of these techniques is to follow an eigenstate of the system, constraints on timing and field strength stability are usually low, especially for trapped systems. In this paper we present a detailed example of a technique to adiabatically transport a single atom between different waveguides on an atom chip. To ensure that all conditions are fulfilled, we carry out fully three-dimensional simulations of the system, using experimentally realistic parameters. We also detail our method for simulating the system in very reasonable time scales on a consumer desktop machine by leveraging the power of graphics-processing-unit computing.

We analyze a tight-binding model of ultracold fermions loaded in an optical square lattice and subjected to a synthetic non-Abelian gauge potential featuring both a magnetic field and a translationally invariant SU(2) term. We consider in particular the effect of broken time-reversal symmetry and its role in driving nontrivial topological phase transitions. By varying the spin-orbit coupling parameters, we find both a semimetal-insulator phase transition and a topological phase transition between insulating phases with different numbers of edge states. The spin is not a conserved quantity of the system, and the topological phase transitions can be detected by analyzing its polarization in time-of-flight images, providing a clear diagnostic for the characterization of the topological phases through the partial entanglement between spin and lattice degrees of freedom.

We introduce the notion of a ${\displaystyle \mathcal{PT}}$-symmetric dimer with a ${\chi }^{\left(2\right)}$ nonlinearity. Similarly to the Kerr case, we argue that such a nonlinearity should be accessible in a pair of optical waveguides with quadratic nonlinearity and gain and loss, respectively. An interesting feature of the problem is that because of the two harmonics, there exist in general two distinct gain and loss parameters, different values of which are considered herein. We find a number of traits that appear to be absent in the more standard cubic case. For instance, bifurcations of nonlinear modes from the linear solutions occur in two different ways depending on whether the first- or the second-harmonic amplitude is vanishing in the underlying linear eigenvector. Moreover, a host of interesting bifurcation phenomena appear to occur, including saddle-center and pitchfork bifurcations which our parametric variations elucidate. The existence and stability analysis of the stationary solutions is corroborated by numerical time-evolution simulations exploring the evolution of the different configurations, when unstable.

We investigate the fluorescence spectrum of a two-level system driven by a monochromatic classical field by the Born-Markovian master equation based on a unitary transformation. The main purpose is to understand the effects of counter-rotating-wave terms of the driving on spectral features of the fluorescence. We have derived an analytical expression for the fluorescence spectrum, which is different from Mollow's theory, while Mollow's result on resonance is the limiting case of ours in moderately weak driving regimes. Our results demonstrate precisely that the counter-rotating-wave terms of the driving play an important role in the fluorescence spectrum for intense driving: (i) the counter-rotating coupling suppresses the red sideband in the Mollow triplet and it enhances the blue one in explicitly contrast to the well-known equal intensity of the sideband in Mollow's theory, (ii) the higher-order Mollow triplets appear as a characteristic spectral feature arising from counter-rotating-wave terms of the driving, and (iii) a significant frequency shift of the sidebands is observed, which depends on both the detuning and driving strength.

We examined the propagation of bending nonanalytical points encoded on temporally smooth Gaussian-shaped optical pulses in fast- and slow-light media, using an optical ring resonator. For bending nonanalytical points, the pulse function is continuous, but the first-order derivative is discontinuous, in contrast to traditional nonanalytical points in which the pulse function itself is discontinuous. The nonanalytical points were neither advanced nor delayed, but appeared at the same instance as they entered the ring resonator, in good accordance with the fact that the information velocity was equal to the light velocity in vacuum or the background medium. The differences between the traditional discontinuous and the present bending nonanalytical points were investigated.

It is shown that complexities in a problem of elastic scattering of a photon on a pair of Rayleigh particles (two small metallic spheres) are similar to the complexities of the classic problem of three bodies in celestial mechanics. In the latter problem, as is well known, the phase trajectory of a system becomes a nonanalytical function of its variables. In our problem, the trajectory of a virtual photon at some frequency could be considered such as the well-known Antoine set (Antoine's necklace) or a chain with interlaced sections having zero topological dimension and fractal structure. Such a virtual “zero-dimensional” photon could be localized between the particles of the pair. The topology suppresses the photon's exit to the real world with dimensional equal-to-or-greater-than units. The physical reason for this type of photon localization is related to the “mechanical rigidity” of interlaced sections of the photon trajectory due to a singularity of energy density along these sections. Within the approximations used in this paper, the effect is possible if the frequency of the incident radiation is equal to double the frequency of the dipole surface plasmon in an isolated particle, which is the only character frequency in the problem. This condition and transformation of the photon trajectory to the zero-dimensional Antoine set reminds of some of the simplest variants of Poincaré's catastrophe in the dynamics of some nonintegrable systems. The influence of the localization on elastic light scattering by the pair is investigated.

We report a frequency measurement of the $2{}^1{P}_1\to 3{}^1{D}_2$ transition in ${}^4$He at 668 nm. Doppler-free saturation spectroscopy is performed in an rf discharged sealed-off cell using an external cavity diode laser stabilized by an optical frequency comb. The measured absolute frequency is 448 791 399.113 (268) MHz with a relative uncertainty of $6\times {10}^{-10}$. Our result is more precise than the previous measurement by one order of magnitude. The ionization energy of the $2{}^1{P}_1$ state using this result and the theoretical value of the $3{}^1{D}_2$ state is derived to be 814 709 148.133 (269) MHz, in agreement with our previous determinations derived from other precisely measured transitions. We also derive the ionization energy of the $2{}^1{S}_0$ state and the energy separation between $3{}^1{D}_2$ and $3{}^3{D}_1$ states using this result and other transitions. These determinations should be essential for comparison with the theoretical calculations.

We revisit the problem of quantum state reconstruction of light beams from the photocurrent quantum noise. As is well known, but often overlooked, two longitudinal field modes contribute to each spectral component of the photocurrent (sideband modes). We show that spectral homodyne detection is intrinsically incapable of providing all the information needed for the full reconstruction of the two-mode spectral quantum state. Such a limitation is overcome by the technique of resonator detection. A detailed theoretical description and comparison of both methods is presented, as well as an experiment to measure the six-mode quantum state of pump-signal-idler beams of an optical parametric oscillator above the oscillation threshold.

We derive an analytical expression for the propagation of a quantum particle at asymptotic long distances and times from the potential where the particle was initially confined. We obtain that the particle is described by an evolving resonance in time domain that possesses a peaked shape characterized by the resonance parameters of the dominant resonance coefficient of the decaying wave function. The above situation corresponds to an unexplored postexponential regime that represents the ultimate fate of a decaying particle.

It is shown that the usual model for confined one-center systems can be extended to two-center systems and the theory is used to treat ${\text{H}}_2$ and ${\text{HeH}}^{2+}$. It is convenient to use spheroidal coordinates for the two-center problems and we use this formalism to compare hydrogenlike systems confined spheroidally with the more standard spherical confinement. A discussion of confinement by both penetrable and impenetrable barriers is given together with a technique for describing the effect of the external region.

We study a bosonic gas of hard spheres by using the exact zero-temperature path-integral ground-state (PIGS) Monte Carlo method and the equations of superfluid hydrodynamics. The PIGS method is implemented to calculate for the bulk system the energy per particle and the condensate fraction through a large range of the gas parameter $n{a}^3$ (with $n$ the number density and $a$ the $s$-wave scattering length), going from the dilute gas into the solid phase. The Maxwell construction is then adopted to determine the freezing at $n{a}^3=0.264\pm 0.003$ and the melting at $n{a}^3=0.290\pm 0.003$. In the liquid phase, where the condensate fraction is finite, the equations of superfluid hydrodynamics, based on the PIGS equation of state, are used to find other relevant quantities as a function of the gas parameter: the chemical potential, the pressure, and the sound velocity. In addition, within Feynman's approximation, from the PIGS static structure factor we determine the full excitation spectrum, which displays a maxon-roton behavior when the gas parameter is close to the freezing value. Finally, the equations of superfluid hydrodynamics with the PIGS equation of state are solved for the bosonic system under axially symmetric harmonic confinement obtaining its collective breathing modes.

We suggest a way to obtain a near-perfect nonmagnetic invisibility cloak structure. Within the particular example of cylindrical geometry and transverse magnetic polarization of light, we show that such a cloak can operate for a cylindrical object of very large diameter, keeping the total scattering cross section limited by $2\lambda /\pi$. We also demonstrate that proposed cloak does not require extremely high values of dielectric permittivity, in contrast to the cloaks designed according to a transformation optics technique.