Spin conservation in high-order-harmonic generation using bicircular fields

We present an alternative theoretical model for a recent experiment [A.Fleischer et al., Nature Photon. 8, 543 (2014)] which used bichromatic, counter-rotating high intensity laser pulses to probe the conservation of spin angular momentum in high harmonic generation. We separate elliptical polarizations into independent circular fields with definite angular momentum, instead of using the expectation value of spin for each photon in the conservation equation, and we find good agreement with the experimental results. In our description the generation of each individual harmonic conserves spin angular momentum, in contrast to the model proposed by Fleischer et al. Our model also correctly describes analogous processes in standard perturbative optics.Comment: Final changes from published version. Updated license to CC-BY-NC-S

Disorder-induced effects in high-harmonic generation process in fullerene molecules

This work aims to exploit the extreme nonlinear optical response of inversion symmetric fullerene molecules subjected to various types of disorders to reveal Anderson localization effects on high harmonic generation spectra. We show that the disorder-induced effects are imprinted onto molecules' high-harmonic spectrum. Specifically, we observe a presence of strong even-order harmonic signals already for relatively small disorders. The odd-order harmonics intrinsic for disorder-free systems are generally robust to minor disorders. Both diagonal and off-diagonal disorders lift the degeneracy of states, opening up new channels for interband transitions, leading to the enhancement of the high-harmonic emission. For the second harmonic signal we obtain a law describing the dependence on the diagonal disorder strength, enabling the usage of the harmonic spectrum as a tool in measuring the disorder type and the strength.Comment: 8 pages, 9 figure

Intense high-order harmonic generation in giant fullerene molecule C240_{240}

In this work the extreme nonlinear optical response of a giant fullerene molecule C$_{240}$ in strong laser field is studied. The investigation of high-order harmonic generation in such quantum nanostructure is presented modeling the C$_{240}$ molecule and its interaction with the laser field in the scope of the tight-binding mean-field approach. Electron-electron interaction is modeled by the parametrized Ohno potentail, which takes into account long-range Coulomb interaction. The essential role of many body Coulomb interaction in determining of harmonics intensities is demonstrated. We also consider vacancy-deffected molecule C$_{240}$. The presence of a single vacancy breaks the icosahedral symmetry leading to the emergence of intense even-order harmonics. We examine the dependence of moderate harmonics on laser frequency that shows the multiphoton resonant nature of high harmonics generation. The dependence of cutoff harmonics on both laser intensity and frequency are examined too.Comment: 9 pages, 10 figure

Reconciling Conflicting Approaches for the Tunneling Time Delay in Strong Field Ionization

Several recent attoclock experiments have investigated the fundamentalquestion of a quantum mechanically induced time delay in tunneling ionizationvia extremely precise photoelectron momentum spectroscopy. The interpretationsof those attoclock experimental results were controversially discussed, becausethe entanglement of the laser and Coulomb field did not allow for theoreticaltreatments without undisputed approximations. The method of semiclassicalpropagation matched with the tunneled wavefunction, the quasistatic Wignertheory, the analytical R-matrix theory, the backpropagation method, and theunder-the-barrier recollision theory are the leading conceptual approaches putforward to treat this problem, however, with seemingly conflicting conclusionson the existence of a tunneling time delay. To resolve the contradictingconclusions of the different approaches, we consider a very simple tunnelingscenario which is not plagued with complications stemming from the Coulombpotential of the atomic core, avoids consequent controversial approximationsand, therefore, allows us to unequivocally identify the origin of the tunnelingtime delay.<br

Interpreting Attoclock Measurements of Tunnelling Times

Resolving in time the dynamics of light absorption by atoms and molecules, and the electronic rearrangement this induces, is among the most challenging goals of attosecond spectroscopy. The attoclock is an elegant approach to this problem, which encodes ionization times in the strong-field regime. However, the accurate reconstruction of these times from experimental data presents a formidable theoretical challenge. Here, we solve this problem by combining analytical theory with ab-initio numerical simulations. We apply our theory to numerical attoclock experiments on the hydrogen atom to extract ionization time delays and analyse their nature. Strong field ionization is often viewed as optical tunnelling through the barrier created by the field and the core potential. We show that, in the hydrogen atom, optical tunnelling is instantaneous. By calibrating the attoclock using the hydrogen atom, our method opens the way to identify possible delays associated with multielectron dynamics during strong-field ionization.Comment: 33 pages, 10 figures, 3 appendixe

Recursive formulation of the multiconfigurational time-dependent Hartree method for fermions, bosons and mixtures thereof in terms of one-body density operators

The multiconfigurational time-dependent Hartree method (MCTDH) [Chem. Phys. Lett. {\bf 165}, 73 (1990); J. Chem. Phys. {\bf 97}, 3199 (1992)] is celebrating nowadays entering its third decade of tackling numerically-exactly a broad range of correlated multi-dimensional non-equilibrium quantum dynamical systems. Taking in recent years particles' statistics explicitly into account, within the MCTDH for fermions (MCTDHF) and for bosons (MCTDHB), has opened up further opportunities to treat larger systems of interacting identical particles, primarily in laser-atom and cold-atom physics. With the increase of experimental capabilities to simultaneously trap mixtures of two, three, and possibly even multiple kinds of interacting composite identical particles together, we set up the stage in the present work and specify the MCTDH method for such cases. Explicitly, the MCTDH method for systems with three kinds of identical particles interacting via all combinations of two- and three-body forces is presented, and the resulting equations-of-motion are briefly discussed. All four possible mixtures of fermions and bosons are presented in a unified manner. Particular attention is paid to represent the coefficients' part of the equations-of-motion in a compact recursive form in terms of one-body density operators only. The recursion utilizes the recently proposed Combinadic-based mapping for fermionic and bosonic operators in Fock space [Phys. Rev. A {\bf 81}, 022124 (2010)] and successfully applied and implemented within MCTDHB. Our work sheds new light on the representation of the coefficients' part in MCTDHF and MCTDHB without resorting to the matrix elements of the many-body Hamiltonian with respect to the time-dependent configurations. It suggests a recipe for efficient implementation of the schemes derived here for mixtures which is suitable for parallelization.Comment: 43 page

Disorder-induced effects in high-harmonic generation process in fullerene molecules

The objective of this article is to investigate the profound nonlinear optical response exhibited by inversion symmetric fullerene molecules under the influence of different types of disorders described by the Anderson model. Our aim is to elucidate the localization effects on the spectra of high harmonic generation in such molecules. We show that the disorder-induced effects are imprinted onto molecules’ high-harmonic spectrum. Specifically, we observe a presence of strong even-order harmonic signals already for relatively small disorders. The odd-order harmonics intrinsic for disorder-free systems are generally robust to minor disorders. Both diagonal and off-diagonal disorders lift the degeneracy of states, opening up new channels for interband transitions, leading to the enhancement of the high-harmonic emission. The second harmonic signal has a special behavior depending on the disorder strength. Specifically in the case of diagonal disorder, the second harmonic intensity exhibits a quadratic scaling with the disorder strength, which enables the usage of the harmonic spectrum as a tool in measuring the type and the strength of a disorder