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We present a grid-based implementation of the time-dependent configuration-interaction singles method suitable for computing the strong-field ionization of small gas-phase molecules. After outlining the general equations of motion used in our treatment of this method, we present example calculations of strong-field ionization of helium, lithium hydride, water, and ethylene that demonstrate the utility of our implementation. The following companion paper specializes to the case of spherical symmetry, which is applied to various atoms.

We investigate the role of nuclear motion and strong-field-induced electronic couplings during the double ionization of deuterated water using momentum-resolved coincidence spectroscopy. By examining the three-body dicationic dissociation channel, D$^{+}$/D$^{+}$/O, for both few- and multi-cycle laser pulses, strong evidence for intra-pulse dynamics is observed. The extracted angle- and energy-resolved double ionization yields are compared to classical trajectory simulations of the dissociation dynamics occurring from different electronic states of the dication. In contrast with measurements of single photon double ionization, pronounced departure from the expectations for vertical ionization is observed, even for pulses as short as 10~fs in duration. We outline numerous mechanisms by which the strong laser field can modify the nuclear wavefunction en-route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate-dependence to the strong-field ionization rate, intermediate nuclear motion in monocation states prior to double ionization, and near-resonant laser-induced dipole couplings in the ion. These results highlight the fact that, for small and light molecules such as D$_2$O, a vertical-transition treatment of the ionization dynamics is not sufficient to reproduce the features seen experimentally in the strong field coincidence double-ionization data., Comment: 14 pages, 13 figures

Polyatomic molecules in strong laser fields can undergo substantial nuclear motion within tens of femtoseconds. Ion imaging methods based on dissociation or Coulomb explosion therefore have difficulty faithfully recording the geometry dependence of the field ionization that initiates the dissociation process. Here we compare the strong-field double ionization and subsequent dissociation of water (both H$_2$O and D$_2$O) in 10-fs and 40-fs 800-nm laser pulses. We find that 10-fs pulses turn off before substantial internuclear motion occurs, whereas rapid internuclear motion can take place during the double ionization process for 40-fs pulses. The short-pulse measurements are consistent with a simple tunnel ionization picture, whose predictions help interpret the motion observed in the long-pulse measurements., 9 pages, 5 figures

The recent observation of high-lying Rydberg states of excitons in semiconductors with relatively high binding energy motivates exploring their applications in quantum nonlinear optics and quantum information processing. Here, we study Rydberg excitation dynamics of a mesoscopic array of excitons to demonstrate its application in simulation of quantum many-body dynamics. We show that the $\mathbb{Z}_2$-ordered phase can be reached using physical parameters available for cuprous oxide (Cu$_2$O) by optimizing driving laser parameters such as duration, intensity, and frequency. In an example, we study the application of our proposed system to solving the Maximum Independent Set (MIS) problem based on the Rydberg blockade effect., Comment: 9 pages, 4 figures

We simulate the pump-probe experiments of lasing in molecular nitrogen ions with particular interest in the effects of rotational wave-packet dynamics. Our computations demonstrate that the coherent preparation of rotational wave packets in ${\mathrm{N}}_{2}{}^{+}$ by an intense short nonresonant pulse results in a modulation of the subsequent emission from ${B}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}\ensuremath{\rightarrow}{X}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ transitions induced by a resonant seed pulse. We model the dynamics of such pumping and emission using density-matrix theory to describe the ${\mathrm{N}}_{2}{}^{+}$ dynamics and the Maxwell wave equation to model the seed pulse propagation. We show that the gain and absorption of a delayed seed pulse is dependent on the pump-seed delay, that is, the rotational coherences excited by the pump pulse can modulate the gain and absorption of the delayed seed pulse. Further, we demonstrate that the coherent rotational dynamics of the nitrogen ions can cause lasing without electronic inversion.

Air lasing in the nitrogen molecular ion is not well understood because the complex physics responsible for gain is interwoven with pulse propagation in an extreme environment. Here we use a short gas jet to limit the interaction length, thereby removing the propagation effects. We report on several mechanisms that contribute to the decay of gain in different conditions, and experimentally isolate two decay timescales: the decay of long-term gain due to collisional state mixing, and short-term gain that cannot be explained by population inversion. To test the former, we control the inelastic electron scattering rate by varying the gas concentration while keeping the propagation length fixed, and predict the change of the decay using a model of collisional state mixing. We show that the same mechanism causes the decay of rotational wave packets in the states of the ion. Finally, we simulate the complex modulations of gain due to rotational wave packets and the propagation of the probe pulse through the evolving rotationally excited and inverted medium.

We demonstrate an efficient approach for enhancing the spectral broadening of long laser pulses and for efficient frequency redshifting by exploiting the intrinsic temporal properties of molecular alignment inside a gas-filled hollow-core fiber (HCF). We find that laser-induced alignment with durations comparable to the characteristic rotational time scale T R o t A l i g n enhances the efficiency of redshifted spectral broadening compared to noble gases. The applicability of this approach to Yb lasers with (few hundred femtoseconds) long pulse duration is illustrated, for which efficient broadening based on conventional Kerr nonlinearity is challenging to achieve. Furthermore, this approach proposes a practical solution for high energy broadband long-wavelength light sources, and it is attractive for many strong field applications.

We show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields., High Intensity Lasers and High Field Phenomena 2020, High-field Re-scattering Physics, Relativistic Nonlinear Phenomena, Intense Pulse Propagation and Filamentation (HM1B) Session, November 16–20, 2020, Washington, DC, United States, Series: OSA Technical Digest

Using machine learning, we explore the utility of various deep neural networks when applied to high harmonic generation scenarios. First, we train the neural networks to predict the time-dependent dipoles and spectra of high harmonic emission from reduced-dimensionality models of di- and triatomic systems based on sets of randomly generated parameters (laser pulse intensity, internuclear distance, and molecular orientation). These networks, once trained, are useful tools to rapidly simulate the high harmonic spectra of our systems. Similarly, we have trained the neural networks to solve the inverse problem—to determine the molecular parameters based on high harmonic spectra or dipole acceleration data. The latter types of networks could then be used as spectroscopic tools to invert high harmonic spectra in order to recover the underlying physical parameters of a system. Next, we demonstrate that transfer learning can be applied to our networks to expand the range of applicability of the networks with only a small number of new test cases added to our training sets. Finally, we demonstrate neural networks that can be used to classify molecules by type: di- or triatomic, symmetric or asymmetric. With outlooks toward training with experimental data, these neural network topologies offer a novel set of spectroscopic tools that could be incorporated into high harmonic generation experiments.

The strong coupling between intense laser fields and valence electrons in molecules causes distortions of the potential energy hypersurfaces which determine the motion of the nuclei and influence possible reaction pathways. The coupling strength varies with the angle between the light electric field and valence orbital, and thereby adds another dimension to the effective molecular potential energy surface, leading to the emergence of light-induced conical intersections. Here, we demonstrate that multiphoton couplings can give rise to complex light-induced potential energy surfaces that govern molecular behavior. In the laser-induced dissociation of H2+, the simplest of molecules, we measure a strongly modulated angular distribution of protons which has escaped prior observation. Using two-color Floquet theory, we show that the modulations result from ultrafast dynamics on light-induced molecular potentials. These potentials are shaped by the amplitude, duration and phase of the dressing fields, allowing for manipulating the dissociation dynamics of small molecules., Conical intersections, a hallmark of polyatomic molecules, can be induced with light, leading to new reaction pathways. Here, the authors show that intense fields can create complex, beyond-conical intersections even in diatomics, resulting in an unexpected angular distribution of fragment ions.

The XUV field emitted by impulsively aligned ethylene molecules during high-order harmonic generation is retrieved as a function of molecular orientation. The results can be ascribed to multielectron contributions to the harmonic emission.

We make an ab initio calculation of the bending distribution functions for low lying vibrational states of the CO2 molecule in its ground electronic state. These functions have their maximum values at a non-linear geometry, and the value zero at linearity, despite the fact that the potential surface has its minimum value at linearity. These functions are in accord with experimental distribution functions inferred by analysis of Coulomb Explosion Imaging experiments. Thus in a femto-second ‘snapshot’ of a room temperature ensemble of gas phase rotating-vibrating CO2 molecules, none would be linear. The same can be said for any triatomic molecule.

Synopsis Using theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating H 2 + .

We have trained a deep (convolutional) neural network to predict the ground-state energy of an electron in four classes of confining two-dimensional electrostatic potentials. On randomly generated potentials, for which there is no analytic form for either the potential or the ground-state energy, the neural network model was able to predict the ground-state energy to within chemical accuracy, with a median absolute error of 1.49 mHa. We also investigate the performance of the model in predicting other quantities such as the kinetic energy and the first excited-state energy of random potentials.

We discuss the seminal three-step model and the recollision picture in the context of high harmonic generation in molecules. In particular, we stress the importance of multi-electron correlation during the first and the third of the three steps of the process: (1) the strong field ionization and (3) the recombination. We point out how accurate account for multi-electron correlations during the third, recombination, step allows one to gauge the importance of pre-collision: the term coined by J. H. Eberly to describe unusual pathways during the first, ionization, step., Comment: 28 pages, 11 figures

We measure the ellipticity dependence of high-harmonic generation (HHG) from unaligned gas-phase ensembles of the five-membered ring molecules 2,3-dihydrofuran (${\mathrm{C}}_{4}{\mathrm{H}}_{6}\mathrm{O}$), furan (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O}$), and thiophene (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S}$). As is normally the case, the HHG emission is suppressed for increased ellipticity since the recollision wave packet leading to HHG is steered away from the parent ion for large ellipticity. However, through comparison with computations of the first step in HHG, namely, strong-field ionization (SFI), we show that the observed differences in the ellipticity dependence for the three molecular species reflect the lateral momentum distributions of the continuum electron responsible for HHG, which in turn provides information about the particular orientation between the molecular axis and the laser field that maximizes the SFI probability. Strikingly, and contrary to the conventional wisdom in the strong-field community, we find that for furan and thiophene the most probable orientation for SFI occurs when the electric field of the laser is aligned near a nodal plane of the corresponding ionizing orbital. 2,3-dihydrofuran, on the other hand, follows the standard expectation that the most probable orientation for SFI occurs when the electric field is aligned away from any nodal plane.

The encoding of quantum information in photonic time-bin qubits is apt for long-distance quantum communication schemes. In practice, due to technical constraints such as detector response time, or the speed with which copolarized time-bins can be switched, other encodings, e.g., polarization, are often preferred for operations like state detection. Here, we present the conversion of qubits between polarization and time-bin encodings by using a method that is based on an ultrafast optical Kerr shutter and attain efficiencies of 97% and an average fidelity of $0.827\ifmmode\pm\else\textpm\fi{}0.003$ with shutter speeds near 1 ps. Our demonstration delineates an essential requirement for the development of hybrid and high-rate optical quantum networks.

Inducing and controlling lasing in the open air is an intriguing challenge. Several recent experiments on laser filamentation in air have demonstrated generation of population inversion and lasing at the 391 nm and 428 nm lines in the nitrogen molecular ion [1-8], corresponding to transitions between the ground vibrational state of the second excited electronic state, B2Σ+ u , and the ground and first excited vibrational state of the ground electronic state, X2Σ+g, see figure. Importantly, lasing at these transitions appears to be a very general effect, arising during filamentation of virtually any incident radiation, from visible to mid-infrared.

Ionization of an atom or molecule by a strong laser field produces sub-optical cycle wave packets whose control has given rise to attosecond science. The final states of the wave packets depend on ionization and deflection by the laser field, which are convoluted in conventional experiments. Here, we demonstrate a technique enabling efficient electron deflection, separate from the field driving strong-field ionization. Using a mid-infrared deflection field permits one to distinguish electron wave packets generated at different field maxima of an intense few-cycle visible laser pulse. We utilize this capability to trace the scattering of low-energy electrons driven by the mid-infrared field. Our approach represents a general technique for studying and controlling strong-field ionization dynamics on the attosecond time scale., Comment: 5 Pages, 4 figures

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both “above threshold” (dissociating) and “below threshold” (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving “hot” neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO–O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled “inner region” to a loosely coupled “outer region” where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.

The influence of cyclic electron delocalization associated with aromaticity on the high-order-harmonic generation (HHG) process is investigated in organic molecules. We show that the aromatic molecules benzene $({\mathrm{C}}_{6}{\mathrm{H}}_{6})$ and furan $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O})$ produce high-order harmonics more efficiently than nonaromatic systems having the same ring structure. We also demonstrate that the relative strength of plateau harmonics is sensitive to the aromaticity in five-membered-ring molecules using furan, pyrrole $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{NH})$, and thiophene $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S})$. Numerical time-dependent Schr\"odinger equation simulations of total orientation-averaged strong-field ionization yields show that the HHG from aromatic molecules comes predominantly from the two highest $\ensuremath{\pi}$ molecular orbitals, which contribute to the aromatic character of the systems.

A number of techniques have been developed to characterize the attosecond emission from high-order-harmonic sources. These techniques are broadly classified as ex situ, where the attosecond pulse train photoionizes a target gas in the presence of an infrared field, and in situ, where the measurement takes place in the medium in which the attosecond pulses are generated. It is accepted that ex situ techniques measure the characteristics of the electromagnetic field, including the phase of the recombination transition moment of the emitting atom or molecule, when the phase of the second medium is known. However, there is debate about whether in situ techniques measure the electromagnetic field, or only the characteristics of the recolliding electron before recombination occurs. We show numerically that in situ measurements are not sensitive to the recombination phase, when implemented in the perturbative regime as originally envisioned, and that they do not measure the electromagnetic phase of the emission.

We propose and experimentally demonstrate a scheme to generate optically controlled delays based on off-resonant Raman absorption. Dispersion in a transparency window between two neighboring, optically activated Raman absorption lines is used to reduce the group velocity of broadband 765 nm pulses. We implement this approach in a potassium titanyl phosphate (KTP) waveguide at room temperature, and demonstrate Raman-induced delays of up to 140 fs for a 650-fs duration, 1.8-THz bandwidth, pulse. Our approach should be applicable to single-photon signals, offers wavelength tunability, and is a step toward processing ultrafast photons.

We use differential holography to overcome the forward scattering problem in strong-field photoelectron holography. Our differential holograms of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ molecules exhibit a fishbonelike structure, which arises from the backscattered part of the recolliding photoelectron wave packet. We demonstrate that the backscattering hologram can resolve the different nuclear dynamics between ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ with subangstrom spatial and subcycle temporal resolution. In addition, we show that attosecond electron dynamics can be resolved. These results open a new avenue for ultrafast studies of molecular dynamics in small molecules.

In the initial article [Phys. Rev. Lett. 110, 044301 (2013), arXiv:1208.4611] it was claimed that human hearing can beat the Fourier uncertainty principle. In this Comment, we demonstrate that the experiment designed and implemented in the original article was ill-chosen to test Fourier uncertainty in human hearing., Comment: 2 pages, 1 figure, accepted to Phys. Rev. Lett

We have demonstrated a generally applicable tabletop approach utilizing a 266nm femtosecond laser pulse pump, 800nm pulse probe, coupled with Coulomb explosion imaging (CEI). We have investigated two simple chemical reactions in C2H2 + simultaneously: proton transfer and C=C bond-breaking, triggered by multiphoton ionization to excited states. Too and fro proton migration results are in excellent agreement with new ab initio trajectory simulations which predict isomerization timescales and pathways., 29th International Conference on Photonic, Electronic and Atomic Collisions, ICPEAC 2015, 22 July 2015 through 28 July 2015

The absorption of broadband photons in atomic ensembles requires either an effective broadening of the atomic transition linewidth, or an off-resonance Raman interaction. Here we propose a scheme for a quantum memory capable of storing and retrieving ultrafast photons in an ensemble of two-level atoms by using a propagation medium with a time-dependent refractive index generated from aligning an ensemble of gas-phase diatomic molecules. The refractive index dynamics generates an effective longitudinal inhomogeneous broadening of the two-level transition. We numerically demonstrate this scheme for storage and retrieval of a weak pulse as short as 50 fs, with a storage time of up to 20 ps. With additional optical control of the molecular alignment dynamics, the storage time can be extended about one nanosecond leading to time-bandwidth products of order $10^4$. This scheme could in principle be achieved using either a hollow-core fiber or a high-pressure gas cell, in a gaseous host medium comprised of diatomic molecules and a two-level atomic vapor at room temperature., Comment: 7 pages, 6 figures. Fixed typos

The first high-resolution molecular movie of proton migration in the acetylene cation is obtained using a tabletop multiphoton pump-probe approach—an alternative to demanding free-electron-lasers and other VUV light sources when ionizing from the HOMO-1., Series: Springer Proceedings in Physics