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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.

Time-resolving and controlling coupled electronic and nuclear dynamics at conical intersections on the sub-femtosecond to few-femtosecond time scale is among the challenging goals of attosecond physics. Here we present numerical simulations of time-resolved photoelectron spectroscopy of such dynamics in NO2, where the coupled electron–nuclear motion at the 2A1/2B2 conical intersection is steered on the sub-laser-cycle time scale by a nearly single-cycle, waveform controlled mid-infrared laser pulse. For a rigorous description of the photoionization dynamics, we employ ab initio energy- and geometry-resolved photoionization matrix elements obtained with the multichannel R-matrix method, using a multiconfigurational description of the molecule and a newly developed algorithm to generate photoionization dipoles that are phase consistent on the level of both the neutral and the ionic states. We find that for sufficient molecular alignment, the time- and energy-resolved anisotropy parameters of the photoelectron angular distributions provide a particularly clear picture of both the ultrafast natural molecular dynamics at the conical intersection and its modifications by the control pulse. In particular, changes in the electronic and nuclear configurations induced by the control pulse lead to the appearance of non-vanishing odd anisotropy parameters in the photoelectron spectra. These are absent in the spectra obtained without the control pulse and therefore provide sensitive, background-free diagnostic of the control.

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

Light can be used to modify and control properties of media, as in the case of electromagnetically induced transparency or, more recently, for the generation of slow light or bright coherent XUV and X-ray radiation. Particularly unusual states of matter can be created by light fields with strengths comparable to the Coulomb field that binds valence electrons in atoms, leading to nearly-free electrons oscillating in the laser field and yet still loosely bound to the core [1,2]. These are known as Kramers-Henneberger states [3], a specific example of laser-dressed states [2]. Here, we demonstrate that these states arise not only in isolated atoms [4,5], but also in rare gases, at and above atmospheric pressure, where they can act as a gain medium during laser filamentation. Using shaped laser pulses, gain in these states is achieved within just a few cycles of the guided field. The corresponding lasing emission is a signature of population inversion in these states and of their stability against ionization. Our work demonstrates that these unusual states of neutral atoms can be exploited to create a general ultrafast gain mechanism during laser filamentation., Comment: 9 pages, 4 figures, 10 pages supplementary material, 8 supplementary figures

Light is used to modify or control properties in many quantum systems, leading to phenomena such as electromagnetically induced transparency, the generation of slow light or bright coherent XUV radiation. Using light fields, with strengths comparable to the Coulomb field which binds valence electrons in atoms, particularly unusual quantum states can be created which describe a nearly free electron oscillating in the laser field yet still loosely bound to the core1, 2. We demonstrate that such states can arise not only in isolated atoms3, but also in gases at a few atmospheric pressures, guiding intense laser pulses, where they can act as a gain medium. This gain is created within just a few cycles of the guided field. Using shaped pulses4 with 5–10 fs risetimes, we create pulse conditions, within a laser filament, where these states can persist, and we observe in the corresponding emission, the signatures of these states. This work demonstrates a general, new, ultrafast gain mechanism during filamentation, in which any driven atom can achieve inversion intra-pulse, using pulse shaping techniques.

Today laser pulses with electric fields comparable to or higher than the electrostatic forces binding valence electrons in atoms and molecules have become a routine tool with applications in laser acceleration of electrons and ions, generation of short wavelength emission from plasmas and clusters, laser fusion, etc. Intense fields are also naturally created during laser filamentation in the air or due to local field enhancements in the vicinity of metal nanoparticles. One would expect that very intense fields would always lead to fast ionization of atoms or molecules. However, recently observed acceleration of neutral atoms [Eichmann et al. (2009) Nature 461:1261–1264] at the rate of 10 15 m/s 2 when exposed to very intense IR laser pulses demonstrated that substantial fraction of atoms remained stable during the pulse. Here we show that the electronic structure of these stable “laser-dressed” atoms can be directly imaged by photoelectron spectroscopy. Our findings open the way to visualizing and controlling bound electron dynamics in strong laser fields and reexamining its role in various strong-field processes, including microscopic description of high order Kerr nonlinearities and their role in laser filamentation [Béjot et al. (2010) Phys Rev Lett 104:103903].

In the version of this Letter originally published, the units of the bottom three values in the Fig. 1d legend were incorrect; they should have been W cm–2. This has now been corrected.

we present the results of experimental and theoretical investigations for remote initiation of lasing in nitrogen and air by femtosecond filaments generated by laser pulses with different wavelengths, polarization and applying adaptive control methods

We discuss the connection between strong-field ionization, saturation of the Kerr response, and the formation of the Kramers-Henneberger (KH) atom and long-living excitations in intense external fields., High Intensity Lasers and High Field Phenomena, March 18-20, 2014, Messe Berlin, Berlin

Two-component mixtures in optical lattices reveal a rich variety of different phases. We employ an exact diagonalization method to obtain the relevant correlation functions in hexagonal optical lattices to characterize those phases. We relate the occupation difference of the two species to the magnetic polarization. `Iso'-magnetic correlations disclose the nature of the system, which can be of easy-axis type, bearing phase segregation, or of easy-plane type, corresponding to super-counter-fluidity. In the latter case, the correlations reveal easy-plane segregation, involving a highly-entangled state. We identify striking correlated supersolid phases appearing within the superfluid limit.

We present a generalized model for the intensity dependent response of atoms in strong IR laser fields, describing deviations in the nonlinear response at the frequency of the driving field from the standard model (linear Kerr effect + plasma defocusing) based on the formation of the stable KH states. According to our numerical simulations, shaping the driving laser pulse allows one to reveal a prominent resonance structure in the Kerr response of an individual atom, which can only be explained by population transfer into excited KH states., CLEO®/Europe-IQEC 2013, May 12th to 16th, 2013, Munich, Germany

Solitons are among the most distinguishing fundamental excitations in a wide range of non-linear systems such as water in narrow channels, high speed optical communication, molecular biology and astrophysics. Stabilized by a balance between spreading and focusing, solitons are wavepackets, which share some exceptional generic features like form-stability and particle-like properties. Ultra-cold quantum gases represent very pure and well-controlled non-linear systems, therefore offering unique possibilities to study soliton dynamics. Here we report on the first observation of long-lived dark and dark-bright solitons with lifetimes of up to several seconds as well as their dynamics in highly stable optically trapped $^{87}$Rb Bose-Einstein condensates. In particular, our detailed studies of dark and dark-bright soliton oscillations reveal the particle-like nature of these collective excitations for the first time. In addition, we discuss the collision between these two types of solitary excitations in Bose-Einstein condensates., 9 pages, 4 figures

Using the example of the molecular ion and its isotopes, we show how the Coulomb explosion triggered by ionization of the molecule can be used to reconstruct the emission times and identify the physical mechanism contributing to the emission of near- and below-threshold harmonics (BTH). The key idea is that recombination responsible for the harmonic emission halts Coulomb explosion at different times, depending on the final vibrational state of the molecule. As a consequence, interferences between different vibrational channels show up, enabling enhanced temporal resolution in the harmonic spectrum. The reconstruction of the emission times for BTH shows that they are associated with the so-called short trajectories and that the time–energy mapping for such harmonics is possible, similar to the well-known mapping for the plateau harmonics.

Interaction of a laser pulse with a centrally symmetric medium, such as an isotropic gas of atoms, leads to the generation of harmonic emission which contains exclusively odd harmonics of the incident field. This result is the consequence of both the central symmetry of the medium and the temporal symmetry of the oscillating electric field, , where ωl is the laser frequency. In the case of oriented heteronuclear molecules, the spatial symmetry no longer holds and both odd and even harmonics become allowed. Here we show, by solving the time-dependent Schrodinger equation for H, D, and T, that even-order harmonic generation is also possible for sufficiently long infrared (IR) laser pulses in homonuclear molecules. The appearance of even harmonics is a signature of the coupled electron-nuclear dynamics and reflects field-induced electron localization initiated by the strong laser field, which breaks the spatial symmetry in the system. The analysis of even harmonics generated by pulses of different durations might therefore provide information on correlated electron-nuclear dynamics and charge migration in more complex un-oriented molecular ensembles.

The dissociative excitation of CO was studied in the molecular frame as a function of probe laser intensity, ellipticity and polarization with respect to the molecular bond at laser wavelengths of 800 nm and 1350 nm. This allowed the identification of the main excitation pathway consisting of tunnel ionization from HOMO-2 followed by a parallel dipole transition from the second excited state B to the predissociating, third excited state C. Recollision excitation was shown to play a negligible role. Using laser induced impulsive alignment, the strong field induced coupling at 800 nm and 1350 nm of the ionic states B and C could thus be controlled by the laser polarization. This leads to a suppression of the fragmentation yield of up to 70% when the laser polarization was perpendicular to the molecular axis compared to parallel polarization. We have performed simulations of various ionization channels of CO2. Our simulations reflect the experimental findings and show that dissociation of CO is induced by tunnelling from deeper molecular orbitals HOMO-1, HOMO-2, HOMO-3, followed by laser driven hole dynamics in the ion.