High-order Harmonic Generation In Atoms Research Articles

Manipulating helical phase of HHG vortex with atomic states

By deploying the strong-field approximation theory, we perform numerical simulation on atomic high-order harmonic generation (HHG) of hydrogen and HHG vortices generated by hydrogen layer. Our results show that the atomic HHG spectra demonstrate peak-shifting and the helical phase of the HHG vortex can be manipulated with initial states of atoms, all ionized by oppositely polarized bicircular ω and 2ω Laguerre–Gaussian fields. The HHG peaks shift to higher frequency by ω if the initial state is switched from ψ100 to ψ211 , and the HHG peaks shift to lower frequency by ω if the initial state is switched from ψ100 to ψ21−1 , the final state is always the ground state ψ100 . The helical phase patterns of the HHG vortices are investigated in connection with the atomic HHG peak-shifting and the angular momentum conservation law. In addition, we observe that the fourth HHG harmonic is strongly dependent on the 2ω field but weakly on the ω field for an initial atomic state of magnetic quantum number −1.

Tunable spectral continuous shift of high-order harmonic generation in atoms by a plasmon-assisted shaping pulse

We delve into the phenomenon of high-order harmonic generation within a helium atom under the influence of a plasmon-assisted shaping pulse. Our findings reveal an intriguing manipulation of the frequency peak position in the harmonic emission by adjusting the absolute phase parameter within the frequency domain of the shaping pulse. This phenomenon holds potential significance for experimental setups necessitating precisely tuned single harmonics. Notably, we observe a modulated shift in the created harmonic photon energy, spanning an impressive range of 1.2 eV. This frequency peak shift is rooted in the asymmetry exhibited by the rising and falling edges of the laser pulse, directly influencing the position of the peak frequency emission. Our study quantifies the dependence of this tuning range and the asymmetry of the laser pulse, offering valuable insights into the underlying mechanisms driving this phenomenon. Furthermore, our investigation uncovers the emergence of semi-integer order harmonics as the phase parameter is altered. We attribute this discovery to the intricate interference between harmonics generated by the primary and secondary return cores. This observation introduces an innovative approach for generating semi-integer order harmonics, thus expanding our understanding of high-order harmonic generation. Ultimately, our work contributes to the broader comprehension of complex phenomena in laser-matter interactions and provides a foundation for harnessing these effects in various applications, particularly those involving precise spectral control and the generation of unique harmonic patterns.

Time-dependent multiconfiguration self-consistent-field and time-dependent optimized coupled-cluster methods for intense laser-driven multielectron dynamics

We reviewed the time-dependent multiconfiguration self-consistent-field (TD-MCSCF) method and the time-dependent optimized coupled-cluster (TD-OCC) method for first-principles simulations of high-field phenomena such as tunneling ionization and high-order harmonic generation in atoms and molecules irradiated by a strong laser field. These methods provide a flexible and systematically improvable description of the multielectron dynamics by expressing the all-electron wavefunction by configuration interaction expansion or coupled-cluster expansion, using time-dependent one-electron orbital functions. The time-dependent variational principle plays a key role in deriving these methods while satisfying gauge invariance and Ehrenfest theorem. The real-time/real-space implementation with an absorbing boundary condition enables the simulation of high-field processes involving multiple excitation and ionization. We present a detailed, comprehensive discussion of such features of the TD-MCSCF and TD-OCC methods.

Frequency shiftand control ofhigh-order harmonicsof H atom driven by anasymmetric laser pulse

A scheme of the large frequency shift for high-order harmonic generation (HHG) produced by atomic gas driven by an asymmetric laser pulse is proposed in the tunneling ionization regime. By numerically solving the three-dimensional time-dependent Schrodinger equation in the dipole approximation, we theoretically investigate the characteristics of HHG emitted from hydrogen atom driven by the laser pulse with different rising and falling times. Our results show that the HHG spectra of atomic H in cutoff region present a strong redshift and blueshift. The shift can be adjusted by varying the rising time or falling time of the laser pulse. The time frequency analysis, reveals that the reason for the frequency shift comes from different contributions from the rising time or falling time in the asymmetric laser pulse. If the contributed harmonics during the falling time is larger than that during the falling time, the red shift of HHG occurs. otherwise the blue shift appears. Therefore, by shaping the laser pulse waveform, the frequency of atomic HHG for a given order in the cutoff region in the tunneling ionization regime is tunable, which can cover the frequency range from the odd order to the adjacent even order.

Tunable spectral shift of high-order harmonic generation in atoms using a sinusoidally phase-modulated pulse

High-order harmonic generation (HHG) from an atom illuminated by a sinusoidally phase-modulated pulse is investigated by solving the time-dependent Schrödinger equation. The spectral shift that occurs in atomic HHG can be achieved easily using our laser pulse. It is shown that the photon energy of the generated harmonics is controllable within the range of 1 eV. The shift of the frequency peak position is rooted in the asymmetry of the rising and falling parts of the laser pulse. We also show that by varying the phase parameters in the frequency domain of the laser one can adjust and control the shift in atomic harmonic spectra.

Resolving the quantum dynamics of near cut-off high-order harmonic generation in atoms by Bohmian trajectories.

We present an ab initio study of the quantum dynamics of high-order harmonic generation (HHG) near the cutoff in intense laser fields. To uncover the subtle dynamical origin of the HHG near the cutoff, we extend the Bohmian mechanics (BM) approach for the treatment of attosecond electronic dynamics of H and Ar atoms in strong laser fields. The time-dependent Schrödinger equation and the self-interaction-free time-dependent density functional theory are numerically solved accurately and efficiently by means of the time-dependent generalized pseudospectral method for nonuniform spatial discretization of the Hamiltonian. We find that the most devoting trajectories calculated by the BM to the plateau harmonics are shorter traveling trajectories, but the contributions of the short trajectories near the cutoff are suppressed in HHG. As a result, the yields of those harmonics in the region near the cutoff are relatively weak. However, for the last few harmonics just above the cutoff, the HHG intensity becomes a little higher. This is because the HHG just above the cutoff arises from those electrons ionized near the peak of the laser pulse, where the ionization rate is the highest. In addition, the longer Bohmian trajectories return to the core with lower energies, these trajectories contribute to the below-threshold harmonics. Our results provide a deeper understanding of the generation of supercontinuum harmonic spectra and attosecond pulses via near cutoff HHG.

Control of Polarization States of Atomic High-order Harmonic Generation

Control of Polarization States of Atomic High-order Harmonic Generation

Same-period emission and recombination in nonsequential double-recombination high-order-harmonic generation

Non-sequential double recombination (NSDR) high-order harmonic generation (HHG) is studied in a molecular system. We observe a unique molecular two-electron effect with a characteristic cutoff in the HHG spectrum at higher energies than what was previously seen for NSDR HHG in atoms. The effect is corroborated with a classical model where it is found that the effect is sensitive to the molecular potential and originates from same period emission and recombination (SPEaR) of two electrons. The effect persists for intermediate nuclear distances of $R \gtrsim 8.0$ a.u.

High-order harmonic generation enhanced by coherent population return

We present computations of high-order harmonic generation in atoms by using the coherent population return technique. This approach consists in two steps: firstly, the active medium is prepared in a coherent superposition of two bound states using a relatively long laser pulse which has a photon energy close to the energy gap between the states and then, the system is driven by an ultrashort infrared laser pulse. We demonstrate, by employing the numerical solution of the time-dependent Schrodinger equation in one and three dimensions, that using this scheme it is possible to enhance the harmonic conversion efficiency by several orders of magnitude. Our numerical results show that, on one hand the increase in ionization efficiency is responsible of an enhancement in the harmonic signal and on the other hand, the coherent preparation of the medium produces a much richer structure of the harmonic emission.

Quantum-classical correspondence in circularly polarized high harmonic generation

Using numerical simulations, we show that atomic high order harmonic generation (HHG) with a circularly polarized laser field offers an ideal framework for quantum-classical correspondence in strong field physics. With an appropriate initialization of the system, corresponding to a superposition of ground and excited state(s), simulated HHG spectra display a narrow strip of strong harmonic radiation preceded by a gap of missing harmonics in the lower part of the spectrum. In specific regions of the spectra, HHG tends to lock to circularly polarized harmonic emission. All these properties are shown to be closely related to a set of key classical periodic orbits that organize the recollision dynamics in an intense, circularly polarized field.

Berry phases of quantum trajectories of optically excited electron–hole pairs in semiconductors under strong terahertz fields

The quantum evolution of particles under strong fields can be essentially captured by a small number of quantum trajectories that satisfy the stationary phase condition of the Dirac–Feynmann path integral. The quantum trajectories are a key concept in understanding extreme nonlinear optical phenomena, such as high-order harmonic generation (HHG) and high-order terahertz sideband generation (HSG). In contrast to HHG in atoms and molecules, HSG in semiconductors can have interesting effects due to nontrivial ‘vacuum’ states of band materials. We find that, in a semiconductor with non-vanishing Berry curvature in its energy bands, the cyclic quantum trajectories of an electron–hole pair under a strong elliptically polarized terahertz field can accumulate a Berry phase. Taking monolayer MoS2 as a model system, we show that the Berry phase appears as a Faraday rotation angle in the pulse emission from the material under short-pulse excitation. This finding reveals an interesting transport effect in the extreme nonlinear optics regime.

Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of an atom can be generated by a circularly polarized laser pulse. The harmonic spectrum shows a clear cutoff with an energy Ip + 2Up. In particular, the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity. These results are verified by the classical model theory and the time-frequency analysis of a harmonic spectrum.

Carrier-envelope phase-dependent effect of high-order sideband generation in ultrafast driven optomechanical system

We analyze the features of the output field of a generic optomechanical system that is driven by a control field and a nanosecond driven pulse, and find a robust high-order sideband generation in optomechanical systems. The typical spectral structure, plateau and cutoff, confirms the nonperturbative nature of the effect, which is similar to high-order harmonic generation in atoms or molecules. Based on the phenomenon, we show that the carrier-envelope phase of laser pulses that contain huge numbers of cycles can cause profound effects.

High-order harmonic generation in fullerenes with icosahedral symmetry

We investigate high-order harmonic generation in a sequence of icosahedral fullerenes using the extension of the so-called three-step or Lewenstein model to the molecular case. The high harmonic spectra obtained at midinfrared wavelengths show modulations in the plateau region. These modulations are due to the coherent superposition of the contributions from the atomic centers in the molecule to the dipole moment. Results of test calculations, in which the interference effects between the atomic contributions are deliberately neglected, show a ``flat'' plateau, as it is known from high-order harmonic generation in atoms. It is shown that the characteristic modulations are present in ensembles of aligned fullerenes as well as randomly oriented ones. We further show that in the case of the larger fullerenes the radius can be determined from the positions of the interference minima in the high harmonic spectra using a spherical model.

Control of harmonic generation using two-colour femtosecond–attosecond laser fields: quantum and classical perspectives

We investigate high-order harmonic generation in atoms and molecules using two-colour laser pulses: an infrared (IR) laser pulse of a few cycles superposed on an attosecond ultraviolet (UV) or vaccum ultraviolet laser pulse. We adapt a standard semiclassical model to this process. We show that enhancement and additional control of harmonics via time delay between these pulses are possible because of a different ionization mechanism from the one-colour case. The electron is moved to the continuum via one-photon absorption of a UV photon (instead of tunnelling through the potential barrier) at a specific phase of the IR pulse (determined by the time delay between two pulses), with a non-zero initial velocity, leading to new possibilities for the control of harmonic generation.