High-order Harmonic Generation Cutoff Research Articles

Extension of high-order harmonic generation cutoff from laser-ablated tin plasma plumes.

The high-order harmonic spectra from laser-ablated tin plasma plumes are investigated experimentally and theoretically at different laser wavelengths. It is found that the harmonic cutoff is extended to ∼84 eV and the harmonic yield is greatly improved by decreasing the driving laser wavelength from 800 nm to 400 nm. Appling the Perelomov-Popov-Terent'ev theory with the semiclassical cutoff law and one-dimensional time-dependent Schrödinger equation, the contribution of the Sn3+ ion to harmonic generation accounts for the cutoff extension at 400 nm. With the qualitative analysis of the phase mismatching effect, we reveal the phase matching caused by the dispersion of free electrons is greatly optimized in the 400 nm driving field relative to the 800 nm driving field. The high-order harmonic generated from laser-ablated tin plasma plumes driven by the short laser wavelength provides a promising way to extend cutoff energy and generate intensely coherent extreme ultraviolet radiation.

Investigation of the multielectron dynamics of CO in intense laser fields by the effective potential analysis of natural orbitals

We simulated the multielectron dynamics of CO in multicolor pulses of a near-IR fundamental ω and harmonics up to 4ω by using the multiconfiguration time-dependent Hartree-Fock method. The dynamics were analyzed in terms of effective potentials that govern the time evolution of molecular orbitals. The effective potential for the highest occupied orbital has a hump barrier responsible for anisotropic ionization of CO and the shape of a hump significantly changes at different internuclear distances. The electronic response became nonadiabatic when a 4ω-field was included, resulting in the enhancement in ionization and the extension of the high-order harmonic generation cutoff.

Higher-order harmonics generation based on near-field scattered laser pulse in Au-Si core-shell nanospheres

• Calculated scattered near-field from the core-shell nanosphere using Mie Theory. • Enhancement of scattered near-field around 800 nm. • Production of higher harmonics via Au-Si core-shell nanosphere. • Increasing the incident intensity beyond the damage threshold of metal core. We present theoretical studies of high-order harmonic generation (HHG) from a hydrogen atom located around metal-insulator core-shell nanosphere using 800 nm Ti: sapphire laser field. Near-fields scattered by a core-shell nanosphere are calculated using the Mie scattering approach. In order to control the scattered field of the Si film coated gold nanoparticles, two parameters (i) the size of core (ii) the shell thickness, which enable to shift the peak of the scattered field to the near infrared region, must be considered. We show that the amplification and inhomogeneity of local fields play an important role in the HHG cut off increment.

Double-plateau structure and the effect of the carrier-envelope phase on high-order harmonic generation from a Rydberg atom in a few-cycle laser pulse

Recent studies have demonstrated that in a few-cycle laser pulse, a coherent Rydberg atom—an atom in a superposition of the ground and highly excited states—can generate high-order harmonic generation (HHG) spectra with high conversion efficiency and high cutoff energy, making it a potential procedure for producing attosecond pulses. In this study, we theoretically report two interesting findings that can be realized experimentally: the nontrivial dependence of HHG cutoff on the laser carrier-envelope phase (CEP) and the double-plateau structure in the spectrum when the CEP ranges from 75° to 120°. The second effect has not been reported for a Rydberg atom in the previous studies focusing only on CEP of 0°. Finally, using classical simulation and time-frequency analysis, we explain the influence of the CEP on the cutoff energy and, especially, the origin of the double-plateau structure. Unlike the first plateau generated by the recombination of an electron escaping from the Rydberg state but returning to the ground state, the second one arises from the ionization from the ground state. Consequently, by controlling the laser CEP, the electron dynamics can be embedded in the structure of plateaus in the HHG spectra.

Calculation of high-order harmonic generation in laser-produced lithium plasma.

We report on detailed calculations of high-order harmonic generation (HHG) spectra from Li, Li+, and He using the Lewenstein model. Our model well describes the HHG experiment in Li plasma [J. Phys. B45, 065601 (2012)JPAPEH0953-407510.1088/0953-4075/45/6/065601]. The cutoff position, in the case of neutral lithium atoms (43th harmonic of 800nm radiation), corresponds to 3.5×1014 W/cm2 laser intensity, as numerically-simulated in the current work. The difference from the experimental value can be well explained by the measurement errors and uncertainty in determining the laser intensity, which is usually around 25%. We found that Li+ ions do not contribute to HHG plateau region of the spectra and that those ions start to contribute only at very high intensities. Our calculations show that the application of materials possessing higher ionization potential does not necessarily leads to the extension of HHG cutoff.

Simultaneous control of harmonic yield and energy cutoff of high-order harmonic generation using seeded plasmonically enhanced fields

We study high-order harmonic generation (HHG) driven by seeded plasmonic-enhanced fields. On one hand, plasmonic-enhanced fields have shown a great potential to extend the HHG cutoff, an instrumental pre-requisite for the generation of attosecond pulses. On another hand, the use of XUV seeds appears to have a considerable potential to improve the HHG conversion efficiency, which is typically modest when a unique fundamental laser pulse is employed. By mixing these two sources, we show it is possible to, simultaneously, boost the HHG cutoff and to increase the harmonic photon flux. The combination of these features potentially enables to generate intense and spectrally broad attosecond pulse trains.

Quantum mechanical study of high-order harmonic generation from HeH2+ molecule in homogeneous and plasmonic-enhanced laser fields

High-order harmonic generation (HHG) from molecular ion HeH2+ in two initial states— (ground state) and (first excited state)—in homogeneous and plasmonic-enhanced laser fields is investigated theoretically. The electron ionization and the enhancement of HHG yield with ground and first excited states as the initial states is studied, and a strong orientation effect for electron ionization in state, which can be used to reach a longer lifetime for the electron in the first excited state during the high harmonic emission, is discovered. In order to investigate the effect of the plasmonic field on the cutoff position of HHG spectra, first the enhancement of laser field in a nano-antenna is calculated and then the interaction of the enhanced field with HeH2+ molecule is studied by solving the time-dependent Schrödinger equation. According to the results, when an HeH2+ molecule in the initial state of is irradiated by a plasmonic polarized laser field with polarization normal to the molecular axis, the HHG yield and cutoff position are enhanced, while the excited state has a long lifetime during the HHG process.

Enhancing high-order harmonic generation by sculpting waveforms with chirp

We present a theoretical analysis showing how chirp can be used to sculpt two-color driving laser field waveforms in order to enhance high-order harmonic generation (HHG) and/or extend HHG cutoff energies. Specifically, we consider driving laser field waveforms composed of two ultrashort pulses having different carrier frequencies in each of which a linear chirp is introduced. Two pairs of carrier frequencies of the component pulses are considered: ($\ensuremath{\omega}, 2\ensuremath{\omega}$) and ($\ensuremath{\omega}, 3\ensuremath{\omega}$). Our results show how changing the signs of the chirps in each of the two component pulses leads to drastic changes in the HHG spectra. Our theoretical analysis is based on numerical solutions of the time-dependent Schr\odinger equation and on a semiclassical analytical approach that affords a clear physical interpretation of how our optimized waveforms lead to enhanced HHG spectra.

Plasmonic nanostructure assisted HHG in NIR spectrum and thermal analysis

We study plasmonic nanoparticle assisted high-order harmonic generation (HHG), illuminated by near infrared (NIR) laser sources, and the effect of the geometry of some different dimers on HHG cutoff frequency is evaluated. Dimers are installed on different dielectric substrates and the electric field enhancement factors are simulated. We demonstrate that NIR femto-fiber sources are good options for the HHG process. Such sources can induce significant inhomogeneous electric fields in the nanogaps; and consequently, high harmonic cutoff orders more than 250 will be obtained. Moreover, by time dependent thermal analysis of Au nanoparticles exposed to NIR ultrafast high power lasers, we could determine the temperature distribution in the nanoparticle and substrate.

High-order harmonic generation driven by inhomogeneous plasmonics fields spatially bounded: influence on the cut-off law

We study high-order harmonic generation (HHG) in model atoms driven by plasmonic-enhanced fields. These fields result from the illumination of plasmonic nanostructures by few-cycle laser pulses. We demonstrate that the spatial inhomogeneous character of the laser electric field, in a form of Gaussian-shaped functions, leads to an unexpected relationship between the HHG cutoff and the laser wavelength. Precise description of the spatial form of the plasmonic-enhanced field allows us to predict this relationship. We combine the numerical solutions of the time-dependent Schrödinger equation (TDSE) with the plasmonic-enhanced electric fields obtained from 3D finite element simulations. We additionally employ classical simulations to supplement the TDSE outcomes and characterize the extended HHG spectra by means of their associated electron trajectories. A proper definition of the spatially inhomogeneous laser electric field is instrumental to accurately describe the underlying physics of HHG driven by plasmonic-enhanced fields. This characterization opens up new perspectives for HHG control with various experimental nano-setups.

Extending the high-order harmonic generation cutoff by means of self-phase-modulated chirped pulses

In this letter we propose a complementary approach to extend the cutoff in high-order harmonic generation (HHG) spectra beyond the well established limits. Inspired by techniques normally used in the compression of ultrashort pulses and supercontinuum generation, we show this extension can be achieved by means of a nonlinear phenomenon known as self-phase-modulation (SPM). We demonstrated that relatively long optical pulses, around 100 fs full-width half maximum (FWHM), non linearly chirped by SPM, are able to produce a considerable extension in the HHG cutoff. We have also shown it is possible control this extension by setting the length of the nonlinear medium. Our study was supported by the numerical integration of the time-dependent Schrödinger equation joint with a complete classical analysis of the electron dynamic. Our approach can be considered as an alternative to the utilization of optical parametric amplification (OPA) and it can be easily implemented in usual facilities with femtosecond laser systems. This technique also preserves the harmonic yield in the zone of the plateau delimited by Ip + 3.17Up law, even when the driven pulses contain larger wavelength components.

Double-electron recombination in high-order-harmonic generation driven by spatially inhomogeneous fields

We present theoretical studies of high-order harmonic generation (HHG) driven by plasmonic fields in two-electron atomic systems. Comparing the two-active electron and single-active electron approximation models of the negative hydrogen ion atom, we provide strong evidence that a double non-sequential two-electron recombination appears to be the main responsible for the HHG cutoff extension. Our analysis is carried out by means of a reduced one-dimensional numerical integration of the two-electron time-dependent Schr\"odinger equation (TDSE), and on investigations of the classical electron trajectories resulting from the Newton's equation of motion. Additional comparisons between the negative hydrogen ion and the helium atom suggest that the double recombination process depends distinctly on the atomic target. Our research paves the way to the understanding of strong field processes in multi-electronic systems driven by spatially inhomogeneous fields.

Generation of Isolated Attosecond Pulse from Asymmetric Molecular Ions by Introducing Half-Cycle-Like Laser Fields**Supported by the National Natural Science Foundation of China under Grant No 11404204, the Key Project of the Ministry of Education of China under Grant No 211025, the Research Fund for the Doctoral Program of Higher Education of China under Grant No 20111404120004, and the Natural Science

We propose an efficient method for the generation of an isolated attosecond pulse from the asymmetric molecular ions HeH2+ by adding a half-cycle-like field (HCLF) to the fundamental driving laser field. The high-order harmonic generation (HHG) is investigated by numerically solving the time-dependent Schrödinger equation. By performing the time-frequency distributions and the electronic wave packet probability densities, we find that the optimizing combined field is not only useful for extending the HHG cutoff, but also for simplifying the recombination channels through controlling the electron localization. In addition, by adjusting the intensity of the HCLF, a dominant short quantum path is selected to contribute the HHG spectrum. As a result, a 75-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.

Carrier-wave Rabi-flopping signatures in high-order harmonic generation for alkali atoms.

We present a theoretical investigation of carrier-wave Rabi flopping in real atoms by employing numerical simulations of high-order harmonic generation (HHG) in alkali species. Given the short HHG cutoff, related to the low saturation intensity, we concentrate on the features of the third harmonic of sodium (Na) and potassium (K) atoms. For pulse areas of 2π and Na atoms, a characteristic unique peak appears, which, after analyzing the ground state population, we correlate with the conventional Rabi flopping. On the other hand, for larger pulse areas, carrier-wave Rabi flopping occurs, and is associated with a more complex structure in the third harmonic. These characteristics observed in K atoms indicate the breakdown of the area theorem, as was already demonstrated under similar circumstances in narrow band gap semiconductors.

Generation of isolated sub-20-attosecond pulses from He atoms by two-color midinfrared laser fields

We propose an efficient method for the generation of ultrabroadband supercontinuum spectra and isolated ultrashort attosecond laser pulses from He atoms with two-color midinfrared laser fields. High-order harmonic generation (HHG) is obtained by solving the time-dependent Schr\"odinger equation accurately by means of the time-dependent generalized pseudospectral method. We found that the optimizing two-color midinfrared laser pulse allows the HHG cutoff to be significantly extended, leading to the production of an ultrabroadband supercontinuum. As a result, an isolated 18-attosecond pulse can be generated directly by the superposition of the supercontinuum harmonics. To facilitate the exploration of the ultrashort attosecond generation mechanisms, we perform both a semiclassical simulation and a wavelet time-frequency transform.

Extension of high-order harmonic generation cutoff via control of chirped laser pulses in the vicinity of metal nanostructure media

By using a suitable chirping field, an ultrashort pulse was obtained in the vicinity of the metal nanostructure.

Carrier-envelope-phase control of plasmonic-field enhanced high-order harmonic generation

Plasmonic field-enhanced high-order harmonic generation (HHG) is investigated theoretically using the solutions of the three-dimensional time-dependent Schrödinger equation and a (semi)classical three-step model. Plasmonic field is modeled by a spatially inhomogeneous field with a time-dependent cosine squared pulse envelope. The dependence of the HHG yield on the carrier-envelope phase (CEP) is investigated. It is shown that the position of the cutoff of the HHG spectra is very sensitive to the value of CEP and that for the inhomogeneous field the dependence on the CEP is modulo. This enables both the CEP control of the plasmonic field-enhanced HHG and the determination of the CEP modulo by the measurement of the HHG cutoff position. Contrary to the homogeneous field case, for inhomogeneous field the difference between the maximum and minimum HHG cutoff (for the CEP in the interval from 0 to) remains substantial even for pulses longer than 10 optical cycles.

High-order harmonic generation from graphene: Strong attosecond pulses with arbitrary polarization

We explore high-order harmonic generation (HHG) from a graphene sheet exposed to intense femtosecond laser pulses based on the Lewenstein model. It is demonstrated that the HHG cutoff frequency increases with graphene size up to the classical limit for distant diatomic systems. In contrast to two-center systems, the cutoff frequency remains constant with increasing power of the harmonics as the graphene diameter extends beyond maximal electron excursion. It is shown that the extended nature of the graphene sheet allows for strong HHG signals at maximum cutoff for linearly as well as circularly polarized laser pulses, the latter opening for generation of strong circularly polarized attosecond pulses.

Frequency-selected enhancement of high-order-harmonic generation by interference of degenerate Rydberg states in a few-cycle laser pulse

We demonstrate that the frequency-selected enhancement of high-order-harmonic generation (HHG) can be achieved by a few-cycle laser pulse interacting with a coherent superposition state, which is prepared by the ground state and two degenerate Rydberg states. The degenerate states have the same orbital radius and hence have a large overlap in the electronic density distribution. By controlling the relative phase between the two degenerate states, the constructive or destructive interference of them can markedly change the initial density distribution of the Rydberg electron, thereby we can manipulate the characteristics and the conversion efficiency of HHG. Specifically, a significant enhancement in the continuous harmonics near HHG cutoff can be obtained, hence an intense isolated pulse with a duration less than 100 attoseconds is straightforwardly generated. On the other hand, since there exists a specific dependence of the harmonic efficiency on the relative phase of the two degenerate states, one can expect that the relative phase may be probed by examining the corresponding harmonic intensity. In practice, we may apply a weak static electric field in the whole dynamic process to obtain an asymmetry electron density distribution at a large radius; hence similar HHG results can be obtained.

Quantum-orbit analysis of high-order-harmonic generation by resonant plasmon field enhancement

We perform a detailed analysis of high-order harmonic generation (HHG) in atoms within the strong field approximation (SFA) by considering spatially inhomogeneous monochromatic laser fields. We investigate how the individual pairs of quantum orbits contribute to the harmonic spectra. We show that in the case of inhomogeneous fields, the electron tunnels with two different canonical momenta. One of them leads to a higher cutoff and the other one develops a lower cutoff. Furthermore, we demonstrate that the quantum orbits have a very different behavior in comparison to the homogeneous field. We also conclude that in the case of the inhomogeneous fields, both odd and even harmonics are present in the HHG spectra. Within our model, we show that the HHG cutoff extends far beyond the semiclassical cutoff as a function of inhomogeneity strength. Our findings are in good agreement both with quantum mechanical and classical models.