Effects In High Harmonic Generation Research Articles

Propagation effects in high-harmonic generation from dielectric thin films

A theoretical investigation is conducted of high-order harmonic generation (HHG) in silicon thin films to elucidate the effect of light propagation in reflected and transmitted waves. The first-principles simulations are performed of the process in which an intense pulsed light irradiates silicon thin films up to $3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ thickness. Our simulations are carried within the time-dependent density functional theory (TDDFT) with the account of coupled dynamics of the electromagnetic fields and the electronic motion. It was found that the intensity of transmission HHG gradually decreases with the thickness, while the reflection HHG becomes constant from a certain thickness. Detailed analyses show that transmission HHG has two origins: the HHG generated near the front edge and propagates to the back surface and the HHG generated near the back edge and emitted directly. The dominating mechanism of the transmission HHG is found to depend on the thickness of the thin film and the frequency of the HHG. At the film thickness of $1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$, the transmission HHG with the frequency below 20 eV is generated near the back edge, while that with the frequency above 20 eV is generated near the front edge and propagates from there to the back surface.

Doping effects in high-harmonic generation from correlated systems

Using the one-dimensional Hubbard model, which is commonly used for describing, e.g., high-${T}_{c}$ superconducting cuprates, we study high-harmonic generation (HHG) from doped, correlated materials. Doping is modeled by changing the number of electrons in the lattice from the conventional half-filling case. For relatively small Hubbard $U$, i.e., small electron-electron correlation, we find little to no effect of doping on the dynamics and the HHG spectra. For increasing $U$ the degree of doping has a marked effect on the dynamics and spectra. We explain these findings through the quasiparticle-based doublon-holon picture. The dynamics are separated into two types, first, doublon and holon movement, and, second, doublon-holon pair creation and annihilation. Doping results in all configurations containing doublons or holons. Those quasiparticles can move at no extra cost in energy regardless of the correlation level. This motion at no energy cost increases the high-harmonic gain for low- and medium-harmonic orders. We discuss that in the high-$U$ limit, antiferromagnetic ordering becomes increasingly unlikely with increasing doping rates and explain an associated drop in the high-order harmonics relative to the case of half-filling.

Signatures of Multiband Effects in High-Harmonic Generation in Monolayer MoS_{2}.

High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS_{2}, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multiband recombination dipoles.

Edge-state and bulklike laser-induced correlation effects in high-harmonic generation from a linear chain

We explore the significance of laser-induced correlation effects for high-order harmonic generation (HHG) in a linear chain model of a generic band gap material for both single-IR-pulse and VUV-pump--IR-probe HHG schemes. We examine various pump-pulse excitations, and categorize signatures of laser-induced correlation effects into two classes related to edge-state and bulklike effects. The relative importance of the edge-state ef- fects decreases with increasing system size, while the bulklike effects remain. We identify regimes where these effects may alter the harmonic yield by an order of magnitude, but also regions where they may be neglected. We characterize the underlying laser-induced dynamics of the correlational mechanisms, using both an electronic population description in a static field-free basis and a comparison to a band-structure picture with laser-induced time-dependent energy shifts. From this we provide a guideline on when to account for laser-induced electron correlations and give an estimate of the timescale in which the laser-induced electron-electron interactions can cause correlational changes in the harmonic spectra. We find this timescale to be a few tens of femtoseconds.

Intense-laser-driven electron dynamics and high-order harmonic generation in solids including topological effects

A theory for laser-driven electron dynamics and high-harmonic generation in bulk solids with two lattice sites per unit cell of arbitrary dimension is formulated. In tight-binding approximation, such solids can be described by $2\times 2$ Bloch-Hamiltonians. Our theory is able to fully capture topological effects in high-harmonic generation by such systems because no simplifications beyond tight-binding, dipole approximation, and negligible depletion of the valence band are made. An explicit, analytical expression for the electron velocity is given. Exemplarily, the theory is applied to the Su-Schrieffer-Heeger chain and the Haldane model in strong laser fields.

Nonlinear propagation effects in high harmonic generation in reflection and transmission from gallium arsenide.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

Phase matching effects in high harmonic generation at the nanometer scale.

Plasmon resonances are known to amplify the electromagnetic fields near metallic nanostructures, providing a promising scheme to generate extreme-ultraviolet harmonics using low power drivings. During high-order harmonic generation (HHG), the driving and harmonic fields accumulate a phase difference as they propagate through the target. In a typical set-up -a laser focused into a gas jet- the propagation distances amount to several wavelengths, and the cumulative phase-mismatch affects strongly the efficiency and properties of the harmonic emission. In contrast, HHG in metallic nanostructures is considered to overcome these limitations, as the common sources of phase mismatch -optical density and focusing geometry- are negligible for subwavelength propagation distances. We demonstrate that phase matching still plays a relevant role in HHG from nanostructures due to the non-perturbative character of HHG, that links the harmonic phase to the intensity distribution of the driving field. Our computations show that widely used applications of phase matching control, such as quantum path selection and the increase of contrast in attosecond pulse generation, are also feasible at the nanoscale.

Spin-valley dynamics of electrically driven ambipolar carbon-nanotube quantum dots

An ambipolar n–p double quantum dot defined by potential variation along a semiconducting carbon-nanotube is considered. We focus on the (1e,1h) charge configuration with a single excess electron of the conduction band confined in the n-type dot and a single missing electron in the valence band state of the p-type dot for which lifting of the Pauli blockade of the current was observed in the electric-dipole spin resonance (Laird et al 2013 Nat. Nanotechnol. 8 565). The dynamics of the system driven by periodic electric field is studied with the Floquet theory and the time-dependent configuration interaction method with the single-electron spin-valley-orbitals determined for atomistic tight-binding Hamiltonian. We find that the transitions lifting the Pauli blockade are strongly influenced by coupling to a vacuum state with an empty n dot and a fully filled p dot. The coupling shifts the transition energies and strongly modifies the effective g factors for axial magnetic field. The coupling is modulated by the bias between the dots but it appears effective for surprisingly large energy splitting between the (1e,1h) ground state and the vacuum (0e, 0h) state. Multiphoton transitions and high harmonic generation effects are also discussed.

High-Harmonic Generation Enhanced by Dynamical Electron Correlation.

We theoretically study multielectron effects in high-harmonic generation (HHG), using all-electron first-principles simulations for a one-dimensional model atom. In addition to the usual plateau and cutoff (from a cation in the present case, since the neutral is immediately ionized), we find a prominent resonance peak far above the plateau and a second plateau extended beyond the first cutoff. These features originate from the dication response enhanced by orders of magnitude due to the action of the Coulomb force from the rescattering electron, and, hence, are a clear manifestation of electron correlation. Although the present simulations are done in 1D, the physical mechanism underlying the dramatic enhancement is expected to hold also for three-dimensional real systems. This will provide new possibilities to explore dynamical electron correlation in intense laser fields using HHG, which is usually considered to be of single-electron nature in most cases.

High harmonic generation with long-wavelength few-cycle laser pulses

We report the extension of hollow-core fibre pulse compression to longer wavelengths. High-energy multi-cycle infrared pulses are generated via optical parametric amplification and subsequently broadened in the fibre. 2.5-cycle pulses at the Signal wavelength (1.4 µm) and 1.6-cycle pulses at the Idler wavelength (1.8 µm) in the sub-millijoule regime have been generated. New compression schemes can be applied at 1.8 µm and beyond. In this manner, 1.6-cycle carrier envelope phase stable pulses were generated by linear propagation in the anomalous dispersion regime of bulk glass which surprisingly enables compression below its third-order dispersion limit. Furthermore, a dispersion-free way of controlling the carrier envelope phase is demonstrated. Moreover, we experimentally confirm the increase in high-harmonic cut-off energy with driving laser wavelength and demonstrate the beneficial effect of few-cycle pulses which enable higher saturation intensities on target compared to multi-cycle pulses. It will be an ideal tool for future synthesis of isolated attosecond pulses in the sub-keV regime. With this laser source, we revealed for the first time multi-electron effects in high harmonic generation in xenon.

Separation of target structure and medium propagation effects in high-harmonic generation

We calculate high-harmonic generation (HHG) by intense infrared lasers in atoms and molecules with the inclusion of macroscopic propagation of the harmonics in the gas medium. We show that the observed experimental spectra can be accurately reproduced theoretically despite the sensitivities of the HHG spectra to the experimental conditions. We further demonstrate that the simulated (or experimental) HHG spectra can be factored out as a product of a ‘macroscopic wave packet’ and the photo-recombination transition dipole moment where the former depends on the laser properties and the experimental conditions, while the latter is the property of the target only. The factorization makes it possible to extract target structure from experimental HHG spectra, and for ultrafast dynamic imaging of transient molecules.

Role of multiphoton excitation and two-electron effects in high harmonic generation of H 2: A TDDFT calculation

Using two different TDDFT methods, we study the role of electronically excited states and two-electron dynamics in high harmonic generation (HHG) of H 2. The two methods produce slightly different electronic structures as reflected in the calculated ionization potentials. They nevertheless give similar HHG spectra. The difference between the two methods increases with the laser intensity, while their predictions remain qualitatively consistent. Our results suggest that two-electron dynamics can extend the HHG cutoff. Specifics of such extension depends on the internuclear distance and the laser intensity. We propose an ion excitation plus tunneling ionization mechanism to explain these extensions. The involvement of excited states is further revealed when we analyze each harmonic as a function of the internuclear distance. We see resonant peaks that are due to multiphoton excitation. These peaks exist above the ionization threshold as well.

Polarization effects in high-harmonic generation in the presence of static-electric field

We study the ellipticity dependence of the high-order harmonic generation (HOHG) when a static electric field is presented. Through investigating the corresponding classical trajectories of the electron for the harmonic generation, it is found that there exists two different mechanisms. For circularly polarized incident light the HOHG originates mainly from the recombination of the electrons to the nucleus after multiple returns. We also find that for the elliptical incidence HOHG, the application of a static electric field can generate harmonics far beyond the usual cutoff.

Wave-packet recollision and pulse-shape effects in high-harmonic generation

The recollision model of harmonic generation in a time-varying field has two elements: the emission due to a recolliding wave packet, and the quasiperiodic emission of wave packets in successive cycles. By discussing these issues separately, we explain the origin of blueshifting, and the ‘‘smearing’’ of the harmonic spectrum. We find that such a model shares many of the qualitative features of the semiclassical theories of harmonic generation. @S1050-2947~97!09401-8#

Nonadiabatic Effects in High-Harmonic Generation with Ultrashort Pulses.

Nonadiabatic Effects in High-Harmonic Generation with Ultrashort Pulses.