Theory Of High Harmonic Generation Research Articles

Theory of high-harmonic generation in solids

High-harmonic generation (HHG) in bulk crystals exposed to intense mid-infrared lasers with photon energies below the bandgap is investigated theoretically. A three dimensional, two-band model that considers both interband and intraband currents is used. It is shown that the interband current is the dominant mechanism for HHG in solids. A physical interpretation of interband HHG – similar to atomic HHG – is provided by saddle point analysis. The effects of dephasing time and driving field wavelength on the harmonic specrum are investigated.

Two states hydrogenlike model for high-order harmonic generation and an isolated attosecond pulse generation in a He+ ion

We present an analytic quantum theory of high harmonic generation by low frequency laser fields for a two-state hydrogenlike atom model. In the case of an initial coherent superposition of two states, the theory clearly explains why high conversion efficiency can be obtained. Furthermore, we apply this model to generate an intense isolated attosecond pulse by the proposed combined field. By mixing an infrared chirped pulse with a fundamental chirped pulse, a supercontinuum spectrum can be formed. The long quantum trajectory is suppressed, and the short one is enhanced and an intense isolated 95-as pulse is obtained by He+ ion. Our simulation shows that the produced pulses with good signal-to-noise ratio are obtained without any phase compensation. In particular, these results are analyzed by using both classical and time-frequency analyses.

Probing Polar Molecules with High Harmonic Spectroscopy

We bring the methodology of orienting polar molecules together with the phase sensitivity of high harmonic spectroscopy to experimentally compare the phase difference of attosecond bursts of radiation emitted upon electron recollision from different ends of a polar molecule. This phase difference has an impact on harmonics from aligned polar molecules, suppressing emission from the molecules parallel to the driving laser field while favoring the perpendicular ones. For oriented molecules, we measure the amplitude ratio of even to odd harmonics produced when intense light irradiates CO molecules and determine the degree of orientation and the phase difference of attosecond bursts using molecular frame ionization and recombination amplitudes. The sensitivity of the high harmonic spectrum to subtle phase differences in the emitted radiation makes it a detailed probe of polar molecules and will drive major advances in the theory of high harmonic generation.

Theory of High Harmonic Generation for Probing Time-Resolved Large-Amplitude Molecular Vibrations with Ultrashort Intense Lasers

We present a theory that incorporates the vibrational degrees of freedom in a high-order harmonic generation (HHG) process with ultrashort intense laser pulses. In this model, laser-induced time-dependent transition dipoles for each fixed molecular geometry are added coherently, weighted by the laser-driven time-dependent nuclear wave packet distribution. We show that the nuclear distribution can be strongly modified by the HHG driving laser. The validity of this model is first checked against results from the numerical solution of the time-dependent Schrödinger equation for a simple model system. We show that in combination with the established quantitative rescattering theory this model is able to reproduce the time-resolved pump-probe HHG spectra of N(2)O(4) reported by Li et al. [Science 322, 1207 (2008)].

Extracting Continuum Electron Dynamics from High Harmonic Emission from Molecules

We show that high harmonic generation is the most sensitive probe of rotational wave packet revivals, revealing very high-order rotational revivals for the first time using any probe. By fitting high-quality experimental data to an exact theory of high harmonic generation from aligned molecules, we can extract the underlying electronic dipole elements for high harmonic emission and uncover that the electron gains angular momentum from the photon field.

Use of three detuned lasers to generate isolated attosecond pulses

The dynamics of a one-dimensional atom driven by three-laser fields is investigated. The total electric field is made up of a fundamental laser field of intensity W cm−2 and wavelength λ = 820 nm and two weak lasers with larger wavelengths. The intensity of the two weak fields is with k = 0.25. The frequencies of the weak fields are and , with and . The three lasers have a Gaussian envelope of 72 fs FWHM. It is shown, by numerical computation and using the semiclassical theory of high-harmonic generation, that the atom interacting with this combined field is able to emit an isolated attosecond burst of radiation.

S-matrix theory of high harmonic generation and ionization of coherently ro-vibrating linear molecules by intense ultrashort laser pulses

A theory of high harmonic generation and ionization of coherently rotating and vibrating linear molecules by a delayed pair of intense ultrashort laser pulses is presented. It correlates the nuclear motions in real time with the modulation of the harmonic emission and ionization signals as a function of the time-delay between the exciting and the probing pulses. An illustrative analytical example of the excitation and detection of the clock-motion associated with the “0–1” vibrational oscillation of a diatomic molecule is also given.

A Model Study of High Harmonic Generation from Ions

AbstractWe have studied the influences of atomic parameters and laser parameters with various atomic potentials and laser intensities on high harmonic generation (HHG) by solving the time‐dependent 1‐D Schrödinger equation with a soft‐core model potential. For these purposes we examine harmonic spectra for the cases of deeply bound initial states of ions in the hydrogen isoelectronic sequence. The harmonic signals for the lower order harmonics rapidly decrease so that the harmonic spectrum reveals a ‘well’ structure in the region of lower order harmonics, which is not common in the ordinary high harmonic spectrum. In order to explain the origin of this feature we also calculate HHG spectra for the cases where the excited states are initially populated. These studies naturally require considerations of quantum effects that are normally neglected, by which we under stand the limit of the semi‐classical theory of HHG.

High harmonic generation beyond the electric dipole approximation

A generalization of the analytical theory of high harmonic generation in the long wavelength limit and in the single active electron approximation is developed taking into account the magnetic dipole and electric quadrupole interaction. Quantum mechanical and classical theories are found to be in excellent agreement, which allows one to explain the influence of multipole effects in terms of an intuitive picture. For Ti:S lasers ( 0.8 &mgr;m) multipole contributions are found to be small below an intensity of about 10(17) W/cm(2), at which harmonic radiation with photon energies of several keV is generated. This promises the extension of high harmonic generation well into the sub-nm wavelength regime.

Quantum theory of high harmonic generation as a three-step process

Fully quantum treatment explicitly presents the high harmonic generation as a three-step process: (i) above threshold ionization (ATI) is followed by (ii) electron propagation in a laser-dressed continuum. Subsequently (iii) stimulated (or laser assisted) recombination brings the electron back into the initial state with emission of a high-energy photon. Contributions of all ATI channels add up coherently. All three stages of the process are described by simple, mostly analytical expressions that allow a detailed physical interpretation. A very good quantitative agreement with the previous calculations on the harmonic generation by H-minus ion is demonstrated, thus supplementing the conceptual significance of the theory with its practical efficiency. The virtue of the present scheme is further supported by a good accord between the calculations in length and velocity gauges for the high-energy photon.