Harmonic Emission Process Research Articles

Mechanisms of Below-Threshold Harmonic Generation in Atoms

Most previous studies have focused on high-order harmonic generation beyond the ionization threshold; mechanisms of below-threshold harmonics are less understood. We schematically study the harmonic emission process in this region by numerically solving the time-dependent Schrödinger equation of an atom in laser fields. We show that, besides the quantum path interference mechanism recently identified, the effects induced by the Coulomb potential also have a critical impact on these harmonics. These mechanisms can be distinguished in the structure of harmonic spectra by changing the laser wavelength and peak intensity. We find that the long quantum orbits can influence lower-order harmonics at a higher laser intensity. In addition, we show that the intensity-dependent steps of harmonic yield can disappear for certain harmonic orders, due to the trapping in the Rydberg states before recombination, which can explain recent experimental observations.

Theoretical study of Čerenkov-type second-harmonic generation in periodically poled ferroelectric crystals

We study theoretically the Čerenkov-type second-harmonic generation by a Gaussian beam in quadratic nonlinear media with periodically reverted ferroelectric domains. In particular, we consider the role of fundamental beam width on the harmonic emission process. We show that varying the beam width of the fundamental wave not only affects the strength of the emitted Čerenkov harmonic signal but also changes the sensitivity of the emission process to wavelength tuning.

Efficient continuum generation exceeding 200 eV by intense ultrashort two-color driver

A temporal gating on the high-order harmonic emission process is achieved using an intense 20 fs, 1.45 microm pulse (IR) in combination with an intense 13 fs, 800 nm pulse [visible (VIS)]. Exploiting this two-color gating scheme, a coherent continuous emission extending up to 160 eV using Ar gas and 200 eV using Ne gas is efficiently generated. The IR pulse contributes to significantly extending the harmonic emission to higher photon energies, whereas the VIS pulse improves the conversion efficiency of the process. These results indicate the possibility to produce bright attosecond pulses approaching the soft X spectral region.

Three-dimensional time-profile analysis of high-order harmonic generation in molecules: Nuclear interferences inH2+

We describe a numerical method used previously [Phys. Rev. A 70, 011404(R) (2004)] for solving the three-dimensional time-dependent Schr\"odinger equation for ${\mathrm{H}}_{2}^{+}$ (with fixed nuclei) in interaction with an intense, arbitrary oriented laser pulse. In this approach, we use the prolate spheroidal coordinate system, and expand the time-dependent wave function in a complex basis of Laguerre polynomials and Legendre functions. Our results indicate that ionization, excitation, and harmonic generation are strongly influenced by the orientation of the molecular axis with respect to the laser polarization axis. We evaluate the contribution of each nucleus to harmonic generation, as this permits a quantitative and nonambiguous assessment of interference effects as a function of molecular orientation. A time-profile analysis, using a Gabor transform of the harmonic spectrum around certain harmonics, shows that every half-cycle high-order harmonics are emitted by each nucleus when the electron wave packet returns for a recollision with the molecular core, thus confirming the strong field recollision model in molecules. In general, each nucleus emits both odd and even harmonics, but even harmonics are destroyed by interferences between contributions of each nucleus. These interferences are shown to be maximum at certain harmonic orders as a function of molecular orientation. A comparison of acceleration and dipole formulations of the harmonic emission process is made in order to assess the use of high-order harmonic generation for electron wave-function imaging.

Harmonic-generation experiments on the Mark III free electron laser

An understanding of the harmonic-emission process in an FEL oscillator is important so that the properties of the harmonic emission can be tailored for maximum usefulness. In scientific applications one might wish to minimize the bandwidth of the harmonic radiation, while in high-powered FEL oscillators one might wish to minimize the energy in a particular harmonic to avoid mirror damage. In previous work we showed that the bundwidths of the harmonics are considerably larger than predicted by simple theory. Numerical simulation of the harmonic emission process has led to a better understanding of the bandwidths. As the fundamental power changes during a micropulse, the harmonics go through multiple cycles of bunching and overbunching, leading to multiple peaks in the harmonic micropulse and spectral broadening. This oscillatory dependence of harmonic power on instantaneous fundamental power has also been predicted elsewhere for the optical klystrom. In addition to our bandwidth measurements, we have measured spatial profiles, small-signal rise times, and ratios of coherent/incoherent harmonic emission. The small-signal rise time for the nth harmonic (n = 4−6) is a factor of n smaller than the fundamental rise time, as theory predicts.

An analysis of plasma emission by correlation radiometry

Correlation radiometry is proposed as a method of distinguishing between collective and individual particle processes in plasma radiation. Several schemes of correlation radiometry are investigated and the appropriate one is selected to examine correlation between radiation emitted at axially separated points in a beam-plasma discharge column at the harmonics of the electron cyclotron frequency. The correlation measurements establish the usefulness of correlation radiometry and add to the understanding of the collective nature of the cyclotron harmonic emission process.