skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Pathway of bichromatic high-order harmonic generation

Abstract

We present a theoretical study of the pathway of harmonic generation in bichromatic linearly polarized laser fields. A 'harmonic-collapse' phenomenon is observed in the power spectrum for a particular value of the amplitude ratio of two components of the external field. We employ an offset frequency shift to the additional field to distinguish the harmonic pathway and then to explain the harmonic-collapse phenomenon. The harmonic intensity as a function of the relative amplitude ratio and, furthermore, the fine structure of the harmonic spectrum can be well understood on the basis of a pathway analysis.

Authors:
;  [1]
  1. Department of Physics, Wuhan University, Wuhan 430072 (China)
Publication Date:
OSTI Identifier:
20982424
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. A; Journal Volume: 75; Journal Issue: 3; Other Information: DOI: 10.1103/PhysRevA.75.035801; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
74 ATOMIC AND MOLECULAR PHYSICS; AMPLITUDES; FINE STRUCTURE; FUNCTIONS; HARMONIC GENERATION; LASER RADIATION; SPECTRA; SPECTRAL SHIFT

Citation Formats

Cai, Jun, and Qiao, Haoxue. Pathway of bichromatic high-order harmonic generation. United States: N. p., 2007. Web. doi:10.1103/PHYSREVA.75.035801.
Cai, Jun, & Qiao, Haoxue. Pathway of bichromatic high-order harmonic generation. United States. doi:10.1103/PHYSREVA.75.035801.
Cai, Jun, and Qiao, Haoxue. Thu . "Pathway of bichromatic high-order harmonic generation". United States. doi:10.1103/PHYSREVA.75.035801.
@article{osti_20982424,
title = {Pathway of bichromatic high-order harmonic generation},
author = {Cai, Jun and Qiao, Haoxue},
abstractNote = {We present a theoretical study of the pathway of harmonic generation in bichromatic linearly polarized laser fields. A 'harmonic-collapse' phenomenon is observed in the power spectrum for a particular value of the amplitude ratio of two components of the external field. We employ an offset frequency shift to the additional field to distinguish the harmonic pathway and then to explain the harmonic-collapse phenomenon. The harmonic intensity as a function of the relative amplitude ratio and, furthermore, the fine structure of the harmonic spectrum can be well understood on the basis of a pathway analysis.},
doi = {10.1103/PHYSREVA.75.035801},
journal = {Physical Review. A},
number = 3,
volume = 75,
place = {United States},
year = {Thu Mar 15 00:00:00 EDT 2007},
month = {Thu Mar 15 00:00:00 EDT 2007}
}
  • We perform a dynamical symmetry analysis (DSA) of the high-order harmonic generation (HHG) spectrum of an atom interacting with a bichromatic laser field. Within the framework of the conventional Hermitian quantum mechanics (QM), the HHG spectrum calculated using a single Floquet state, or any finite number of Floquet states, is invariant under the inversion of the relative phase of the two-frequency components, {phi}{yields}-{phi}. The asymmetry with respect to the phase inversion seen in the simulated HHG spectra is obtained in the conventional QM only when the Floquet spectrum is continuous and ionization is taken into consideration. However, when the Hamiltonianmore » is complex scaled the description is different. Even a single eigenstate of the complex scaled Floquet operator is enough to describe the breaking of the {phi}{yields}-{phi} symmetry in the HHG spectra. We find that there is a direct correlation between the strength of the asymmetry with respect to the relative phase inversion and the magnitude of the ionization rate. For illustration purposes, the DSA is accompanied by the results obtained for a one-dimensional effective single-electron model Hamiltonian mimicking xenon atom interacting with strong laser field.« less
  • We analyze the dynamical symmetries and the selection rules relevant to the process of production of high-order harmonics as a result of irradiating an atom with strong, continuous bichromatic laser radiation of similar frequencies {omega} and {omega}+{delta}{omega}. We show that when the two laser frequencies are close to one another (small {delta}{omega}), the spectrum becomes very dense and new selection rules are obtained. Such a dense spectrum of harmonics could be a source for the production of attosecond light pulses. We demonstrate our results numerically on a one-dimensional model Hamiltonian of a Xe atom and show how a pulse ofmore » length 700-as could be produced.« less
  • Numerical solutions of the time-dependent Schroedinger equation (TDSE) for a two-dimensional H{sub 2}{sup +} molecule excited by a bichromatic ultrashort intense circularly polarized laser pulse with frequencies {omega}{sub 0} and 2{omega}{sub 0} and relative carrier envelope phase {phi} are used to explore the generation of high-order elliptically polarized harmonics as a function of internuclear distance R. Optimal values of {phi} and R for efficient and maximum molecular high-order harmonic generation (MHOHG) are determined from a classical model of collision with neighboring ions and confirmed from the TDSE nonperturbative simulations. Maximum elliptically polarized harmonic energies of I{sub p}+13.5U{sub p} are found,more » where I{sub p} is the ionization potential and U{sub p}=I{sub 0}/4m{sub e{omega}0}{sup 2} is the ponderomotive energy at intensity I{sub 0} and frequency {omega}{sub 0}. The polarization properties of MHOHG, phase difference {delta}, ellipticity {epsilon}, and orientation angle {phi} are presented as well. The high efficiency of the proposed MHOHG scheme should be useful for production of elliptically polarized attosecond extreme ultraviolet pulses.« less
  • It is proposed to use bichromatic laser fields to increase the efficiency of generation of coherent short-wavelength radiation by atoms in the process of above-threshold tunnel ionisation. It is shown in the framework of a semiclassical model that the use of a relatively weak static field, along with the basic pump radiation, results in the increase of the fraction of recombining photoelectrons and in the rise of their kinetic energy at the moment of recombination. It is shown that the experimental realisation of this effect is possible with the use of high-power CO{sub 2} laser radiation (instead of the staticmore » field) and ultrashort pulses of the basic pump component with wavelength {lambda} {approx} 1 {mu}m. Numerical calculations in the framework of a quantum mechanical model confirm the conclusions of the semiclassical model and show that the addition of long-wavelength radiation enables tripling the maximum generation frequency. Under specific intensity of CO{sub 2} laser radiation, the so-called multi-plateau structure was discovered near the high-frequency edge of the generated radiation spectrum. An interpretation of this effect is given in the framework of a semiclassical model. (this issue is dedicated to the memory of s a akhmanov)« less