Phase-locked high-order-harmonic and sub-100−aspulse generation from stretched molecules

Abstract

High harmonic generation from diatomic molecules in a linearly polarized intense laser field is investigated and the emission time of the harmonics is discussed with the time-frequency analysis method. It is shown that high harmonic generation from molecules at equilibrium distance is similar to that from atoms. Only the harmonics in the cutoff are synchronized, i.e., well phase-locked, whereas the other harmonics are not phase-locked. For the molecule stretched well beyond its equilibrium distance, the harmonics exhibit distinct time-frequency characteristics. The harmonic spectrum can be extended to ${I}_{p}+8{U}_{p}$, where ${I}_{p}$ and ${U}_{p}$ are the ionization and ponderomotive potential, and the harmonics with energies below ${I}_{p}+3.17{U}_{p}$ are not phase-locked and the harmonics with energies beyond ${I}_{p}+3.17{U}_{p}$ are well phase-locked. Thus a large range of harmonics which are well phase-locked are produced, and a train of clean attosecond (as) pulses with a single $90\text{\ensuremath{-}}\mathrm{as}$ pulse in each half optical cycle can be generated with a multicycle laser pulse. Using a few-cycle laser pulse, an isolated attosecond pulse with a duration of about $95\phantom{\rule{0.3em}{0ex}}\mathrm{as}$ is obtained.