Identifying spatially asymmetric high-order harmonic emission during the falling edge of an intense laser pulse

Abstract

Two different induced effects of a laser falling edge on high-order harmonic generation are resolved by solving numerically full-dimensional electronic time-dependent Schr\"{o}dinger equation beyond the Born-Oppenheimer approximation. The harmonic spectrum of H$_2^+$ and T$_2^+$ isotopes are compared to see the effects of a 4-cycle falling edge of a 800 nm, 15-cycle trapezoidal laser pulse of $I=$3 $\times 10^{14}$ Wcm$^{-2}$ intensity on harmonic emission spectrum. The harmonic emission at the laser falling part is negligible for H$_2^+$ due to ionization suppression, but considerable for T$_2^+$. The falling edge of the laser pulse induces two effects on the HHG in T$_2^+$. The first well-known effect is non-adiabatic frequency redshift of generated odd-order harmonics. The second unknown one is spatially asymmetric harmonic emission which appears as even harmonic orders. In order to clarify this new effect, spatial distribution of HHG and resolving HHG into different components are demonstrated. The asymmetric emission would appear for both atoms and molecules as long as harmonic emission of either rising or falling edge of an intense trapezoidal or non-trapezoidal laser pulse is dominant.