Spatial coherence of trajectory-resolved higher-order harmonics generated from an argon-filled gas cell using single- and two-color laser pulses

Abstract

An experimental study on spatial coherence of electron trajectory-resolved higher-order harmonic generation (HHG) in an argon-filled gas cell (length \ensuremath{\sim}15 mm) using single-color (\ensuremath{\lambda} \ensuremath{\sim} 800 nm) and two-color laser pulses (\ensuremath{\lambda} \ensuremath{\sim} 800 nm + 400 nm) is performed. The contribution of both a long and short electron trajectory is observed in HHG using single-color laser pulse, whereas a dominant contribution of short trajectory is observed in HHG using two-color laser pulses. The spatial coherence of trajectory-resolved higher-order harmonics is investigated using a double slit experiment. The fringe visibility is higher (\ensuremath{\sim}0.7) at low gas cell pressure, for both short and long trajectory harmonics. At higher gas pressure ($\ensuremath{\ge}50$ mbar), the fringe visibility of short trajectory harmonics (\ensuremath{\sim}0.7) is significantly higher than the long trajectory harmonics (\ensuremath{\sim}0.3--0.4). In the case of two-color laser pulses, the fringe visibility (\ensuremath{\sim}0.75) is found to be further higher than the single-color short trajectory harmonics and is nearly independent of gas pressure (\ensuremath{\sim}10--100 mbar). A theoretical model is used to explain the observed result and the intensity dependent dipole coefficient for short and long trajectory is estimated, which is in good agreement with the other reported results. The study provides an easy tool to study and control the role of different electron trajectories responsible for higher-order harmonic generation and also the spatial coherence property of higher-order harmonics generated from gas cell.