Attosecond Synchronization of High-Harmonic Soft X-rays

TLDR

Subfemtosecond pulses can be produced by superposing multiple high harmonics of an intense laser, and adding more harmonics should shorten the pulse if they are emitted simultaneously. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter. We found that unsynchronized high harmonics limit pulse duration, but by controlling ultrafast electron dynamics we improved synchronization to produce 130‑attosecond pulses.

Abstract

Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.