Dynamic Chirp Control and Pulse Compression for Attosecond High-Order Harmonic Emission

We propose a scheme to compensate dynamically the intrinsic chirp of the attosecond harmonic pulses. By adding a weak second harmonic laser field to the driving laser field, the chirp compensation can be varied from the negative to the positive continuously by simply adjusting the relative time delay between the two-color pulses. Using this technique, the compensation of the negative chirp in harmonic emission is demonstrated experimentally for the first time and the nearly transform-limited attosecond pulse trains are obtained.

Figure 1

(a) Calculated phase of the 25th harmonic in argon (b) Calculated attosecond chirp for the 19th to 29th harmonics as functions of the time delay between the fundamental and the SH pulses.

Figure 2

Schematic of the experimental setup.

Figure 3

Measured harmonic spectrum from the argon cell.

Figure 4

(a) and (b) Measured spectral phase difference between the adjacent harmonics from the 19th to the 29th harmonics. (c) and (d) Reconstructed waveforms of the attosecond harmonic pulse trains. In (a) and (c) the two-color laser pulses are polarized parallelly, while in (b) and (d) they are polarized orthogonally.

Figure 5

(a) The XUV supercontinuum (upper panel) and the corresponding chirp (lower panel). (b) The calculated attosecond pulse duration (solid lines) and the corresponding TL pulse duration (dashed lines) as functions of central photon energy, obtained with (red lines) and without (blue lines) using a properly delayed weak 400 nm laser pulse.