Optimization of Attosecond Pulse Generation

The generation of attosecond pulses by superposition of high harmonics relies on their synchronization in the emission. Our experiments in the low-order, plateau, and cutoff regions of the spectrum reveal different regimes in the electron dynamics determining the synchronization quality. The shortest pulses are obtained by combining a spectral filtering of harmonics from the end of the plateau and the cutoff, and a far-field spatial filtering that selects a single electron quantum path contribution to the emission. This method applies to isolated pulses as well as pulse trains.

Figure 1

(a) Measured (normalized) temporal profiles for 10 harmonics (17 to 35) generated in Ne at $4.7\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (intensity at focus). (b) Normalized phase-locking parameters: measurements ${\tilde{\gamma }}_t^{\mathrm{e}\mathrm{x}\mathrm{p}}$ (red dots) and simulations ${\tilde{\gamma }}_t^{\mathrm{s}\mathrm{g}\mathrm{l}}$ (red dashed line) at $4.7\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$; TDSE calculations from 10 for harmonics 41 to 59 (blue line) and simulations ${\tilde{\gamma }}_t^{\mathrm{s}\mathrm{g}\mathrm{l}}$ (blue dashed-dotted line) at $6\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. (c) Measured normalized spectra for harmonic 19 at $z=-0.3b$ (red dashed dots), 0 (green line), and $0.2b$ (blue dashed line).

Figure 2

Harmonic intensities (a) and emission times (b) obtained by generating in Xe (detection in Ar) at $3\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (red squares) and $6\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (green triangles), and by generating in Ar (detection in He) at $9\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (blue circles). In (b), the solid lines are the short trajectory emission times given by the single-atom calculations, and the dashed line indicates the zero of the laser field.

Figure 3

Emission time of low-order harmonics generated in Ne (detection in Ar) at $2\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (red circles), $3\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (green triangles), $3.5\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (blue squares), and $4\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (magenta stars).