Figure 1

Electric field of the THz pulse at the focus (black solid lines) and after 1 mm propagation in the presence of a 5.2 fs laser pulse, with a peak intensity of $6\times {10}^{14}$ W/cm${}^2$ (red dashed line) and 10${}^{15}$ W/cm${}^2$ (blue dotted line). The upper insets show the incident laser field for the $6\times {10}^{14}$ W/cm${}^2$ case. (b) Central portion of the THz pulse’s electric field in the focus (black solid line), and after 1 mm propagation in a medium with 0% (green dash-dotted line), 4.2% (red dashed line), and 22% (blue dotted line) ionization, indicating the strong distortion of the THz field due to ionization right after the peak of the IR pulse. o. c. referring to the optical cycles of the IR laser.Reuse & Permissions

Figure 2

(a) Harmonic spectra from a single atom, generated at the beginning (black solid line) and at the end (red dashed line) of the cell, obtained by using a 5.2 fs IR laser pulse combined with the 100 MV/cm THz pulse. The spectrum on the bottom (blue dash-dotted line) shifted five orders of magnitude downward shows the spectrum generated by the same IR pulse but without the THz field. (b) The resulting attosecond pulse from the single-atom response for the case without the THz field, and (c) the propagated and radially integrated harmonic field intensity at the exit of the interaction region (also in the case with only the IR field present). (d) shows the attosecond pulse from the single-atom response and (e) the propagated and radially integrated harmonic field intensity (resulting power) at the exit of the interaction region for the case with the THz pulse. The attosecond bursts were synthesized by selecting harmonic orders $⩾$81 from the harmonic spectra. o. c. referring to the optical cycles of the IR laser.Reuse & Permissions

Figure 3

(a) Harmonic spectra from a single atom, generated at the beginning and at the end of the cell obtained by using an 8 fs IR laser pulse combined with the 100 MV/cm THz pulse. (b) The resulting attosecond pulse from the single-atom response obtained by selecting harmonics $⩾$81, and (c) the propagated and radially integrated harmonic field intensity at the exit of the interaction region showing a clean SAP. The inset in logarithmic scale shows a contrast of almost 10${}^2$ between the main pulse and the second most powerful one. o. c. referring to the optical cycles of the IR laser.Reuse & Permissions

Figure 4

Radially integrated spectral intensity of the propagated field after different lengths of the gas cell for the (a) 5.2 fs and (b) 8 fs laser pulses combined with the 100 MV/cm THz pulse. The spectra shown for $z=1$ mm correspond to the temporal shapes shown previously in Figs. 2e and 3c, respectively.Reuse & Permissions

Figure 5

Spectral intensity of the propagated harmonic field as a function of radial and axial ($z$) coordinate for harmonic 81 (a), 101 (b), and 121 (c) calculated for generation with the 8 fs laser pulse.Reuse & Permissions

Figure 6

(a),(b),(c) Harmonic field intensity of the generated single-atom emission at different axial ($z$) and radial ($r$) coordinates, and (d),(e),(f) the intensity of the propagated field at the same coordinates. o. c. referring to the optical cycles of the IR laser.Reuse & Permissions

Figure 7

Generated harmonic bursts at the single-atom level (top row) and the propagated and radially integrated harmonic field intensities at the exit of the interaction region (bottom row) for different generating laser pulses having 10 fs (a) and (d), 12 fs (b) and (e), and 15 fs duration (c) and (f), assisted by the THz field.Reuse & Permissions

Figure 8

Radially integrated spectral intensity of the propagated field after different lengths of the gas cell for 25, 35, and 50 Torr gas pressure, showing the detrimental effect of high gas pressure on the phase matching of cutoff harmonics.Reuse & Permissions