Fig. 1. Sketch of the proposed targets: “sine” wire (top), “triangle” wire (middle), and “square” wire (bottom). The targets are irradiated at the open end by an intense femtosecond laser pulse. The general propagation direction of the laser-induced discharge pulse is shown by black arrows: it propagates from the irradiated end of the wire to the opposite one along the wire surface.

Fig. 2. (a1)–(a4) Results of 2D PIC simulation for a straight wire target irradiated by a laser pulse with maximum intensity I_max = 10²² W/cm² and FWHM duration τ_las. = 12.5 fs: B_z component of the electromagnetic field at moments of time 27, 54, 81, and 135 fs, respectively. (b) Dependence of the amplitude of the total electric current flowing through the wire cross-section at x ≈ 30 µm on the maximum laser intensity at the focal spot; the data for different laser pulse durations are shown by markers of different colors. The points appear to closely follow the power laws represented by solid lines. (c) Dependence of the duration of the discharge pulse at FWHM on the laser duration at FWHM for various intensities of the laser driver, shown by markers of different colors. The points appear to closely follow the linear dependence represented by the black solid line. (d) Evolution of the current profile as it propagates along the wire. The dashed line shows an approximation by the sum of two exponential functions.

Fig. 3. Electromagnetic fields emitted by the “sine” target with geometry defined by Eq. (1): (a1)–(a4) B_z component in the plane of the target, i.e., at z = 0.64 mm, at moments of time 1.0, 2.0, 3.0, and 4.0 ps, respectively; (b1)–(b4) B_x component in the plane y = 0.64 mm at the same moments of time; (c1)–(c4) B_z component in the plane y = 0.64 mm at the same moments of time. The projections of the target on the considered planes are shown as solid black lines. For clarity of the field profiles, the energy dissipation of the discharge current pulse is not taken into account in these plots.

Fig. 4. Electromagnetic fields emitted by the “sine” target with geometry defined by Eq. (1) on the edges of the simulation box: (a1)–(a4) E_y component on the edge x = 1.28 mm at moments of time 4.27, 4.37, 4.47, and 4.57 ps, respectively; (b1)–(b4) B_z component on the edge x = 1.28 mm at moments of time 4.27, 4.37, 4.47, and 4.57 ps, respectively; (c1)–(c4) B_x component on the edge z = 1.28 mm at moments of time 3.4, 4.2, 5.0, and 5.8 ps, respectively; (d1)–(d4) E_y component on the edge z = 1.28 mm at moments of time 3.4, 4.2, 5.0, and 5.8 ps, respectively. For clarity of the field profiles, the energy dissipation of the discharge current pulse is not taken into account in these plots.

Fig. 5. Power spectral density (PSD) for targets with different geometries: (a) “sine” target with geometry defined by Eq. (1); (b) “sine” target with geometry defined by Eq. (4); (c) “triangle” target with geometry defined by Eq. (2); (d) “square” target with geometry defined by Eq. (3). The blue curves correspond to the PSD in the forward direction, i.e., θ = 0°, the red curves correspond to the PSD in the perpendicular direction, i.e., θ = 90°, and the green curves correspond to the PSD in the backward direction, i.e., θ = 180°. The subtle peak in the region 0.1–1 THz indicated by the black arrow in (a) results from the “slow” decay of the amplitude of the discharge current pulse with characteristic decay time τ_dec..

Fig. 6. (a1)–(c1) Electromagnetic fields emitted by the “sine” target with geometry defined by Eq. (1): (a1) B_z component in the plane of the target, i.e., at z = 0.64 mm, at 4.0 ps; (b1) B_x component in the plane y = 0.64 mm at the same moment of time; (c1) B_z component in the plane y = 0.64 mm at the same moment of time. (a2)–(c2) The same, but for the “sine” target with geometry defined by Eq. (4). (a3)–(c3) The same, but for the “triangle” target with geometry defined by Eq. (2). (a4)–(c4) The same, but for the “square” target with geometry defined by Eq. (3). The projections of the targets on the considered planes are shown as solid black lines. For clarity of the field profiles, the energy dissipation of the discharge current pulse is not taken into account in these plots.

Fig. 7. Time-integrated energy fluence of THz radiation through the edges of the simulation box for three-period sine targets with (a) a₁ = a₂ = 20 µm (a) and (b) a₁ = 34 µm, a₂ = 8.5 µm. The target is located at the center of the simulation box at (x, y, z) = (0, 0, 0). The current pulse duration is ∼50 fs, and its amplitude increases linearly from zero to 10⁵ A on the first period, stays constant at 10⁵ A on the second period, and decreases linearly from 10⁵ A to zero on the third period. The fluence is normalized per value emitted on the second period when the current amplitude is constant.

Fig. 8. Results of analytical estimates of the main frequency emitted by the “sine” target: (a) and (c) dependence of the normalized frequency aω/c on the geometrical parameter ϰa for different directions θ and two different propagation velocities 0.9c and 0.99c, respectively; (b) and (d) dependence of the normalized frequency aω/c on the direction θ for different values of the geometrical parameter ϰa and two different propagation velocities 0.9c and 0.99c, respectively. The green and magenta filled circles show points corresponding to the geometrical parameters of the sine targets considered in the numerical study, with green and magenta corresponding to the “sine” targets with geometries defined by Eqs. (1) and (4), respectively; their absolute frequency values are marked on the axes with the corresponding color.