Ultrafast Plasma Electron Dynamics: A Route to Terahertz Pulse Shaping

Intense ultrafast laser interaction with solid-density plasma can lead to relativistic transient electron dynamics that are favorable for the generation of high-field terahertz (THz) pulses. We investigate the temporal characteristics of such THz pulses. Single-shot electro-optic measurements enable us to record the complete temporal profiles of the THz pulses emanating from planar $\mathrm{Cu}$ and aligned $\mathrm{Cu}$-nanorod-array plasma. The temporal properties of the THz pulses exhibit several transients. Fully relativistic two-dimensional particle-in-cell simulations corroborate experimental observations and reveal that these features originate from electron microbunch emission from the target. Our results demonstrate that target nanostructuring provides a route to control such ultrafast electron dynamics and hence THz-pulse properties in the time domain.

Figure 1

(a) A schematic diagram of the experimental setup used for single-shot electro-optic detection. THz pulses are generated by high-intensity laser-plasma interaction on a $\mathrm{Cu}$ target. An echelon mirror (E) is used to replace the temporal delay line required in an EO-sampling experiment. (b) The echelon mirror (E) consists of several steps and splits a plane wave front into several beamlets with a delay ($\mathrm{\Delta }t$) between the beamlets. It finally generates a train of beamlets at the focus, which interact with the THz pulse inside the $\mathrm{Zn}\mathrm{Te}$ crystal to capture its temporal profile in a single shot.

Figure 2

SEM images of $\mathrm{Cu}$ nanorod arrays at different magnifications: (a) cross-section view; (b) top view.

Figure 3

(a) The experimentally recorded THz field generated from a plane $\mathrm{Cu}$ target as a function of the driver-laser intensity. (b),(c) The THz field generated from an aligned nanorod target of diameter 200 nm with various lengths at laser intensity (b) $8\times {10}^{17}{\mathrm{W}/\mathrm{cm}}^2$ and (c) $3.5\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$. The THz field is calculated for each pixel of a resized probe image of size $30\times 400$ pixels for a better grasp on the spatiotemporal distribution of the THz field. (d)–(f) The average THz field profiles generated from the spatiotemporal profiles of the THz field in (a)–(c), respectively. The numbers in the legend in (d) represent the laser intensity in ${10}^{18}{\mathrm{W/cm}}^2$, whereas the numbers in the legends in (e) and (f) represent the nanorod length in micrometers.

Figure 4

(a) The simulation geometry. The red dot represents the position of the numerical probe. (b)–(f) The magnetic field ($B_z$) distribution on the front of the target obtained from PIC simulation at different times after the interaction. The driver laser is filtered out using a spectral filter based on fast Fourier transformation.

Figure 5

(a) The temporal profile of the THz field generated in PIC simulation from plane targets of different thicknesses. A low-pass Butterworth filter is applied to extract the THz field in the frequency range $\le 15$ THz. (b) The electron energy spectrum from simulation as a function of the interaction time detected between $0.5\lambda$ and $1\lambda$ in front of the target. The color axis represents $dN/dE$, in arbitrary units. (c) The angular spectrum from simulation as a function of the interaction time. The color axis represents $dN/d\theta$, in arbitrary units.

Figure 6

The angular distribution of the THz waveform generated in PIC simulation for three different target thicknesses: (a) $10\lambda$, (b) $15\lambda$, and (c) $20\lambda$. A low-pass Butterworth filter is applied to extract spectral components $\le 15\mathrm{THz}$.

Figure 7

The evolution of the electron density (normalized to the critical density) in PIC simulation, at different times. A large number of electron ejections from the target are observed at times of 32 fs, 109 fs, and 238 fs after the interaction. However, in the intermediate time intervals, the electron ejection is significantly reduced.

Figure 8

The electron phase space ($P_x/m_{e}c$-$x$) from PIC simulation at six different times after the interaction shows electron recirculation. The dotted lines mark the positions of the target surfaces.

Figure 9

(a) The temporal profile of the THz pulses at three different laser intensities recorded in PIC simulation in the specular-reflection direction. The numbers in the legend represent the driver-laser intensity in units of ${10}^{18}{\mathrm{W}/\mathrm{cm}}^2$. The angular distribution of the time-dependent THz field generated from PIC simulation at the driver-laser intensities (b) $0.77\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$ and (c) $8.58\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$.

Figure 10

(a) The simulation geometry for the nanorod target. (b) An enlargement of the target, to show the aligned nanorod target on the planar surface. The THz waveform generated from the nanorod target in the specular-reflection direction at laser intensity (c) $0.77\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$ and (d) $8.58\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$, using 2D PIC simulation. The numbers in the legend represent the nanorod length in micrometers. (e),(f) The time-resolved electron spectrum from a 5-$\mu \mathrm{m}$-long nanorod target at two different laser intensities, $0.77\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$ and $8.58\times {10}^{18}{\mathrm{W}/\mathrm{cm}}^2$, respectively, obtained from simulation. The color axis represents $dN/dE$, in arbitrary units.