Demonstration of Coherent Terahertz Transition Radiation from Relativistic Laser-Solid Interactions

Coherent transition radiation in the terahertz (THz) region with energies of sub-mJ/pulse has been demonstrated by relativistic laser-driven electron beams crossing the solid-vacuum boundary. Targets including mass-limited foils and layered metal-plastic targets are used to verify the radiation mechanism and characterize the radiation properties. Observations of THz emissions as a function of target parameters agree well with the formation-zone and diffraction model of transition radiation. Particle-in-cell simulations also well reproduce the observed characteristics of THz emissions. The present THz transition radiation enables not only a potential tabletop brilliant THz source, but also a novel noninvasive diagnostic for fast electron generation and transport in laser-plasma interactions.

Figure 1

Schematic of the top view of the experimental setup. The THz collection lens and windows are not shown. The lower inset sketches the three types of targets used. Pyroelectric detector (PD), pinhole (PH).

Figure 2

(a) Experimentally measured (blue circle dashed) and simulated (black solid) frequency spectra of the THz emission at $-75°$ from the metal foils without the PE layer. (b) Measured THz intensity (blue circle) as a function of the THz polarizer angle and the corresponding fitting curve (black dashed) with the cosine-squared function. (c) THz radiation angular distributions obtained from the experiment (blue circle), numerical simulation (black dashed), and theory model (red solid), which are normalized by the THz intensity at 75°. (d) Fast electron angular distributions obtained from experimental measurements (blue solid) and numerical simulations (black dashed). The red arrows in (c) and (d) indicate the laser incidence.

Figure 3

Measured THz radiation intensity at $-75°$ (blue circle) and calculated diffraction modification factor $D$ for mass-limited targets with different sizes. The solid and dashed curves are obtained with different parameters ($\gamma$, $\lambda$), where $\gamma$ is the relativistic Lorentz factor of electrons and $\lambda$ the radiation wavelength in $\mu \mathrm{m}$.

Figure 4

(a)–(b) Measured THz intensity at 75° (black square), 45° (red diamond), $-60°$ (green triangle), and $-75°$ (blue circle) from the metal-PE targets as a function of the thickness of the PE layer. (c) Measured THz intensity at $-75°$ from the single-layer PE targets with different thickness. (d) Comparison of the typical signals recorded by the THz detector from the single-layer PE (red dashed) and the PE-metal (black solid) targets with a $40\mu \mathrm{m}$ PE layer.

Figure 5

(a) Measured ion energy spectra from $500\times 500\mu \mathrm{m}$ (blue solid) and $2000\times 2000\mu \mathrm{m}$ (red dashed) mass-limited Cu targets. (b) Ion energy spectra from the metal-PE targets with different PE thickness. The maximum energy of ions from the 40 and $75\mu \mathrm{m}$ thick PE layer becomes less than the lower detection limit (1 MeV), which is caused by the shielding Al filters in front of IP.