Probing the conductivity of compressed plastic using THz time-domain spectroscopy*

Understanding the electrical conductivity of hydrocarbons at HED conditions has important implications for understanding the thermodynamic evolution of planets like Uranus and Neptune. Recent work has revealed that under shock loading, carbon and hydrogen atoms separate, with carbon forming into nanodiamonds [1]. While ab initio simulations suggest that after phase separation metallic hydrogen also forms [2], direct evidence of the presence of metallic hydrogen has yet to be observed.

We present single-shot THz time-domain spectroscopy measurements of shock compressed plastic. THz time-domain spectroscopy is a useful tool for determining electrical conductivity [3]. Because THz frequencies are much slower than electron-electron and electron-ion interactions, the THz field probes the quasi-DC electric response of the plastic. Furthermore, single-shot THz detection enables the probing of material properties during irreversible processes, making it an appealing tool for interrogating matter driven to extreme conditions. By observing changes in the THz signal reflected off the sample, we infer changes in the electrical properties of the plastics under shock compression.

[1] D. Kraus et al., Nat. Astron., 1, 606-611 (2017).

[2] D. Kraus et al, Phys. Rev. Res., 5, L022023 (2023).

[3] B. K. Ofori-Okai et al, Phys. Plasmas, 31, 042711 (2024).