Terahertz (THz) waves offer a distinctive diagnostic method for detecting high energy density matter. However, realizing the THz time-domain spectral (THz-TDS) diagnosis of matter states under extreme conditions in large high-energy density devices remains a significant obstacle. To address this requirement, we designed and implemented an optical pump-THz single-shot detection system driven by a strong femtosecond laser. The system possesses the capability of THz single-shot detection under extreme conditions and diagnosis of irreversible processes with extreme transience using THz-TDS diagnosis under intense laser pumping.

We developed an integrated optically pumped terahertz (THz) single-shot detection system that utilizes a 45 TW Ti∶sapphire femtosecond laser with a pulse width of 30 fs, central wavelength of 800 nm, and spot diameter of approximately 38 mm. The laser pulses were initially directed to realize second harmonic generation (SHG) via KDP crystals and then separated into fundamental and SHG using a dichroic mirror (DM). The SHG was reflected into the pump time-delay line (TD2) and focused by a lens to ensure complete pumping of the target object with a focus size of approximately 2 mm in diameter consistent with the THz focus size. Meanwhile, the fundamental frequency laser transmitted by the DM was divided by the beam splitting mirror (BS) with 90% of the energy used as the driving laser of the lithium niobate wafer. An intense THz pulse was generated by collinear optical rectification effect, and an off-axis parabolic mirror (OAP) was utilized to focus it onto the target object. The THz pulses transmitted through the target object were focused by the OAP and reflected by indium tin oxide (ITO) to reach the surface of the ZnTe crystal. Moreover, 10% of the transmitted energy of the THz probe laser was directed into the time-delay line (TD1) incident with the surface normal of the reflective echelon at 14° and encoded time information into a one-dimensional space. The outgoing laser was spatiotemporal coincident with the THz on the surface of the ZnTe crystal. Finally, the orthogonal detection scheme was utilized to probe the THz waveform.

We present the design and implementation of an intense-field optical pump-THz time-domain spectroscopy single-shot detection system for measuring the irreversible non-equilibrium transient processes in high-energy and low-repetition-rate pumps of large laser devices. The system employs a reflective echelon and orthogonal detection scheme to detect pulses generated through the collinear optical rectification of a lithium niobate wafer with a diameter of 3 inch. The system consists of a THz generation-intense laser pumping module and a THz time-domain spectral single-shot detection module integrated into separate optical breadboards. The former can be placed in a vacuum chamber, and the latter in an atmospheric environment, making it easy to move and install and suitable for different laser-device application scenarios. The THz pulses have an energy of 7 µJ at 800 nm 1 J laser energy, can be easily adjusted, and have a detection capability of a 30 nm free-standing gold foils transmission spectrum at room temperature. We verify that the waveform obtained by the single-shot detection is the same as that obtained by traditional scanning. Based on this device, the variation of conductivity in the THz band of 30 nm free-standing gold foils with pumping delay measured under the 0.8 MJ/kg laser energy density of a 400 nm pump contributes to the further understanding of the generation and evolution of the warm dense state of gold.

With the advent of intense femtosecond lasers, it has become possible to investigate the state of matter in extreme conditions. The maturation of THz time-domain spectroscopy technology also provides a new tool for diagnosing extreme non-equilibrium states. To meet the demands of THz emission and state diagnosis under such extreme conditions, an intense-field optical pump-THz time-domain spectroscopy single-shot detection system with a simple THz path was designed and fabricated. The system was employed to measure the transient THz conductivity of 30 nm thick free-standing gold foils pumped by a 400 nm laser pulse. The obtained results serve as a potent platform for further exploration of irreversible processes including extreme condition THz emission-detection and the diagnosis of non-equilibrium states of matter under extreme conditions.