Recently,  application of Chinese terahertz free-electron laser (CTFEL) was highlighted for the investigation of the THz dynamics of electronic materials.

This research was conducted by Prof.Xu Wen from the Institute of Solid State Physics, Hefei Institute of Physical Science (HFIPS) together with the Chinese Academy of Engineering Physics. And related outcomes were highlighted as the cover paper on Chinese Physics B.

In the process, the first monochromatic picosecond (ps) THz pump-probe system (Fig. 1) was developed by utilizing the unique features of the THz pulse structure (especially the ps pulse width of the micro-pulses) of the CTFEL. Basing on this facility, scientists measured the electron energy relaxation time,which had been one of the key physical parameters for an electronic material,  in a semiconductor, and studied the dynamic electronic properties of high mobility n-GaSb wafer with different radiation frequencies of the CTFEL at room temperature.

The measured electron energy relaxation time was in consistent with what was obtained from, e.g., the four-wave mixing measurement after fitting with the theoretical model (Fig. 2).
 
CTFEL achieved saturation output and was put into operation in Chengdu at the end of 2017, marking that the terahertz (THz) science and technology in China has officially stepped into the era of free-electron laser (FEL).

Compared to other ultrafast optoelectronic techniques, the monochromatic ps THz pump-probe showed the following advantages: 1) It could measure the momentum and energy relaxation dynamics of the free electrons while not involving photo-generated carriers and associated excitonic effects; 2) The monochromatic THz pump and probe could be achieved so that the Fourier transformation was not required for the analysis of the experimental data. And 3) when combining with the continuous frequency tunability of the free-electron laser, the selective frequency could be realized in the pump-probe measurement.

 It proved that "monochromatic ps THz pump-probe" basing on free-electron laser could provide a new technique for the measurement of the dynamic characteristics of the electronic and optoelectronic materials, which would expand the field of the application for pulsed THz free-electron lasers.

Link to the paper:  Picosecond terahertz pump-probe realized from Chinese terahertz free-electron laser

Fig. 1 Schematic diagram of ps THz pump-probe system based on the CTFEL(Image by WEN Hua)

Fig. 2 The transmission intensity as a function of delay time for different pumping frequencies as indicated. The smooth curves are the results of theoretical fitting for 1.6 THz and 2.4 THz and the coarse curves are the raw data of the experimental results. (Image by WEN Hua)