Fig. 1. The generation and detection experimental setup of the high-average-power strong-field THz system. (a) Schematic diagram of the optical system. (b) The echelon-TPFP module. (c) A brief statistic of the previously reported THz lithium niobate sources with THz pulse energy as a function of the operating repetition rate. The dashed line depicts the levels of the different THz output average powers.

Fig. 2. The THz characteristics of the strong-field 1-kHz THz source based on the grating-TPFP. (a) The designed angle of incidence (AOI) and dispersion (AOD) for the transmission grating. (b) The THz pulse energy (circle) and optical-to-THz conversion efficiency (triangle) vary with pumping energy, measured at a relative humidity of ∼40%. The points are original data, while the curves are third-order polynomial fitted results. The insets are the THz focal spot (upper left) and the laser spot (lower right) measured at the input surface of the lithium niobate crystal. (c) The normalized THz temporal waveform. The vertical dashed lines show the calculated locations of the first four reflected pulses in the GaP crystal. (d) The corresponding normalized THz spectrum (solid), together with the estimated THz spectrum (dashed), where the influence of multi-reflection in detection crystal is removed semi-empirically.

Fig. 3. The THz characteristics of the strong-field 1 kHz THz source based on the echelon-TPFP. (a) The THz pulse energy (circle) and efficiency (triangle) vary with pumping energy, where the points are original data and the curves are third-order polynomial fitted results. The upper left inset shows the normal incidence of the echelon mirror, and the lower right inset shows the THz focal spot. The relative humidity is ∼45% when measured. (b) The laser beam spot at the input surface of the lithium niobate crystal, which is 1.61 mm behind the imaging plane. The x axis is elongated to better show the minor segments. (c) The laser spot at the imaging plane, determined by its high contrast ratio.

Fig. 4. The THz characteristics of the high-average-power 50 kHz THz source based on the grating-TPFP with an expanded pumping laser. (a) Normalized THz temporal waveforms at 1 kHz and 50 kHz repetition rates, respectively. (b) Corresponding normalized THz spectra. (c) THz focal spot and (d) laser beam spot measured at the input surface of the lithium niobate crystal. (e) The THz power curves and (f) corresponding efficiency curves measured at different repetition rates. The points are the original data, and the curves are the third-order polynomial fitted results. The top purple axis shows the pumping power at 50 kHz repetition rate, calculated by multiplying pumping energy by the repetition rate. All the THz data in this figure are measured at a relative humidity of ∼13%.

Fig. 5. Influence of the pumping fluence and Gaussian spot profile on THz efficiency. (a) The schematic diagram and configuration of the model. The center of the pumping laser spot is 1.5 mm from the crystal edge. The angle of the crystal is 63°. (b) and (c) are the small and large input Gaussian spots for the two grating systems without/with the expander, respectively. (d) The THz energy curves and (e) conversion efficiency curves for grating systems with small/large spots and the echelon system. The points are the measured results, and the curves are the simulated results.

Fig. 6. Magnetization dynamics in CoFe thin film induced by the strong-field THz pulse. (a) Schematic diagram of THz pump-probe experiment and the polarization configuration of THz and laser pulses. (b) The normalized Zeeman torque signal. (c) The differential and normalized signal (solid) of the original signal in (b), compared with the EOS signal (dashed) measured by the GaP crystal. The vertical dashed lines show the calculated locations of the first three reflected pulses.

Fig. 7. Reflection and transmission of THz pulses in a thin film.

Fig. 8. The THz temporal waveforms. (a) The waveforms for grating-TPFP and echelon-TPFP systems. (b) The corresponding THz spectra.

Fig. 9. The THz output of the high-average-power system at the relative humidity of ∼40%. (a) The THz energy curves and (b) the THz conversion efficiency curves at the repetition rates of 1 kHz and 50 kHz, respectively. The right y axis in (a) shows the corresponding THz power at the repetition rate of 50 kHz.