Fig. 1. Schematic of the THz TDS. BS, beam splitting mirror; S, Fe/Pt film; B, external magnetic field; OAP, off-axis parabolic mirror; Si, silicon wafer; FS-EOS, free-space electric-optical sampling consisting of an electro-optic crystal (ZnTe), a quarter-wave plate (λ/4), a Glen prism (GP), and two balanced photodiodes (PD1 and PD2).

Fig. 2. (a) THz emission from a sample of Fe(7 nm)/Pt(4 nm) with laser pulse energy of 1.4 mJ under a beam spot diameter of 6 mm in a saturating magnetic field of −B=−200Oe. (b) The normalized THz amplitude spectra of Fe(7 nm)/Pt(4 nm) thin film. The central frequency of the radiation THz signal is about 0.4 THz, and the frequency range of Fe(7 nm)/Pt(4 nm) is 0.1–2 THz. Inset: the peak amplitude of the wave form varies as the intensity of the pump pulse changes. The black point is the peak-to-peak value (normalization) of the THz signal, and the red line is the linear fitting data. The intensity of THz radiation is proportional to the intensity of the pump pulse.

Fig. 3. (a) THz emission from a sample of Fe(7 nm)/Pt(4 nm) with a saturating magnetic field of +B=+200Oe. The phase of the THz pulse is inverted as the orientation of B is reversed. (b) The same sample in (a) is turned over as shown in (b) inset, where the polarity of THz emission has been inversed. That means THz emission can be reliably attributed mainly to the super-diffusive transient spin current with ISHE.

Fig. 4. (a) Electric field of the emitted THz radiation from Fe(5 nm)/Pt(2 nm) as a function of time with different external magnetic fields from −100 to +100Oe. The traces are vertically shifted with respect to each other. (b) Amplitude (red point) of the THz signal with different external magnetic fields and MOKE signal of Fe(5 nm)/Pt(2 nm) (blue point).