Fig. 1. Optical path diagram of ultrafast THz s-SNOM system

Fig. 2. Schematic of the principle of spintronic THz emission at the nanoscale

Fig. 3. Modulation and demodulation to achieve THz transmitter signal measurements.（a）Schematic of distribution of the scattered electric field signal at the tip of the needle in the ultrafast THz s-SNOM system；（b）dependence of the distance between the tip and the sample（d）on time（t）；（c）dependence of the far-field and near-field scattering signals on the distance between the tip and the sample（d）；（d）higher-order signal distributions of far-field scattered signals and near-field scattered signals

Fig. 4. Spintronic THz emission second-order signals based on THz s-SNOM. (a) THz time-domain waveforms with and without nitrogen environment; (b) corresponding frequency-spectral distribution

Fig. 5. Higher-order THz emission signals. (a) THz time-domain waveform signals from 2nd to 5th order; (b) dependence of the peak-to-peak radiation value on the change in order; (c) corresponding frequency-spectral distribution

Fig. 6. Pump power dependence. (a) THz time-domain waveforms at different pump powers; (b) dependence of time-domain waveform peak-to-peak value on power

Fig. 7. Surface ablation characterisation of magnetic nanofilms. (a) Mechanical diagram of atomic force microscope; (b) static THz time-domain spectral scattering scanning imaging map

Fig. 8. Comparison of second-order signal amplitude and frequency spectra of THz emission based on ultrafast THz s-SNOM. (a) W/CoFeB/Pt; (b) InAs; (c) GaAs