Fig. 1. Experimental setup of the THz time-domain spectroscopy experiment using THz-slicing technology in conjunction with superradiant THz sources. The dashed boxes indicate the THz-slicing module and the different ultrafast experimental schemes: far-field THz-TDS in (a), time-domain THz HHG in (b) and time-domain THz s-SNOM in (c). In the THz-slicing module, a glass rod is used to stretch the laser pulse. The CDR and the stretched pulse interact in the ZnTe crystal, while the half-wave plate (HWP) and the GT prisms are set to a cross-polarized arrangement. The synchronously sliced pulse is directed to the ultrafast experiments as probe pulse. (a) In the far-field THz-TDS scheme, the multicycle THz pulse is focused on a 2 mm thick ZnTe crystal and is characterized by EO sampling. (b) In the time-domain THz HHG scheme, the multicycle THz pulse is focused onto the sample, with two BP filters introduced in front and behind the sample. The generated THz higher harmonics are then again characterized by EO sampling in a 2 mm thick ZnTe crystal. (c) In the time-domain THz s-SNOM scheme, the multicycle THz pulse is focused onto the sample area underneath the oscillating cantilever tip of the near-field microscope and the scattered THz radiation is measured by EO sampling in a 2 mm thick ZnTe crystal. The weak near-field contribution is distinguished from the huge far-field background by demodulation techniques.

Fig. 2. (a) Autocorrelation function of the stretched pulse (solid blue curve), sliced pulse (solid black curve), and a Gaussian fit (dashed red curve). (b) Part of the THz waveforms of undulator pulses measured for different levels of timing jitter adjusted by the setting of the Synchrolock device. Inset: The zoom-in view of the red rectangular area at one zero-crossing position. (c) Full THz waveform and (d) spectrum of undulator pulses at 300 GHz. Measurements were performed through a band pass filter centered around 300 GHz.

Fig. 3. Normalized spectra of the 3rd harmonic signal under different pump field strength with a 900 GHz bandpass filter placed right behind the sample. The maximum pump field strength (shown as 100% in Fig. 3) corresponds to 13 kV/cm. Inset: THG field strength plotted as a function of the fundamental pump field strength. A linear fit with a slope of 1.67 can be deduced from the logarithmic scale plot.

Fig. 4. THz s-SNOM near-field signals from an Au sample. (a) THz waveforms demodulated at Ω, 2Ω, and 3Ω, and (b) their corresponding power spectra. The inset in (b) shows a sketch of the THz s-SNOM experiment on the 200 nm thick patterned Au structures on a silicon substrate.

Fig. 5. THz s-SNOM of a topological insulator (TI) n-Bi₂Se₃ flake deposited onto a high-resistivity float-zone silicon substrate in the time-domain. (a) Schematic of the THz s-SNOM set-up; (b) topography image of the studied 190 nm thick n-Bi₂Se₃ flake. (c) Time-domain near-field responses recorded on both the Si substrate (blue) and the TI flake (red) when applying the THz-slicing technique. Two dashed lines indicate the time delay of t₁ = 6.0 ps and t₂=12.6 ps, respectively; (d,e) near-field images NF_2Ω(x, y, t) recorded for t equal (d) 6.0 ps and (e) 12.6 ps, respectively.