Fig. 1. THz frequency modulation based on photo-induced planar pattern^[42]. (a) Formation of planar light induced line grating patterns on the surface of silicon prisms; (b) optical microscope image; (c) modulation performance as a function of the period of the pattern; (d) dependence of spectral minimum frequency on structural periodicity

Fig. 2. Demonstration of several types of modulations with various photo-excited patterns^[45]. (a) Experimental setup for THz modulation. (b) light excitation modes of different patterns

Fig. 3. THz modulation of PI-FZP^[51]. (a) Experimental setup; (b)‒(e) measured two-dimensional radiation intensities showing the THz beam profile for different PI-FZP patterns

Fig. 4. Generation of THz special beams. (a) Vortex beam^[52]; (b) vector beam^[53]

Fig. 5. THz imaging based a single-pixel detector with optically controlled STM^[63]. (a) Typical schematic of imaging system; (b) THz imaging result

Fig. 6. Theoretical investigation on all-optical Si-based STM^[117]. (a) Theoretical model; (b) modulation depth variation curve with Si thickness; (c) modulation depth variation curve with pump laser wavelength; (d) modulation depth variation curve with laser reflectivity; (e) modulation depth variation curve with carrier lifetime

Fig. 7. All-optical THz modulator based on SiNT-Si^[121]. (a) Scanning electron microscope (SEM) results of SiNT; (b) test results of laser reflectivity; (c) schematic of all-optical THz modulator based on SiNT-Si; (d) THz time-domain spectra; (d) modulation depth variation curves with laser power

Fig. 8. All-optical THz modulator based on MPA-Si^[122]. (a)(b) SEM results and schematic of all-optical THz modulator based on MPA-Si; (c) results of laser reflection measurement; (d) comparison results of modulation depth obtained with different laser wavelengths

Fig. 9. All-optical THz modulator based on TPA-Si^[123]. (a) Preparation procedure; (b) SEM result of TPA-Si surface; (c) SEM result of TPA-Si cross section; (d) measured results of effective lifetime of carriers; (e) results of laser reflection measurement; (f) schematic of all-optical THz modulator based on TPA-Si

Fig. 10. All-optical THz modulator based on S-passivated GaAs^[125]. (a) Structure diagram; (b) results of dynamic response characterization; (c) performance comparison of all-optical THz modulators