Fig. 1. Structure chart of ultrafast all-optical terahertz modulation. (a) Schematic illustration of hybrid structure combining Ge film with inverse-designed metasurface at the pump light of 800 nm for terahertz modulation. (b) Processed inverse-designed metasurface structure without Ge film covering on the face. (c) Processed inverse-designed metasurface structure covering 200 nm thick Ge film on the face. The scale bar in the pictures is 50 μm.

Fig. 2. Sketched scheme of inverse design of ultrafast terahertz modulator based on EIT effect utilizing the PSO algorithm and finite-difference time-domain method. (a) General design cycle of PSO algorithm. At this optimization procedure, we set the target function by using coupled harmonic oscillator model. The number of particles is set as 10, and the number of iterations is 100. (b) The fitness function tends to convergence with the increasing iterations. After 60 iterations, the fitness is convergent, and the result is what we want. (c) The transmittance of the terahertz wave at the last design is described in the red line, and the black line shows the target transmittance. All terahertz transmittance is obtained through finite-difference time-domain method.

Fig. 3. Experimentally light-activated Ge-controlled terahertz EIT modulation for varying pump fluences and simulation results of EIT resonance modulation with different conductivities of Ge film. (a) Experimental modulation of terahertz EIT amplitudes for various pump fluences ranging from 0 to 2.2  mJ/cm2. (b) The experimentally normalized resonance amplitude, defined by the difference between transmission at dip and transmission at peak, is changed following various pump fluences. (c) Simulated modulation of terahertz EIT amplitudes for different conductivities of Ge film ranging from 10 to 1000 S/m. (d) The simulated normalized resonance amplitude is changed following different conductivities.

Fig. 4. Temporal evolution dynamics of ultrafast terahertz modulation behaviors under the pump fluence 2.2  mJ/cm2. (a) Contour map of experimental terahertz transmission as a function of pump-probe time delay. (b) Top: Experimental terahertz transmission spectra at different pump-probe time delay from −5 to 0 ps. The state of EIT resonance switches from on to off. Bottom: Experimental terahertz transmission spectra at different pump probe time delay from 1 to 5 ps. The state of EIT resonance switches from off to on.

Fig. 5. Simulated near-field distributions as a function of Ge film conductivity ranging from 10 to 1000 S/m. E-field magnitudes are obtained at the position 1 μm above the metasurface.

Fig. 6. (a) Schematic illustration of one-unit inverse-designed metasurface combined with 200 nm thick Ge film (the red part), including a cut wire and numerous crosses around it. The material of inverse-designed metasurface is gold with thickness of 200 nm (the golden part). The whole unit structure is mirror symmetric. The periods of one-unit metasurface are px=110  μm in the x direction and py=180  μm in the y direction. (b) Sketch of a half of one-unit metasurface. Each of cross consists of two orthogonal rectangles with length cy=5  μm and width cx=3  μm. The parameters of the cut wire in this sketch are w=10  μm and L=63  μm.

Fig. 7. Experimentally measured group delay spectra of hybrid structure combining a reverse-designed metasurface with Ge film as a function of pump fluence ranging from 0 to 2.2  mJ/cm2.

Fig. 8. Negative differential transmission spectra of pure 200 nm thick Ge film are experimentally measured at the maximal amplitude of terahertz time-domain pulse signal as a function of optical-pump terahertz time delay. Experimental data points are fitted with equation in the main text and the delay time constants τ of pure Ge film at various pump fluences are extracted. The delay time constant is found to increase with the pump fluence increasing.

Fig. 9. Modulations of the EIT resonance amplitude as a function of pump-probe time delay evaluated from the transmission spectra of Fig. 4 in the main text.