Fig. 1. Device schematics and working principle. (a) Schematic of the fabrication of the galvanic cell device. (b) The crystal structure of FLG before and after Li intercalation. LiC₁₂ periodically repeats the layer along the z-axis with the stacking sequence of AAαAAα, with A and α being the fictitious graphene and Li-ion layer, respectively. The direction of electron transfer from Li atoms to graphene layers is described by the deep blue arrows. Black balls represent carbon atoms, and light gray balls represent Li-ions. (c) Schematic of the graphene band structure before and after Li intercalation.

Fig. 2. (a) Raman spectra of FLG before (black line) and after (red line) Li intercalation. (b) Four-probe I versus V curves for FLG before (black line) and after (red line) Li intercalation.

Fig. 3. Demonstration of visible optical modulation of FLG with Li intercalation. (a) and (b) are visible images of FLG and LiGIC, respectively. The scale bar is 5 mm. The FLG and Li metal are marked with blue and red dotted boxes, respectively. (c) Schematic interband electronic transitions of the Li-intercalated graphene for visible absorption. (d) Optical transmittance as a function of wavelength in the visible and near-infrared range for FLG (black line) and LiGIC (red line). (e) Optical reflectivity as a function of wavelength in the visible and near-infrared range for FLG (black line) and LiGIC (red line). (f) Optical absorptivity as a function of wavelength in the visible and near-infrared range for FLG (black line) and LiGIC (red line).

Fig. 4. Demonstration of infrared optical modulation of FLG with Li intercalation. (a) Cross-sectional illustration of infrared transmission characterization by an infrared camera. The optical device is placed on a hot plate. (b) Thermal images of FLG and LiGIC, and their positions with the blue dotted boxes. The hot temperatures are set to 30°C and 40°C. The scale bar is 5 mm. (c) Schematic intraband and interband electronic transitions of Li-intercalated graphene for mid-infrared absorption. (d) Optical absorptivity as a function of wavelength in the infrared range for BaF₂ (blue line), FLG (black line), and LiGIC (red line). (e) Optical transmittance as a function of wavelength in the infrared range for BaF₂ (blue line), FLG (black line), and LiGIC (red line).

Fig. 5. Demonstration of terahertz optical modulation of FLG with Li intercalation. (a) Terahertz time-domain signals of Z-cut (blue line), FLG (black line), and LiGIC (red line). (b) The spectra of the transmitted THz signals of FLG (black line) and LiGIC (red line) obtained through Fourier transform time-domain signals and normalization with the reference signal. The inset shows schematic intraband electronic transitions of the Li-intercalated graphene for THz absorption.