Fig. 1. Schematic illustration of an electrically tunable VO2 ladder device. The dynamic modulation of a “ladder” device can be attained by electrically triggering the phase change of VO2 at different currents, and holography and optical encryption can be achieved via programmable “ladder” units. An optical image of the fabricated ladder chip is shown on the left.

Fig. 2. Simulation and experimental demonstration of an electrically controlled “ladder” THz modulator. (a) Schematic diagram of the “ladder” design metasurface with the following parameters: P=102μm, L=96μm, l=74μm, w1=8μm, and w2=6μm. (b) Optical microscopy image of the fabricated sample. (c) Image of the fabricated “ladder” design PCB chip. (d) Measured THz transmission spectra at different bias currents. (e) Simulated and (f) fitted THz transmission spectra at different VO2 conductivities. (g) Modulation depth colour map under different currents. (h) Electric field distributions of the “ladder” metasurface at 0.51 THz with different VO2 conductivities.

Fig. 3. Electrically controlled dynamic “ladder” design for holography and encryption. (a) Schematic of the static and dynamic pixels. (b) Transmission spectra of dynamic pixels in insulating and metallic states and static pixels. (c) Optical images of the fabricated holographic “ladder” design metasurface and enlarged images of static and dynamic pixels. (d) Design schematic of the “ladder” design hologram, and its flow chart includes the following processes. I: The Rayleigh-Sommerfeld (RS) formula is used to calculate the hologram G(m,n) of the target character “C”. II: The complex amplitude G(m,n) is binarized into the binary hologram A(m,n) according to Eq. (2). III: The static or dynamic pixel is matched according to the obtained binary hologram. (e) Simulation results at different VO2 states. (f) Experimental results at 0 and 360 mA, consistent with the simulation results. (g) Optical path schematic and corresponding experimental equipment for the THz focal plane imaging system.

Fig. 4. Broadband imaging and electrical modulation process. (a) Measured holographic images at 360 mA and encrypted images at 0 mA at frequencies of 0.47, 0.52, 0.58, 0.64, and 0.7 THz. (b) Measured electrically controlled holographic images at 0.51 THz under currents of 235, 238, 245, 250, and 300 mA.

Fig. 5. Thermodynamics study of electrically heated metasurface. (a) Periodically applied current (upper panel) and the measured transmittance (bottom panel). (b) Zoomed-in single period of applied current (upper panel) and the measured transmittance (bottom panel). (c) Temperature plot along the line passing through the center of the metasurface in y-direction at 3.5 s (blue curve), 4 s (green curve), and 4.5 s (black curve). (d) Temperature evolution of the central point (blue curve) and the averaged value of the metasurface (green curve). (e), (f) Temperature distribution in yz plane (e) and xy plane (f) at t=3.5s.