Fig. 1. Schematic diagram of the overall effect of Huygens metasurface. All metal layers are 50 nm thick gold, fabricated on both sides of 100 μm SiC substrates. 2DEG with a thickness of 3 nm is provided by AlGaN/GaN HEMT, which is composed of a GaN layer with a dielectric constant of 8.9 and a thickness of 1.5 μm, and an AlGaN layer of 25 nm. The carrier concentration at room temperature is 1.125  × 1013  cm−2, electron mobility 2248  cm2/(V·s), and square resistance 250.3  Ω/□. The applied gate voltage controls the carrier concentration of 2DEG and flexibly handles the current distribution of the top and bottom units on the metasurface.

Fig. 2. Electromagnetic properties of the Huygens metasurface. (a)–(c) Magnetic field and surface current distribution at (a) 333 GHz with 1014  cm−2 carrier densities, (b) 333 GHz with 2  ×1010  cm−2 carrier densities, and (c) 425 GHz with 2  ×1010  cm−2 carrier densities. (d) Transmission spectra at different carrier concentrations. (e) Phase difference of the top and bottom current. (f) Normalized intensity of electric and magnetic fields at 333 GHz.

Fig. 3. Fabricated Huygens metasurface and its experimental results of spectrum characteristics. (a) Photograph of the top and bottom units. (b) Photograph of a portion of the fabricated Huygens metasurface. (c) Image of the assembled Huygens metasurface. (d) Frequency-dependent transmission curves at different gate voltages. (e) Modulation efficiency of Huygens metasurface.

Fig. 4. Measurement results of THz Huygens metasurface modulation speed. (a), (b) Demodulated THz waveforms received by the detector. (c) Demodulated waveforms at different THz carrier frequencies with 1 GHz modulation speed. (d) Demodulated waveform amplitudes at different carrier frequencies with distinct output powers.

Fig. 5. Schematic of the device we designed. (a) Composition of the HEMT region of the top unit. (b) Model diagram of the 4  ×  1 arrays. (c) SRR in the bottom unit. (d) Cross section and dimension label of the HEMT in the yoz-plane and size label of the HEMT.

Fig. 6. ZMS and YES of the THz modulator vary with incident wave frequency at different carrier concentrations. (a) Calculation results of the real part of ZMS. (b) Calculation results of the imaginary part of ZMS. (c) Calculation results of the real part of YES. (d) Calculation results of the imaginary part of YES.

Fig. 7. THz-TDS experimental system. (a) Schematic and (b) photo of the test system.

Fig. 8. Dynamic experimental system. (a) Schematic of the system. (b) Photo of the system. (c), (d) Partial detail drawings of the system.