# Metal-Organic Hybrid Metamaterials for Spectral-Band Selective Active Terahertz Modulators
## 3 Results and Discussion  
Figure 2a shows the simulation results of the transmission amplitude spectrum of the metamaterials using the COMSOL Multiphysics software. The simulations show two obvious transmission dips located at the frequencies of 0.35 THz and 0.85 THz. To obtain the simulation results, the electric-conductivity values that corresponded well to the experimental results, shown in Figure 2b, were selected. The transmission amplitude spectra of the PCBM/metamaterial/Si hybrid structure, as shown in Figure 2b, were measured by varying the laser power of the optical beam for photoexcitation. The polarization of the incident THz waves was parallel to the metal lines present in the center of the unit structure. Similar to the simulations, the experimental results also showed two obvious transmission dips, located at the frequencies of 0.35 THz and 0.85 THz, with the exception that the dip values at both the frequencies were slightly different. Figure 1: **(a)** Microscopic image of the sample with PCBM/metamaterial/Si hybrid structure. **(b)** Geometry and dimensions of an individual unit cell. Yellow and dark-green parts are metamaterial and Si substrate, respectively. **(c)** Schematic side view in section through the X–Z plane. **(d)** An energy-level diagram showing the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the PCBM/Si hybrid structure. **(e)** Experimental configuration for THz transmission measurements on a sample. As shown in Figure 2a,b, the transmission rapidly decreased as the laser power of the optical beam increased over the entire THz-frequency ranges. As the laser power of the optical beam for photoexcitation increased, the concentrations of both photo-excited electrons and holes increased, and subsequently, the electric conductivity of the silicon substrate and the PCBM thin layer increased steadily. This phenomenon was caused by efficient charge separation owing to the photo-induced electron transfer from the excited states of the Si substrate into the PCBM layer, changing the tri-layer hybrid structures from insulating to metallic [13]. The measured transmission spectra showed two obvious resonant dips located at 0.35 THz and 0.85 THz, as shown in Figure 2b. According to the electric-field distribution, as shown in the left inset of Figure 2b, the transmission dip located at 0.35 THz may have been due to a fundamental inductive-capacitive (LC) resonance mode, since the electric field was concentrated in the small volume near the gaps at the center of the metamaterial units, and the circular current flow appeared on the metallic ring (not shown here). In addition, the second-order LC resonance mode, which is similar to the horizontal electric quadrupole mode, appeared at a frequency of 0.85 THz. Furthermore, the transmission of the spectral band located between the two resonance frequencies could be manipulated by varying the amount of photoexcitation. In principle, the range of spectral bands to be transmitted can be engineered by manipulating the structures and dimensions of metamaterials. Figure 3 shows the intensity values obtained from the simulations (Figure 3a) and the experimental results (Figure 3b) at the two resonant frequencies of \(f_{1}\) = 0.35 THz and \(f_{2}\) = 0.85 THz and, in addition, at the peak frequency (\(f_{3}\) = 0.47 THz) of the spectral band located between the two resonant frequencies. Unlike the two frequencies at resonance, the peak frequency of the spectral band was dramatically varied with increasing the laser power of optical beam. This means that the hybrid structures of organic/metamaterial/semiconductor could be a promising structure for realizing actively controllable band-pass filters or spectral-band engineering. Figure 2: (**a**) Simulation results of the transmission amplitude for the tri-layer sample.