# Metal-Organic Hybrid Metamaterials for Spectral-Band Selective Active Terahertz Modulators
## 2 Methods  
As shown in Figure 1a, the metamaterial units, which consist of a planar array of SRR elements, were fabricated on a Si substrate with a high resistivity of up to 1.0 \(\times\) 10\({}^{5}\)\(\Omega\cdot\)cm by a conventional photolithographic technique. The unit elements were arranged with periods of \(P_{x}\) = 100 \(\upmu\)m and \(P_{y}\) = 100 \(\upmu\)m. A single unit cell of the metamaterial structure, shown in Figure 1b, had a 100-nm-thick metallic layer (2 nm Cr adhesive layer and \(\sim\)100 nm Au film), 4 \(\upmu\)m line width, outer dimensions of \(d_{x}\) = \(d_{y}\) = 74 \(\upmu\)m, and split gap spacing of g = 4 \(\upmu\)m. The SRR was a suitable structure to independently control the two resonance modes, fundamental LC resonance and dipolar resonance, by adjusting the polarization of incident THz waves.  
As shown in Figure 1c, an organic thin layer of PCBM molecules (\(>\)99.5%), purchased from Sigma-Aldrich (product number: 684449), was deposited on the metamaterial/Si hybrid structure using a spin coating method. By controlling the organic concentration and the rotation speed range of a spin coater, a PCBM thin layer with a thickness of approximately 200 nm was fabricated. To improve the modulation efficiency, the thermal annealing process of the fabricated samples was carried out on a hot plate at temperatures above 200 \({}^{\circ}\)C for 1 h. The dynamics of photo-excited carriers, especially at the interface between the PCBM and Si substrate, considerably depended on the relationship between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels shown in Figure 1d. According to the energy level diagram, the photo-excited electrons in the Si substrate were injected into the PCBM layer, and the photo-excited holes diffused within the Si layer.  
The transmission through the samples was measured by a THz time-domain spectroscopy (THz-TDS) system. Pulsed THz waves were generated by a p-type InAs crystal mainly due to an anisotropic photocurrent in the surface electric field and were detected by a photoconductive antenna method [27; 28; 29]. As shown in Figure 1e, the THz waves were incident onto the surface area of the samples restricted by a 3-mm-diameter pinhole, while the optical beam from a cw diode laser with a center wavelength of 785 nm was incident at an angle of 45\({}^{\circ}\) on the same area. The transmitted time-domain signals were measured under the conditions of both with and without photoexcitation, and the transmission amplitude spectra were obtained by carrying out a fast Fourier-transform method. Here, the THz transmission measurements were conducted by changing the power of the incident optical beam.  
Numerical simulations were performed by finite element modeling using the COMSOL Multiphysics software. Accurate simulation results could be obtained based on an incident plane wave of THz radiation using the COMSOL wave optics module. Owing to the periodic nature of the metamaterial elements, relatively easier simulations were possible because they could be performed in a unit cell, shown in Figure 1b, with periodic boundary conditions. To quantitatively understand the carrier dynamics in the PCBM/metamaterial/Si hybrid structures, the simulations, which were conducted to efficiently match the experimental results, were performed for different values of electric conductivity of the PCBM and Si layers.