## 4 Results and Discussion  
In this paper, the electromagnetic behavior of quasi-periodic selective (multichannel band gaps\(\backslash\)multi-Bragg peaks) photonic structures is theoretically studied at THz frequencies. Our objective is to tune the position and the number of the resonance peaks, as well the position and the width of the PBGs. These tunable properties open the way to applying the proposed structures as filters for the communication field, sensors or micro-cavities for LASER equipment. The photonic structures (Figure 1) are made of a GaSe semiconductor material (with an indirect band gap) and sinusoidal plasma material (with permittivity \(\varepsilon_{p}\)). If this study is experimentally validated in the future, we intend to insert two transparent polymer film materials at the top and bottom of the structure (two red lines in Figure 1), which are intended to protect the plasma material and prevent it from escaping and spreading outside the structure. This polymer material should be transparent, and we assume that that has no effect on the propagation of waves. The most transparent and commercial polymers are Polycarbonate, PMMA or Acrylic, Polyethylene Terephthalate (PET), Amorphous Copolymer (PETG), Polyvinyl Chloride (PVC), Liquid Silicone Rubber (LSR), Cyclic Olein Copolymers (COC), Polyethylene (PE), Ionomer Resin, Transparent Polypropylene (PP), Fluorinated Ethylene Propylene (FEP), Styrene Methyl Methacrylate (SMMA), Styrene Acrylonitrile Resin (SAN), Polystyrene (General Purpose--GPPS) and MABS (Transparent ABS) materials. The proposed structure is designed to confine the light at different resonant frequencies. The PC lattice is built according to the Fibonacci sequence, where the multilayered stacks are recursively generated by the substitution rule \(\sigma_{\text{F}}(\text{H},\text{L}):\text{H}\rightarrow\text{HL}\,,\text{L} \rightarrow\text{H}\) ([21; 25]). According to the recursive substitution rule above, the fractal Fibonacci structure \(\text{S}_{\text{k}+1}\) can be written as \(S_{l+1}=S_{l}S_{l-1}\), where \(l\) is the generation number of the Fibonacci sequence. In the input of the PC (the left side of the structure), we consider an incident plane wave coming at an angle \(\theta\) from the z direction. The refractive indices of the boundary limits are equal to 1 (Air). The plasma (layers L) and GaSe (layers H) materials have refractive indices n\({}_{\rm P}\) and n\({}_{\rm H}\) = 2.43, respectively. The plasma-dependent refractive index is given in Equation (10). The electromagnetic properties (such as reflectance and electric field) of the tunable photonic band gaps (PBGs) were determined and simulated using the transfer matrix method (TMM). In this part, the effect of the iteration number (\(l\)) of the Fibonacci sequence on the reflectance spectrum is studied. Figure 2 shows the reflectance spectra for different iteration numbers (\(l\)) of the Fibonacci (F(1,1)) sequence. In this step, the thickness of the layers is still not deformed (\(h\) = 0), and the incidence angle of wave \(\theta\) varies from 0 to 90 degrees. The classic Fibonacci sequence F (1, 1) exhibits a multichannel sub-band gap at different values of the iteration number. These relevant gaps are due to the Bragg scattering. In Figure 2, we notice that the bandwidth of each PBG decreases with the increase in the Fibonacci order (\(l\)), whereas the number of gaps increases according to this order. This is due to the confinement of light within the stacks introduced by the increase in the number of cavities (local defects) when increasing the number of layers (due to the increase in the Fibonacci sequence) of the whole structure. Therefore, the auto-similarity of defects increases with the Fibonacci order. Here, and compared with the conventional periodic structures, it is clear that the increase in the fractal order of the quasi-periodic structure has a significant effect on the bulk material properties and the transmission of waves.