Figure 1 illustrates the structure of proposed THz reflecting metasurface imposed on a gold layered polydimethylsiloxane dielectric material (εr = 2.35) with the unit cell dimension of 100×100×10 um3. The reflecting structure is designed with three phases to attain the desired characteristics.
In the first phase, a conventional slotted square loop element is constructed as shown in Fig. 2(a) for 0.82 THz. The initial design is simulated with the finite element method, and its essential characteristics are analysed using an equivalent circuit model [11] as displayed in Fig. 2(b).
The equivalent circuit model for slotted square loop is developed using (1)-(7) [12],
$${L}_{1}=\frac{{X}_{L11}}{{Z}_{0}}=\text{cos}\theta F (p,g,,\theta )$$ 1
$$U=\frac{{X}_{L21}}{{Z}_{0}}=\frac{k}{p}\text{cos}\theta F (p,l,,\theta )$$ 3
$${L}_{2}=\frac{{X}_{L22}}{{Z}_{0}}=U+\frac{s}{l+g}{L}_{1}$$ 4
$$L=\frac{{C}_{1}}{{Y}_{0}}=4\text{sec}\theta F (p,d,,\theta )$$ 5
$$M=\frac{{C}_{2}}{{Y}_{0}}=4\text{sec}\theta F(d-s,s,,\theta )$$ 6
$${C}_{1}=\frac{C}{{Y}_{0}}=(1.75 L+0.6 M){}_{eff}$$ 7
In (1), L1 denotes the inductance of the outer square loop with g width. L2 in (4) denotes the inductance of inward squares (with l sides) influenced by L1. The capacitance value from (7) is attained from equations (5) and (6).
To improve the bandwidth and angular stability compared with the conventional design, four slits on the corner of the inner square are introduced as presented in Fig. 3(a) and the corresponding equivalent circuit model is shown in Fig. 3(b).
In third phase, a three-legged slit is introduced to increase the angular stability and polarization insensitivity as shown in Fig. 4(a). Further, Fig. 4(b) depicts its equivalent circuit model. Table 1 presents the structural parameter values of the proposed design.
TABLE I Dimensions of the Proposed Unit Cell
Parameter
|
d
|
p
|
S
|
g
|
L
|
W
|
G
|
Dimension (um)
|
85
|
100
|
15
|
15
|
19.42
|
2
|
5
|