Strictly speaking, the absorption of a periodic structure is given by (1).

However, my guess is that people might use (2) if they happen to have an opaque substrate like silicon for visible light, because in this case T=0.

You can calculate and draw the absorption curve by Lumerical (FDTD), or calculate by equation (1). Transmission and Reflection can calculate by Fresnel equations. If Transmission equals zero, you can use equation (2).

Equation 1

The intensity IO of the beam incident to the surface of the solid medium must equal the sum of the intensities of the transmitted, absorbed, and reflected beams, denoted as IT, IA, and IR respectively, or

IO = IT + IA + IR

T: Transmissivity A = IA /I0 Absorptivity R = IR /I0 Reflectivity

T~1 : Transparent T~0 : Opaque

T + A + R = 1

The intensity IO of the beam incident to the surface of the solid medium must equal the sum of the intensities of the transmitted, absorbed, and reflected beams, denoted as IT, IA, and IR respectively, or

IO = IT + IA + IR

T: Transmissivity A = IA /I0 Absorptivity R = IR /I0 Reflectivity

T~1 : Transparent T~0 : Opaque

T + A + R = 1

two common ways to calculate absorption in metamaterial absorbers are

1- using numerical analysis, in this case, the common formula is using A=1-R-T, the R and T are obtained using numerical results

2- using circuit modeling and transmission line and create impedance matching condition. both R and T can be calculated using this model. for example, here, R=|(Zin-Z0)/(Zin+Z0)|^2 in which Z0 is air impedance and Zin is input impedance can be calculated using the circuit model.

The absorptance is obtained through the equation

A(w) = 1 − T(w) − R(w), where T(w) and R(w) represent transmittance and reflectance, respectively.

Since, the perfect absorber can be discerned by reducing reflectance and transmittance to zero.

When the thickness of the bottom continuous (substrate) much than skin depth of electromagnetic waves, The transmittance can be effectively eliminated and the absorptance equation is A(w) = 1 − R(w).

Dear Son

This formula is true

But inside the skin, must be taken into consideration refractive index of blood and scattering inside the skin layer

Hi dear, My question is what are the formulas of equivalent L,C of multiple ring SRR split ring resonator? I need the formulas to use it in MATLAB and then design antenna in CST

When you design a absorber with a reflector layer (ground)at end(such as gold, copper, and etc) you don’t have the transmittion coefficient becouse the incident wave can’t propagation into metal. In this situation, the formula (1) is utilized. In other situation (when you don’t have reflector layer (metal)) usually the formula (2) is used.