## 5 Design and Characterization of Optically-Controlled Microwave FSS  
Coplanar slot based MTS structures have been designed by \(550\times 550\)\(\upmu\)m\({}^{2}\) square metallic patches with a 100 \(\upmu\)m gap with surrounding planar ground plane in Figure 1. Metallic parts are fabricated by optical lithography of a 2 \(\upmu\)m-thick layer of gold deposited on a 250 \(\upmu\)m thick HR Si substrate after a SiN dielectric layer deposition and etching at edges of conductive FSS parts.  
The beam spot cross section of collective optical illumination of 2.5 cm\({}^{2}\) at a wavelength of 805 nm and a maximum incident power of 10 W is applied at the top face of the MUT. In dark environment, MTS-based MUT is expected to demonstrate a microwave bandpass filtering behaviour centred at a frequency of 46 GHz. Reflection coefficient simulations have been achieved by electromagnetic (EM) full-wave analysis (Ansys-HFSS software), as shown in Figure 8 where conduction losses of the HR-Silicon substrate play a significant role in the frequency response.  
Under optical illumination, magnitude and phase values of \(\Gamma\) obtained from \(S_{21}\) measurements reveal impedance mismatching response of MTS structures operated by light absorption through photoconductive effect operation occurring in the substrate opened ring area formed by the gap between the metallic patches and surrounding ground plane.  
As reported in Figure 8, dual magnitude decrease and null-phase shifting of the reflection coefficient \(\Gamma\) indicate the progressive device microwave response transition from a FSS to short-circuit type in the optical power range of 0 to 7 W at an optical wavelength of \(\lambda=805\) nm, directly attributed to a surface impedance \(Z_{s}\) modification of FSS structure by optical control. Experimental results have been then validated by a complementary full-wave analysis (Ansys-HFSS software) integrating the photogenerated plasma ring layer with experimental complex data obtained in Figure 1. Free carriers presence during light excitation affect the magnitude of \(\Gamma\) through its real part, and the resonance frequency shift from its imaginary part as shown in Figure 8. Resistive losses in silicon are responsible of the detected frequency notch as we consider a perfect dielectric substrate for dark environment representation in EM simulations.  
FSS surface impedance \(Z_{s}\) is then directly extracted from input impedance \(Z_{in}=Z_{D}.Z_{s}/\left(Z_{D}+Z_{s}\right)\) with \(Z_{D}\) modelling the impedance of the thin SiO2 dielectric layer (2 \(\upmu\)m) covering the HR-Silicon substrate (\(\varepsilon_{r}=11.9\) and thickness of 250 \(\upmu\)m) inducing a resonant frequency shift between \(\Gamma\) and \(Z_{s}\).  
Figure 8: Measured (MD) and Simulated (SD) magnitude \(\Gamma(dB)\) and phase \(\angle\Gamma(rad)\) values of reflection coefficient under different optical illumination power from FSS structures on HR Silicon substrate with a thickness of 250 \(\upmu\)m.  
As reported in Figure 9, when increasing optical power, plasma conductivity enhancement \(\Delta\sigma\) tends to shunt FSS structures, where a decrease of the real part of \(Z_{s}\) as well as a low slope of the imaginary part of the surface impedance \(Z_{s}\) respectively is measured. Identical frequency-dependent behaviour of \(Z_{s}\) is observed through EM simulations by changing plasma conductivity value \(\Delta\sigma\) in the gap region.