## 1 Introduction  
Metasurfaces (MTS) can act today as a new generation of frequency-selective surface (FSS) which have significantly contributed to advanced modern communication systems enhancements. Applications of MTSs at microwaves encompass those of FSS, namely, phase shifting elements for reflector antennas [1; 2; 3; 4; 5; 6], filtering interfaces in radomes to reduce RCS levels in military platforms [7], as well as new generation of antennas [8; 9; 10]. Today, a complementary frequency and phase agility in MTS design is expected in next future industrial systems, as in 5G communications where a dynamic frequency allocation at end user level is required. In this area, main research works have been largely devoted to electronic solutions suggestion, such as by local electrical switching [11] from planar active devices (diodes) networks, or by substrate material permittivity change under specific electrical bias, using phase change materials.  
Optical control becomes a disruptive alternative providing many advantages such as ultrafast activation speed, and EMI-protected external control. In this context, for the first time, this paper describes the development of a novel accurate free space experimental set-up dedicated to microwave reflection/transmission Fresnel coefficients measurements. This environment also incorporates optical illumination system for frequency response change demonstration of microwave FSS structures under light excitation.  
Our approach was born from research works on exploitation and exaltation of photoconductive properties of semiconducting materials, in microwave switching devices when submitted to photons flux bringing an energy by absorption superior to their band gap energy \(E_{g}\).  
In such materials, under light excitation, at first order of approximation, photogenerated free carriers either electrons or holes in the illuminated area change locally material conductivity by a value of \(\Delta\sigma\). As material absorption coefficient determining light penetration depth value \(\alpha\) in the material is dependent to the wavelength \(\lambda\) of the incident optical signal, coupled parameters such as optical wavelength and material thickness must be carefully chosen for photoconductive effect existence [12; 13; 14; 15; 16]. In our case, NIR light appears as a suitable wavelength illumination for light/matter interaction exaltation in silicon material, which also acts as standard substrate material for integrated electronic devices manufacturing.  
This publication presents experimental microwave response results of optically-controlled FSS structures Figure 1 assumed by planar capacitive square metallic patterns supported by a HR silicon substrate whose relative complex permittivity expression following [PERSON] model (1) is a function of the angular frequency \(\omega\), \(\sigma\) is the substrate conductivity, \(\epsilon_{\mathrm{inf}}\) being the high frequency substrate permittivity, and \(\epsilon_{0}\) the vacuum permittivity.  
\[\epsilon_{r}=\epsilon_{\mathrm{inf}}+i\frac{\sigma_{0}+\Delta\sigma}{\omega \epsilon_{0}} \tag{1}\]  
Through light/matter interaction, the photon energy absorption produces a material conductance change \(\Delta\sigma\) (2), which is strongly dependent on carriers dynamics such as lifetime \(\tau\) and mobility \(\mu_{n,p}\), in association with the incident photon flux \(\Phi(t)\), quantum efficiency \(\eta\) and effective volume of the light/matter interaction area \(wA\)[17].  
\[\Delta\sigma=q\frac{\eta\tau(\mu_{n}+\mu_{p})\Phi}{wA} \tag{2}\]