In recent years, metamaterials have attracted great attention due to their unique properties^[1-5]. Thus, some actively tunable composite metamaterial devices based on a split-ring resonator (SRR) have been investigated in terahertz region ^[6-12]. These structures with better device performances are highly desired for sensing applications, especially in bio-medical region^[13].

Ionic solution, such as calcium chloride (CaCl₂), plays an essential role in many biochemical processes^[14-16]. So it is necessary to study its optical properties for practical application. However, few works has been done about it in terahertz region. Here, we firstly measured the dielectric responses of CaCl₂ solutions with different concentrations (up to 3 mol/L) for further sensing simulation.

In this work, a novel terahertz photo-excited tunable metamaterial switch is investigated. Its resonant frequencies can be modulated by variation of external light intensity. Furthermore, the gap of this structure can be equivalent to a capacitor, and LC resonance occurs as a result, which is sensitive to its surrounding dielectric environment. Therefore, this structure can also achieve a sensing application by changing the concentration of the injected ionic solution.

The schematic diagram of the proposed structure’s unit cell is shown in Fig. 1 (a). It consists of a split ring copper resonator on the top layer on a polyimide layer. The permittivity of the polyimide is set to be 3.5. The geometric parameters of the structure are shown as follows: p_x=p_y=120 μm, l_x=60 μm, l_y=90 μm, g₁=4 μm, g₂=18 μm, w₁=6 μm, w₂=12 μm, w₃=3 μm, h=6 μm. The thickness of the copper lays t₁ is 0.4 μm, while the thickness of the polyimide layer t₂ is 2 μm. We filled the two gaps on the top of the material with silicon. The configuration of testing is shown in Fig. 1 (b). When irradiated by an external pump laser on the SRR array, light-generated carriers existed in the silicon region. The structure is illuminated by terahertz wave whose electric field is along the y direction. As the power of external light increases, the carrier concentration on Si increases, and it will lead to the increase of its conductivity ^[8-9]. The silicon conductivity can reach the magnitude of 10⁵ S/m, which is close to metallic conductivity when the light power is high enough. The CST studio simulation results with various silicon conductivities are presented in Fig. 2. For the structure illuminated by terahertz wave whose electric field was perpendicular to the top two split gaps, the resonant dip located in 1.139 THz when the conductivity of the silicon was 1 S/m. With the increase of the silicon conductivity, the frequency of the resonant dip gradually shifted to 0.8 THz as the silicon conductivity reached 300000 S/m which was equivalent to the laser light with an intensity of 6111 μm/cm^{2 [17]}.The frequency of the resonant dip varied from 1.14 to 0.8 THz as the silicon conductivity increased from 1 to 300000 S/m, which is shown in Fig. 2(a). For the structure illuminated by terahertz wave whose electric field was parallel to the top two split gaps, as shown in Fig. 2(b), the two resonant dips located in 0.645 THz and 1.716 THz when the conductivity of the silicon was 1 S/m, and slightly shifted to one resonant dip in 1.256 THz as silicon conductivity varied to 300000 S/m. Figure 3 (a-e) show the electric field and surface current density distributions of the proposed structure when terahertz wave was perpendicular or parallel to the top two split gaps at the resonant frequencies. As the silicon conductivity increased and reached the magnitude of 10⁵ S/m, which was close to metallic conductivity, the silicon component became a conducting region allowing the surface current at the silicon-copper interface to bridge the split gap, and thus the two top gaps of the SRRs were short-circuited ^[8].

The dielectric responses of CaCl₂ solutions with different concentrations (up to 3 mol/L) were measured by a typical terahertz time domain spectroscopy system in the range of 0.2~1.5 THz, which are shown in Fig. 4. The real parts (ε′) and imaginary parts (ε″) of the complex permittivity can be calculated for further sensing simulation ^[18]. Figure 5 shows the simulation results of CaCl₂ with different concentration without light irradiation. The result in pure de-ionic water is simulated as a reference. As the concentration increased from 0 to 3 mol/L, the resonant dip presented red-shifted from 1.009 to 0.975 THz, which indicated that this structure can be an efficient sensor for further application. The simulation results of frequency shifts for different CaCl₂ concentrations are illustrated in Fig. 6. The growth of frequency shift shows positive correlation with the increasing of molar concentration. The sensitivity in terms of molar concentration of CaCl₂ yields 11.4 GHz per M, which indicated that this structure can be an effected sensor monitoring concentrations of ionic solution. The sensitivity can be further optimized by method such as changing the geometry of the pattern and substrate configuration.

In this work, a terahertz photo-excited tunable metamaterial sensor is investigated, and the resonant frequency of this switch can be modulated by variation of external light intensity and changing the permittivity of the surrounding material. This sensor is composed of a hybrid metal-semiconductor structure and a flexible polyimide substrate. Silicon is filled in the gaps of the structure. Simulation results reveal that the conductivity of the semiconductor component can be tuned by changing the external pump light’s power, resulting in resonant peak shift of the composite metamaterial structure. The electric field and current density distributions of this structure under different resonant frequencies are also analyzed. The physical mechanism of this device has been further discussed. Moreover, the resonant peak will be red-shift as the permittivity of CaCl₂ increases, and the sensitivity is 11.4 GHz per M. This work will contribute to qualitative and quantitative study in trace sensing in terahertz region, especially for non-destructive testing of low-density or thin-film biological samples.