Terahertz (THz) waves exhibit many unique properties, including strong penetrating power, perfect application security, high signal-noise ratio and low transmission loss. The application prospect is very broad [1^–3]. However, because the interactions of THz waves with most natural materials are very weak, the development of THz devices is limited [4^–6]. A perfect absorber is one of these. When metamaterial emerged, things changed dramatically. A metamaterial is one kind of artificially synthesized electromagnetic material that consists of periodic arrays of sub-wavelength unit structures [7^–9]. The interaction of metamaterial with THz waves triggers strange responses. Since the first metamaterial absorber (MMA) with three silver slots periodically arranged on a dielectric substrate was proposed by Landy et al. [10] in 2008, a huge number of functional devices have been designed in different application fields. Furthermore, the working frequency has gradually expanded to microwave, terahertz, infrared and visible light regions [11^–14]. In addition, the resonant band has been extended from a single band to multiple bands [15^–19].

In previous studies, a large portion of MMAs rely on fixed unit cells to achieve specific resonances [20, 21]. That is to say, the only way to change the performance of the device is by adjusting structural parameters. Obviously, it is very passive and extremely inconvenient. With the rapid development of science and technology, a dynamically tunable MMA has become the research focus of scholars all over the world [22^–24]. Multiple excitation methods, including optical [25], electrical [26], thermal and mechanical stimulations [27, 28], have been used to change the properties of active functional materials. Concretely, Seren et al. [29] introduced silicon material into a resonance unit. Modulation depths of 38% and 91% for the two absorption peaks were obtained in THz range. Ji et al. [30] added silicon and germanium simultaneously to an open-loop resonator. Under light excitation, a multi-frequency tunable THz switch was formed. Zhang et al. [31] proposed a bifunctional THz absorption modulator with a hybrid VO₂ and graphene configuration. When VO₂ changes from dielectric to metal, the device switches from a six-band absorber to a broadband absorber. Liu et al. [32] achieved a THz absorber with an absorption modulation depth of 95% as phase transition happens in VO₂. Pan et al. [33] built a perfect square ring-shaped THz absorber with graphene patterns deposited on a SiO₂/Au/Si layer. By changing the Fermi level of graphene, the resonance frequency adjustment of the three band was realized. Yuan et al. [34] put silicon into a combination resonator. Under light excitation, single-frequency absorption switched to dual-frequency absorption. Zamzam et al. [35] proposed a double-layer structure with horizontal and vertical strips placed in graphene to realize a four-band switchable THz absorber. Although the work above effectively promoted the development of a terahertz absorber, there are still problems that need to be solved, such as the deficiency of high modulation depth and the lack of a multifunction dynamically tunable characteristic. Also, there are few research reports on dynamic and continuous adjustment of the absorption amplitude at a fixed resonant frequency.

Inspired by these ideas, we proposed a dual-function dynamically tunable MMA with a metal-dielectric-metal structure. Surface metal patterns are composed of a double open ring resonator and a metal strip with an opening filled with photosensitive silicon in the center. The device is polarization sensitive because of its C2 symmetry pattern. By adjusting the incident polarization angle, we can easily and conveniently modulate the absorption amplitude. By means of laser pumping on photosensitive silicon, we can control absorption frequency switching. Thus, by introducing two degrees of regulation freedom, absorption amplitude modulation and resonant frequency switching are simultaneously realized. More importantly, dynamic and continuous adjustment of the absorption amplitude is obtained at a fixed resonant frequency, and the modulation depth reaches 100%. Furthermore, we also used the device in a testing application. The results indicate that the device possesses a good sensing property, which is outstanding among the existing reports on multi-band MMA.

The configuration of the proposed MMA is depicted in Fig. 1. It is formed of three layers, a patterned Cu layer, a polyimide layer, and a fully reflective Cu layer. The patterned Cu layer consists of a double-opening resonant ring and a metal strip with an opening in the center. A fully reflective layer is also formed by Cu, and its conductivity is σ = 5.8 × 10⁷ S/m. Photosensitive silicon is used to fill the central opening of the metal strip, and the relative dielectric constant is ε_r = 11.7. Polyimide is used to fabricate an intermediate layer, and the relative dielectric constant is ε_r = 3.5. The optimized geometrical parameters of the unit cell are as follows: p = 60 μm, r = 23 μm, ω = 4 μm, g =4 μm, h =0.6 μm, t =5 μm. Simulations were carried out using frequency domain solvers in computer simulation technology (CST). The incident THz wave propagated along the z direction. The initial boundaries along the x-direction and y-direction are an electric field and magnetic field, respectively. The angle between direction of polarization electric field and x-direction was marked φ. An 800 nm laser with intensity of 600 μJ/cm² was selected as the excitation pulse for its high excitation sensitivity. Without an excitation pulse, the conductivity of Si was taken to be σ_Si =1 S/m. With an excitation pulse, σ_Si was calculated as 3 × 10⁵ S/m [36]. Changes in φ and σ_Si will affect the structure, which is the main reason for the achievement of dynamic tuning.

Figure 1. Schematic diagram of the proposed metamaterial absorber (MMA).

It is well known that the absorption A(ω) of the absorber can be expressed as A(ω) = 1 − T(ω) − R(ω), where T(ω) = |S21|² and R(ω) = |S11|² are transmission and refection respectively. As scattering parameters, S11 and S21 can be directly obtained from the numerical simulation. In the considered frequency region of 0.9 THz^–4.2 THz, the skin depth value of copper can be calculated to be between 0.032 μm and 0.069 μm. That is to say, the fully reflective layer is thick enough that transmission of incident waves is suppressed. Therefore, the overall absorption can be expressed as A(ω) = 1 − R(ω), because T(ω) is considered zero. To actually fabricate the structure, one possible way is to patch doped silicon thin film on top of the polyimide substrate with the radio-frequency plasma enhanced chemical vapor deposition method in a relative low temperature environment (120 ℃–180 ℃). Then, based on alternate use of positive and negative photoresists, we can obtain the structure with a two-step lithography process.

First, we considered the absorption spectrum without an excitation pulse. It was found that there are two separate absorption peaks in Fig. 2(a). For clarity, the peak at 1.545 THz with a corresponding absorption amplitude of 93.8% is marked peak Ⅰ, and the one at 3.21 THz with a corresponding absorption amplitude of 99.4% is marked peak Ⅱ. According to the definition, the full-width at half-maximum (FWHM) is 0.09 THz for peak Ⅰ and 0.44 THz for peak Ⅱ, respectively. Then, an 800 nm laser with an intensity of 600 μJ/cm² was used as an excitation pulse, and the absorption spectrum is given in Fig. 2(b). Obviously, two separate absorption peaks still exist, although the amplitude decreased slightly. However, the resonant frequency of the two peaks, especially peak Ⅱ, obviously changed. Peak Ⅰ experienced a slight red shift, and the resonant frequency switched from 1.545 THz to 1.525 THz. Peak Ⅱ experienced a big red shift, and the resonant frequency switched from 3.21 THz to 2.79 THz. When the energy from the excitation pulse was irradiated on the surface of photosensitive silicon, the carrier concentration significantly increased. The conductivity increase of photosensitive silicon changes the inherent structure of the proposed MMA, which is the main reason for the frequency shift.

Figure 2. Absorption spectrum of the metamaterial absorber (MMA): (a) without an excitation,(b) comparison between spectrums with and without an excitation pulse.

In order to better reveal the physical mechanism of the absorption peaks, surface currents and electric fields at resonance points were investigated. As shown in Figs. 3(a), 3(b), 3(e), and 3(f), surface currents are symmetrically distributed on both sides of the double open ring resonator, and the electric field is concentrated on the double gaps. It is a typical characteristic that peak Ⅰ is caused by electric dipole resonance with or without an excitation pulse. Similarly, the surface currents and electric fields of peak Ⅱ are depicted in Figs. 3(c), 3(d), 3(g), and 3(h). We can clearly see that the main cause for peak Ⅱ is different. Without an excitation pulse, it is caused by sextupole resonance. Correspondingly, it is caused by quadrupole resonance with an excitation pulse.

Figure 3. Calculated surface current and electric field at different resonance points: (a) surface current and (b) electric field at 1.545 THz, (c) surface current and (d) electric field at 3.21 THz, (e) surface current and (f) electric field at 1.525 THz, (g) surface current, and (h) electric field at 2.79 THz.

As shown in Fig. 4, modulation of the absorption on different polarization angles was also investigated. The angle between the direction of the polarization electric field and x-direction was marked φ. As φ gradually increases, absorption amplitudes of the two peaks change sequentially, but resonance frequencies remain constant. In detail, as φ increases from 0° to 45°, absorption amplitudes of the two peaks decrease, and peak Ⅱ disappears when φ = 45°. As φ increases from 45° to 90°, absorption amplitudes of the two peaks gradually increase. Due to its C2 symmetry design, the device experiences an inverse changing process. As φ increases from 90° to 135°, the absorption amplitudes of the two peaks decrease again, and peak Ⅰ disappears when φ = 135°. As φ increases from 135° to 180°, the absorption amplitudes of the two peaks increase again. Once more, the device experiences another inverse changing process. Here, to analyze the issue clearly, modulation depth is defined as D = (A_max − A_min) / A_max, where Amax stands for the maximum value of the absorption amplitude and A_min is the minimum one at resonant frequency. So, as φ increases from 0° to 180°, a modulation depth of 100% is obtained for both peaks. All the evidence above indicates that changing of the polarization angle cannot break the original resonant mode or excite new resonance. It just affects the strength of the effective electric field, which leads to the changing of resonance intensity.

Figure 4. Absorption spectra at different polarization angles. (a) Without and (b) with an excitation pulse. Absorption color map (c) without and (d) with an excitation pulse.

Finally, we studied the sensing performance of the proposed MMA in monitoring the refractive index changes of different surroundings. Here, a characteristic parameter, sensitivity S, must be introduced. The sensitivity S is one of the most important criteria commonly used to evaluate sensing performance. It is defined as S = Δf / Δn, where Δf is the resonant frequency drift distance and Δn indicates the change of the refractive index. The sensitivity S represents the frequency drift per unit change of the refractive index, and the unit is THz/RIU [37]. Figure 5(a) shows the numerically tested results without an excitation pulse, and Fig. 5(c) is the one with an excitation pulse. Obviously, as the refractive index gradually increases, both peak Ⅰ and peak Ⅱ have a clear trend of red shift with or without an excitation pulse. That is to say, either peak can be used to distinguish the surroundings with different refractive indexes. The variation rules of frequency with the refractive index are demonstrated in Fig. 5(b) and 5(d). It was calculated that the sensitivities corresponding to peak Ⅰ and peak Ⅱ are S₁ = 0.5 THz/RIU and S₂ = 0.9 THz/RIU without an excitation pulse. Similarly, they are S’₁ = 0.35 THz/RIU and S’₂ = 0.475 THz/RIU with an excitation pulse.

Figure 5. Sensing performance: (a) absorption spectra under different refractive index conditions and (b) variation rules of frequency with the refractive index without an excitation pulse, (c) absorption spectra and (d) variation rules with an excitation pulse.

Table 1 shows a comparison of the proposed MMA with some reported ones. The results indicate that our device not only has a dual-function tunable characteristic, but also has the highest modulation depth of up to 100%. More importantly, the proposed MMA simultaneously possesses fine sensing performance. All the evidence indicates that our work is an effective exploration of the design and realization of future multifunctional THz devices.

TABLE 1. Comparison of proposed MMA with some reported results.

References	[38]	[39]	[40]	[41]	This work
Frequency Range (THz)	0.3–2.5	0.5–3.2	1.5–2.5	4.0–6.0	0.9–4.2
Absorption Peaks	2	3	3	4	2
Dynamical Tunability	YES	NO	YES	NO	YES
Modulation Depth	86%	-	100%	-	100%
Sensitivity (THz/RIU)	0.83	1.60	0.19	0.47	0.90

In summary, a dual-function dynamically tunable MMA with a classical sandwich structure is proposed. Two resonance peaks can be achieved at frequency points of 1.545 THz and 3.21 THz. By regulating the conductivity of photosensitive silicon a with pump laser, resonance frequency switching for both peaks is achieved. By adjusting the incident polarization angle by rotating the device, absorption amplitude tuning is obtained. More importantly, dynamic and continuous adjustment of absorption amplitude is obtained at a fixed resonant frequency, and the modulation depth reaches 100%. Also, the sensing performance of the proposed MMA was studied when it was used as a refractive index sensor. The results indicate that our device not only has a dual-function tunable characteristic and the highest modulation depth, but also simultaneously possesses fine sensing performance. Our work discussed in this paper can provide important reference values for future multifunctional THz devices.

The authors declare no conflicts of interest.

Data underlying the results presented in this paper are not publicly available at the time of publication, which may be obtained from the authors upon reasonable request.

National Natural Science Foundation of China (Grant No. 62075052); the Talents Project of Harbin Science and Technology Innovation (Grant No. 2016 RAQXJ025).