Perfect metasurface absorbers play a significant role in imaging, detecting, and manipulating terahertz radiation. We utilize all-dielectric gratings to demonstrate tunable multi-band absorption in the terahertz region. Simulation reveals quad-band and tri-band absorption from 0.2 to 2.5 THz for different grating depths. Coupled-mode theory can explain the absorption phenomenon. The absorption amplitude can be precisely controlled by changing the pump beam fluence. Furthermore, the resonant frequency is sensitive to the medium’s refractive index, suggesting the absorber may be of great potential in the sensor detection field. The experimental results exhibit a high detectivity of pesticides.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Perfect metasurface absorbers (PMAs) have attracted much attention because of their extensive applications in fields such as imaging, sensing, and communications [1–5]. PMAs play an essential role in many devices that work in the THz range, including modulators and cloaking devices [6,7]. In 2008, Landy et al. first proposed a PMA with a near-perfect absorption amplitude, which manifests an impedance match with the surrounding air by simultaneously promoting electric and magnetic resonance . Subsequently, numerous PMA designs were proposed, including narrowband absorber, multi-band absorber and broadband absorber [9–13]. More recently, different metasurface designs have been extensively investigated with a working wavelength range from microwaves to visible light [14–25]. Typical PMAs are based on a metal-dielectric-metal structure. A periodic metal resonator on the top can cause local plasmon resonance that significantly enhances the electric field, while a metal block at the bottom prevents electromagnetic waves from transmission the absorber. However, these methods generally suffer from a complicated design or limited bandwidth, and the fabrication process, as well as the metal patterns, is easily corroded and oxidized, which is not conducive for practical application.
Recently, PMAs that have been proposed can generally be categorized into narrowband absorbers and broadband absorbers according to their absorption bandwidth [26–32]. Nevertheless, absorbers with just broadband or single narrowband still have certain limitations in applications such as imaging and spectroscopy [33,34]. Therefore, high-performance multi-band or dual-band absorbers are more desirable. Multi-band absorption can be obtained by combining different geometries of subwavelength plasmonic resonators on a single layer or stacking multi-layer metastructures with different geometries separated by dielectric layers [35–39]. However, the structures of these multi-band absorbers composed of metallic materials are generally complicated and difficult to fabricate [40,41]. Therefore, doped silicon has been introduced into PMAs because of its low thermal conductivity and low intrinsic loss [42–46]. Such a design of multi-band absorbers using all-dielectric materials are rarely reported.
Similarly, the use of PMAs for the detection of chemical and biological residues is increasing. The rapid detection of pesticide residues is crucial to food safety. Currently, common methods for detecting pesticide residues are mainly based on instrumental detection methods, such as gas chromatography and high-performance liquid chromatography [47–49]. However, the detection preprocessing steps are complex, time taking, and costly. Thus, it is imperative to develop new analytical methods for pesticide detection. Recent research has focused on PMAs as a sensor for the detection of pesticide residues [30,50].
The proposed PMA based on the silicon grating in this study can achieve quad-band or tri-band perfect absorption in the THz regime. The maximum absorption of the quad-band absorber is 98.03%, and the minimum absorption is greater than 91%. Similarly, the maximum absorption of the tri-band absorber is 99.99%, and the minimum absorption is greater than 91%. The average absorption amplitude is 96.1% at the resonance frequencies, and the maximum quality factor (Q) can reach 12.64 and 12.46, respectively. This multi-band absorber also has the significant advantage of absorption tunability via optical excitation. The modulation depth can reach 51.91% when the absorber covered a layer with a lower carrier concentration of silicon for a pump beam energy of 1800 µJ/cm2. Finally, the feasibility of the proposed absorber in sensing applications is analyzed. We demonstrate that the absorber can be used for chlorpyrifos detection, and the experimental results show a good linear relationship. Owing to the simple structure and excellent performance, we believe that the proposed PMAs based on the silicon grating have potential applications in the regions of refractive index sensors, optical inspection equipment, pesticide detection, and multispectral thermoelectric voltage measurements.
2. Structure and design
Two multi-band absorbers for terahertz frequencies, i.e., the quad-band and the tri-band selective absorbers, are investigated in this paper to explain the tunable multi-band selective absorption effects. The structural design of the perfect absorber, based on one-dimensional all-dielectric gratings, can be prepared with dielectric materials such as N-type silicon. The gap cavity can be infiltrated by microfluidic analytes for THz sensing. The related geometric parameters are the grating period p, the width of the grooves w, the structure thickness h, and the grating height d. The quad-band and the tri-band selective absorbers are composed of the same structure, but the etching depth is slightly different. The structure diagram of the absorber is shown in Fig. 1. In our simulations, p = 320 μm, w = 40 μm, h = 300 μm, and d = 108/160 μm; the values are constant unless otherwise specified. Illuminated with parallel light, the incident electric field polarized along the x-axis. The array of periodical silicon gratings was etched from a phosphorus-doped (N-type) silicon wafer via a standard lithography process. We used CST Microwave Studio (a commercial electromagnetic solver) to design and optimize the grating structure, in which the permittivity of the highly doped silicon was descripted by using the Drude dispersion model 
3. Results and discussion
Figure 2(a) and (b) show the simulated absorption spectrum for the quad-band (d = 108 μm) and tri-band (d = 160 μm) selective absorbers under normal incidence, respectively. The absorption amplitude is calculated using finite-difference time-domain (FDTD) simulations and calculates the absorption by $A = 1 - R - T$.
As shown in Fig. 2(a) and (b), the numerically calculated transmission of the grating structure is nearly zero, i.e., T ≈ 0, in the simulation frequency range of 0.2–2.5 THz. Consequently, the absorption $A = 1 - R$. As shown in Fig. 2(a), the quad-band absorber has four strong resonant absorption peaks, with grating height d = 108 μm. The peak absorption amplitude is higher than 90% at the resonant frequencies of approximately 0.49, 1.05, 1.73 and 2.04 THz with absorption efficiency rates of 91.79%, 95.95%, 97.89% and 98.03%, respectively. The four resonant modes were labeled as M1, M2, M3 and M4. Moreover, it is worth noting that the full-width at half-maximum (FWHM) of the resonant peaks is extremely narrow, 0.218, 0.089, 0.156 and 0.161 THz. The corresponding Q (ratio of resonance frequency to FWHM) values are calculated as 2.25, 11.84, 11.07 and 12.64. For the triband absorber shown in Fig. 2(b), three resonant absorption peaks with absorption amplitudes above 90% are achieved at the frequencies of about 0.36, 1.37 and 2.18 THz, and the absorption was nearly 91.50%, 97.55% and 99.99%, respectively. The three resonant modes were labeled as M1’, M2’ and M3’. We calculated the FWHM and Q values for the tri-band absorber, the FWHM are 0.184, 0.200, 0.175, and the Q values are 1.97, 6.82 and 12.46, respectively. The higher Q values leads to a better sensing performance.
The scanning electron microscope (SEM) image of the absorber is demonstrated in Fig. 2(c), and the inset is the false color SEM image of the side wall of the structure. To compare with the simulation results, Fig. 2(d) shows the experimental absorption spectra of the proposed quad-band absorber. The absorber can receive four resonant absorption from 0.2 to 2.5 THz, and the peak absorbance occurs at 0.57, 1.09, 1.541 and 2.04 THz with absorption efficiency rates of 83.09%, 96.07%, 88.86% and 99.99%, respectively. The resonance frequency deviation is less than 0.19 THz, and the minimal bias is 0.0062 THz. The experimental results agree with the simulation spectrum, which validates our numerical model. Angle deviation of the incident wave during the experiment and the error in size caused by the fabrication process is the cause of the difference between the simulation and experimental results.
To provide useful guidance for real applications, the influence of geometric parameters on the absorption spectrum is investigated in this section. In Fig. 3, we show the absorption spectrum as a function of the grating period p, the width of the grooves w, the thickness of the structure h, and the grating height d. The basic parameters for all plots are obtained from Fig. 1(a).
According to Fig. 3(a), it is apparent that all the four resonant absorption peaks are red-shifted with the period of the substrate p increasing, and the resonance position of the M1 absorption peak is slightly affected by p. The resonance modes of M2 and M4 occur near p = 125 μm, and the resonance of M3 disappears at p = 190 μm. As shown in Fig. 3(c), the width of the grooves was also investigated, and the absorption curve shifts slightly with the increase of w. The M1 and the M2 modes show a slight blue shift, while the M3 and M4 modes eventually couple when w increases to 60 μm. Similarly, the influence of geometric parameters for the tri-band absorber is shown in Fig. 3(b) and (d). As the quad-band absorber spectrum, all the three resonant absorption peaks shift slightly as p and w increase.
To determine the absorption sensitivity as a function of the structural thickness h, the thickness of the tri-band absorber’s thickness was further analyzed. As shown in Fig. 3(e), the absorption spectrum hardly changes with varying h. This phenomenon is due to the high boron doping; the structure exhibits metallic properties, while the structure’s thickness is sufficient to stabilize the absorption.
The scanned result of grating height d, the most critical factor affecting absorption, is shown in Fig. 3(f). As Fig. 3(f) illustrates, the absorption peaks shift towards lower frequencies as d increases from 80 μm to 180 μm.
To develop a qualitative understanding of the multi-band perfect absorption mechanism, we investigated the electric field distribution |E| and the magnetic field distribution |H| of the resonant frequency at 0.49, 1.05, 1.73 and 2.04 THz. The magnitudes of the electric and magnetic field distributions for the quad-band absorber under normal incidence are shown in Fig. 4(a–d) and (e–g), respectively.
As shown in Fig. 4, the electric field distribution |E| and magnetic field distribution |H| are concentrated in the vacuum gap between the Si grating and the top of the structure. The electric field’s presence on the top of the structure indicates that the Si structure produces the surface plasmon resonance (SPR). Simultaneously, the electric field is periodically confined in the vacuum gap between the Si grating along the z-axis, which indicates that the incident electromagnetic wave excited different orders of vertical Fabry-Perot-like gap plasmonic resonance (GPR) modes [52–54]. Apparently, according to Fig. 4(e–h), the magnetic field is highly localized in the vacuum gap between the Si grating and the top of the structure, resulting in the incident energy at the resonance frequency strongly trapped in the absorber with little energy reflected, resulting in near-perfect absorption.
Furthermore, the absorption phenomenon can be explained by the coupled-mode theory (CMT). CMT is used to calculate the reflectivity and the field distribution of the structure, with a sufficient order of the Fourier expansion to ensure convergence. This algorithm has been tested for a series of absorbers. According to CMT, the absorption intensity is given by 
3.1 Photo-excitation of the all-dielectric absorber
In practical applications, to attain resonant frequencies, most absorbers need to be tunable. The tunability and absorption performance of the structure are studied herein. As described in Fig. 1, the absorption performance of the proposed absorber can be adjusted by changing the structural geometric parameters. However, this method requires structural reconfiguration and increases cost. However, tunable multi-band selective absorption can be achieved by adjusting the incident pump beam. To develop an understanding of the design of an absorbers’ optical tunability, we use the tri-band absorber to perform full-wave electromagnetic simulations. Figure 6(a) shows the absorption spectrum with different incident pump beam energies. The effects of a normal incident pump beam were modeled by modifying the conductivity of the top and the sidewall of the grating portion, as depicted in the inset of Fig. 6(a). The thickness of the doped layer using the penetration depth of 800 nm light in silicon was determined to be approximately 10 µm . The excited carrier density is calculated by 
Where Rsi is the reflectivity of the 800 nm light, feff is the incident fluence, α = 1020 cm−1 is the linear absorption coefficient, Eph is the pump beam’s photon energy, β = 6.8 cm GW−1 is the two-photon absorption coefficient parameter, and τ = 35 fs is the FWHM pulse width of the optical excitation. The excited carrier density can be calculated according to Eq. (3). The simulated results for the carrier density under optical excitation are shown in Fig. 6(b). For instance, we set the top and the sidewall of the grating portion with carrier densities of 5.428 × 1018 cm−3 to fit the absorption amplitude under 2000 µJ/cm2 excitation shown in Fig. 6(b). According to Fig. 6(a), the absorption changes with the increase of the pump beam energy. The absorption change of the first absorption peak is pronounced.
Here We introduced the modulation depth that allowed us to investigate the tunable property of the absorber more directly and visually. The modulation depth of the THz metamaterial absorber can be defined as:
Where Amax and Amin stand for maximum or minimum for the absorption under different incident pump beam energies. According to the Eq. (4), the modulation depth of the three absorption peaks under different pump beam energies is shown in Fig. 6(c). However, as Fig. 6(c) indicates, the modulation depth of the first absorption peak reaches 6.32% at 3000 µJ/cm2, while the modulation depth of other peaks is less than 6%. Although the result is not as good as expected, the results still indicate that this absorber design could achieve photo-excitation.
If a higher modulation depth can be attained, there will be more significant applications in spectral imaging systems, communication, and non-destructive sensing at THz frequencies. We proposed covering a layer of phosphorus-doped silicon with thickness t = 10 µm on the top of the gratings (see the inset of Fig. 7(b)). For experiments, the carrier concentration of Si was 5 × 1015 cm−3. The absorption spectrum of the absorber coated N-type silicon is shown in Fig. 7(a) (black line). According to Fig. 7(a), the absorber obtained three resonance frequencies at 0.3472 THz, 1.2994 THz and 1.902 THz with an absorption efficiency rate of 92.41%, 99.69% and 92.41%, respectively. As noted, we simulated the absorption of the structure under the pump beam and observed a change in the absorption. According to Fig. 7(a), the absorption performance of the first two peaks has changed in varying degrees. In contrast, the third absorption peak disappeared, and new absorption peaks appear at higher frequencies. The modulation depth of the three absorption peaks under different pump beam energies is shown in Fig. 7(c). The modulation depth of the proposed absorber can be up to 51.91%. It is noteworthy that we use a lower pump fluence energy to achieve a higher modulation depth. The proposed absorber can be applied to spectral imaging systems, communications and THz frequency non-destructive testing of tunable multi-band absorbers.
3.2 Sensing capability of the structure
To test the reported sensing performance of the quad-band and tri-band perfect absorber, we calculated the absorption spectrum under different environmental refractive indices with the other parameters fixed, as shown in Figs. 8(a) and 9(a). The sensing capability is generally described in terms of sensitivity (S) and figure of merit (FOM) as defined by [26,30]
According to Eqs. (5) and (6), we can evaluate the sensing performance of several modes of quad-band and tri-band absorbers (see Table 1). We investigated the perception ability of the design perfect absorber with different surrounding refractive indices. Figures 8(a) and 9(a) show the variation of the resonance frequency as a function of the surrounding refractive index from 1.30 to 1.40, with a step interval of 0.02. The geometric parameters are the same as discussed above. It can be seen that the absorption spectrum of all the corresponding resonant redshifts gradually varies with the absorption efficiency as the surrounding refractive index increases. The specific information of peak absorption and peak frequency shift change with refractive index is shown in Figs. 8(b–e) and 9(b–d). As shown in Figs. 8(b–e) and 9(b–d), the resonance frequency varies linearly as a function of the change in the surrounding’s refractive index. The corresponding slope of the curve (shown in Figs. 8(b–e) and 9(b–d) by the black line) represents the sensitivity (S) of the absorber.
Furthermore, it can be seen from Figs. 8(a) and 9(a) that a slight spectral shift will cause a sizeable optical intensity variation. We can use Eqs. (7) and (8) to evaluate the sensing performance of several modes of quad-band and tri-band absorber (see Table 1). The corresponding slope of the curve (shown in Figs. 8(b–e) and 9(b–d) by the red line) represents the sensitivity (S*) of the absorber. This slope is much higher than other recently reported values [58,59]. The above results prove the feasibility of the proposed absorber for sensing applications.
3.3 Application in pesticide detection
First, the chlorpyrifos solution with a concentration of 10 mg L−1 was prepared by dissolving 1 mg chlorpyrifos power in 100 mL of petroleum ether. Petroleum ether and chlorpyrifos powder were used as solvent and solute, respectively. Petroleum ether was used to dilute the solution to the different concentrations (20, 40, 50, 70, 90, and 100 ppt). Finally, the chlorpyrifos solutions were uniformly mixed by a centrifugal oscillator.
Figure 10(a) shows a schematic of the absorber as a sensor. Before measurement, the absorber was cleaned by sonication in pure ethyl alcohol for 3 min. As the schematic shows, the chlorpyrifos was prepared at different concentrations (20, 40, 50, 70, 90, and 100 ppt) and was drop coated onto the absorber’s surface by pipette and dried before spectral measurement in the reflection mode. Even though the absorber’s resonance frequencies are different from the characteristic peaks of chlorpyrifos (1.47 THz, 1.93 THz, and 2.73 THz), the dielectric properties of the absorber have changed due to the addition of the pesticide, which results in the absorption change.
In order to observe the change in the dynamics of the absorption amplitude in detail, the absorption was partially magnified (as shown in the inset of Fig. 10(b–d)), where the absorption amplitudes with the different concentrations are represented by the blue circle, and the red line indicates the linear absorption amplitude. The absorption spectrum indicates that the peak spectral intensity is in resonance with chlorpyrifos for concentrations ranging from 20 to 100 ppt. According to the result, as the chlorpyrifos concentration increases, the resonance frequency amplitude gradually decreases. The regression coefficient of the three resonance peaks is 0.6693, 0.9466 and 0.9348. The results demonstrate the linearity of the analysis for the second and third absorption peaks. Thus, the absorption amplitude of the last two absorption peaks can be used as an important indicator for detecting the concentration of chlorpyrifos. Moreover, a high degree of accuracy was achieved when a multi-band absorber was used as a sensor. In summary, the proposed absorber has potential applications in areas such as pesticide detection.
In this study, we designed a tunable all-dielectric multi-band absorber based on one-dimensional all-dielectric gratings. Quad-band and tri-band absorption peaks with a maximum absorption rate of 99.99% and the lowest above 91.5% are obtained from 0.2 to 2.5 THz at normal incidence, with the highest quality factor reaching a value of 12.6 in simulation. The multi-band absorption mainly originates from the SPR and GPR modes. Moreover, we demonstrated the dynamic response of the absorber under optical pump beam excitation. By changing pump beam energy of the incident light, the modulation depth of the proposed absorber can reach 6.32% at 3000 µJ/cm2. We proposed covering a layer of N-type silicon (of different carrier concentrations) to achieve a greater modulation depth. The modulation depth of the proposed absorber can be up to 51.91%. Finally, the simulation results show that the resonant frequency and absorption amplitude are easily tunable to the environment’s refractive index. The sensitivity and FOM can reach up to 2.77 /RIU and 15.86 /RIU when changing the refractive index of the environment. The absorber performs a unique advantage in pesticide detection. Therefore, considering the impressive absorption properties, the dynamic optical response, and the high sensitivity to the refractive index, the absorber has potential applications in the fields of THz sensing, modulators and imaging applications.
Natural Science Foundation of Shaanxi Province (2020JZ-48); National Natural Science Foundation of China (61975163, 11704310, 61575158); Key Laboratory of Engineering Dielectrics and Its Application (Harbin University of Science and Technology), Ministry of Education (KEY1805); Youth Innovation Team of Shaanxi Universities.
The authors declare no conflicts of interest.
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