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Abstract
A flexible terahertz (THz) metamaterial biosensor is theoretically and experimentally investigated. The metamaterial unit cell of the periodic structure array was simply composed of three non-overlapping cut wires with different length parameters on a flexible thin-film (parylene-C) to improve sensitivity. The biosensor sample was fabricated using a lithography process and characterized by a THz time-domain spectroscopy (TDS) system. The metamaterial exhibited multi-resonance dips in transmission spectrum at 0.6-2.0 THz, which can self-correct errors in biosensing. Numerical results show that the Q-factor, figure of merit (FOM) and sensitivity can change in dynamic ranges with the geometric parameters (space and width) of three-cut-wire metamaterial. When space distance was 40 µm and other parameters were default, the sensitivity, FOM and Q-factor reached 710 GHz/RIU (Refractive Index Unit), 9, and 20, respectively. Therefore, through proper design and preparation, the metamaterial can be applied to biochemical detection.
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1. Introduction
With the outbreak of novel coronavirus, many researchers paid more attention to the biosensing field. The detection scheme of virus, protein, Deoxyribo Nucleic Acid (DNA), and microorganism had become a hot research direction of biosensing technology. Metamaterials with hypersensitivity sensing properties provided a new solution for biological sensing detection [1,2,3]. Metamaterials were a new type of materials composed of periodic or aperiodic subwavelength metal structures and dielectric materials [4,5,6], which could be designed according to a special need. The transmission and reflection spectra of metamaterial were sensitive to the change of the dielectric constant of the material layer above the metal structure. For the biosensor, many researchers had proposed solutions [7,8,9], which were applied in the infrared band and the visible band, respectively. Due to the high photon energy of infrared, it could cause damage to the samples in biosensing detection. Meanwhile, different structures or single, composite, and 2D material structures were also utilized to explore the terahertz biosensing performances [10,11,12,13,14,15].
Nevertheless, terahertz (THz) wave had the advantages of low photon energy, strong directivity, and fingerprint spectrum [16], which could avoid damage to biological samples. In the past decade, researchers have developed different THz metamaterials to detect the bio-sample [17,18,19,20,21,8]. For example, Keshavarz, A et al. presented a terahertz metamaterial, which composed of an H-shaped resonance structure to detect three kinds of viruses [6], and the Q-factor reached 9.17. The sensitivity was 540 GHz/RIU (Reflective Index Unit). The figure of merit (FOM) was 2.86. Additionally, the researchers used THz metamaterial biosensors to detect other biological samples, such as protein [22,23,24], DNA [25,26,27], Ribonucleic Acid (RNA) [28] and so on [29,30,31,32]. The work of researchers confirmed the possibility of terahertz metamaterial biosensor. Meanwhile, there were some shortcomings, such as low detection sensitivity, complex detection process, and the lack of specific detection.
In the preparation of materials, the THz metamaterial on hard substrate (high resistance silicon) [8] was popular in last two decades. Although the preparation of hard substrate metamaterials enhances the stability of the device and reduces the difficulty of fabrication, it prevents metamaterial from surface detection and high signal-to-noise ratio. Compared with the metamaterials on hard substrate, Polyimide (PI) [33,34], polyethylene terephthalate (PET) [35] and parylene-C film [18] are the main flexible substrate materials. PI flexible film has the disadvantages of low transparency and easy-to-produce voids, which will cause additional material loss to the metamaterial devices. PET flexible film has many disadvantages, such as high hardness and embrittlement. The parylene-C film has the advantages of small dielectric constant ($\varepsilon = $2.57) and good film-forming quality, therefore, it is especially suitable for the application research of metamaterial substrate. As a result, when bio-samples were detected by metamaterial biosensor on the thin-film substrate, there are obvious insensitivity (sensitivity increased by multiple), FOM and full-width half-maximum (FWHM). Low FOM and sensitivity were the essential defects of the high dielectric constant substrate materials.
Besides, the unit structure of the metamaterial was complex. To achieve better signal transmission, researchers use the method of optimizing the geometry of metamaterials, including single geometry (split-ring resonators (SRR), cut metal wire), composite structure (superposition of single geometry), two or three-dimensional (2D or 3D) material structures (the combination of graphene and geometry) [13,14,36]. As an emerging research field based on the 2D metamaterials, 3D structures have great development potential. However, due to the complexity of design, simulation and manufacturing, and considering the uncertainty of graphene preparation process, 3D structures did not be discussed. Compared with split-ring resonators, cut-metal-wire structures only contain straight lines, which are compatible with traditional semiconductor technology and therefore can reduce the preparation process and enhance the performance, having become an ideal research target. While gaining the advantages of geometry, researchers are also striving to obtain more resonance dips in a specific frequency range, which can achieve error self-correction in biosensing. In terms of the resonance principle of multiple resonance dips, it mainly uses the coupling effect between metamaterials, especially in composite structures rather than single geometries. And it uses the Fano resonance or electromagnetically induced transparency (EIT) effect to produce multiple resonance peaks, which brings the performance advantage of high sensitivity, however, the stability of the device and the advantage of free adjustment of single resonance peak are difficult to be guaranteed. The position of the resonance dip is at a loss for independent and free control in the ideal THz waveband, which is a great obstacle to develop terahertz metamaterial biosensor.
To sum up, it found that the current sensors have potential to be optimized from the aspects of the geometry of the metamaterial, such as the selection of the substrate material, the selection of the resonance peak of the transmission spectrum and the detection of biological samples. For example, the metamaterial of the high resistance silicon substrate hardly meets the requirements of surface detection, and the detection method of single resonant frequency offset was lack of accuracy. It was well known that the sensitivity increased with the frequency increasing. When the detection object was attached, multi-resonance dips cause multiple different frequency shifts. multi-resonance dips are used to indicate a single detection object, and the average value of the multiple frequency movements can reduce the experimental error caused by other reasons. Therefore, the metamaterial sensor with multi-resonances dips, high performance, free adjustment and meeting the needs of surface detection has become our research focus.
Here, we experimentally and numerically explored a novel terahertz metamaterial biosensor with multiple resonance dips and a simple structure on flexible substrate. The metamaterial unit cell of the periodic array was composed of non-overlapping metal cut-wires (CWs) structures, rather than cross ones, for free adjustment of single resonance dip. The metal structures were fabricated on the flexible thin film (parylene-C) for smaller substrate dielectric constant to improve sensitivity. In terms of the method of generating multiple resonance dips, we adopt the method of superposition of metal CWs structures, which has the advantage of meeting the requirement that a single resonance dip could be adjusted without affecting the transmission performance of other CWs structures. CWs lengths were carefully designed to achieve uniform distribution of dips in the investigated THz scale. Meanwhile, simple structure not only reduce the preparation cost and process difficulty but also improve the ability of qualitative and quantitative detection of test samples. To characterize the biosensing properties of terahertz metamaterials, we mainly discussed the Q-factor, FOM, and sensitivity. The sensing properties of terahertz metamaterial were optimized by changing the structural parameters (CWs width and space) of three-cut-wire metamaterial. The influence of CWs width, and CWs pitch on the biosensing performances were investigated in detail to overcome the problems of low Q-factor, and low sensitivity brought by the hard substrate.
2. Materials and methods
The schematic illustration for the investigation of the multi- resonant dips terahertz metamaterial biosensor in the proposed planar metamaterial structure was shown in Fig. 1(a). The flexible metamaterial biosensor was composed of two-layer structures [37,38]. The top layer consisted of a periodic subwavelength metal plane three-cut-wire structure. The structural unit of the metamaterial was composed of three CWs with different length parameters as Fig. 1(b). The CWs were made up of gold with a thickness of a = 100 nm. The three metal lengths of CWs were different, L1 = 120 µm, L2 = 80 µm, and L3 = 60 µm, respectively. The width of the three CWs was fixed at D1 = D2 = D3 = 10 µm. The distance between the shortest metal CWs and the middle short metal CWs was termed as W1. The distance between the longest short metal CWs and the middle short metal CWs was termed as W2. It was originally defined as W1 = W2 = 15 µm. The substrate consists of a flexible material thin film (parylene-C). The unit cells were arranged in the periods of PX = PY = 140 µm and the thickness, H = 10 µm. The incident terahertz wave was x in the direction of the electric field and the y in the direction of the magnetic field as shown in Fig. 1 (a). The optical micrograph of a unit was shown in Fig. 1(c). The whole flexible metamaterial on parylene-C thin film with a diameter of 4-inches shown in Fig. 1(d), which is fabrication results of flexible metamaterial terahertz biosensor.
Fig. 1. Schematic diagram of the metamaterial structure. (a) The illustration of the proposed metamaterial biosensor and testing model. (b) The structure parameters of three-cut-wire metamaterial. (c) The fabricated metamaterial unit structure. (d) The whole flexible metamaterial on 4-inch wafer-scale.
The performances of the flexible metamaterials biosensor were explored using the finite-difference time-domain method. The full-field electromagnetic wave was set as light sources. A lossy metal model was adopted for Au with conductivity $\mathrm{\sigma } = 4.6\textrm{*}{10^7}s{m^{ - 1}}$, and the dispersion relation was considered into its material attribute. In the numerical modeling, the periodic boundary conditions were applied along with the x and y directions while the perfectly matched layer boundary condition was set in the z-axis direction. The plane wave irradiated the structure with the electric field along the x-axis direction. All the simulation work is carried out in the frequency field. The transmission spectra of the structure, the electric field and surface current distributions were recorded through different field monitors. Based on transmission spectra, the metamaterial performances of the unit-cell were calculated and analyzed.
Notably, with the thickness of the Au structure layer is not more than 100 nm, at 0-3 THz range, the real part and imaginary part of the metal refractive index change is not obvious, which causes the dispersion of Au structure layer subtle, compared with absorption in THz regime. Therefore, dispersion of Au layer did not be discussed. In fact, the dielectric constant or refractive index of Au material changes slightly with wavelength or frequency, that is to say, in the simulation, its dispersion relation has been taken into account. It is just the final result of the simulation output.
In the aspect of device preparation, the chemical vapor deposition (CVD) technology was applied to prepare the flexible substrate film. The preparation processes were optimized to ensure the flatness and film quality of parylene-C film. Hereafter, the standard lithography technology was used to fabricate the cut-wires structure of the metamaterial metal (Cr 10 nm/Au 100 nm). The parylene-C with the three-cut-wire structures was peeled off from the silicon wafer and was clipped as a 20 mm×20 mm sensing unit. The main processes included the pretreatment of the flexible substrate, photoresist coating, exposure, development, metal deposition, lift-off, dicing, etc. The illustration of same fabrication process was presented in our previous research work [18].
Due to the special sensitivity of the metamaterial structure to the change of the dielectric constant of the material layer above the metal structure, special attention should be paid to the cleanliness of the environment of preparation and test. The whole preparation process needs to be completed in a cleanroom. It is worth noting that the stretch of parylene-C on the flexible substrate may deviate from the experimental results. Finally, the terahertz time-domain spectrum system was used to detect the transmission spectrum of the flexible metamaterial biosensor. To reduce the influence of the bending and polarization of the flexible substrate chip on the detection structure, a test holder which could fix the flexible substrate was designed and fabricated.
Before the fabrication, we explored the biosensing performance of the three-cut-wire metamaterial biosensor to obtain better structural parameters with the FDTD method. To understand the evolution of multi-resonance dips in a metamaterial structure and then discuss the sensing performance, we investigated the transmission response and induced electric field distribution of the three-cut-wire metamaterial biosensor to variable dielectric constant of the material layer (tested sample) above the metal structure (Fig. 2(a)). Therefore, different surface dielectric constants (ɛ=1, 1.74, 1.796, 1.85, 1.904, and 1.96) was set in FDTD processes to simulate the permittivity (or refractive index) of biological elements in the microenvironment. In general, the refractive index of liquid biological samples is between 1.33 and 1.6 or the dielectric constant is between 1.7689 and 2.56. Therefore, dielectric constant 1.796, 1.85, 1.904 and 1.96 were mainly used. As shown in Fig. 2(a), the position of all resonance dips changed with different dielectric constants, which illustrated that the frequency-shift increase with the differences of dielectric constant increasing. In Fig. 2(b), the transmission spectra of simulation results and experimental measurement of the metamaterial structure at ɛ=1 were presented by red lines and black lines, respectively. The results illustrated measured transmission spectra matched well with that of simulation results, which indicates that the simulation model was correct. Therefore, these simulation models were used in other cases such as ɛ=1.796, 1.85, 1.904, and 1.96.
Fig. 2. The resonance frequency shifts vary by permittivity change, with metal structure at a = 100 nm, L1 = 120 µm, L2 = 80 µm, L3 = 60 µm, D1 = D2 = D3 = 10 µm, W1 = W2 = 15 µm, PX = PY = 140 µm and H = 10 µm. (a) The transmission spectrum at six different dielectric constants of material layer above the metal structure versus frequency; (b) The simulated (red line) and measured (black line) transmission spectra of the metamaterial at ɛ=1.
To improve sensitivity and accuracy of the metamaterial biosensor, a multi-scale self-similar array structure was used to obtain the multi-resonant dips structure with highly efficient and versatile. The transmission view of the self-similar multi-scale array was presented in Fig. 3. These structures had the same period (PX = PY = 140 µm), polarization direction and wire spacing (W1 = W2 = 15 µm). The frequency position of the resonant dipole was 0.89 THz and the intensity was -18.60 dB when the metal length of CWs was L1 = 120 µm as shown in Fig. 3(a). The frequency position of the resonant dipole was 1.27 THz and the intensity was -12.23 dB when the metal length of CWs was L2 = 80 µm as Fig. 3(b). The frequency position of the resonant dipole was 1.50 THz and the intensity was -7.71 dB when the metal length of CWs was L3 = 60 µm as Fig. 3(c). When three metal wires of different sizes were combined to form a metamaterial structure, three resonance dipoles were generated in the metamaterial with f1 = 0.801 THz, f2 = 1.15 THz, and f3 = 1.48 THz in Fig. 3(d). The frequency and intensity of the resonance dipole will change because of the electromagnetic coupling between the CWs.
Fig. 3. Transmission views of three dipole arrays with different THz resonant dip and the combined multi-scale self-similar array at (a) L1 = 120 µm, (b) L2 = 80 µm, (c) L3 = 60 µm, and (d) combination of L1 = 120 µm, L2 = 80 µm and L3 = 60 µm. With other metal structure parameters at a = 100 nm, D1 = D2 = D3 = 10 µm, W1 = W2 = 15 µm, PX = PY = 140 µm and H = 10 µm.
3. Results and discussion
3.1 Surface electric field analysis
Under the action of the electric field, the free electrons moved along the CW, which broke the electric neutral characteristics in the metal structure. Therefore, the resonance effect caused by the CW of the three-cut-wire structure was the electric dipole resonance effect. Additionally, the coupling effect between units in the element was reduced by a reasonable structure. As shown in Fig. 4, this was the transmission spectrum of the three-cut-wire metal CW structure. According to the simulation transmission spectra, there were three resonance dips at 0.801 THz, 1.15 THz and 1.48 THz for the metamaterial. To better understand the resonance mode of the three-cut-wire metal CWs structure, the simulation diagrams of z-component of electric field in the structure at different resonance frequencies of the three-cut-wire structure shown in Fig. 4, also to illustrate the sensitivity of the structure. The results, which exhibited a dipolar resonance at 0.8 THz by the CW (120 µm), were excited directly by the incident electric field. The results that exhibited a dipolar resonance at 1.15 THz by the CW (80 µm) were excited directly by the incident electric field. The results illustrated a dipolar resonance at 1.48 THz by the CW (60 µm) was excited directly by the incident electric field. Due to the coupling effect between CWs, the resonance dip of the corresponding frequency was also caused by the shorter CW. The reason for resonance frequency was explained in the theoretical analysis. The biosensing performances of flexible terahertz metamaterial with three-cut-wire structure were explored. Meanwhile, the influence of geometric parameters on the performances of biosensors through controlling variables also was investigated.
Fig. 4. The diagrams of the z-component of electric field distributions in the metal structure at (a) f= 0.801 THz, (b) f= 1.15 THz, and (c) f= 1.48 THz, with metal structure at a = 100 nm, L1 = 120 µm, L2 = 80 µm, L3 = 60 µm, D1 = D2 = D3 = 10 µm, W1 = W2 = 15 µm, PX = PY = 140 µm and H = 10 µm.
3.2 Effect of CWs spacing and width on the biosensing properties of metamaterials
The sensing performances of the biosensor were mainly characterized by sensitivity, Q-factor, and FOM. Q-factor represents the sharpness of resonance dip, which affected the identification of frequency shift. The theoretical analysis shows that the higher the Q-factor, the better biosensing performance; The sensitivity represents the detection ability of bio-samples; Also, to better characterize the performance of flexible metamaterial biosensors, the FOM should be introduced. Here, Q-factor, sensitivity, and FOM were defined as
The f represents the frequency of the resonance dips, and the full-width at half-maxima (FWHM) stands for the half-width of the resonance dip. The f1 and f2 represent the frequencies of resonance dips caused by the different surface permittivity. The results showed that the refractive index of the simulated surface layer was different. According to the definition, the unit of sensitivity S is the refractive index unit (GHz/RIU). The software automatically obtained FWHM and resonant frequency.
To fully understand the influence of the structural parameters of three-cut-wire metamaterials on the resonant dip frequency, the contour and color diagrams for various CWs width and space parameters were plotted in Fig. 5, with other constant metal structure parameters at a = 100 nm, L1 = 120 µm, L2 = 80 µm, L3 = 60 µm, PX = PY = 140 µm, and H = 10 µm. Default CWs width and space parameters are D1 = D2 = D3 = 10 µm and W1 = W2 = 15 µm. The frequency and parameter change along the x and y-axis and the strength of the signal were indicated through the color bar. The change of resonance dips with the CWs width parameter was obvious in Fig. 5(a). The transmission dips intensity also increased gradually. Similarly, in Fig. 5(b), the change of resonance dips with CWs spacing was obvious. The shift of resonance dips was affected by the change distance among CWs. Therefore, to obtain metamaterial biosensors with high-performance, structural parameters of metamaterial were investigated. As shown in Fig. 6(a) and Fig. 7(a), the influence of CWs spacing (W1 = W2) on sensitivity, Q-factor, and FOM of metamaterial structure was in turn. The results showed that different CWs spacing played a great influence on high frequency (f = 1.48 THz). When the distance was 40 µm, the sensitivity, FOM and Q-factor reached 710 GHz/RIU, 9, and 20, respectively. Compared with that at low frequency (f = 0.801 THz), the resonance dips were blind to the sensitivity of the CWs’ space change because the increase of the distance between the CWs could weak the coupling effect between the CWs, reduce the high-frequency energy loss, decrease the FWHM value, which increases the sensitivity. The influence of the width (D1 = D2 = D3) on the sensitivity, Q-factor, and FOM is shown in Fig. 6(b). The results illustrated that the CWs width had little effect on the performance of the sensor. The absence of enhancing the coupling between the CWs caused the sensitivity and Q-factor to be difficult to sensitive in CWs width changing. Meanwhile, to directly illustrate the influence of structural parameters on the transmission spectrum, a transmission spectrum is added. The results are shown in Fig. 7. under different cut wire space, CWs width and period of metamaterial sensing structure under refractive index of background 1.
Fig. 5. Contour plot of simulated transmission for different CWs width and space, with other constant metal structure parameters at a = 100 nm, L1 = 120 µm, L2 = 80 µm, L3 = 60 µm, PX = PY = 140 µm, and H = 10 µm. Default CWs width and space parameters are D1 = D2 = D3 = 10 µm and W1 = W2 = 15 µm. The color bar shows the magnitude of transmission intensity. (a) The contour plot represents the magnitude of transmission intensity change caused by different metamaterial cut-wire widths, as 2 µm < D1 = D2 = D3 < 5 µm; (b) The Contour plot represents the magnitude of transmission intensity change caused by different cut-wire spaces, as 9 µm < W1 = W2 < 50 µm.
Fig. 6. The effect of cut wire space (9 µm < W1 = W2 < 50 µm) and width (2 µm < D1 = D2 = D3 < 5 µm) changes on sensor performance, with other constant metal structure parameters at a = 100 nm, L1 = 120 µm, L2 = 80 µm, L3 = 60 µm, PX = PY = 140 µm, and H = 10 µm. Default CWs width and space parameters are D1 = D2 = D3 = 10 µm and W1 = W2 = 15 µm. (a) Q-factor, FOM, sensitivity changing with the wire space; (b). Q-factor, FOM, sensitivity change with the cut wire width.
Fig. 7. Transmission spectrum under different parameters. (a) The transmission spectrum under different cut wire space; (b) The transmission spectrum under different CWs width; (c) The transmission spectrum under different period of metamaterial sensing structure.
3.2 Effect of polarization directions on the biosensing properties of metamaterials
The structure had a small change in Q-factor in the range from 0° to 30°of polarization angle. When it was greater than 30°, the Q-factor became smaller until it disappeared. The different polarization directions would affect the resonance by altering the interaction between the CWs and free spaces. The opposite side of the CWs was similar to the two poles of the parallel plate capacitor. When the vertical E between the parallel CWs was applied, the resonance intensity is the strongest. When the polarization angle gradually tended to be parallel, the resonance intensity gradually decreased until it disappeared. In the experimental verification process, to control the polarization direction, we made a mold for fixing the metamaterial device and controlled the polarization angle by rotating the abrasive tool.
3.2 Model analysis
To further understand the electromagnetic response of the multi-resonance, The electromagnetic response of the multi-resonant array can be understood with temporal coupled-mode theory (TCMT) [37], which can be directly extended to the coupling model of the multi-resonant concept between CWs. The full multi-resonant structure is created by repeating this fractal-like generation procedure, resulting in a multiscale geometry with self-similar characteristics and with as many resonances as fractal interactions. The detailed information also presented in Ref. [37], which can effectively explain the multi-resonant coupling in this experiment, such as amplitude of charge oscillation, the asymmetry of the different order mode.
4. Conclusion
A kind of flexible THz metamaterial with a multi-resonance dips structure was designed, fabricated, and investigated. The unit cell of the periodic structure array was composed of three non-overlapping cut wires with different length parameters. The dielectric constant of flexible substrate (parylene-C thin-film) is relatively smaller than hard substrate, which can improve the sensitivity of the sensor. The numerical and experimental results demonstrated that the three-cut-wire metamaterial could produce three different resonance dips (0.801 THz, 1.15 THz, and 1.48 THz), which can self-correct errors in biosensing. The structural parameters (space and width) of the CWs would affect the performance of the biosensor based on flexible metamaterial. The distance between the CWs had a great influence on the sensitivity of the high-frequency resonance dip (1.48 THz), with a range of 0-700 GHz/RIU. Especially, when space distance was 40 µm and other parameters were default, the sensitivity, FOM and Q-factor reached 710 GHz/RIU (Refractive Index Unit), 9, and 20, respectively. The low-frequency resonance dip of the three-cut-wire metamaterial was sensitive to polarization. With a polarization angle greater than 30 degrees, the performance of the biosensor would decline. On the contrary, biosensor performances were insensitive to CWs width changes, which directly reduces the process requirements in the production of metamaterials. Structure size of the three-cut-wire metamaterial belonged to the micron level, which provided the advantages of simple fabrication, high sensitivity, and stable structure. The electromagnetic response range was uniform and very suitable for the application of biosensors. Therefore, the results provided suggestions for the future optimization of flexible substrate biomaterial biosensors.
Funding
Young and Middle-aged Talents Program of the State Ethnic Affairs Commission (2019); Beijing Municipal Natural Science Foundation (4181001); National Natural Science Foundation of China (61774175).
Acknowledgment
The authors would like to acknowledge Mr. Jin Li in Beijing Daheng photoelectric technology co. LTD for the assistance in the terahertz time-domain spectroscopy test.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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