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Abstract
Terahertz (THz) refers to electromagnetic waves with frequency from 0.1 to 10 THz, which lies between millimeter waves and infrared light. This paper proposes an ultra-thin metasurface absorber which is perfectly suited to be the signal coupling part of terahertz focal plane array (FPA) detector. The absorptance of the proposed metasurface is higher than 80% from 4.46 to 5.76 THz (25.4%) while the thickness is merely 1.12 µm (0.018 λ). Since the metasurface absorber will be applied to terahertz FPA detector which requires planar array formation, it is divided into meta-atoms. Each meta-atom consists of the same unit cell layout, and air gaps are introduced between adjacent meta-atoms to enhance the thermal isolation, which is crucial for FPA detector to obtain desired imaging results. Due to the symmetrical layout of meta-atoms, absorptance keeps stable for different polarized waves, moreover, good absorptance could also be achieved for incidence angles range of ± 30 °. Spectral measurements show good agreement with the simulation. As a result, features of ultra-thin thickness, polarization insensitivity, and high absorptance make the proposed metasurface absorber well suited to highly efficient coupling of terahertz signals in FPA detector.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz band (0.1-10 THz) lies between millimeter wave and infrared light, which is also called the terahertz gap since it is the least explored electromagnetic waveband by far [1]. Terahertz technology has great potential in security inspection [2], biomedicine [3], diagnostics [4], remote sensing [5,6] and the next generation of mobile communications [7]. Several methods have been proposed to detect terahertz waves, including micro-bolometer [8–11] which is originated from infrared detection technology. Micro-bolometer is composed of two parts in general, namely the signal coupling part and the signal measurement part [11]. Signal coupling is accomplished through absorber where the terahertz radiation is converted to heat, and temperature change is caused. Bridge circuit that contains thermo-sensitive material is then used to measure the temperature change and export the corresponding electric signal. Therefore, the absorber impacts the performance of terahertz detectors directly. Conventional absorbers are films made of natural materials such as Vanadium Oxide and GaAs, although they can obtain a certain absorption effect in wide terahertz band, the absorptance is relatively low (< 40%) [12–16]. Since the absorption characteristics of natural materials are mainly determined by their intrinsic lattice structures, it is difficult to further improve the absorptance, meanwhile, the frequency band where absorption peak occurs could not be adjusted flexibly.
The concept of metasurface provides a potential solution to overcome the limitations of natural absorbing materials. As the two-dimensional version of metamaterial, metasurface is composed of several unit cells periodically arranged in a plane [17,18]. By properly designing the size, shape, and layout of the unit cells, the metasurface can macroscopically present different electromagnetic properties [19–22], so as to realize the manipulation of electromagnetic waves in amplitude [23,24], phase [25,26], polarization [27,28] and such, making it an artificial material with electromagnetic parameters that can be flexibly designed. Therefore, the absorption peak of a metasurface absorber can be tuned flexibly to satisfy the demands of terahertz detectors. Moreover, compared to absorbers made from natural materials, the absorptance of metasurface absorbers can keep above 80% around the absorbing peak.
Different from ordinary metasurface absorbers, the metasurface absorber employed as the signal coupling part of micro-bolometer should not only perform high absorptance, but also generate the highest temperature increment after absorbing the same electromagnetic energy (that is thermal response) [2,19,29]. Thinner absorber will produce higher thermal response with the same material, so ultra-thin metasurface absorber is necessary for micro-bolometer. Besides, since the absorber is expected to be the signal coupler of focal plane array (FPA) detector which is a planar array, it needs to be divided into separated meta-atoms and thermal isolation should be enhanced by introducing air gaps between different meta-atoms. Several metasurface absorbers have been proposed, a metasurface absorber with two narrow absorption bands at 2.34 THz and 4.74 THz has been reported in [30], and its thickness was 3.4 µm, however, the multi-band design amplifies the disadvantages of process errors, making it difficult to fabricate accurately. A wideband metasurface absorber with multi-layer resonate rings has been presented in [31], the absorbing band is from 4.46 to 5.76 THz but the thickness reaches 6 µm (0.042 λ), the thermal effect would be greatly reduced. Another metasurface absorber based on fractal pattern was studied in [32], it has achieved the absorptance higher than 80% from 2.47 to 5.5 THz, however, the thickness of 11.3 µm (0.113 λ) also means it can hardly produce any thermal effect. The features of ultra-thin thickness, absorption bandwidth, absorptance, and meta-atom formation bring challenges to the design and fabrication of metasurface absorber for micro-bolometer.
In this paper, we propose an ultra-thin metasurface absorber which is suitable for signal coupling of terahertz FPA detector, with a thickness of merely 1.12 µm (0.018 λ). The absorber is fabricated on SiO2 substrate through semiconductor technology process, in order from bottom to top, there are three layers, namely ground layer (Au), dielectric layer (Si3N4), and unit cell layer (Au). To increase the absorption bandwidth, the composite form of unit cells with different sizes is adopted; to fit the application scenario of array detector, the metasurface is divided into meta-atoms, and air gaps are introduced between adjacent meta-atoms to enhance thermal isolation. In addition, the shape and layout of composite unit cells are designed symmetrically to maintain stable absorptance for incident waves with different polarizations and incidence angles. According to simulated and measured results, the absorptance is higher than 80% from 4.46 to 5.76 THz where the peak is up to 94.3% and occurs at 4.6 THz. For incident waves with different polarizations, the absorptance is stable in general with only a few frequencies dropping to 70%. Moreover, the absorptance can be maintained above 80% for different incidence angles in the range of ± 30 °. Structures, simulations and discussions, fabrication and measurements of the proposed metasurface absorber will be introduced in the following sections.
2. Design and characteristics of structures
Absorptance and absorption bandwidth are two important performance indicators of metasurface absorber, here absorption bandwidth refers to operating bandwidth where the absorptance can stay above a specific value. When the thickness and structure are determined, the design principle of metasurface absorber will be achieving higher absorptance in band as wide as possible, meanwhile, the absorber is expected to have characteristics of polarization insensitivity and angular stability for different incident waves. The absorptance A(ω) can be calculated by the following formula:
where R(ω) represents reflectance and T(ω) represents transmittance. Considering the metasurface absorber as a 2-port network, so R(ω) and T(ω) can be expressed by the scattering matrix parameters:In Eq. (2), the cross-polarization is also considered. The S2:y,1:x refers to the transmission coefficient from the x-polarization of port 1 to the y-polarization of port 2, and the same applies to other S parameters; x-polarization is perpendicular to y-polarization. Therefore, the absorptance of metasurface absorber is determined by reflection coefficients and transmission coefficients. The reflection coefficients S1:x,1:x, S1:y,1:x reflect the impedance matching between free space and metasurface, the transmission coefficients S2:x,1:x, S2:y,1:x represent the power that goes through the metasurface. The designing objective is to obtain all S parameters close to 0, in which case, all of the incident energy is absorbed and converted to heat, perfectly suited to the requirement of micro-bolometer. The design and characteristics of proposed elementary unit cell, composite unit cell, meta-atom will be described in order.
2.1 Elementary unit cell
For higher fabrication accuracy, square unit cells are employed as the elementary unit cells. The proposed metasurface absorber is fabricated on substrate of SiO2 through semiconductor technology process. As shown in Fig. 1(a), both the ground layer and unit cell layer are made of gold with conductivity of 4.1×107 S/m. Since the thickness of ground layer is t1 = 0.15 µm as depicted in Fig. 1(c), which is far greater than the skin depth of terahertz wave, there is hardly any terahertz wave can penetrate the ground layer, and the S21 is close to zero. The material of dielectric layer is Si3N4 with the relative permittivity of εr = 7.5 and loss tangent of 0.025.
Fig. 1. The (a) perspective view, (b) top view, and (c) side view of elementary unit cell, symbols a and p represent the unit cell size and periodicity, respectively; symbols t1 and t2 represent the thickness of metal layer and dielectric layer, respectively. Surface current distributions on (d) unit cell layer, (e) ground layer, and (f) electric field distribution on the center section.
Surface current distributions on unit cell layer and ground layer are respectively plotted in Fig. 1(d)-(e) to illustrate the absorbing mechanism of elementary unit cells. Strong coupled currents with opposite directions can be observed on the unit cell and ground layers. Considering the electric field distribution on the center section shown in Fig. 1(f), a current loop is thus formed, which can be effectively considered as a magnetic dipole along x-axis [33]. Since the magnetic field of incident wave is along x-axis too, highly efficient absorption is then achieved.
The strength of magnetic coupling between ground layer and unit cell layer is determined by their distance, that is the thickness (t2) of dielectric layer. According to impedance matching theory, optimal impedance matching between free space and metasurface can be achieved by adjusting the dielectric layer thickness t2, as well as the lowest reflection (S11) and highest absorptance. Therefore, parametric studies on the thickness of dielectric layer are carried out in full-wave simulation. As shown in Fig. 2(a), the highest absorptance close to 100% is realized around 5 THz when the thickness is 0.8 µm.
Fig. 2. Simulated absorption spectra with (a) different dielectric layer thickness (denoted as t2) and (c) unit cell size. (b) The equivalent RLC circuit model of elementary unit cell. (d) The calculated resonance frequency according to Eq. (3).
The equivalent circuit model is derived to analyze the relation between absorption peak and unit cell size. As plotted in Fig. 2(b), the unit layer along electric field direction is equivalent to an inductor Lpatch and the inductance is inversely associated with unit cell size (denoted as a); ground layer, unit cell layer and the dielectric layer between them form an equivalent capacitor Cmetal and its capacitance has a positive relation with unit cell area (represented as a2); the dielectric layer is lossy and equivalent to the resistor Rsub. Thus, the whole elementary unit cell can be regarded as an RLC resonate circuit and its resonance frequency (f0) corresponds to the absorption peak, which can be expressed by:
2.2 Composite unit cell
To obtain broad absorption band while maintaining an ultra-thin thickness, the composite metasurface is adopted. Composite metasurface absorber consists of composite unit cells which are formed by combining elementary unit cells with the same shape in different sizes. The composite unit cell proposed in this paper employs metal squares which have been discussed in section 2.1 as the elementary unit cells. As shown in Fig. 3(a)-(c), there are three sizes and they are indicated with three colors, specifically, the metal squares with side length of a1, a2, and a3 are in yellow, blue, and red, respectively. The distance between centers of two adjacent metal squares is denoted as p, a single composite unit cell is then divided into 36 areas and each contains one elementary unit cell. As mentioned above, the composite metasurface is also fabricated on the SiO2 substrate by semiconductor process. In order from bottom to top, there are three layers, namely ground layer (Au), dielectric layer (Si3N4) and unit cell layer (Au) as shown in Fig. 3(a)(c). The ground layer and unit cell layer have the same thickness (denoted as t1) and are both made of gold. The dielectric layer which is made of Si3N4 has a thickness denoted as t2.
Fig. 3. The (a) perspective view, (b) top view, and (c) side view of a sigle composite unit cell. The surface current distributions on unit cell layer at (d) 4.6 THz, (e) 5.1 THz, and (f) 5.64 THz.
Both the side length of metal squares and the thickness of dielectric layer affect to the absorption performance. By optimizing the side length of metal squares contained in composite unit cell, three absorption peaks can be generated on the spectrum. As for the thickness of dielectric layer, the thinner layer will lead to three separate absorption peaks instead of a broad absorption band; a thicker dielectric layer can perform an absorptance increment, but significantly reduce the thermal response which will badly influence the sensitivity of terahertz micro-bolometer. The trade-off between absorption performance and thermal response has been made, and the optimized dimensional parameters of composite unit cell are listed in Table 1. As a result, the total thickness of composite metasurface is only 1.12 µm which is equivalent to 0.018 λ.
Table 1. Optimized dimensional parameters of proposed composite unit cell
With the optimized parameters listed in Table 1, the simulated absorption spectra of TE and TM-polarized waves under normal incidence are shown with solid lines in Fig. 4(a). It can be noticed that each absorption curve of composite unit cell contains three peaks. For comparison, absorption curves of three different elementary unit cells are also plotted in the same figure with dash lines. Obviously, absorption peaks of the composite unit cell coincide with that of three elementary unit cells approximately, the slight differences in side length can be regarded as the coupling between adjacent elementary unit cells which influence the resonance properties. The results verify the feasibility of broadening absorption bandwidth through composite unit cells. Taking the frequency band with absorptance higher than 80% as a criterion, the absorption band is from 4.46 to 5.76 THz (25.4%) where the peak reaches 94.3% at 4.6 THz. Compared with elementary unit cells, despite the slight decreases of absorption peaks, the bandwidth of composite unit cells has increased by nearly 5 times. Furthermore, since the layout of composite unit cell is designed symmetrically, the absorption spectra under TE and TM-polarized incidence are identical, leading to desirable polarization insensitivity of the metasurface absorber.
Fig. 4. (a) Simulated absorption spectra of composite unit cell with TE and TM-polarized waves under normal incidence (black, red solid curves, respectively), and three absorption curves corresponding to three different elementary unit cells, yellow, blue, green dash lines correspond to cells with side lengths (denoted as a) of 9 µm, 10 µm, 11 µm, respectively. (b) The normalized impedance of the composite metasurface relative to free space, the inserted picture shows a magnified view of normalized impedance within the absorption band.
To further reveal the absorbing mechanism of composite metasurface absorber, we have obtained the surface current distributions of the composite unit cell at three absorption peak frequencies by full-wave simulations. As shown in Fig. 3(d)-(f), the stronger surface currents are mainly distributed on the yellow, blue, and red cells at 4.6 THz, 5.1 THz, 5.64 THz, respectively. According to the parametric study on elementary unit cell size (Fig. 2(c)), it is obvious that these frequencies are the corresponding resonance frequencies of the elementary unit cells. The result shows that the broad absorption band of composite unit cell is composed of three narrow bands corresponding to those elementary unit cells in three sizes, which further verifies the feasibility of our design principle.
The absorbing mechanism is also illustrated from the perspective of impedance matching. The normalized impedance of composite metasurface absorber is calculated through the following formula [30,34,35]:
2.3 Meta-atom
As presented above, with a thickness of merely 0.018 λ, the composite metasurface absorber has an absorptance higher than 80% from 4.46 to 5.76 THz (25.4%) and the peak is up to 94.3%, indicating satisfactory performance. Since the proposed absorber will be employed as the signal coupler of terahertz FPA detector, planar array formation should be addressed. To this end, air gaps with the width of 30 µm are introduced between adjacent meta-atoms. These air gaps have broken the structure design, especially for the ground layer, which is divided from an infinity plane into square patches with certain areas. Moreover, the gaps between adjacent square patches are equivalent to capacitors which will slightly influence the resonance characteristics of metasurface. To remain the absorbing performance and obtain the smallest meta-atom, we set a 2 × 2-array composite unit cell as one meta-atom, as depicted in Fig. 5(a)(b).
Fig. 5. The (a) top view, (b) side view of a single meta-atom, the structure in red frame is a composite unit cell. (c) Simulated TE and TM-polarized absorption spectra of meta-atom with air gaps under normal incidence (blue, red curves, respectively). The simulated absorption spectra of different (d) polarization angles and (e) incidence angles, the polarization angle is denoted as φ and the incidence angle is θ.
After the separation of composite metasurface absorber by grouping unit cells and introducing air gaps, the absorptance is recalculated in full-wave simulation as shown in Fig. 5(c). It’s obvious that the separation barely changes the absorption spectra except for some burrs in band over 6 THz, the absorptance in absorption band has been perfectly maintained.
In the study of polarization insensitivity, incident waves with different polarization angles (denoted as φ) are applied in simulation. Here the polarization angle φ = 0 ° for TE polarization and φ = 90 ° represents TM polarization. Due to the rotation symmetry of the meta-atom, polarization angles of φ and 90 ° - φ have obtained the same absorption spectra, therefore, only results of φ varying from 0 to 45 ° are depicted in Fig. 5(d). When the polarization angle gradually increases from 0 to 15, 30, and 45 °, the absorption band remains stable and the absorptance slightly decreases to 73% at 4.8 THz, the rest part of absorption band still has absorptance higher than 80%, indicating good polarization insensitivity.
The incidence angle (denoted as θ) here is defined as angle between the propagation direction and normal direction of the metasurface, therefore, for normal incidence, the incidence angle is θ = 0 °. Parametric studies of different incidence angles are carried out and the results are plotted in Fig. 5(e). With the increase of incidence angle, the absorption band maintains stability while some burrs occur outside the band. When θ < 30 °, the absorptance remains above 80%; as θ is further increased to 45 °, the absorptance drops to 70% at a few frequencies. Despite the slight fluctuation of absorptance for different incidence angles, the composite metasurface absorber realizes good performance within a wide range of incidence angles.
3. Fabrication and measurements
After the design and optimization of meta-atom based metasurface absorber, fabrication and measurements are carried out for purpose of performance verification. A 4-inch SiO2 wafer with thickness of 300 µm is employed as the substrate and the partition of areas is shown in Fig. 6(a). After the whole fabrication process, one wafer can yield 12 square metasurface absorber chips and each of them contains 96 × 96 meta-atoms, the size of a single square metasurface absorber chip is 2 cm × 2 cm.
Fig. 6. (a) The schematic diagram of tape-out and array scale. (b) Fabrication processes of the meta-atom based metasurface absorber.
Semiconductor technology process is adopted to fabricate the meta-atom based metasurface absorber, including magnetron sputtering, photolithography, chemical vapor deposition (CVD), dry etching, etc. As depicted in Fig. 6(b), firstly, a metal film with the thickness of 0.15 µm is deposited by magnetron sputtering onto a cleaned 4-inch SiO2 substrate, air gaps are then introduced by photolithography and etching through lithography reticles to form separated ground layer; secondly, dielectric layer of Si3N4 is prepared on the ground layer using CVD, and air gaps are again introduced by photolithography and etching; finally, another metal film with thickness of 0.15 µm was deposited onto the dielectric layer, and pattern of the composite unit cells is formed on this film through photolithography and etching process.
The picture of wafer with meta-atom based metasurface absorbers is shown in Fig. 7(a). After dicing, square chip containing metasurface absorber with an area of 2 cm × 2 cm is obtained as shown in Fig. 7(b), each absorber is composed of 96 × 96 meta-atoms as mentioned above, and the microphotograph of a single meta-atom is shown in Fig. 7(c) to exhibit more details. There is a slight deviation in alignment for different layers of the meta-atom based metasurface absorber. However, the error is less than 2 µm, as will be seen later, such level of error has little impact on absorbing performance in spectral measurements.
Fig. 7. (a) Image of the wafer with meta-atom based metasurface absorbers. (b) Square metasurface absorber chip composed of a 96 × 96-meta-atom array. (c) Microscopic image of a single meta-atom (in red frame).
The measurements of 4 selected square absorber chips are carried out by Fourier Transform Infrared (FTIR) spectrometer (Vertex 80V), the specific measurement settings are described in Supplement 1. The measured absorption spectra under TE-polarized incident wave are plotted in Fig. 8 with solid lines, and the simulated curves are added into the same figure for comparison. Taking square absorber chip1 for instance (Fig. 8(a)), the measured bandwidth with absorptance higher than 80% is from 4.5 to 5.78 THz, which has good agreement with the simulation result (4.46 - 5.76 THz). The measured absorbing peak is 92.3% at 5.1 THz while the peak is 94.3% at 4.6 THz in simulation, delicate difference of absorption peak position is mainly caused by fabrication errors. The measured results verify that the metasurface absorber realizes good absorbing performance while possessing ultra-thin structure at the same time.
Fig. 8. Absorption spectra (blue solid lines) of meta-atom based metasurface absorber chips tested by Fourier Transform Infrared (FTIR) spectrometer, part (a)-part (d) represent chip 1 to chip 4, respectively, the simulated spectra are plotted in red dash lines for comparison.
Comparisons between this work and relevant research in terms of absorption bandwidth and thickness are listed in Table 2. Taking absorptance higher than 80% as a criterion, the absorber in [31] has a fractional bandwidth of 30.1%, but it is much thicker than the metasurface absober proposed in this paper (0.018 λ). The thinnest metasurface absorber is presented in [3](0.009 λ), but its fractional bandwidth is too narrow. By comprehensive comparison, the meta-atom based metasurface absorber proposed in this paper is perfectly suited to be the signal coupling part of micro-bolometer and has potential to further improve the performance of terahertz FPA detector.
Table 2. Comparison between proposed metasurface and relevant research
4. Conclusion
The design of ultra-thin meta-atom based metasurface absorber and its absorbing mechanism are presented in this paper. Maintaining the thickness as merely 1.12 µm, the absorptance higher than 80% can be achieved from 4.46 to 5.76 THz and the peak reaches 94.3% at 4.6 THz. Besides, the proposed absorber has characteristics of polarization insensitivity and angular stability. It can keep stable absorbing performance when the polarization angle varies from 0 to 90 ° or the incidence angle varies within ± 30 °. The metasurface absorber is fabricated on SiO2 substrate and 12 square absorber chips with an area of 2 cm × 2 cm have been obtained after dicing, each of them contains 96 × 96 meta-atoms. Measurements of the absorption spectrum are carried out by FTIR spectrometer (Vertex V80), simulated and measured results show good agreement, verifying that the proposed design is suited to be the signal coupling part of micro-bolometer, and has the potential to further enhance the performance of terahertz FPA detector.
Funding
Beijing Nova Program (Z201100006820130).
Disclosures
The authors declare no conflicts of interest.
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.
Supplemental document
See Supplement 1 for supporting content.
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