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Abstract
Increasing interest in perfect absorption of metasurface has initiated a discussion on the implementation of ultra-broadband coherent perfect absorption (CPA). Here, we present a mirror symmetric coherent absorption metasurface (CAMS) with polarization independence based on resistive thin films and annular metal patterns to force the fulfillment of ultra-broadband CPA in terahertz (THz) regime, controlling the interplay between electromagnetic waves and matter. By incorporating internal and external ring-shaped films with attached phase-delay lines, the desired phase response can be obtained, laying the foundation for implementing ultra-broadband coherent absorption. Simultaneously, by building a metal-medium composite structure superseding the dielectric substrate, additional promotion of the coherent absorptivity over the operation frequencies is realized. Manipulating the phase difference of two back-propagation coherent beams, the coherent absorptivity at 8.34-25.07 THz can be tailored successively from over 95.7% to as low as 38.1%. Moreover, with the incident angle up to 70° for the transverse electric wave, the coherent absorptivity is still over 74.8% from 8.34 THz to 25.07 THz. And for the transverse magnetic wave, at 6.67-24.2 THz, above 81.3% coherent absorptivity is visible with the incident angle increased from 0° to 60°. Our finding provides an interesting approach to designing ultra-broadband coherent absorption devices and may serve applications in THz modulators, all-optical switches, and signal processors.
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1. Introduction
Perfect absorption of electromagnetic energy by a material, ascribed to the interference between the incident electromagnetic waves and the target object, underlies numerous significant applications, including sensing [1], photovoltaics [2], photo detection [3], and optical switching [4]. Formerly, by virtue of the internal electromagnetic properties of traditional absorbing materials, the energy of incident electromagnetic waves is converted into other forms of energy, resulting in the implementation of absorbing electromagnetic waves. Nevertheless, to achieve broadband absorption, it is imperative for the conventional materials that multilayer materials are employed to fulfill the impedance match of different frequencies, which inevitably leads to the increase of material thickness [5]. Obviously, it is extraordinarily difficult to accomplish ultra-broadband absorption with thin and light conventional materials.
Due to advances in material science, a design concept for electromagnetic properties of the metamaterials offers a novel platform for the development of high-performance absorbing materials. Electromagnetic metamaterials including two-dimensional metamaterials (i.e. metasurfaces) are artificially engineered composites with patterned subwavelength components, which possess unusual features not available by naturally existing materials [6]. One or more desired responses can be yielded at frequencies from microwave to optical via the construction of metamaterials [7–9], giving rise to many relevant applications consisting of perfect absorbers [10], perfect lens [11], thermal emitters [12], photovoltaic cells [13], and so on.
Especially, building up metamaterial absorbers (MMAs) is one of feasible approaches for the realization of perfect absorption. Hitherto, various MMAs have been extensively studied and experimentally verified in microwave band [14,15], terahertz (THz) region [16,17], and infrared region [18,19]. Nevertheless, it is remarkable that the reported MMAs suffer from some demerits including narrow bandwidth, dependence on the polarization effect for the incident waves, etc. Significantly, breaking the bottleneck of narrow operating band and further realizing high-efficiency absorption is of great meaning to support more potential applications in many fields. To this end, multilayer MMAs can be constructed, which offers an effective channel to realize high absorption in a wide range. Employing a quadrangular frustum pyramid with multilayer metal-dielectric films in the presented MMA, Ding et al. demonstrated the ultra-broadband absorption induced by the overlapping of multi-frequencies resonances in the frequency region of 7.8-14.7 GHz [20]. Lin et al. confirmed that the ultra-broadband absorption can be achieved in tungsten/germanium anisotropic nano-cones [21]. A pyramid-shaped MMA with two-dimensional plasmonic subwavelength structures, performing excellent absorption effect at full infrared waveband, was reported by Liang et al. [22]. Whereas the absorbance relying on the original structure is fixed for these MMAs, consequently, leading to the limitation of many applications that involve tunable absorption. Additionally, if its performance maintains broadband and high absorption, the proposed MMA either becomes bulky or demands sophisticated craft. Notably, the ineluctable decreasing of electromagnetic absorption in connection with reduced-thickness MMAs can be conquered via the exploitation of coherent resonant electromagnetic effect enhancing the absorption.
Remarkably, a coherent absorber provides an effective platform, which is termed an optical system associated with a laser by time reversal by Chong et al. [23]. In such a system, coherent perfect absorption (CPA) is stemmed from the interaction of absorption and interference, which, consequently, indicates that the perfect absorption of electromagnetic energy can be fulfilled through the control of the interference of multiple incident waves. So far, most studies have shown that CPA effect can be implemented in graphene films [24], metal-insulator-metal plasmonic waveguides [25], and planar metamaterials [26] in the linear regimes. Moreover, CPA of nonlinear waves has been reported in an epsilon-near-zero plasmonic waveguide [27]. Coherent absorption of nonlinear matter waves was realized and experimentally demonstrated in an atomic Bose-Einstein condensate [28]. Of particular significance here is additionally designed flexibility afforded via the second coherent beam. In contrast to MMA due to strong coupling, the absorption tunability without tuned material introduced can be attained by virtue of the interplay of wave interference.
A slab CPA was implemented in a silicon wafer, in which the absorption at CPA resonance can be modulated successively via the variation of relative phase for the incidence fields [29]. Huang et al. put forward a multi-band three-layered metasurface, where CPA resonances were observed at four frequencies, with coherent absorptivity revealing distinct dependence on the phase difference of two oppositely directed incident beams [30]. The realization of CPA in an all-dielectric fishnet metasurface was reported by Zhu et al., which demonstrated the tunability of coherent absorptivity through the phase modulation [31]. Nevertheless, like the majority of coherent-enhancement schemes, the fulfillment of CPA is confined to the discrete resonant frequency.
The devices with broadband CPA are desirable for many telecommunication applications, such as optical modulators and logic elements. And these devices can be attractive as the basis for all-optical switches and terminators for telecommunications-related applications. As well, they can be employed for enhanced interferometers [32,33]. Employing the multi-sized and multilayer circular patch pairs integrated on both sides of the dielectric substrate, a coherent absorber with multi-band CPA was put forward. Ulteriorly, the broadband CPA can be accomplished via the optimization of the radius of multi-sized patterns and the metasurface thickness [34]. Owing to the extraordinary properties of epsilon-near-zero (ENZ) material for the boost of light-matter interaction [35], which provides a general and flexible channel for perfect absorption, renders the possibility of broadband CPA. Herein, Kim et al. [36] put forward a strategy for broadband CPA using indium tin oxide as an epsilon-near-zero film in the near-infrared region. On basis of a mirror-symmetric graphene structure, broadband CPA over 1-10 GHz is achieved by Zhang et al. [37]. However, another tough proposition worth exploring is the possibility of further boost coherent absorption performance.
In this work, we first put forward a dual ring-shaped polarization insensitive coherent absorption metasurface (CAMS) with mirror symmetrical resistive thin films. Whereas, the coherent absorption of such a metasurface is limited to the narrow operating band. In view of the factors to achieve CPA, the bandwidth of CPA can be increased through the elements with linear phase response, containing utilizing phase-delay lines [38], multilayer structures [39], etc. Hence, after then, the phase-delay lines are assembled into the internal and external resistive films, enabling ultra-broadband coherent absorption with coherent absorbance larger than 90% from 7.6 THz to 26.51 THz (the relative bandwidth is 110.87%). Subsequently, erecting a multilayer dielectric-metal stacked structure, as a result, the coherent absorptivity raises up to 95.7%, covering from 8.34 THz to 25.07 THz (the relative bandwidth is 100.1%). And more critically, the coherent absorptivity over the operation frequencies can be modulated by altering the relative phase between the two input beams as well as the relative intensity of the two input beams. Combined with scattering matrix theory and electric field analysis, the coherent absorption nature is illustrated, that is the interaction of destructive self-interference and coherent wave interference. Additionally, the coherent absorption effect for oblique incidence is investigated. This work is intended to provide an alternative approach for the implementation of ultra-broadband coherent absorption, rather than a complementary theoretical derivation of CPA through the scattering matrix method. As well, the use of phase-delay lines may contribute to the achievement of linear and circular polarization conversion devices.
2. Theoretical framework: scattering matrix method
Utilizing the scattering matrix method in a simple theoretical model portrayed in Fig. 1, we derive the indispensable conditions for CPA. There is a target metasurface placed at z = 0 in the free space. Such a metasurface is illuminated by two coherent counter-propagating parallel beams emitted from two ports (forward (+z) and backward (-z) ports). In this system, E1 and E2 as well as O1 and O2, assuming a monochromatic time harmonic convention ejωt with an angular frequency ω, denote the input and output fields in the ± z directions, respectively. The output fields (O1 and O2) of the target metasurface are related to the input fields (E1 and E2) by a scattering matrix [40]
Fig. 1. Schematic of the theoretical model. Two counter [1] propagating waves illuminate a metasurface at z = 0.
In Eq. (1), the scattering matrix [S] is denoted by
In Eq. (2), for the incident field E1, r1 and t1 refer to the complex reflection and transmission coefficients of the proposed metasurface. Similarly, the complex reflection and transmission coefficients r2 and t2 are associated with E2.
For realizing CPA, the destructive interference of the output waves in both z < 0 and z > 0 is a prerequisite. In other words, each outgoing wave component vanishes (O1 = O2 = 0), generating CPA, i.e. [S] = 0. Therefore, the necessary conditions for CPA can be obtained as follow
The principle of the reversibility of |t1|=|t2|=|t| can be used. Besides, On account of such a CAMS possessing the reciprocity and spatial symmetry under investigation, thus, |r1|=|r2|=|r|, the coherent absorptivity is expressed as follows
That is, two input coherent beams are of the same amplitude. And
with n being an arbitrary integer (see Supplement 1 for details).3. Design process and simulations
The challenge remains concerning the implementability of ultra-broadband coherent absorption. The designed steps of the CAMS are outlined in Figs. 2(a)-(c), where the lossy polyimide (permittivity is 3.5 and loss tangent is 0.0027 [41]) is employed as the medium in the presented metasurface and the mirror-symmetrical resistive films of 150 nm thickness is deposited on two sides of the dielectric plate (the resistance value of the resistive film is Rf = 55 Ω). Furthermore, the external and internal patterns of the proposed CAMS shown in Fig. 2(f) are given in Figs. 2(d) and (e), respectively. The numerical simulations of electromagnetic effects for such a metasurface are performed by commercial software High Frequency Structure Simulator (HFSS). The Floquet ports are set in the plane vertical to the z-direction.
Fig. 2. (a)-(c) Schematics depiction of evolution routine for the proposed CAMS, (d) the external pattern of the CAMS, (e) the internal pattern of the CAMS, (f) schematic diagram of the CAMS, the each unit is connected by the 4-µm long, 1-µm wide and 460 nm thick strips. The feature sizes of the proposed CAMS: a = 9.87 µm, b = 3.33 µm, c = 4.75 µm, d = 2.94 µm, e = 0.95 µm, l = 30 µm, m = 7.6 µm, v0 = 10.45 µm, v1 = 13.3 µm, h0 = 460 nm.
To achieve ultra-broadband coherent absorption, we first characterize a metasurface integrated with two ring-shaped resistive films on the sides of the medium designed by following the evolution routine depicted in Fig. 2. The simulated reflection, transmission, and absorption coefficients with normal incident wave are provided in Fig. 3(a), the reflection |r| get close to the transmission |t| in the frequency regions 8.88-14.03 THz and 21.15-23.81 THz. Besides, the absorptivity under a single beam illuminating reaches nearly 50% at the operating frequencies. Consequently, the generation of the destructive self-interference from the presented metasurface itself opens up the feasibility to realize CPA. When such a metasurface is illuminated via a coherent beam from the opposite side, remarkably, two coherent counter-propagating beams possess the same phase and intensity, fulfilling CPA. Thus, it can be observed in Fig. 3(d) that the phase difference of reflection and transmission coefficients is around ±π at 8.88-14.03 THz and 21.15-23.81 THz, which satisfies the conditions of CPA. As displayed in Fig. 3(d), the coherent absorptivity is higher than 90% in the operating regime. To further promote the coherent absorption effect and keep polarization independence, the phase-delay bars are integrated into the surface units. As delineated in Figs. 3(b) and (e), the relationship between |r| and |t| as well as Δφ1, meet the standard of CPA, as a consequence, the elevation of coherent absorption performance is accomplished with the devised metasurface exhibiting ultra-broadband coherent absorption with the relative bandwidth of 55.75% from 12.25 THz to 21.72 THz. Significantly, we are interested in the more high-efficiency coherent absorption that is highly compelling in many potential applications. Taking a further step, The full CPA-enhanced metasurface is shown in Fig. 2(f), where the external phase-delay lines are introduced to optimize the output intensity and the relative phase of reflection and transmission coefficients. For the case of an input beam, such a CAMS performs ultra-broadband absorption with absorbance closed to 50%, signifying the potential of the implementation for ultra-broadband CPA (in Fig. 3(c)). And when launching the counter-propagating beam, the absorption is improved by the interference caused by two coherent beams. It can be seen in Fig. 3(f) that the coherent absorptivity is over 90% at 7.6-26.51 THz (the relative bandwidth is 110.87%). As mentioned above, it is a practical strategy for the improvement of coherent absorption effect to employ attached phase-delay lines.
Fig. 3. (a)-(c) The reflection, transmission and absorption spectra of evolution routine for the proposed CAMS, (d)-(f) the coherent absorption spectra and the related phase difference (Δφ1) of reflection and transmission coefficients.
The contrast of the presented CAMS with and without phase-delay lines, as well as the electric field distributions at 13.37 THz and 19.05 THz are delineated in Fig. 4 to explain the mechanism leading to such a spectral widening effect. As shown in Fig. 4(c), a distinct enhancement for the electric field can be found around two ring-shaped patterns, especially around the external film with a maximum electric field intensity of 1.8 × 106 V/m. Instead in Fig. 4(d), when such a metasurface is equipped with phase-delay lines, the electric field is accumulated inside of the internal pattern while the electric field outside the external film is slightly weakened, revealing that the inherent dissipation is not destroyed, i.e. the premise of realizing CPA is still maintained, which is the presented metasurface exhibiting the destructive self-interference. Meanwhile, the phase difference (Δφ1) both accords with the demand for CPA (in Fig. 4(b)). That is to say, the phase condition of CPA is still met with the phase-delay lines integrated into this metasurface. Accordingly, One can observe in Fig. 4(a) that the designed CAMS, no matter with phase-delay lines or without that, exhibits CPA at 13.37 THz. However, comparing with the distributions of the electric field at 19.05 THz in Figs. 4(e) and (f), the electric field of the CAMS without phase-delay lines is stronger than that of the CAMS with that, manifesting the CPA condition may not be established. Besides, the phase difference (Δφ1) is apparently greater than -π. Based on the analysis of scattering matrix method, only when these CPA conditions are met, CPA will be achieved. In other words, the CPA cannot appear at 19.05 THz. As mentioned above, by utilizing the phase-delay lines, the coherent absorption performance cannot deteriorate in the specific operation frequencies and can simultaneously further enhanced in other target frequencies.
Fig. 4. (a) and (b) The comparison of coherent absorptivity as well as Δφ1 for the CAMS with and without phase-delay lines, the electric field distributions for the CAMS without phase-delay lines at (c) 13.37 THz and (e) 19.05 THz, the electric field distributions for the CAMS with phase-delay lines at (d) 13.37 THz and (f) 19.05 THz.
However, whether the coherent absorption performance can be elevated another step is suspended in front of us a crucial issue. Through the integration of annular metal and multiple dielectric films, significantly, the whole thickness of such a composite structure coincides with that of the medium for the CAMS, thereby enabling the linearization of phase response for the reflection and transmission. The diagrammatic sketches of structural evolvement are given in Figs. 5(a)-(f).
Fig. 5. (a) Schematic of the CAMS, (b) the layer-by-layer view of the devised CAMS, (c) the metal-integrated composite medium, (d) the layer-by-layer diagram of the composite medium, (e) schematic of the metal ring (gold), (f) schematic of the CAMS with metal loops. The feature sizes of the proposed CAMS with metal rings: u1 = 9.5 µm, u2 = 11.4 µm, w = 70 nm, h1 = 60 nm, h2 = 200 nm, h3 = 60 nm.
Similarly, the output intensities under an input beam are portrayed in Fig. 6(a) which shows that the reflection, transmission, and absorptivity are in conformity with the conditions of CPA. Simultaneously, owing to the construction of multiple stacked films, the linearization of phase response can be done, which renders the flatness of Δφ1 (the indigo curve in Fig. 6(b)) enhanced in comparison to that of the CAMS (the light purple curve in Fig. 6(b)). Therefore, through a clear observation in Fig. 6(c), the coherent absorptivity is further increased up to 95.7%, spanning from 8.34 THz to 25.07 THz (the relative bandwidth is 100.1%) (see Supplement 1 for details).
Fig. 6. (a) The reflection, transmission and absorption spectra for the CAMS with metal loops, (b) and (c) the contrast in Δφ1 and coherent absorptivity between the CAMS and the CAMS with metal loops.
Moreover, Fig. 7(a) exhibits the properties of coherent absorptivity dependent on the phase difference Δφ2 between two counter-propagating incident waves, in which the coherent absorptivity can be tailored successively from 95.7% to 38.1% by altering Δφ2. Besides, to see the change of coherent absorptivity very clearly, the coherent absorptivity at different Δφ2 is further marked in Fig. 7(a). When the phase difference is equal to ${\pm} {\textrm{2} / \textrm{3}}\mathrm{\pi }$ and ${\pm} {\textrm{4} / \textrm{9}}\mathrm{\pi }$, at 8.34-25.07 THz, the coherent absorptivity is higher than 43.1% and 67.6%, respectively. The coherent absorptivity is increased to 87.8% in the operating frequencies if phase difference is ${\pm} {\textrm{2} / \textrm{9}}\mathrm{\pi }$. To comprehend more intuitively, the coherent absorption spectra for the CAMS with metal rings under different Δφ2 are depicted in Fig. 7(b), which show that the coherent absorptivity is lower than 38.1% for the case of Δφ2=±π, while for the case of Δφ2 = 0, the coherent absorbance raises up to over 95.7% at 8.34-25.07 THz.
Fig. 7. (a) The color plot of coherent absorptivity as a function of Δφ2 and frequency (the white, indigo, reddish, and yellowish dashed lines represent respectively that the coherent absorptivity is 43.1%, 67.6%, 87.8%, and 95.7%), (b) the coherent absorption spectra for the CAMS with metal loops under different Δφ2.
In addition, Fig. 8(a) displays the properties of coherent absorptivity dependent on the relative intensity |E1|/|E2| between two back-propagation coherent beams, which shows that the coherent absorptivity can be continually adjusted from higher than 95.7% to below 67.7% via the variation of the relative intensity of two input beams. Furthermore, the coherent absorptivity at different relative intensities is further marked in Fig. 8(a), showing that, at 8.34-25.07 THz, the coherent absorptivity is increased from 74.9% to 94.9% with the relative intensity varying from 0.2 to 0.8. The operating bandwidth is gradually decreased with the coherent absorptivity enhanced continually. Additionally, the coherent absorption spectra for the CAMS with metal loops under different |E1|/|E2| are depicted in Fig. 8(b), indicating that the coherent absorptivity is lower than 67.7% for the case of |E1|/|E2|=0 corresponding to the single beam irradiation, while for the case of |E1|/|E2|=1, the coherent absorbance is increased to above 95.7% in the frequency regime 8.34-25.07 THz. And notably, if we use non-identical incident power, the CPA conditions (Eqs.(5)-(7)) of theoretical analysis will not work anymore.
Fig. 8. (a) The color plot of coherent absorptivity as a function of the relative intensity of input beams and frequency (the white, indigo, reddish, gray, and yellowish dashed lines represent respectively that the coherent absorptivity is 74.9%, 85.2%, 91.7%, 94.9%, and 97.6%), (b) the coherent absorption spectra for the CAMS with metal loops under different relative intensities of input beams.
The absorption performance can be affected by the sheet resistance, notably, the largest bandwidth can be achieved with the sheet resistance equal to 377 Ω. Nevertheless, in a metasurface structure, the surface resistance, the substrate thickness, the element spacing all influence the absorption bandwidth [42,43]. Thus, a mean resistance value of resistive thin films is necessary to fulfill high-efficiency absorption. As a key parameter for the fulfillment of nearly perfect ultra-broadband coherent absorption, we demonstrate how the CPA response of such a CAMS changes with the resistance value of the resistive film (Rf) in Fig. 9(a) with 97.3% and 95.7% absorptivity marked via the lavender and white dashed lines. As Rf is increased from 25 Ω to 55 Ω, the coherent absorption performance is ameliorated apparently. And as displayed in Fig. 9(a), with Rf up to 55 Ω, the ultra-broadband coherent absorption over 95.7% is accomplished from 8.34 THz to 25.07 THz (the relative bandwidth reaches up to 100.1%), especially at 8.8-20.2 THz, the coherent absorbance is higher than 97.3% with 78.6% relative bandwidth. When enhancing sequentially the resistance value of the resistive film from 55 Ω to 325 Ω, the coherent absorption effect, particularly in mid-frequencies, deteriorates further. To be more intuitive, the average coherent absorptivity and relative bandwidth for Rf are also investigated in Fig. 9(b), showing the successive decline of the average coherent absorptivity from 95.7% to 86.5% corresponding to the relative bandwidth promoted from 100.1% to 151.4%. Therefore, the high-efficiency coherent absorption with ultra-broadband bandwidth can be realized in a wide range of Rf.
Fig. 9. (a) The contour plot of coherent absorptivity as a function of the resistance value of the resistive film (Rf) and frequency (the indigo and white dashed lines represent that the coherent absorptivity is 97.3% and 95.7%), (b) the average coherent absorptivity and relative bandwidth for such a CAMS with metal rings with varied from 55 Ω to 295 Ω.
Such a metasurface is polarization independent due to symmetry geometry despite the slight effect of the internal individual hexagonal patterns. And the relationship of coherent absorbance associated with polarization angle at normal incidence is demonstrated in Figs. 10(a) and (b), showing that the polarization angle has hardly an impact on the coherent absorption effect. In addition, the CPA performances of such a CAMS at oblique incidence for transverse electric (TE) and transverse magnetic (TM) are investigated in Figs. 10(c)-(f). For the TE wave (in Figs. 10(c) and (e)), the CAMS represents a stable coherent absorption effect for an angle up to 30° at 8.34-25.07 THz. For larger incident angles, the coherent absorptivity commences dropping in the operation frequencies, yet is still over 74.8% with an angle of 70°. In Figs. 10(d) and (f) ((for the TM wave)), with the incident angle increased from 0° to 30°, above 90% coherent absorptivity in the operating frequency region is visible. Nevertheless, the coherent absorption function is decreased with the angle raised to 60°. Fortunately, the coherent absorbance is higher than 81.3% from 6.67 THz to 24.2 THz. It can be concluded that the proposed CAMS possesses a good omnidirectional CPA response.
Fig. 10. The contour plot of coherent absorptivity as a function of the polarization angle and frequency for (a) TE wave and (b) TM wave, the contour plot of coherent absorptivity as a function of the incident angle and frequency for (c) TE wave and (d) TM wave, the absorption spectra of different incident angles for (e) TE wave and (f) TM wave.
Eventually, we sum up the coherent absorption devices reported earlier, listed in Table 1 for comparison. Many works perform brilliant performance, notably at microwave frequencies, the conductive film and graphene can be used to realize high-effective coherent absorption by virtue of the nondispersive conductivity [37,44], strongly beneficial to ultra-broadband applications. Likewise, black phosphorus can be utilized to realize ultra-broadband CPA. In this paper, we adopt a novel approach for the realization of ultra-broadband (8.34-25.07 THz) CPA by combining resistive thin films and phase-delay lines.
Table 1. Comparisons between this work and reported coherent absorption devices
4. Metamaterial fabrication and challenges
The methods of optical lithography are mainly used for metamaterial fabrication, which generally includes wafer cleaning and drying, coating photoresist, prebaking, exposure, post-baking, development, hard baking, and other processes. Among them, it should be noted that exposure and development in the lithography process are directly related to the accuracy of metal nanostructures.
The metal (gold) film can be deposited on a silicon wafer utilizing magnetron sputtering. And then, such a sample is immersed in acetone for 8 hours to fully dissolve the resist and obtain the suspended gold film. Subsequently, a transmission electron microscope copper grid is dipped into the solution for picking up the suspended film. The suspended film is dried in N2 in a cleanroom environment. By applying these steps, a flat and self-supporting gold film can be obtained. Next, the metal ring can be defined by a focused ion beam on the suspending gold film. After electron beam lithography and metal deposition, a metal loop can be patterned on the polyimide film [48,49].
The resistive thin film can be manufactured by the silk screen printing technique through a photo-etched frame with carbon black serum [50].
Remarkably, The difference in the coherent absorption spectra between the experimental measurements and simulated results can attribute to the structure imperfection of the fabricated sample, such as deviation of the permittivity of dielectric during the fabrication as well as the resistance deviation of the resistive thin films and so on [50,51].
5. Conclusion
In conclusion, the design process of ultra-broadband CPA based on the CAMS incorporating dual ring-shaped resistive films with attached phase-delay lines and dielectric-metal composite structure is demonstrated. By varying the phase difference of two counter-propagating waves, at 8.34-25.07 THz, the coherent absorptivity of the CAMS can be continually tailored from above 95.7% to below 38.1%. And the coherent absorptivity can be continually adjusted from higher than 95.7% to below 67.7% via the variation of the relative intensity of two input beams. The theoretical investigations manifest that the CPA of the proposed CAMS stems from the intrinsic absorption by the device destructively interfering and then the significant absorption boost relying on the interference caused by two coherent beams. In addition, it is of especial importance to determine the resistance value of the resistive film, which act a pivotal role in the realization of ultra-broadband coherent absorption. Moreover, the ultra-broadband coherent absorption can be achieved for either TE or TM waves at incident angle up to 80°. As well the polarization insensitivity of this metasurface is investigated. Such a metasurface provides an alternative approach to devise ultra-broadband coherent absorption devices based on the ingredient with linear phase response, enriching the design of CPA devices and offering attractive applications in the domain of THz modulation and signal processing.
Funding
Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX20_0807).
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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