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Abstract
Monitoring and controlling the state of polarization of electromagnetic waves is of significant interest for various basic and practical applications such as linear position sensing and medical imaging. Here, we propose the first conformal digital metamaterial absorber to detect the polarization state of THz incident waves. The proposed polarimeter is capable of characterizing four independent polarization states of (TE, TM, ±45° linear, and RCP/LCP) by observing the reflectivity of the structure with respect to the x- and y-direction. Besides, the proposed structure displays a strong absorptivity above 90% up to the incidence angle of 50° for oblique incident waves with different polarizations. By mere changing the bias voltage of two orthogonal VO2 microwires via two independent computer-programmed multichannel DC network, distinct conditions for reflected waves occurs under excitations of different polarizations, whereby the polarization state of the incident wave may readily be estimated. We believe that the proposed metasurface-based polarimeter can pave the way for polarization detection applications on curved surfaces.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, Terahertz (THz) waves have reached maturity and have witnessed remarkable attention due to their distinct features that make them beneficial in various sensing and imaging technologies [1,2]. Artificial metasurfaces, as periodic arrangements of engineered sub-wavelength structures, can be structured for complex and functional manipulation of electromagnetic (EM) waves [3–5]. They can be exploited to modify the permittivity and permeability of materials by purposefully controlling the local properties of phase, amplitude, and polarization of EM waves. Metamaterials remarkably provide a wider range of wave-matter applications which present them especially appealing in the usages of tunable absorbers [6–8], Graphene-based meta-sensing, and near field imaging [9,10], and amplitude modulation [11]. Beyond the scope of analog metamaterials, digital metamaterials have experienced rapid evolution compared to traditional wave manipulations and outstandingly provide a broader range of wave-matter functionalities which make them fascinating in various practical applications such as smart surfaces, power intensity control, and programmable structures [12–15].
Given that each polarization can represent an independent information in communication systems, the polarization can be considered as one of the fundamental attributes characterizing the features of EM waves, and the measurement of polarization divulges rich information about the nature of scattering, radiation, and absorption phenomena. A polarization-dependent metamaterial absorber is an ideal building block for polarimetric sensing systems which can equip an inspiring platform for solving some crucial THz challenges such as linear position sensing, security scanning, and medical imaging [16–19].
After introducing the first perfect metamaterial absorber (MMA) by Landy et al. in 2008 [20], as a counterpart to conventional absorbers, MMAs have become research hotspots and have also been of great attention for solving EM interference challenges, such as the stealth technologies, radar absorbing and thermal emitter [21–23]. Generally, the maximum absorption is obtained for the normal incidence. However, when a metamaterial absorber is built on a curved surface, its absorption efficiency drops drastically. Accordingly, the flexibility and incident angle insensitivity are two main factors for designing MMAs for curved surfaces that have been followed by some works in recent years [24,25].
The necessity of polarimetric sensors for curved surfaces has led us to design a first conformal MMA for detection of the polarization states of THz wavefronts. Our proposed phase/polarization encoded anisotropic digital metasurface can exhibit the perfect absorptivity in wide incident angles based on tuning the proper electrical resistivity of vanadium dioxide ($\textrm{V}{\textrm{O}_\textrm{2}}$).
$\textrm{V}{\textrm{O}_\textrm{2}}$ is a well-known tunable material that undergoes an ultrafast reversible phase transition from insulator monoclinic to metallic tetragonal phase above the critical temperature around ${T_c}{{ = 340 \textrm{K}}}$. Due to the dramatic changes of electrical and optical properties of $\textrm{V}{\textrm{O}_\textrm{2}}$ across the two phases, it has been identified as a notable substance in tunable metamaterial devices over a broad spectral range and has myriad applications in the THz regions, such as THz waves modulator, angle-insensitive absorber, and programmable ultrafast meta-devices [26–29].
In this paper, a THz polarimetric device is proposed as depicted in Fig. 1, which comprises two perpendicular $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires whose reflection characteristics can be dynamically tuned in a real-time manner independently for two orthogonal x- and y-polarized excitations. The proposed $\textrm{V}{\textrm{O}_\textrm{2}}$- integrated anisotropic metasurface (VIAM) can retain the perfect absorptivity in wide oblique incidences for THz waves that make them suitable for a curved surface. By digitally changing the spatial voltage distribution provided by two independent multichannel DC bias voltages, and observing the reflection properties of scattered beams via two receiving detectors, the state of incident polarization can be determined. To demonstrate the main mechanism of VIAM, the relationship between spatial coding sequences and the polarization of the incident beam is investigated. Besides, we show that increasing the electrical conductivity of $\textrm{V}{\textrm{O}_\textrm{2}}$ plays an effective role in dissipating the incoming EM energy especially when the oblique incident angle increases. We believe that the integrated structural design, ultrafast switching time, and wide coverage of incident-angle characteristics pave the way for many scientific and engineering applications, such as linear position sensing and future practical THz devices.
Fig. 1. a) The schematic diagram of conformal metamaterial absorber for polarization detection application. (b), schematic view of the unit-cell. (c) schematic view of the back of the unit-cell where Vx and Vy are two patches that serve as positive electrodes for applying DC bias voltage.
2. Results and discussions
Figure 1(b) shows the sketch representation of the proposed meta-particle composed of four layers of $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires, polyamide, gold plate, and polyimide from top to bottom respectively. Two pieces of separated patches are located beneath the bottom substrate as positive electrodes which are independently connected to two DC bias voltage networks as shown in Fig. 1(c). The gold plate acts as a negative electrode which is connected to two lateral metalized electrodes for each $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires through two metallic via holes. Through this mechanism, all the biasing wires and networks can be located beneath the structure without interfering with the EM properties of the metasurface. The geometrical dimensions are P=50 $\mu m$, H=17 $\mu m$, and T=40 $\mu m$ respectively. The thickness and the width of the $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires are 1 $\mu m$ and 4 $\mu m$.
The complex permittivity of the $\textrm{V}{\textrm{O}_\textrm{2}}$ thin films can be determined by the Bruggeman effective-medium theory in the THz frequencies as:
In the above equation, $\varepsilon _d$ and $\varepsilon _m$ represent the dielectric constant of the insulator and metallic states, respectively and $V$ implies the volume fraction of metallic phase [30]. Typical $\textrm{V}{\textrm{O}_\textrm{2}}$ thin films show electrical conductivity in the range of $10 \sim 100\,\,\textrm {{S/m}}$ with the relative permittivity of about 9 at room temperature [31]. After occurring the structural transformation, its electrical conductivity reaches as high as an order of ${10^5}$ in the metallic state [32]. We have performed full-wave simulations by the commercial CST software. For the evaluation of reflection properties of the infinite array of VIAM, the periodic boundary conditions are utilized along the x and y directions and the open boundary condition is employed along the z-axis. Meanwhile, linearly and circularly polarized plane waves at different incidence angles illuminate the VIAM structure.
Since the thickness of a ground plane (gold layer) is much larger than the penetration depth of the incident THz wavefronts ($T(\omega ) = 0$), the absorptivity of the VIAM can be calculated by Eq. (2), where $A(\omega )$ and $\Gamma (\omega )$ are the absorption and reflectance, respectively:
When $\textrm{V}{\textrm{O}_\textrm{2}}$ is in the dielectric steady-state phase and having the lowest electrical conductivity, the reflection characteristics of the VIAM is in its highest value. On the other hand, at the intermediate temperatures, the transmitted waves are dissipated due to the significant $\textrm{V}{\textrm{O}_\textrm{2}}$ ohmic losses, and the maximum absorption efficiency is achieved. Therefore, we set ${\sigma _{off}} = 10 S/m$ ( "0" digital state) and ${\sigma _{on}} = 5 \times {10^4} S/m$ ( "1" digital state). The conductivity of $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires along the x- and y-direction are marked by ${\sigma _x}$ and ${\sigma _y}$, and by independently changing the DC bias voltage, four digital states of 0/0, 0/1, 1/0, and 1/1 correspond to ${\sigma _{off}}$/${\sigma _{off}}$, ${\sigma _{off}}$/${\sigma _{on}}$, ${\sigma _{on}}$/${\sigma _{off}}$, ${\sigma _{on}}$/${\sigma _{on}}$ will be attained where the binary codes before and after the slash symbol indicates the digital codes for the $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires along the y and x-directions, respectively.
Under the normal incidence, numerical simulations have been conducted for different polarized incident waves, and the reflection spectra of the proposed VIAM for four different digital states are illustrated in Fig. 2. The absorptivity of VIAM is novelty employed to characterize the four independent polarization states (TE, TM, $\pm {45^\circ }$ linear, and RCP/LCP). Consequently, the proposed structure demonstrates a wideband polarization-dependence of absorption for distinct independent polarization, thus, the high contrast can be realized for sensing applications. Consider the TM polarized waves as an example (depicted in the first row of Table 1), since the VIAM has anisotropic structural nature, just those $\textrm{V}{\textrm{O}_\textrm{2}}$ films which are parallel to x-direction are excited and the reflection amplitude response for cross-polarization is zero. Therefore, the receiving detector towards the y-direction receives nothing for all the four spatial code distributions. Fig. 2(a) shows the reflection coefficient under the excitation of TM polarized waves for the receiving detector on x- direction. In this case, since the reflectivity on y- direction is zero, we do not show its diagram. Observe in Table 1 that the polarization cross-talk between two orthogonal directions is negligible, and changing the digital state of $\textrm{V}{\textrm{O}_\textrm{2}}$ microwire in one orthogonal direction does not affect the electromagnetic response in the other direction for the LP incident THz wave.
Fig. 2. Simulated results of reflectivity of the designed VIAM structure under the excitation of a) TM polarized THz incident waves. b) TE polarized THz incident waves. c,d) Circular polarized THz incident waves for receiving detector on X and Y directions respectively. e,f) $+45^\circ$ linear polarized THz incident waves for receiving detector on X and Y directions respectively.
Table 1. Reflectivity for four digital states at the frequency of 2.4 THz and the corresponding distinct polarization states. $R_{x}$ and $R_{y}$ denote the reflection amplitude received by detector on the x and y-direction, respectively.
For another example, consider RCP THz incident waves illuminating the structure (Fig. 2(c) and Fig. 2(d)). For a digital state of 0/0, all the EM energy reflected by RCP, and according to the relationship between CP and LP polarization, we observe that half of the EM radiation reflected towards x-direction, and the remaining EM energy reflected towards y-direction. For the digital state of 1/1, all the incident THz wave energy is effectively absorbed by the proposed VIAM due to the ohmic losses of $\textrm{V}{\textrm{O}_\textrm{2}}$ films. For the digital states of 0/1, a quarter of the incident wavefronts is reflected by RCP, and the other quarter of the wave energy is reflected by LCP, and these two wavefronts have the same phase difference. Therefore, half of the incident THz wavefront is reflected towards x-direction and the remaining wave energy is absorbed by the structure. In the same way, for the 1/0 digital state, half of the EM energy absorb by the structure and the remaining THz wavefront is reflected toward y-direction. As a final comment, all the conditions presented in Table 1 are distinct, and by observing the reflected energies through x- and y-polarization, one can readily determine the state of the incident THz wavefronts polarization. It should be noted that due to the nature of the structure, our designed VIAM can not differentiate between LCP and RCP as well as +$45^\circ$ LP and $-45^\circ$ LP.
When an MMA is fabricated on a curved surface, only those meta-atoms at the center of the absorber are illuminated at normal incidence, while, the other meta-particles on the curved surface are illuminated at oblique incidence. However, by increasing the incident angle, the degradation of absorptivity is inevitable [17]. For instance, under TE and TM oblique incidence, the reflection coefficients can be obtained by:
Figure 3(b) depicts the absorption spectra of the designed VIAM under excitation of circular polarization. The near-unity absorption can be achieved up to $50^{\circ }$ for the oblique incident THz waves. Due to the nature of the structure, for incident waves with four different polarizations, always the anti-parallel currents are effectively excited on $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires, leading to a wide-angle absorption charactristics. Accordingly, our proposed structure shows a wideband absorptivity above 80% in $1.6-3$ THz for oblique incidences up to $50^{\circ }$ for different cases of incident polarization (LP, $\pm {45^\circ }$ linear, and CP).
Fig. 3. The simulated result of absorptivity for different cases of a) (TE, TM, $\pm {45^\circ }$ linear)-polarized oblique incident THz wavefronts. b) RCP and LCP oblique incident THz wavefronts, when the electrical conductivity of $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires are 50000 S/m.
In the following paragraph, we survey two of the potential applications of our VIAM structure. A metamaterial-based polarimetric sensor reflects one polarization while absorbs the other, which renders them a perfect building block for the barcode of a THz encoder system. Linear position sensors identify the location of a barcode when it has linear movement with respect to the sensor head. Position sensing is highly favorable in numerous industrial usages, like integrated circuit assembly equipment, laser scanners, and printers. Therefore, our VIAM structure has the potential in terahertz barcode location identification [18]. Moreover, our structure can be used on curved surfaces which makes it highly desirable for practical applications. As another application, our structure can be used as a compact In-line polarimeter [17]. The measurement of polarization manifests rich information, for instance about the arrangement of materials, the handedness of chiral molecules, and commonly about the character of scattering, emission, and absorption. Our polarimeter benefits from the flexibility, simplicity, and compactness of a metasurface structure which makes them highly suitable for industrial polarimeters. Our structure does not employ the surface plasmon propagation which significantly reduces optical losses and, moreover directly measurement the state of polarization can be feasible.
3. Method
In this section, we provide a brief introduction to the current fabrication technologies for realizing the proposed $\textrm{V}{\textrm{O}_\textrm{2}}$-based polarimetric sensor [33,34]. This process can follow the steps below: (a) The fabrication process begins with a polyimide film as a flexible substrate; (b) 500 nm-thick gold layer is deposited on polyimide substrate through e-beam evaporation; (c) 17 $\mu m$-thick polyimide is deposited on the substrate through sputtering deposition; (d) After depositing the photoresist on the polyimide through photolithography, the desired microwires-like patterns can be obtained using an electron beam lithography; (e) Through magneton sputtered technique, 1 $\mu m$ thick $\textrm{V}{\textrm{O}_\textrm{2}}$ thin films can be deposited on the substrate, and after the lift-off process, patterned $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires are annealed at 450 degrees Celsius. Focused ion beam (FIB) drilling techniques is the suitable method for fabricating metallic via holes with nanometers and micrometers diameters [35,36]. After drilling the holes through polyimides, they can be filled with gold through the e-beam evaporation and lift-off process.
In the following paragraph, we survey some works for THz polarization component measurement [37–47]. Polarization-dependent THz measurements need an exact specification of the magnitude and direction of the electric field. A commonly used approach is to detect the changes in the THz polarization state employing two polarizers in the crossed-Nicol arrangement [39–41]. However, in this approach, the extinction ratio of the polarizers limits the sensitivity of the polarization angles. In the THz frequency band, wire-grid polarizers (WGPs) generally are employed for this goal. Some attempts have been proposed to enhance the extinction ratio of these polarizers [42]; but still, the poor extinction ratio has confined the sensitivity in THz polarization measurements. An electro-optic (EO) sampling method with probe-polarization modulation has been reported in [43] which allows users to identify the THz electric-field vectors with arbitrary directions with angular accuracy and precision. Moreover, real-time identification of the polarization of the THz waves by simultaneous detections of the two orthogonal polarization components have been presented in [44,45] by splitting terahertz beams, or the circularly polarized probe laser pulses [46] by employing beam splitters and determine the two electric-field amplitudes through two detectors concurrently. An approach to quickly and accurately determine the polarization direction of coherent terahertz electromagnetic waves produced by femtosecond laser pulses has been reported in [47]. Therefore, both fabrication and measurement method for our VIAM structure are feasible within the current fabrication technologies.
4. Conclusion
In summary, we have introduced the first conformal metamaterial absorber for polarization detection that can characterize the four independent polarization states of (TE, TM, $\pm {45^\circ }$ linear, and RCP/LCP). Our proposed VIAM structure displays a strong absorptivity up to the incidence angle of $50^{\circ }$. By encoding the VIAM in four distinct digital states through changing the independent DC bias voltages for two orthogonal $\textrm{V}{\textrm{O}_\textrm{2}}$ microwires, our designed structure is successfully characterized the polarization of THz incident wavefronts with an ultrafast switching time between distinct digital states. We believe that the presented conformal metasurface-based polarimeters may find great potentials for solving some crucial THz challenges, such as linear position sensing, security scanning, and medical imaging.
Disclosures
The authors declare no conflicts of interest.
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