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Abstract
We present a multitasking tailored device (MTD) based on phase change material vanadium dioxide (VO2) and photoconductive semiconductor (PS) in the terahertz (THz) regime, thereby manipulating the interaction between electromagnetic waves and matter. By altering the control multitasking device, its room temperature, or pump illumination, we switch the function of absorption or polarization conversion (PC) on and off, and realize the tuning of absorptivity and polarization conversion rate (PCR). Meanwhile, the construction of cylindrical air columns (CACs) in the dielectric provides an effective channel to broaden the absorption bandwidth. For the MTD to behave as a polarization converter with VO2 pattern in the insulating phase (IP), exciting the PS integrated to the proposed device via an optical pump beam, the PCR at 0.82-1.6 THz can be modulated continuously from over 90% to perfectly near zero. When the PS conductivity is fixed at 3×104 S/m and VO2 is in the metal phase (MP) simultaneously, the MTD switched to an absorber exhibits ultra-broadband absorption with the absorptivity over 90% at 0.68-1.6 THz. By varying the optical pump power and thermally controlling the conductivity of VO2, at 0.68-1.6 THz, the absorbance of such a MTD can be successively tuned from higher than 90% to near null. Additionally, the influences of the polarization angle and incident angle on the proposed MTD are discussed. The designed MTD can effectively promote the electromagnetic reconfigurable functionalities of the present multitasking devices, which may find attractive applications for THz modulators, stealth technology, communication system, and so on.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Manipulating the electromagnetic waves can be particularly significant in responding to increasingly complex electromagnetic circumstances and communication demands. Thereinto, the polarization converter and absorber are two essential devices in the manipulation of electromagnetic waves. Polarization converter, flexibly regulating the polarization state, exhibits numerous potential applications such as circularly polarized antennas [1,2], radomes [3,4], sensors [5,6]. Absorber, absorbing or harvesting the energy of electromagnetic waves, is of momentous significance to decrease radar cross section, thereby fulfilling the electromagnetic stealth [7–9]. Up to now, extensive linearly-polarized converters [10–12] and circularly-polarized converters [13–15] have been reported. Meanwhile, abundant research has been put forward on the fulfillment of multi-band [16–18], broadband [19–21] absorbers. Nevertheless, owing to single function as well as fixed operating frequency, these devices mentioned above appear fatigued to meet the growing requirements of current technological evolution.
Vanadium dioxide (VO2) is a well-known phase change material, exhibiting thermal-induced phase transition from the insulating phase (IP) to the metal phase (MP) when the critical temperature reaches nearly 68 °C. Below 68 °C, VO2 acts as an insulator with a monoclinic structure. Conversely, once the temperature is above the critical temperature, atomic rearrangement of VO2 leads to the metal phase with a tetragonal lattice [22], thereby generating transformation in physical properties. It is stressed that the variation in the optical and electrical properties of VO2 is attributed to the trigger for the phase transition with abrupt several-orders-of-magnitude changes [23], which can be manipulated through thermal control [24]. Additionally, the insulator-metal transition can be triggered by light [25], external bias voltage [26], and lattice strain [27]. By virtue of these extraordinary features, VO2, as a promising candidate, applied in diverse advanced contexts, such as optical switches [28,29], modulators [30,31], thermal sensors [32,33], and so forth, has been reported. Meanwhile, there have been some reports of VO2 fabrication through direct current magnetron sputtering (DCMS) [34–36] and pulsed laser deposition [37,38].
Semiconducting materials incorporated into the metamaterial, via the application of an optical pump beam, can be switched between an insulating state as well as a conducting state, thereby altering the metamaterial response [39,40]. Consequently, there is currently substantial research in employing photoconductive semiconductor (PS) combined with metamaterial to implement tuned absorption. The semiconductors (silicon) are embedded into critical parts of metallic split-ring resonators, leading to the modification of optical response by excitation of the semiconductors with a pump beam [41–43].
One strategy for realizing function reconfiguration exploits the ability of graphene. This approach has been explored via graphene-based metasurfaces. A graphene-based broadband absorber, integrated with annular and cross-shaped patterns, was reported by Feng et al. [44]. The most noteworthy feature of such an absorber is that the absorbance can be tailored from 1% to as high as 99% via the enhancement of the Fermi energy level from 0 eV to 0.7 eV. A multitasking device constructed by graphene-based metasurface is capable of performing steerable absorption with absorptivity over 80% at 1.59-4.54 THz as well as polarization conversion (PC) with polarization conversion rate (PCR) higher than 90% at 2.11-3.63 THz by altering the chemical potential of graphene [45].
Advances toward the employment of VO2 and graphene in multifunctional devices have been put forward. Through a configuration of VO2 and graphene, Zhu et al. [46] put forward a switchable THz device with broadband and multi-band absorption. A multitasking device integrated with the VO2-graphene metamaterials was reported by Liu et al. [47], where the dynamically switchable dual-broadband absorption can be achieved while the absorption intensity can be tailored consecutively from 5.2% to 99.8% by electrically controlling the graphene. Theoretically presented by Li et al., a reconfigurable metamaterial functional device, realizing the switch between an absorber and a transmittance modulator, is generated on basis of VO2 and two-dimensional graphene [48]. Therefore, to accommodate the developing THz systems, compact and reconfigurable devices such as dynamic regulation and compatibility of multiple functions will be an inevitable trend.
In this work, by ingenious design, the absorptivity and PCR of the reported MTD based on the PS-VO2 pattern can be independently tuned by, respectively, manipulating the phase transition of VO2, as well as the optical pump illumination. Remarkably, the cylindrical air columns (CACs) are embedded into the medium of the MTD, improving the absorption performance. When the MTD acts as an absorber, the ultra-broadband absorption can be realized from 0.68 THz to 1.6 THz. In contrast, the proposed MTD operating as a polarization converter still exhibits strong PC effect with above 90% PCR at 0.82-1.6 THz. And more critically, the absorption frequencies of such a device can totally cover the operating frequencies of PC mode. The field explanation, the distributions of surface currents and the power loss, as well as the impedance match theory are demonstrated to illustrate the operation principle. Moreover, the absorption performance and the PC for oblique incidence, as well as the polarization insensitivity in the absorption mode, are also investigated. Such a MTD possesses the merits of multitasking collaborative processing, wide operating frequencies, and flexible design, enriching the design of multifunctional devices and providing the potential applications in stealth technology, electromagnetic shielding, communication system, etc.
2. Design processes and simulations
2.1 Structure and analysis of the MTD based on gold-VO2 pattern
Our design, which is composed of a hybrid gold-VO2 pattern and a dielectric slab on the bottom metallic layer (gold with conductivity of 4.561×107 S/m [49]), provides a novel source of the realization of a multitasking device with high-efficiency absorption and PC. The reported MTD equipped with the gold-VO2 resonator (see Figs. 1(d) and (e)) is schematically depicted in Fig. 1(a). Thereinto, such a MTD can be acted respectively as an absorber or a polarization converter due to VO2 in MP and IP, shown in Figs. 1(b) and (c). The lossy polyimide with permittivity of 3.5 and loss tangent of 0.0027 [50] is chosen as the medium in this MTD. Both the gold-VO2 layer and the metallic plate are 500 nm thick.
Fig. 1. (a) Schematic depiction of the MTD based on the gold-VO2 pattern, such a MTD operating in (b) PC mode and (c) absorption mode; (d) and (e) the detailed diagrams of the gold-VO2 pattern. The feature sizes of the proposed MTD: r1 = 65 µm, r2 = 80 µm, r3 = 50 µm, r4 = 75 µm, d1 = 8 µm, d2 = 5.5 µm, d3 = 8 µm, d4 = 8 µm, θ1 = 60°, θ2 = 26°, b = 5 µm, h = 47.6 µm, l = 150 µm.
In our calculation, the software High Frequency Structure Simulator (HFSS) is employed to obtain the electromagnetic response of the proposed MTD. The Master-Slave boundary conditions are set to calculate this infinite periodic unit and the floquet port setting upon this MTD is used to the simulation that the electromagnetic waves along the -z-direction is vertically incident on the target matter from infinity.
In the case of the presented MTD behaving as a polarization converter, the co-polarization and cross-polarization reflection coefficients can be expressed by [51]
In Eqs. (1) and (2), Eyi is the electric field (EF) of the y-polarized incident wave, Eyr and Exr are the y-polarized EF and x-polarized EF of the reflected waves. Therefore, the PCR can be obtained via Eqs. (1) and (2) [52].
For the EF polarized along the y-axis, the physical principle of such a device as a polarization converter is described in Fig. 2(a). The incident EF Ei can be decomposed into the component Eiv along the v-axis and Eiu along the u-axis, indicated in Eq. (4). Accordingly, the EF of the reflected wave is described as Eq. (5), in which ruu and rvv refer to the reflection coefficients along the u-axis and v-axis, respectively.
On account of the anisotropy for the devised MTD under the PC mode, the phase difference occurs between Eru and Erv, as shown in Eq. (6)
When ${r_{uu}} \approx {r_{vv}}$ and $\Delta \phi \approx 180^\circ$, the polarization direction for the EF of the reflected wave will be converted to the x-axis, consequently, causing the actualization of PC. To verify this, the reflected amplitudes ruu and rvv as well as the phase difference are presented in Fig. 2(b). As one expected, the reflection amplitudes of ruu and rvv are approximately equal and their corresponding phase difference is roughly 180°, which results in high-efficiency and ultra-broadband PC in the frequency regime 0.55-1.14 THz.As VO2 is in MP, such a MTD converted to absorption mode is performed. The absorptivity of the MTD can be calculated through
In Eq. (7), R is reflectivity and T is transmissivity. By virtue of the back metal plate interdicting the transmission of electromagnetic waves, it is possible to ignore the transmissivity. Therefore, the absorptivity merely relevant to the reflected coefficients of co-polarization ryy and cross-polarization rxy can be described by
Fig. 2. (a) Schematic view of the conversion from co-polarization to cross-polarization; (b) the calculated reflection and phase difference.
In Fig. 3, the absorptivity, PCR, output intensity of reflection for the devised MTD assembled by the gold-VO2 pattern are obtained under the electromagnetic waves normally incident. In the case of the MTD integrated by the insulating VO2 film, such a device operates as a polarization converter. As plotted in Figs. 3(a) and (c), the PCR over 90% at 0.55-1.14 THz originates from the co-polarization reflection coefficient lower than 0.3 and the cross-polarization reflection coefficient higher than 0.9 in the target frequencies. When VO2 transforms into an IP state, the PC property is weakened, accompanied by the output intensity of the cross-polarization reflection coefficient decreased (see Fig. 3(d)). In this case, it can be seen in Fig. 3(b) the proposed MTD operating as an absorber exhibits dual-band absorption at 0.44 THz and 1.16 THz with absorptivity of 88.25% and 93.04%.
Fig. 3. Schematic depiction of the MTD based on the gold-VO2 pattern, accompanied by the simulated (a) PCR and (c) output intensity for VO2 in IP as well as (b) absorptivity and output intensity for VO2 in MP.
As can be seen notably in Fig. 3(a), there are three strong resonant peaks with the PCR closed to 100% at 0.61 THz, 0.77 THz, and 1.09 THz. Hence, the mechanism of the proposed MTD with the gold-VO2 sheet operating in the PC mode can be elucidated via the current distributions depicted in Figs. 4(a)-(f) at these frequencies. When the electromagnetic waves with the EF Ei along y-direction are incident to the target device, one can be observed from the currents at 0.61 THz in Figs. 4(a) and (d) that anti-parallel currents generated between the upper metal pattern and the metal ground will induce magnetic moment m, thereby exciting induced magnetic field (MF) H. The MF H can be decomposed into horizontal component Hx and perpendicular component Hy. Noted that the MF component Hy is parallel to the EF component Ei, as a consequence, the PC from co-polarization wave to cross-polarization wave is realized. Similarly, the opposite currents shown in Figs. 4(b) and (e) give rise to equivalent magnetic moments m1 and m2 (the currents along the x-axis and y-axis are marked respectively by 1 and 1’ as well as 2 and 2’). Therefore, the cross-coupling will occur between the induced MF H1 and the EF Ei. Nevertheless, the component Hx of the MF excited at 0.61 THz and the induced MF H2 at 0.77 THz are vertical to the incident EF Ei, indicating that there is no cross-coupling between Ei and Hx as well as and H2 [53]. Additionally, it can be seen from the current distributions at 1.09 THz in Figs. 4(c) and (f) that the electric resonance is excited due to the currents along the arc-shaped pattern parallel to those on the back metal ground. Thus, the combination of the electric and magnetic coupling is responsible for the realization of high-efficiency and ultra-broadband PC.
Fig. 4. The surface current distributions of the front arc-shaped metal at (a) 0.61 THz, (b) 0.77 THz, (c) 1.09 THz; the surface current distributions of the back metallic ground at (d) 0.61 THz, (e) 0.77 THz, (f) 1.09 THz.
The switching from the PC state to the absorption state in the MTD is accomplished owing to the merit of VO2 that the conductivity can be altered via thermal control. It is stressed that the conductivity of the VO2 pattern is positively related to the room temperature during this process. The VO2 pattern in diverse room temperatures leads to the absorption feature of the MTD plotted in Fig. 5(a), showing the operational absorption switching effect. Furthermore, in Fig. 5(b), we depict the three-dimensional color mapping plot for the absorptivity as a function of σ(VO2) and frequency. It is considerably clear to grasp the universal tendency in the absorption nature under different VO2 conductivities. As shown in Fig. 5(b), the conductivity of VO2 is up to 5×104 S/m, enabling strong absorption of over 80% from 0.4 THz to 1.18 THz. And the absorption performance has slight deterioration with the further increase of the VO2 conductivity. Note that, by virtue of the extraordinary temperature-sensitive property of VO2, the over 80% absorbance running from 0.4 THz to 1.18 THz can be modulated successively to near zero by varying the working environment temperature. Obviously, the simulations manifest the switching between the PC and absorption states, achieved by altering the conductivity of VO2 under thermal control.
Fig. 5. (a) The absorption spectra of the MTD under different VO2 photoconductivity levels; (b) three-dimensional color mapping plot of calculated absorptivity as a function of frequency and VO2 photoconductivity.
To get a clear cognition of the generative mechanism for the presented device as an absorber, an energy-level diagram representing the plasmon hybridization in VO2 loops is portrayed in Fig. 6(a). In a VO2 loop, the essentially fixed-frequency plasmon of a VO2 sheet can interact with the plasmon of a VO2 cavity, thereby producing induced charges at the inner and outer interfaces of the VO2 loop. Owing to this interaction, the plasmon resonance can be split into two new resonances, i.e., bonding plasmon and antibonding plasmon [54,55].
Fig. 6. (a) An energy-level diagram representing the plasmon hybridization in VO2 loops stemming form the interaction between the VO2 sheet and cavity plasmon; the EF distributions at (b) 0.44 THz and (c) 1.16 THz on the hybrid gold-VO2 pattern.
The extinction spectrum depicted in Fig. 3(b) indicates bonding plasmon at 0.44 THz, accompanied by antibonding plasmon at 1.16 THz. To demonstrate this, the EF distributions at two resonant peaks are displayed in Figs. 6(b) and (c), where the plasmon modes being “++–” at 0.44 THz and “+-+-” at 1.16 THz are clarified by the directions of the EF. Therefore, the plasmon hybridization mode from the VO2 loops provides a strong contribution to the absorption of the MTD.
Besides, the absorption spectra and the power loss density distributions of such a device with the VO2 film in MP and IP are displayed in Fig. 7. One can see from Fig. 7(b) that the loss of energy is mostly focused on the central VO2 structures, accompanied by partial consumption of the external VO2 branches. Remarkably, the internal arc-shaped VO2 resonators play a pivotal role in the absorbance of the low frequencies. Furthermore, as portrayed from the power loss distribution at 1.16 THz in Fig. 7(b), keeping a good absorption effect in high frequencies is ascribed to the lossy nature of the VO2 units integrated into the designed device. As shown in Fig. 7(d), the absorption effect is decreased obviously with VO2 reaching IP. Meanwhile, in comparison with the power loss density distributions for the VO2 film in MP, it can be observed from Figs. 7(e) and (f) that there is nearly no energy loss localized on the VO2 sheets.
Fig. 7. (a) The absorption spectrum and the power loss density distributions at (b) 0.44 THz and (c) 1.16 THz for the VO2 film in MP; (d) the absorption spectrum and the power loss density distributions at (e) 0.54 THz and (f) 1.21 THz for the VO2 film in IP.
2.2 Structure and analysis of the MTD based on PS-VO2 pattern
Here, based on the previous structure incorporated with the gold-VO2 film, the other improved MTD employing a PS-VO2 film and the dielectric slab (lossy polyimide) with inserted CACs is schematized in Fig. 8(a). Notably, the hybrid PS-VO2 structure is regarded as multiple arc resonators, which can behave as a polarization converter with the excitation of an optical pump [41], when VO2 reaches IP (see Fig. 8(d)). In contrast to that, as shown in Fig. 8(e), manipulating the conductivity of VO2, simultaneously without the optical pump beam, can render such a MTD to realize absorption. Besides, through the thermal control, an ultra-broadband absorber can be fulfilled with the Si conductivity fixed at 3×104 S/m, displayed in Fig. 8(f). Remarkably, the schematic of the medium is plotted in Fig. 8(c), where the quantity of the CAC is 30 (5×6). The detailed diagram of the hybrid PS-VO2 unit with 500 nm thickness is presented in Fig. 8(b). The metal gold is still selected as a reflective plate at the bottom of the MTD.
Fig. 8. (a) Schematic depiction of the MTD based on the PS-VO2 pattern and CACs; (b) the diagram of the PS-VO2 pattern; (c) the construction of CACs in the dielectric substrate; such a MTD operating in (d) the PC mode with VO2 in IP, (e) the absorption mode without an optical pump beam, and (f) the absorption mode with an optical pump beam and VO2 in MP. The feature sizes of the proposed MTD based on the PS-VO2 pattern and CACs remain unchanged. The feature sizes of CACs: e = 27.5 µm, n = 2.5 µm.
The implementation for the devised tunable ultra-broadband multitasking device is commenced with a hybrid gold-VO2 pattern, accomplishing the conversion from the ultra-broadband PC mode (0.55-1.14 THz) to the dual-band absorption mode with the insulator-to-metal phase transition setting up (see Figs. 9(a) and (d)). Furthermore, the device based on the mixed gold-VO2 film can exhibit tuned absorption by thermal control. Due to the complexity and variability of the operating environment, the realization of a switchable/tunable polarization converter is indispensable to different applied demands. Hence, the PS is introduced to such a device, leading to the PCR over 90% from 0.55 THz to 1.11 THz when applying the maximum illumination accompanied with the IP VO2 (see Fig. 9(e)). Simultaneously, as shown in Fig. 9(b), when the VO2 is in MP and the conductivity of PS is set as 3×104 S/m, the absorption bandwidth is further promoted, spanning from 0.55 THz to 0.92 THz with the absorptivity over 90% (the relative bandwidth is 50.34%). Significantly, high bandwidth is a compelling feature for an absorber or a polarization converter. Taking a further step, the CACs are embedded into the medium, consequently, the proposed MTD based on the PS-VO2 film still remains strong PC performance with over 90% PCR covering from 0.82 THz to 1.6 THz (see Fig. 9(f)). Meanwhile, one can be seen from Fig. 9(c) that the elevation of absorption bandwidth is realized with the device exhibiting ultra-broadband absorption with the relative bandwidth of 80.7% at 0.68-1.6 THz. It is emphasized that the proposed MTA operating in the absorption mode and the PC mode can be implemented in the common target frequency, i.e., 0.82 THz to 1.6 THz. When the electromagnetic waves are incident on the MTD, the destructive interference between the scattering paths takes place in such a device. As a consequence, the electromagnetic energy is delivered into the devised MTD, further dissipated in the lossy resonator cavity, thereby boosting the absorption performance. Accordingly, in comparison to the MTD without the CACs, the absorption bandwidth is further strengthened with the insertion of the CACs into the dielectric substrate.
Fig. 9. (a) The absorptivity and (d) the PCR spectra of the MTD based on the gold-VO2 film; (b) the absorptivity and (e) the PCR spectra of the MTD based on the PS-VO2 film; (c) the absorptivity and (f) the PCR spectra of the MTD based on the PS-VO2 film and CACs.
Meanwhile, such a MTD incorporating the PS-VO2 pattern can serve as a tunable ultra-broadband absorber or polarization converter simultaneously, thereby fulfilling the regulation of electromagnetic waves. In the case of VO2 in IP, as depicted in Figs. 10(a) and (b), the adjustment of Si conductivity from 20 S/m to 1×106 S/m [56] allows such a device to accomplish the successive conversion of PCR between 0 and above 90% at 0.82-1.6 THz. Hence, using an optical pump beam, we design such an arc-shaped pattern integrated into the MTD, enabling dynamic control over PCR as well as transforming between a reflector and a polarization converter. Oppositely, when the PSs are not excited by the pump beam illumination, as portrayed in Figs. 10(c) and (d), the absorbance at 0.68-1.6 THz can be tailored from near zero to greater than 58% by utilizing the variation of the VO2 conductivity. Ulteriorly, as the PS conductivity is set as 3×104 S/m with simultaneously varying the VO2 conductivity, such a device operates as an absorber, enabling tuned ultra-broadband absorption with the modulation of absorptivity from about 52% to above 90% in the target frequencies (see Figs. 10(e) and (f)). It is apparent that the controllable ultra-broadband absorption can be observed from 0.68 THz to 1.6 THz, indicating strong dependence on the VO2 conductivity at different temperatures. According to the simulations, one can be concluded that by the appropriate settings of optical pump intensity and operating temperature, the proposed MTD can be not only expected to achieve the ultra-broadband PC but also can be switched to the ultra-broadband absorption mode covering totally the target frequencies, moreover, can reflect electromagnetic waves almost perfectly as well.
Fig. 10. (a) The PCR spectra of the MTD with VO2 in IP under different Si photoconductivity levels; (b) three-dimensional color mapping plot of calculated PCR as a function of frequency and Si photoconductivity; (c) the absorption spectra of the MTD without optical illumination under different VO2 photoconductivity levels; (d) three-dimensional color mapping plot of calculated absorptivity as a function of frequency and VO2 photoconductivity; (e) the absorption spectra of the MTD with PS set as 3×104 S/m under different VO2 photoconductivity levels; (f) three-dimensional color mapping plot of calculated absorptivity as a function of frequency and VO2 photoconductivity.
The ideal impedance match between the free space and the target object provides a forceful route to total absorption, which has been reported through the metamaterials [57]. Accordingly, the impedance matching principle is employed to comprehend the physical mechanism underlying the absorption phenomenon. The effective impedance Zeff of an absorber can be represented as [57]:
Further, the absorptivity can be calculated by the relative impedance Zr, as denoted by
in which Zr can be expressed asBy the impedance match theory, the designed MTD satisfies the condition of ${Z_r} = {{{Z_{eff}}} / {{Z_0}}} = 1$, revealing the implementation of the perfect absorption. Specifically, the real part of the relative impedance Zr is nearly equal to unity while the imaginary part of that is close to 0 (Re(Zr) = 1 and Im(Zr) = 0), so that the target structure would be matched. Accordingly, the proposed MTD based on the hybrid gold-VO2 pattern and the PS-VO2 film satisfies the condition of impedance match, realizing the dual-band absorption at 0.44 THz and 1.16 THz as well as ultra-broadband absorption at 0.55-0.92 THz, respectively (see Figs. 11(a) and (b)). Additionally, as shown in Fig. 11(c), owing to the construction of CACs in the dielectric, the promotion of absorption bandwidth is closely related to the impedance match between the MTD and the free space.
Fig. 11. (a) Normalized complex impedance of the proposed MTD based on the gold-VO2 pattern; (b) normalized complex impedance of the proposed MTD based on the PS-VO2 pattern; (c) normalized complex impedance of the proposed MTD based on the PS-VO2 pattern and CACs.
To ulteriorly gain insight into the absorption mechanism, the comparisons of the MF distributions for the MTD with the gold-VO2 sheet and the improved MTD employing the PS-VO2 film as well as CACs in the dielectric are presented in Fig. 12. As shown in Figs. 12(a) and (e), the MF focused on the gold-VO2 sheet further demonstrates the plasmon hybridized states as crucial factors for the high-efficiency absorption. Meanwhile, in Figs. 12(b) and (f), the MF is intensely restricted in the gap underneath the gold-VO2 sheet. And the MF is strongly strengthened between the adjoining unit cells, manifesting the excitation of propagating surface plasmon (PSP) resonance [58]. Besides, it is noteworthy that, in Fig. 12(f), the localization within the medium for the MF indicates the contribution of localized surface plasmon (LSP) resonance [58]. When the devised MTD based on the PS-VO2 film and CACs is implemented, in contrast to the MF distributions of the MTD with the gold-VO2 sheet, the enhancement of the concentration for the MF occurs on the PS-VO2 film (see Figs. 12(c) and (g)). Meanwhile, PSP and LSP resonances are distinctly enhanced (see Figs. 12(d) and (h)). As a result, the ultra-broadband absorption is fulfilled. These results show that the absorption intensity and bandwidth by promoting the plasmonic hybridization is improved.
Fig. 12. At the cross-section of 75 µm in the y-z plane, for the MTD based on the gold-VO2 pattern, the MF distributions at (a) 0.44 THz and (b) 1.16 THz, for the MTD based on the PS-VO2 film and CACs, the MF distributions at (c) 0.44 THz and (d) 1.16 THz; at the cross-section of 20 µm in the y-z plane, for the MTD based on the gold-VO2 pattern, the MF distributions at (e) 0.44 THz and (f) 1.16 THz, for the MTD based on the PS-VO2 film and CACs, the MF distributions at (g) 0.44 THz and (h) 1.16 THz.
The absorption stability under different polarization angles and the strong absorption independent of the incident angles are crucial to practical applications. When such a MTD equipped with the PS-VO2 resonator operates as an absorber, to simplify the exposition, we fixed σ(VO2) = 1×105 S/m and σ(Si) = 3×104 S/m. For the TE and TM polarized waves, the correlations of absorptivity associated with polarization angles under the normal incidence are demonstrated in Figs. 13(a) and (b). The polarization angle has hardly an impact on the efficiency of ultra-broadband absorption. The performance at oblique incidence is investigated in Figs. 13(c) and (d) as well. In comparison to the absorbance in the TE and TM modes, the proposed MTD represents a stable absorption effect with the incident angle raised from 0° to ±50° whether for the TE wave or the TM wave. Nevertheless, as the incident angle continues to change, the absorbance is decreased, which is ascribed to the tangential EF weakening accordingly and the parasitic resonances of the portion of such a structure strengthening sharply [46]. In contrast, the absorption performance of the TE wave deteriorates more than that of the TM wave owing to the x-direction component of the MF rapidly decreasing [59].
Fig. 13. Three-dimensional color mapping plots of calculated absorptivity as a function of frequency and polarization angle for the (a) TE wave and (b) TM wave; three-dimensional color mapping plots of calculated absorptivity as a function of frequency and incident angle for the (c) TE wave and (d) TM wave.
And equally, when the devised MTA acts as a polarization converter, it is significant to expound on the PC for oblique incidence. As displayed in Figs. 14(a) and (b), the incident angle is sensitive clearly to the bandwidth of PC. According to Eqs. (1) and (2) in Ref. [51], when the electromagnetic waves are incident obliquely on the MTD, the propagation phase in the medium can be expressed by $2\beta = 2\sqrt {{\varepsilon _r}} {k_0}h/\cos {\theta _t}$, where ɛr and θt denote the relative permittivity and refraction angle. In contrast to the propagation phase for the normal incidence($2\beta = 2\sqrt {{\varepsilon _r}} {k_0}h$), obviously, the propagation phase is increased with the enhancement of the incident angle. Consequently, the primal interference condition will be broken by the destructive interference created via the additional propagation phase at the surface of such a MTD, thereby further affecting the value of rxy. In particular, with the incident angle enhanced, more drastic variation occurring on the additional propagation phase will lead to the PCR deterioration at higher frequencies [60].
Fig. 14. Three-dimensional color mapping plots of calculated PCR as a function of frequency and incident angle for the (a) TE wave and (b) TM wave.
The build of CACs results in the enhancement of energy dissipation in the medium, broadening the absorption bandwidth. Thus, contour plots of absorptivity, as well as PCR as a function of frequency and radius of CACs for the MTD operating in the absorption mode and PC mode, are depicted in Fig. 15, respectively. When the radius of CACs is increased from 0.4 µm to 5.8 µm, the absorption frequency region is gradually promoted owing to the localized enhancement of energy, with a slight redshift (see Fig. 15(a)). Oppositely, when the MTD acts as a polarization converter, there is an obvious redshift of the operating frequency region with PCR larger than 90% (see Fig. 15(b)). Therefore, optimizing the radius of CACs is of great importance to further boost the absorption effect, which act a pivotal part in the implementation of ultra-wideband absorption.
Fig. 15. (a) Contour plot of absorptivity as a function of frequency and radius of CACs; (b) contour plot of PCR as a function of frequency and radius of CACs.
The contour plots of absorptivity, as well as PCR as a function of frequency and thickness of PS-VO2 film for the MTD operating in the absorption mode and polarization conversion mode, are portrayed in Fig. 16, respectively. It can be seen in Fig. 16(a) that the absorption frequency regime over 90% is first broadened and then deteriorated immediately. In contrast, as the thickness of PS-VO2 film is increased from 500 nm to 1500 nm, such a device operates in the polarization conversion mode, whose operation frequencies above 90% almost remain unchanged (seen in Fig. 16(b)). Consequently, the thickness of PS-VO2 film is an important factor to realize high-efficiency absorptivity and PCR.
Fig. 16. (a) Contour plot of absorptivity as a function of frequency and thickness of PS-VO2 film; (b) contour plot of PCR as a function of frequency and thickness of PS-VO2 film.
Finally, we summarize the switchable and multitasking devices reported earlier, which are listed in Table 1 for comparison. Many works show excellent performance, remarkably, the operation performance of this MTD is promoted obviously, which is better than those mentioned in the past.
Table 1. Comparisons between this work and reported switchable and multitasking devices
3. Experimental methods
3.1 Metamaterial fabrication
For the planar metamaterials, the methods of optical lithography are mainly used. As one of the most important steps to fabricate micro nanostructures, optical lithography generally includes wafer cleaning and drying, coating photoresist, prebaking, exposure, post-baking, development, hard baking, and other processes. Among them, exposure and development is the most critical step in the lithography process, which is directly related to the accuracy of metal nanostructures.
The arc-shaped metal resonator pattern can be defined by photolithography, gold can be deposited by E-beam evaporation [41].
The 500 nm VO2 thin films can be deposited by DCMS and post-oxidation. Firstly, in diluted detergent, the glass slides selected as the substrates are wiped with cotton and then cleaned ultrasonically in ethanol and acetone. After that, these substrates are flushed completely and dried via deionized water N2 gas, respectively. Once the preparations of the sputtering chamber (an initial pressure of 2×10−3 Pa) are finished, such substrates are put in a vacuum chamber that is filled with argon. Meanwhile, direct current power of 81 W is used to generate the glow discharge, followed by the cleaning and deposition processes of VO2 thin films. The vanadium metal target is cleaned by pre-sputtering in an argon atmosphere. Subsequently, vanadium layers without oxygen content are deposited on the substrates by DCMS in the pure argon atmosphere, in which the deposition temperature is controlled at 70 °C. And then, the vanadium layers are annealed at 300 °C in an ultra-pure oxygen atmosphere, accomplishing the preparation of VO2 thin films. Finally, the VO2 nanostructures are fabricated by an electron-beam lithography approach [34,35].
The fabrication of the photosensitive area can be accomplished by photolithography, and subsequent reactive ion etching of the Si epilayer outside the photosensitive regions is carried out. Next, the photoresist pattern is removed, resulting in Si photosensitive regions per unit cell [41,70].
3.2 Metamaterial characterization
A powerful THz time-domain spectroscopy system can be utilized to study the experimental characterization of the samples. The THz radiation can be obtained via a non-linear process employing an amplified kHz Ti: Sa laser system that delivers 35 fs pulses at 800 nm central wavelength and maximum energy of 2.3 mJ/pulse. Part of the initial laser beam (the energy is 1.3 mJ) will be focused in ambient air after partial frequency doubling in a betabarium-borate crystal (50 mm thick), producing a two-color filament and subsequently, THz radiation. The entire experimental setup will be placed inside a purge gas chamber to avoid water vapor absorption of the THz radiation [40,41].
The excitation of photocarriers in the silicon can be by an optical pump beam (800 nm). As the pump energy flux increases to about 294.6 µJ/cm2, which denotes the Si conductivity up to 1×105 S/m in the simulation [41].
The temperature of the target device can be manipulated through a hot plate with a hole for THz wave transmission. The actual sample temperature can be measured with a thermometer or by an infrared camera (FLIR Systems i60) to monitor the VO2 phase transition [71,72].
4. Conclusion
In conclusion, the design process of the ultra-broadband MTD on basis of the PS-VO2 film and CACs is demonstrated in the THz region. Such a MTD is ascribed to the merging of the optical tuned property of PS and the phase transition of VO2. An ultra-broadband absorber with over 90% absorptivity can be implemented from 0.68 THz to 1.6 THz, as a result of the PS set as 3×104 S/m and the VO2 in MP. When VO2 reaches IP accompanied by applying the maximum optical pump, an ultra-broadband polarization converter is accomplished with PCR higher than 90% from 0.82 THz to 1.6 THz. Meanwhile, the absorptivity and PCR for such a MTD operating in the absorption mode and the PC mode, respectively, can be continuously adjusted from greater than 90% to as low as zero through the regulation of room temperature and optical illumination. We theoretically investigate the physical mechanism, incident angle, polarization angle, and structural parameter of the proposed MTD in detail as well. Profiting from these ingenious features, the presented MTD may have promising applications in THz modulation, stealth technology, communication system, and so on.
Funding
Jiangsu Agriculture Science and Technology Innovation Fund (JASTIF) (CX(21)3187); 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.
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