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Abstract
Achieving switchable and diversified functionalities in a single metasurface has garnered great research interest for potential terahertz applications. Here, we propose and demonstrate a phase-change metasurface that simultaneously supports broadband electromagnetically induced transparency (EIT) and broadband nearly perfect absorption, depending on the phase state of a phase change material-vanadium dioxide (VO2). The phase-change metasurface is composed of a VO2 nanofilm, a quartz spacer and gold split-square-ring resonators with VO2 nanopads embedded into the splits. When VO2 is in its insulating phase at room temperature, a broadband EIT window (maximum transmittance reaching 83%) with a bandwidth of 0.27 THz (relative bandwidth 30%) can be observed. Alternatively, when VO2 transforms into its fully metallic phase, the EIT functionality will be switched off and instead, the metasurface operates as a broadband absorber with the total absorption exceeding 93% and a bandwidth of 0.5 THz (relative bandwidth 74%). The electric and magnetic field distributions indicate that the broadband EIT stems from the bright-bright mode coupling and the broadband absorption arises from the excitation and superposition of two resonances within a metal-insulator-metal cavity. The design scheme is scalable from terahertz to infrared and optical frequencies, enabling new avenues towards switchable and multifunctional meta-devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Artificial control and manipulation of electromagnetic waves (EMs) in terahertz frequencies has drawn great interest in both fundamental and applied science due to potentially fascinating applications [1,2]. Recently, metasurfaces, planar arrays of subwavelength resonators with only a few hundreds of nanometers of thickness, exhibit unprecedented capability to mold electromagnetic wave with superior performance and flexibility [3–5]. By delicately arranging the resonators and tailoring the amplitude and phase of scattered EM wave, extensive pioneering works have demonstrated various extraordinary phenomena, including anomalous transmission and reflection [6], polarization conversion [7–11], perfect absorption [12–15], electromagnetically induced transparency (EIT) [16–19], perfect lens [20,21], invisible cloaking [22], and hologram [23–25], to name a few. Besides, the ultrathin planar designs of metasurfaces offer a versatile and compatible means for device fabrication and integration compared to their bulk counterparts.
Despite these marvelous achievements, most reported metasurfaces only possess a single and fixed functionality once they are fabricated. With the development of modern integrated systems and applications, a single device with multiple and tunable functionalities is highly desirable [3]. To realize multiple functionalities, the routine approach relies on incorporating different engineered resonators into one supercell [26–28], where each functionality can be independently controlled at different frequencies or different polarization states. Based on this method, multifunctional metasurface lens [29] and holograms [25] have been successfully demonstrated. Nevertheless, such multifunctional devices are inevitably accompanied with static optical response, complex design and large cross-talk. To resolve these problems, people are pursuing more advanced metasurfaces with switchable functionalities. A promising way is to integrate active nanomaterials into metasurfaces, such as graphene [30–32], vanadium dioxide (VO2) [13,32–35], chalcogenide GeSbTe [36], among the others [37]. The optical properties of these active media can be controlled through external stimuli, which enable the active tuning of the performance of the designed meta-device. For instance, metasurfaces-based EIT can be dynamically tuned with silicon pads embedded into the meta-resonator [37]. Perfect absorption can also be tuned in graphene-integrated metasurfaces . Nevertheless, the active nanomaterials in these devices simply act as a switch to turn a single functionality on and off, whereas advanced metasurfaces with completely diversified functionalities over a broadband are still rarely reported, thereby limiting their potential applications for the ever-increasing demands on multi-functionalized EM-device integration and operation with high capacity and speed.
In this paper, we propose and demonstrate a phase-change metasurface with switchable and diverse functionalities in terahertz, employing metallic resonators hybridized with VO2. VO2 is adopted as the active medium because the conductivity of VO2 varies as much as almost four orders of magnitude through phase transition, which is superior for optical switching in terahertz frequency regime [33]. The proposed meta-device is composed of gold split-square-ring resonators (SSRRs) with VO2 nanopads inserted into the splits, a quartz spacer and a VO2 nanofilm. At room temperature, VO2 behaves as an insulator and is transparent to terahertz wave. The designed gold SSRRs interact with incident terahertz wave with simultaneous excitation of two bright mode resonances. When these two bright modes are positioned in close proximity to one another, a broadband EIT window would occur due to frequency detuning and hybridization between these two bright modes [18,38]. When VO2 is heated up with increased conductivity, this broadband EIT window is gradually switched off, which operates as a terahertz modulator. When VO2 reaches its fully metallic state, the SSRRs and the VO2 nano-film would naturally form a metal-insulator-metal (MIM) cavity, which supports two different resonances. These two modes spectrally superimposed with each other, and eventually result in a broadband nearly perfect absorption in terahertz regime. Our design enables tunable and diversified functionalities in a single metasurface, which is robust against previous tunable metasurfaces with merely one function [37,39,40].
2. Design and simulation
The proposed phase-change metasurface is schematically illustrated in Fig. 1(a). A VO2 nanofilm is sandwiched by a quartz substrate and a quartz spacer. On top of the quartz spacer, gold SSRR arrays are arranged is a square lattice with VO2 nanopads inserted into the three splits. Figures 1(b) and 1(c) show the top view and side view of the unit cell, respectively. The geometrical dimensions of the unit cell are px = py = 135 µm, g1 = 30 µm, g2 = g3 = 10 µm, l = 90 µm, w = 1 µm, d = 15 µm, t1 = 350 nm, t2 = 60 µm, and t3 = 500 nm, respectively. It should be noted here that the meaning of d is the distance between the center of the arm of the metallic square and the center of the gap g2. Considering that the size of a terahertz beam in a terahertz optical system, such as terahertz time-domain spectroscope, is around 3∼5 mm in diameter, the full dimension of the device can be 1 cm by 1 cm to ensure full coverage of the beam. This dimension can also be changed with respect to different spot sizes of terahertz beam. The quartz substrate is to support the device with negligible influence of the performance of the device. Thus, the thickness of the quartz substrate can be any value according to different applications. At room temperature, VO2 is an insulator with a low conductivity. In this case, VO2 is transparent to terahertz wave and the gold SSRR can be regarded as a composition of two L-shaped resonators (LSRs) and one U-shaped resonator (USR). Under a normally illuminated terahertz wave with its polarization along x-axis, both LSRs and USRs can be effectively excited. The resonance frequency of the split resonators can be approximately evaluated utilizing the equivalent LC model with ${f_0} = {1 / {(2\pi \sqrt {LC} )}}$, where the gold strips can be treated as inductors and the splits can be viewed as capacitors. At the resonance frequency, surface charges would accumulate at the end the gold strips, forming an enhanced electric field in the capacitive splits. Thus, the dimensions of the LSRs and USRs would affect the effective inductance and capacitance, which modifies the frequency detuning and hybridization of coupled resonators and tunes the EIT performance. Once VO2 goes through the phase transition, the conductivity is increased, which gradually weakens the strength of resonators and leads to an active modulating of the EIT window. When VO2 is in its fully metallic state with a high conductivity, the EIT functionality is completely switched off. Meanwhile, under x-polarized incidence, the MIM cavity formed by the bottom VO2 nanofilm and the SSRRs simultaneously supports two resonance modes. The superposition of these two resonances leads to the highly-efficient broadband absorption of the incident terahertz wave.
Fig. 1. (a) 3D schematic view of phase-change metasurfaces with switchable and diverse functionalities. Incident terahertz wave is normally irradiated with polarization along x-axis. (b) Top view and (c) side view of the unit cell of the metasurface. The optimized geometrical dimensions are px = py = 135 µm, g1 = 30 µm, g2 = g3 = 10 µm, l = 90 µm, w = 1 µm, d = 15 µm, t1 = 350 nm, t2 = 60 µm, and t3 = 500 nm, respectively.
The performance of the proposed phase-change metasurface is numerically investigated with periodical boundary conditions applied to x- and y- axes. Open space boundary conditions are used in both + z and –z directions assuming infinite thickness of the top air and the bottom quartz media, respectively, to avoid any unphysical reflection. The normal incidence is along + z direction with the electric field along x- axis. Gold is modeled as a lossy metal with an electrical conductivity of σ = 4.6 × 107 S/m. Quartz is treated as a lossless dielectric with a permittivity ɛ = 3.78. The dielectric constant of VO2 in terahertz frequencies is described by the Drude Model with a variable conductivity, which is expressed as [33,41]:
3. Results and discussion
Figure 2(a) shows the simulated transmittance spectra of the metasurface when VO2 undergoes its phase transition. When VO2 is in the insulating phase at room temperature with a conductivity of σ = 200 S/m, one can observe a transparent window with a high transmittance between two resonance dips at around 0.71 THz and 1.07 THz, respectively. The full width at half maximum (FWHM) bandwidth of the transparency window spans over 0.27 THz (from 0.78 THz to 1.05 THz), which is sufficiently wide to be justified as a broadband EIT window. During the phase transition process, the transmittance of the transparency window gradually diminishes as the conductivity increases. When VO2 reaches the fully metallic state with a conductivity of σ = 2×105 S/m, the transparency window is completely switched off. In addition, Fig. 2(b) investigates the slow-light effect of EIT by presenting the group delay of the transmitted terahertz wave. The group delay is defined as ${\tau _g} ={-} {{\partial \phi (\omega )} / {\partial \omega }}),$ $\omega = 2\pi f,$ where $\phi (\omega )$ and f are the transmission phase and frequency, respectively [16]. It is observed that large positive group delays are obtained within the EIT window. Interestingly, at 0.91 THz the group delays maintain almost constant with τg = 1.29 ps regardless of VO2 conductivity, enabling a stable EIT control with tunable transmittance. Below or above 0.91 THz, the phase transition of VO2 would change both the transmittance and group delay of the EIT window. These results clearly prove that the proposed phase-change metasurface can realize a broadband switchable EIT effect, which might be beneficial for slow-light applications.
Fig. 2. (a) Transmittance spectra and (b) group delay of the phase-change metasurface with respect to conductivity of VO2. The proposed metasurface functions as a broadband switchable EIT when VO2 is at insulating phase.
On top of EIT, the proposed phase-change metasurface can also operate as a tunable and broadband nearly perfect terahertz absorber when VO2 is at its metallic phase. Figure 3 shows the simulated absorptance spectra with varied conductivity of VO2. As can be seen, the absorptance is relatively small at a low conductivity. Two small absorptance peaks occur when the conductivity of VO2 is 200 S/m, whose resonant frequencies at 0.71 and 1.07 THz coincide with those of the two dips shown in Fig. 2(a). As the conductivity of VO2 increases from 200 to 3×103 S/m and VO2 is in the insulating phase, the absorptance increases in a broadband manner and two absorptance peaks at 0.71 and 1.07 THz are still observable. With the increasing of conductivity, the absorption peaks is vanishing around 1.07 THz while splitting at 0.71 THz. However, starting from the conductivity of 5×103 S/m, two absorptance peaks are observed at 0.43 and 0.93 THz, indicating that VO2 is transforming into its metallic state and the device functions as a broadband absorber. Furthermore, the curves of absorptance with a higher conductivity (>1×104 S/m) only have 2 peaks and are quite smooth at other frequencies while other curves show 3 different absorption peaks with the 3rd peak minor. When the conductivity of VO2 reaches 2×105 S/m at its fully metallic state, the two absorptance peaks are merged into a broadband absorptance between 0.43 THz and 0.93 THz, with the average absorptance exceeding 93%. Therefore, a tunable and broadband terahertz perfect absorber with a relative bandwidth of 74% is achieved using our phase-change metasurface.
Fig. 3. Absorptance spectra of the proposed phase-change metasurfaces with respect to conductivity of VO2. The proposed metasurface functions as a broadband switchable absorber when VO2 is at metallic phase.
To elucidate the underlying mechanisms of broadband EIT and absorber functionalities of the proposed phase-change metasurface, the electric and magnetic field distributions of the metasurfaces under either insulating or metallic phase states of VO2 are rigorously studied. At the insulating phase of VO2, the gold SSRR can be decomposed into two L-shaped resonators (LSRs) and one U-shaped resonator (USR). Figure 4(a) shows the simulated transmittance spectra of the sole LSRs, the sole USRs and their combined SSRRs, respectively. It is clear that the two transmittance dips in Fig. 2(a) originate from the two sub-resonances of LSRs and USRs. The broadband EIT window is due to the coupling between these two bright modes. More intuitively, the resonance dip at 0.71 THz in Fig. 2(a) corresponds to the excitation of a bright dipole mode in the USRs, which is clearly reflected in the associated amplitudes of the total electric field distributions as shown in Fig. 4(b). Likewise, the resonance at 1.07 THz in Fig. 2(a) is due to the excitation of another bright dipole mode in the LSRs, as shown in Fig. 4(c). Within the EIT window at 0.89 THz, the electric field is suppressed, indicating that the metasurfaces weakly interact with the incident terahertz wave as shown in Fig. 4(d). These results confirm that the broadband EIT window stems from frequency detuning and hybridization of USRs and LSRs.
Fig. 4. (a) Transmittance spectra of the sole LSRs, the sole USRs and their combined SSRRs, respectively. The amplitudes of total electric field distributions |E| in the SSRRs at (b) 0.71 THz, (c) 1.07 THz, and (d) 0.89 THz, respectively. Broadband EIT window is achieved due to the coupling between the resonances of LSRs and USRs, when VO2 is at the insulating phase.
Interestingly, in comparison to previous works of broadband absorbers with multiple resonators in one supercell [42] or narrowband absorbers with similar geometric configurations [43]; our design realizes broadband absorption with only a single resonator in one unit cell. To study the working mechanism, the amplitudes of x-component of the total electric field and y-component of the total magnetic field distributions at the two resonant peaks in Fig. 3 (0.50 THz and 0.89 THz) are presented in Fig. 5 with VO2 in its metallic state (σ = 2×105 S/m). At first glance, the amplitudes of x-component of the total electric fields |Ex| shown in Fig. 5(a) indicate that electric dipole-like resonances are excited in the SSRRs at both 0.50 and 0.89 THz for x-polarized incidence. Nevertheless, the amplitudes of y-component of the total magnetic field |Hy| in the cross-section (x-z plane with the y-axis value of Y1 as depicted in Fig. 5(c)) are completely different, as shown in Fig. 5(b). Particularly, at the fundamental resonance of 0.50 THz the magnetic fields are enhanced within the gap regime between the VO2 nanofilm and the top SSRRs. In contrast, at 0.89 THz the magnetic field enhancements are mainly concentrated around the SSRRs as well as the surface of the VO2 nanofilm, and the magnetic fields in the gap are rather weak, proving the existence of a high order resonance. These two resonances are in close proximity to one another and their spectral overlap would result in the broadband absorptance. Besides, at low terahertz frequencies, gold with a conductivity of 4.6×107 S/m that is sufficiently high to be treated as a perfect electric conductor [44], which is mainly used to reflect the terahertz wave. In contrast, VO2 at its metallic phase with a conductivity of 2×105 S/m can be regarded as a dissipative medium, which presents obvious absorption [45]. Hence, VO2 in such a hybrid design would reduce the quality factors of the resonances and also contribute to the broadened absorption bandwidth, which is different from narrow band absorbers with purely gold resonators [14].
Fig. 5. (a) Amplitudes of x-component of the total electric field |Ex| at f = 0.50 THz and f = 0.89 THz in the x-y plane when VO2 is in its metallic states with σ = 2×105 S/m. (b) Amplitudes of y-component of the total magnetic field |Hy| at f = 0.50 THz and f = 0.89 THz in the x-z plane with the corresponding y-axis positions of Y1 depicted in the right schematic (c). Broadband absorption is achieved due to the excitation and superposition of two resonance modes within the MIM cavity, when VO2 is at the metallic phase.
To further investigate the broadband absorption functionality of the metasurface, the absorptance spectra and the optical impedance of the metasurface are plotted in Fig. 6, when the top SSRRs are either hybrid designs (gold with VO2 in the splits) or purely VO2 square-ring resonators (VO2 conductivity 2×105 S/m), respectively. As shown in Fig. 6(a), the purely VO2 square-ring resonator based design presents one absorption peak with maximum absorptance of 59%, while the hybrid design with VO2 inserted into the gold SSRRs exhibits a broadband absorption with the average absorptance beyond 93%. Therefore, the embedding of VO2 into the gold SSRRs significantly modifies the resonance behaviors of the SSRRs and strongly confines the incident terahertz wave in a broadband spectrum. In addition, since impedance matching plays a vital role in the metasurface-based absorbers, Fig. 6(b) depicts the impedance of the proposed metasurface (normalized to the impedance of vacuum) based on the following equation:
where S11 and S21 are the complex reflection and transmission coefficients of the metasurface under x-polarized incidence. Since the metallic VO2 nanofilm is deliberately adopted to reflect all incident terahertz wave, S21 is approximately equal to zero. Clearly, when the splits are filled with metallic VO2 in the hybrid design, both the real and imaginary parts of the impedance match well with the respective vacuum impedance values of one and zero in a broadband spectrum, leading to the nearly perfect broadband absorption. In contrast, when the top SSRRs are purely VO2, the impedance of the metasurface are significantly different from those of vacuum, resulting in the low absorption efficiency.Fig. 6. (a) Absorptance spectra and of (b) impedance of metasurface when the top SSRRs are either hybrid designs (gold with VO2 in the splits) or purely VO2 square-ring resonators (VO2 conductivity 2×105 S/m), respectively. Compare with purely VO2 square-ring resonators, the hybrid design provides stronger confinement of the incident terahertz wave as well as better impedance matching condition, leading to higher broadband absorption efficiency.
The performance of both the broadband EIT and broadband absorption can be flexibly tuned by controlling the geometrical parameters of the metasurface. Among all the parameters shown in Fig. 1(a), (d) and w are two key parameters that are capable of tuning one functionality independently without significant altering the other. Figure 7(a) plots the transmittance of the metasurface for d = 0, 5, 15, 25 µm, with the corresponding VO2 conductivity of 200 S/m (insulating phase). It is observed that the bandwidth of the EIT window is decreased when d changes from 0 to 25 µm. With a larger value of d, the effective length of LSR becomes larger while the effective length of the USR becomes smaller. As a result, the resonance frequencies of the LSRs and USRs get closer in the frequency domain, which merge into one EIT window with a smaller bandwidth. Nonetheless, the value of d should not be too small (e.g., d = 0 µm) as the resonance at the high frequency is mixing with the lattice resonance at 1.14THz ($\lambda = {p_x}\sqrt {{\varepsilon _{sub}}} $) [46], making the transparency window different from a conventional EIT spectral line shape that contains a high transparency window with two transmission dips. Moreover, when d is too small, the two bright modes are separated with a large bandwidth. The two resonances at the low and high frequencies can be regarded as two separated resonances with rather weak coupling, leading to small phase dispersion even with the assistance of the lattice resonance, which is not favorable in EIT applications, such as slow light devices. When VO2 transforms into its metallic state, the absorptance shows almost no change regardless of d, as shown in Fig. 7(b). Under such a circumstance, the value of d affects the bandwidth of the EIT window with slight change of the absorption. Alternatively, Fig. 7(c) plots the transmittance of the metasurface for VO2 at insulating phase with the width w changing from 0.5 to 1, 2 and 5 µm. It is observed that the EIT window changes slightly with different widths, as the resonance behaviors in this case are more related to the length the gold strips. On the contrary, when VO2 is at metallic phase, the width w plays a vital role in the broadband absorption as shown in Fig. 7(d). When w is increased from 0.5 to 1, 2 and 5 µm, the absorptance is decreased. This is due to the fact that smaller width of the resonators would induce lower quality factor of the resonances. When the width is increased, the resonances become narrower and the broadband absorptance is degenerated into two peaks. In addition, the increasing of w would shift the frequencies of two resonances to opposition directions, which decreases the flatness of the absorption spectra. In this regard, the width w can be applied to flatten the absorption curve, while maintaining the broadband EIT. Besides the two parameters discussed above, the effects of other geometrical parameters are not presented here as they are optimized values and their variation would deteriorate the performance of the broadband EIT and absorption.
Fig. 7. (a) Transmittance and (b) absorptance spectra of the phase-change metasurfaces for d = 0, 5, 15, 25 µm, with the corresponding VO2 conductivity of 200 S/m (insulating phase) and 2×105 S/m (metallic phase), respectively. (c) Transmittance and (d) absorptance spectra of the phase-change metasurfaces for w = 0.5, 1, 2, 5 µm, with the corresponding VO2 conductivity of 200 S/m (insulating phase) and 2×105 S/m (metallic phase), respectively.
4. Conclusions
In this paper, we have proposed and demonstrated a phase-change metasurface with switchable and diversified functionalities in terahertz, which can be actively switched from a broadband EIT to a broadband nearly perfect absorption. The phase-change metasurface is composed of VO2-nanopads-integrated gold resonators with a quartz spacer and a VO2 nanofilm at the back. When VO2 is at its insulating phase and is transparent to terahertz wave, the metasurface gives rise to a broadband EIT window (maximum transmittance reaching 83%) with a bandwidth of 0.27 THz. During the phase transition of VO2 with increased conductivity, the metasurface gradually switches off the EIT window and starts to absorb the incident terahertz wave. When VO2 is in its fully metallic state, this meta-device is operating as a broadband nearly perfect absorber with the overall absorptance beyond 93% and a bandwidth of 0.5 THz. The electric and magnetic field distributions indicate that the broadband EIT effect is accomplished by frequency detuning and hybridization of two coupled bright modes supported by the metasurface. On the other hand, the broadband nearly perfect absorption originates from the strong excitation and superposition of two resonances within the MIM cavity. The key geometrical parameters that affect the performance of the metasurface are investigated, which further elucidates the design flexibility of the metasurface. This tunable and multifunctional phase-change metasurface can add value to potential terahertz applications, such as smart modulators, slow light devices and imaging coding. The design principle can also be scaled to other infrared and optical meta-devices with switchable and diversified functionalities.
Funding
Science Challenge Project (TZ2018003); National Natural Science Foundation of China (61905225); CAEP Innovation Grant (CX20200011).
Disclosures
The authors declare no conflicts of interest.
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