Anisotropic optoelectronic devices have emerged as an attractive and appealing research subject due to the recent progress in photonics-based technologies, including communication, sensing, and molecular spectroscopy ^[1–4]. However, it is extremely hard to attain the intrinsic linear dichroism that is directly associated with the intrinsic nature of materials in the terahertz (THz) regime, which hinders the development of abundant anisotropic THz applications. Benefiting from the emergence of metamaterials, a variety of fascinating phenomena can be observed in their designed metal patterns, such as negative refraction ^[5], invisibility cloaking ^[6], hyperbolic media ^[7], perfect absorbers ^[8,9], chiral metamaterials ^[10], and superlenses ^[11], which are barely seen in natural materials. In this sense, it is possible to tailor the artificial metamaterials for accurate manipulation of THz-wave polarization and to exploit these materials with great flexibility.

To date, the integration of metamaterials with actively driven platforms by means of thermal ^[12,13], mechanical ^[14,15], optical ^[16–22], or electrical ^[23–25] control has triggered considerable interest, aiming at controlling and manipulating THz waves. More precisely, these approaches have successfully validated that the custom-built functionalities in the transmission of THz waves can be realized by either strength modulation or resonance frequency shifting. In the case of strength modulation, the near-field coupled metamaterials whose components are electromagnetically interfering with each other are more likely to be actively manipulated than the single metallic dipole resonators, mainly due to the enlarged influence induced by the disturbances of the interface environment. Recently, the Fano resonance-coupled optical-active THz metadevices for ultrasensitive and ultrafast photoactive switching have been realized, but with the drawback of single-frequency modulation ^[26]. When it comes to the tuning of resonance frequency, active control of electromagnetically induced transparency has allowed a slight shifting of a Fano dip ^[27] frequency. A new device that is an electrically controlled connection between meta-atoms with graphene bridges has exhibited a dipole resonance shift up to 0.65 THz ^[28]. However, one significant disadvantage of such devices is their relatively low modulation speed driven by electricity, which may limit their application in ultrafast switching devices and modulating. From an alternative perspective, the polarization control may be viewed as a promising approach to adjust the resonance frequency without sacrificing the all-optical switching merits ^[29], such as sensitivity and high speed. The asymmetric Fano spectral profile and plasmon-induced transparency (PIT) phenomena are two prime examples of Fano resonance manifestation by mimicking quantum phenomena in classical metamaterial systems. In the case of PIT phenomenon, a sharp transparent window in the broad transmission dip is mainly contributed to by the destructive interference between two different modes. Following this definition, the PIT operating in the destructive interference between closed-ring resonators (CRRs) and split-ring resonators (SRRs) ^[30] could be engineered to generate an anisotropic Fano-type resonance. More precisely, this radiative-radiative coupling with a twofold symmetric structure can be endowed with sharp transparent windows at separate frequencies via polarization rotation.

As a dynamic medium, solution-processed MAPbI3 films usually present an astonishing photon-photoconductivity (Δσ) conversion efficiency that determines the high sensitivity switching performance ^[31]. The use of organic-inorganic metal halide perovskite (CH3NH3PbI3) has seen phenomenally rapid progress in the field of photovoltaic cells and photonic devices ^[32–36]. By virtue of high-charge carrier mobility (∼500cm2·V−1·s−1) ^[37–39], long diffusion length (beyond 1 μm), and high photoluminescence (PL) quantum yields (up to 90%) ^[40–45], perovskites have prompted intense research in power conversion efficiencies ^[46,47]. It is worth noting that the rapid recovery of charge carriers in perovskites has greatly facilitated high-speed optoelectronic devices beyond traditional silicon-based devices ^[48–50]. These outstanding properties laid the foundation for the design of photoactive media on resonant structures, with an extremely simple and economical spin-coating process. Herein, we demonstrate an ultrasensitive and high speed all-optical modulation of an anisotropic PIT resonator on the basis of an inexpensive, uncomplicated processing, and astonishing photoconductivity spin-coated MAPbI3 perovskite film. The well optimized PIT resonance metaphotonic array demonstrates an outstanding dynamic response at the transmission windows of 0.86 and 1.12 THz for two perpendicular polarizations, respectively. The present work is an experimental demonstration of an anisotropic active THz metadevice fabricated in a simplified and cost-effective laboratory method.

The schematic architecture of the polarization-dependent metamaterial-perovskite THz device is presented in Fig. 1(a). A periodic array of SRRs and CRRs tailors the PIT resonance at different frequencies determined by incident THz polarizations. The detailed geometrical parameters are revealed in Fig. 1(b), which describes the metamaterial structure along with the unit cell dimensions. In this figure, two pairs of SRRs surround a CRR, with a bar located in the middle. Two aspectant SRRs aligned with CRR in the horizontal (x) direction (L1, L21, and L22) have a different design compared to the other pair of SRRs in the vertical (y) direction (L2, L12, and L11), which is the key point of the anisotropic PIT resonances. The metamaterial array is fabricated using the conventional photolithography technique, where the 127 nm thick gold (Au) structures is thermal-evaporated on a 2 mm thick z-cut quartz substrate as shown by the optical microscope image in Fig. 1(c). Au has excellent chemical stability and ultrahigh conductivity (over 4.56×107S/m), which is often used as a perfect electrical conductor in THz structure arrays. The unit cell in the size of Px=150μm, Py=110μm is repeated in the x and y direction, respectively, eventually forming a rectangular array of 4mm×4mm scale, as shown in Fig. 1(c).

With the metasurface ready, a 55 nm thin layer of solution process MAPbI3 perovskite is spin-coated to obtain the hybrid metadevice ^[51–53]. In the hybrid metaphotonic device system, the properties of the photoactive medium are the dominant factors for the overall dynamical performance. A thin, dense, flat pinhole-free MAPbI3 film is fabricated by spin-coating a perovskite precursor poly(ethylene oxide) (PEO) blend solution, followed by a thermal annealing process. Perovskite precursor solution (10 wt%) is prepared by dissolving methylammonium iodide (MAI) and lead (II) iodide (PbI2) powder with a 1:1 molar ratio in dimethyl sulfoxide (DMSO) at 70°C with vigorous stirring. PEO (3 wt% with respect to the MAPbI3 precursor) is mixed with the precursor solution at 70°C. Before spin-coating, the prepared solution is cooled to room temperature and the metamaterial is cleaned by acetone, 2-propanol and UV−O₃ for 5 min each. Then the solution is spin-coated on the top of the metamaterial at 4000 r/min for 30 s in an N2-filled glovebox. The as-spun device is immediately annealed by a hotplate at 85°C for 10 min, and the transparent film turned brown upon the evaporation of the DMSO. The metasurface is wholly covered and has a uniform surface morphology along with the structure array in Fig. 1(d). The high quality of perovskite film is verified by all the covered gaps between Au structures, which is especially important for near-field coupled resonances. An atomic force microscope (AFM) image inserted in Fig. 1(d) shows a clear stage of 55 nm thick film. In addition, we perform linear absorption and PL spectra tests on the prepared perovskite film. The film we used has a bandgap of 760 nm and an absorption peak at 740 nm (see Appendix A, Fig. 7).

Optical characterization of the hybrid metamaterial-perovskite device is performed by the optical-pump-THz-probe (OPTP) method. Two 1 mm thick nonlinear crystals (ZnTe) are used in the THz generation and detection. The dynamic modulation process is recorded in the form of transient transmission amplitude change when the time delay of optical excitation pulse is step-changed from 0 to 600 ps. A Spectra Physics regenerative amplifier system, which can provide a 1 kHz pulse train of 800 nm (center wavelength) pulses of 100 fs duration (full width at half-maximum), is used as the light source of the whole OPTP setup. A part of 800 nm optical laser beam is frequency-doubled (400 nm) by 1 mm thick β-BaB2O4 crystal for optical pump of MAPbI3 film, whose spot size is 5 mm in diameter, which is large enough to cover the THz beam spot (∼2.2mm in diameter) for a uniform excitation over the surface of the device. A wire-grid THz polarizer is used to project the polarization of THz pulses. The transmission spectra of the active device under different delay times and pump fluence [ES(ω)] are normalized to the transmission spectra of reference substrate [ER(ω)] following the relation of |T(ω)|=|ES(ω)/ER(ω)|.

To investigate the polarization-dependent PIT properties of the designed structure, we numerically simulate and experimentally study the amplitude transmissions of the designed polarization-related metamaterial without perovskite coating, as shown in Fig. 2. To begin with, the numerical simulation results of the typical PIT windows at different frequencies for x-polarized and y-polarized incident waves are displayed in Fig. 2(a). In addition, the experimental data in Fig. 2(b) present that the PIT window is located at 0.81 THz for the x-polarized wave, while at 1.11 THz for the y-polarized wave, in good agreement with the simulated results of Fig. 2(a). As demonstrated in Fig. 1(a), the resonances of CRR and SRR are primarily attributed to their geometrical dimensions via the THz illumination with a polarization along the gap. The strongly coupled electric dipole oscillation of CRR and the weakly coupled fundamental eigenmode of SRR excited by THz electric field are coupled back and forth through the magnetic field. Notably, the resonance frequency of SRR is slightly higher than the design compared to the dipole mode of CRR. In order to quantitatively determine the anisotropic functionality, our meta-atom sample consists of a CRR surrounded by two pairs of SRRs.

The CRR and its corresponding SRRs are designed with different sizes along the x and y axis, respectively. The displacement length in the middle of the CRRs and SRRs is set to be 5 μm to ensure enough coupling strength between each other. The strong coupling effect plays a key role in the giant enhancement of the THz electric field along the gap. Such coupling effects are demonstrated by the simulations of the intrinsic THz spectrum of CRR and SRR resonators ^[54] (see Appendix A, Fig. 8). In view of this, the enhanced mutual coupling between the SRR and CRR resonances exhibits unprecedented sensitivity of the gap environment, opening up new possibilities for ultrasensitive THz modulation beyond natural materials.

The strong resonance feature is still maintained in the experimentally measured transmission spectrum of the MAPbI3 spin-coated sample. As presented in Figs. 3(a) and 3(c), the hybrid metaphotonic device holds a typical PIT resonance transmission spectrum that is similar to the result in Fig. 2(b) (x and y directions) when the pump beam is blocked, despite the influence imposed by the intrinsic conductivity of the perovskite film. In order to characterize such an influence on the PIT window, we take into account a series of the pump fluence of a 400 nm pulse, ranging from 5 to 240μJ/cm2. When the pump fluence is as low as 5μJ/cm2, as displayed in Figs. 3(a) and 3(c), the obvious modulation effect in the x direction and y direction can be clearly visualized in the transmission spectra. Specifically, the sharp PIT window is assigned to the destructive interference between the inductor-capacitor (LC) resonance and dipole resonance, which originate from the SRRs and CRRs, respectively. Once the 55 nm MAPbI3 layer is excited by an increasing pump fluence, the screening effect of the fringing field by the MAPbI3 layer is enhanced, while the PIT strength is gradually suppressed.

In addition, the normalized ratio of transmission amplitude at the resonance peak of 0.86 and 1.12 THz along the x and y direction, respectively, is decreased from 0.44 and 0.60 to 0.37 and 0.45 when the pump fluence changes from 0 to 240μJ/cm2. This observed amplitude modulation of the PIT window is attributed to the reduced capacitance at the gap of CRRs and SRRs throughout the unit cell, which is the result of the optical excitation and photoconductivity of MAPbI3 in the gap. In our study, the resonance peak (y) is not completely switched off, even though the modulation depth is already close to saturation under the pump fluence of 240μJ/cm2. From the simulation result [Fig. 3(d), 1440 S/m], the saturation of the modulation depth is actually derived from the photoconductivity response of the surface photoactive film. Considering that the photoactive film has a thickness of only 55 nm, this problem may be solved by appropriately increasing the thickness of the spin-coated perovskite. The anisotropic modulation is achieved under an ultralow pump fluence (5μJ/cm2), which is markedly different from previous studies ^[20,26,31] (detailed comparison is provided in Table 1). In fact, to improve the sensitivity of this device, we use perovskite as photoactive material and particularly employ a short gap (5 μm) design for the purpose of lowering the threshold to switch off the near-field coupling of LC resonance and dipole resonance. The effect of this design is certified by the simulation analysis shown in Figs. 3(b) and 3(d). The Lorentzian mechanical oscillator is a commonly used theoretical model for explaining the phenomenon of PIT metamaterial resonance ^[55,56]. We analyzed this PIT effect by theoretical calculation, as shown in Appendix A (Fig. 9). The modulation of the PIT peak can only be viewed as the change of the damping rate of SRR arms. Therefore, it indicates that the suppression of the bright mode in SRRs is the major factor of our modulation. Intending to rule out the fake PIT effect that may be hidden in the transmission amplitude ^[57], we extracted the group delay curves from the experimental results as the derivative of phase with respect to circular frequency. Significant slow-light effect is seen in Appendix A (Fig. 10), which could prove the authenticity of the PIT effect exhibited by our device.

From these figures, one can see that the resonance peaks almost disappear when the conductivity of the perovskite film is just set to a relatively low level, namely, 960 and 1440 S/m in the x and y direction, respectively. Importantly, this ultrasensitive (5μJ/cm2) resonant modulation behavior could open up a new direction for the design and construction of low-threshold perovskite-based plasmonic devices in the THz regime. Please note that the overall amplitude reduction in the excited ultrathin 55 nm thick perovskite can be negligible when compared to the modulation effect of the PIT resonance, since the electric field is tightly confined in the 5 μm gaps and enables an extremely sensitive response to the interface environment, resulting in the strong modulation of the PIT resonance. In addition, the experimental and simulation results exhibit a significant discrepancy in the modulation depth along the x polarization. Theoretical calculations show that the x direction is more sensitive than the y direction (the near-field simulation below gives an explanation), but experimentally, the modulation in the x direction is relatively small. This is due to the coupling effect of PIT resonance and optically active phonon mode of MAPbI3 film, which was studied by earlier OPTP characterization at room temperature ^[26,58–60]. In the recent study ^[26], MAPbI3 perovskite has shown an active phonon mode when the pump fluence is greater than 7μJ/cm2. The observed mode coupling of the phonon mode of the perovskite film with the dipole resonance (SRR resonator, Fig. 8 in Appendix A) at 1.0 THz in the former study could provide preliminary evidence for our experimental result (x direction, at ∼0.86THz), which is repeatedly tested. In order to more intuitively reflect the influence of the coupling between the phonon mode and the PIT resonance mode on the device, we verified it by the simulation with the addition of the phonon mode (see Appendix A, Fig. 11).

To clarify the underlying mechanism of the modulation effect in Fig. 3, a commonly used finite-element method is employed to analyze the electric field distributions, considering various conductivities of the perovskite film. As described in Fig. 1(b), a base unit cell is illuminated by the incident THz radiation with an electric field intensity of 1 V/m. Here, the Floquet boundary condition is utilized to describe the continuous metamaterial arrays. In consideration that the influence of resonance is best exemplified by the z component of electric field Ez, which is located 27.5 nm above the upper surface of the substrate, it is shown to characterize resonant behaviors at the center of PIT windows. Since the aforementioned resultant transmission through the perovskite-based anisotropic THz device is a function of frequency, PIT peaks exist at 0.86 and 1.12 THz for x- and y-polarized THz incidences, respectively. Therefore, we restrict our attention to the electric fields with various perovskite film conductivities at both of these frequencies, in order to investigate the change of electric field concentrated at the gaps of the metamaterial structure.

One significant finding from Fig. 4(a) is that the electric field confinement in the gap along the x-polarized THz wave is around 54 times stronger than in the other region, allowing for a giant sensitivity of conductivity change within the gap along the x axis. Particularly, the electric field in the CRR is almost completely suppressed, which identifies and characterizes the feature of a typical PIT effect. Without an optical pump, the perovskite thin film can be considered as a semiconductor with small conductivity of 60 S/m, and the resonators exhibit very strong electric confinement, which is comparable to the case without perovskite, as presented in Fig. 4(b). On photoexcitation, the controllably increased conductivity induced by the pump beam allows for the active tuning under variable coupling strengths with specified polarization direction. It is visible that the electric field enhancement shows a moderate and even strong attenuation when the perovskite conductivity goes up to 240 S/m and 960 S/m, respectively. This confirms the weakening coupling between CRR and SRR, as we predicted in previous sections. The underlying physics can be intuitively understood as follows: the radiative LC mode resonance in the SRR pair is hampered by the photoexcitation enhancement, leading to a weaker circulating current in the SRR pair. Based on this mechanism, we reiterate the designed structure as the image illustrated in Fig. 1 to provide a deeper insight into this process. Two gaps, namely, the gap between the CRR and SRR and the gap in the SRR, would be connected by the perovskite film with a considerably high conductivity. Hence, the circulating current of SRR is difficult to be generated by strong oscillation with the incident THz wave. On the other hand, the electric field confinement of 1.12 THz in the gap along the y-axis direction corresponding to the y-polarized THz incidence shows similar patterns with the change of optically controlled conductivity, as illustrated in Fig. 5. These results contribute to the effective and active modulations of meta-atoms via optical control of the perovskite conductance.

In order to explore the dynamic response of ultrafast modulation aspect, we have experimentally characterized the temporary resonant switching behavior in our proposed perovskite-functionalized metamaterial hybrid device. For illustration, the temporal evolution of the dynamic modulation response in y polarization and the transient transmission amplitude change of solution spin-coated perovskite film on quartz in the absence of Au structure are shown in Fig. 6. Notably, the relative time delay between the pump and the THz probe is controlled by a step motor, and the 400 nm pump pulse is assumed to stimulate the sample at a 0 ps time delay. A main conclusion from Fig. 6(a) is that under a 30μJ/cm2 pump fluence, the PIT amplitude at the frequency of 1.12 THz gradually restores from the state at 0 ps time delay and becomes very close to the original transmission amplitude without photoexcitation at 561 ps. It can be seen that our sample exhibits a faster response speed (<1ns, GHz) than the millisecond response of conventional silicon-based photoactive devices. The heavily curtailed PIT amplitude at a 0 ps time delay implies that the maximum density of photoexcited carriers exists in the perovskite film.

In Fig. 6(a), we use the transmission amplitude of the PIT resonance Et−EonEoff−Eon×100%,(1)to quantify the recovery of device. Eoff is the PIT amplitude without pump, Eon is the PIT amplitude at the time delay of 0 ps, and Et is the PIT amplitude at the time delay of t. Among the time delays of 18, 151, and 561 ps, the calculation results of recovery ratio are 43.3%, 66.7%, and 82.3%, respectively. And the recovery rate of the PIT response is progressively decreased, as revealed in the recombination curve of the nonequilibrium carriers from Fig. 6(b). By step-collecting the change of amplitude (ΔT) throughout the delay line at the peak THz transient signal (T), time-resolved relaxing of photoconductivity in the MAPbI3 film is obtained as −ΔT/T, which is proportional to Δσ. Unlike the PIT resonance, which is more susceptible to interface environment, the light-induced change of perovskite film is relatively small (where the maximum is ∼7% under a 30μJ/cm2 pump fluence) and displays a clear pump power dependence. Extracting the characteristic time from this temporal evolution is essential for the study of other MAPbI3-based devices. At the pump fluence of 20 and 30μJ/cm2, a triple-exponential decay equation, −ΔTT(t)=A1e−tt1+A2e−tt2+A3e−tt3+A0,(2)is used to fit the dynamic curves for the presence of trimolecular processes (Auger recombination). Meanwhile, at the lowest fluence, a double-exponential expression, −ΔTT(t)=A1e−tt1+A2e−tt2+A0,(3)is more suitable for the bimolecular recombination-dominated process. Because the first-order recombination process (i.e., the trap-assisted monomolecular recombination) is reported to last longer than 10 ns, far beyond the delay line in the MAPbI3 sample, we fix the decay time constant t1 at 10 ns for convenience ^[41]. The best-fitting curves are shown as solid lines in Fig. 6(b). Additionally, the trimolecular decay time constant t3 at 20 and 30μJ/cm2 is obtained to be 6.2 and 6.7 ps, respectively. What follows is the bimolecular decay time constant t2, which is 63, 96, and 141 ps for the 5, 20, and 30μJ/cm2 pump fluence, respectively. This indicates that the fast decay process (i.e., the second- and third-order recombination) of photoconductivity in perovskite film is mainly completed within ∼200ps, which is consistent with the result of Fig. 6(a), where the response of PIT resonance is quickly repaired within 151 ps and then changed slowly. It is well known that the trap-assisted monomolecular recombination rate of the MAPbI3 perovskite is exceptionally low, and the carriers’ relaxation curve can be high-quality fitted when just incorporating bimolecular and trimolecular decay components ^[48]. Based on this point—that the first-order decay can be ignored—we can understand the weak effect of a slow recombination process on device response speed. Therefore, it can be concluded that the high-speed response of our sample is caused by the fast recombination process (i.e., the second- and third-order recombination) of nonequilibrium carriers.

In summary, we have experimentally demonstrated the ultrasensitive modulation (5μJ/cm2) and THz transmission modulation characteristics of an active metamaterial device by constructing a perovskite-hybridized anisotropic PIT resonator array with a spin-coated 55 nm MAPbI3 film as the photoactive medium. The high-speed dynamic behavior of all-optical modulation along the x and y polarizations with different PIT frequencies is obtained with a giant sensitivity and a far faster speed compared to silicon-based devices, which implies the excellent potential of easily solution-processed perovskite in application of fast optical response devices. Moreover, we have performed accurate numerical simulations that prove that our resonator structure is sensitive to the surface dielectric environment, a prerequisite for weak-light detection and sensing. Further, the polarization-dependent design has provided a promising alternative to modulate the resonance frequency while preserving the all-optical switching merits (e.g., high sensitivity and ultrafast speed). Therefore, the anisotropic PIT structure can be regarded as a potential candidate for multifunctional subwavelength metamaterial devices operating in the THz range.