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Abstract
Implementation of efficient terahertz (THz) wave control is essential for THz technology development for applications including sixth-generation communications and THz sensing. Therefore, realization of tunable THz devices with large-scale intensity modulation capabilities is highly desirable. By integrating perovskite and graphene with a metallic asymmetric metasurface, two ultrasensitive devices for dynamic THz wave manipulation through low-power optical excitation are demonstrated experimentally here. The perovskite-based hybrid metadevice offers ultrasensitive modulation with a maximum modulation depth for the transmission amplitude reaching 190.2% at the low optical pump power of 5.90 mW/cm2. Additionally, a maximum modulation depth of 227.11% is achieved in the graphene-based hybrid metadevice at a power density of 18.87 mW/cm2. This work paves the way toward design and development of ultrasensitive devices for optical modulation of THz waves.
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1. Introduction
The term terahertz (THz) radiation refers to electromagnetic waves with frequencies in the range from 0.1 to 10 THz, which lie in the spectrum between infrared waves and microwaves [1,2]. Benefitting from the most remarkable properties of THz radiation (including an ultra-broad bandwidth [3], low photon energy [4], and strong penetrability [5]), THz technology has been maturing and demonstrating promise in various fields, e.g., wireless communications [6], aerospace [7], and biological and medical applications [8]. However, the electromagnetic responses of most natural materials when interacting with THz radiation have shown weak performances, and this has resulted in a relative lack of technological development and practical application of THz devices [9–11]. Metasurfaces are artificial electromagnetic media that consist of subwavelength artificial planar microstructures with orderly and reasonably designed sizes, shapes, and arrangements, and these surfaces have led to dramatic progress in THz device development [5,12]. When compared with natural materials, the electromagnetic properties of metasurfaces offer many outstanding advantages, and achievements have been reported in a wide range of applications, including THz imaging [13], switching [14], sensing [15], and slow light devices [16,17]. Because most traditional metasurface-based THz devices cannot fulfill the requirements for dynamic manipulation of THz radiation, development of tunable metasurface devices for use in the THz frequency band has become a research hotspot [18,19].
The emergence of metamaterials has provided a wide field for the development of THz technology, although dynamic modulation of THz devices remains relatively difficult. Over recent decades, multiple tuning methods have been reported, including use of laser pumping [20,21], chemical doping [22], thermal control of superconductors [23], and micro-electro-mechanical systems technology [24,25]. Recently, several works have focused on the tuning of devices through integration of metasurfaces with dynamic materials that include perovskites [26], graphene [27,28], silicon [29,30], and vanadium dioxide [31]. Among these materials, perovskites and graphene offer unique prospects for realization of tunable metasurface devices under external optical pumping stimulation due to their excellent optical and electromagnetic properties. The lead metal-based organic-inorganic perovskite MAPbI3[CH3NH3PbI3] has been regarded as a superior candidate material type because of its excellent ambient stability, photoconductivity, and optical properties [26,32]. In particular, the free carrier density of perovskites can be dynamically adjusted under illuminating the optical pump, making it possible for perovskite hybrid metasurface to achieve the manipulation of THz radiation. Wang et al. investigated a THz free-space modulator using a CsPbBr3 perovskite quantum dot-embedded metasurface, which realized a modulation depth of 88.3% under external optical pumping [33]. Kumar et al. fabricated an ultrafast terahertz photonic device by integrating perovskites with a metasurface and achieved amplitude modulation of 93% at a 250 µJ/cm2 pump fluence [34]. Graphene is a two-dimensional (2D) material that has excellent tunable optoelectronic properties and it has also attracted considerable attention for allowing actively tunable metasurface devices to be realized in the THz band [35–37]. By integrating graphene with metasurfaces, dynamic modulation of the THz waves can be realized in the graphene-based device by changing Fermi energy. Lee et al. fabricated a gate-controlled graphene-based metasurface that realized persistent switching and linear modulation of THz waves by varying the gate voltage [38]. Kim et al. demonstrated an electrically tunable single-layer graphene THz metasurface that achieved a tunable electromagnetically induced transparency-like effect by simply varying its gate voltage [16]. Li et al. proposed a graphene-metal hybrid metasurface that achieved a modulation depth of 23% under a 305 kV/cm THz peak field in their experiments [39]. Although high efficiency modulation of THz waves has attracted extensive study, few experiments have been performed on perovskite and graphene materials and the modulation performance to date is relatively low.
In this paper, we propose the fabrication of two ultrasensitive THz devices through integration of perovskite and graphene with Fano-resonant asymmetric metasurfaces for dynamic control of their transmission spectra. Our experimental results show large-scale intensity modulation at relatively low external optical pump powers. For the proposed perovskite-integrated Fano-resonant asymmetric metasurface (perovskite-FRAM) device, the maximum amplitude modulation depth reaches as high as 190.2% under application of an optical pump power density of only 5.90 mW/cm2. Moreover, theoretically calculated results based on the coupled two-oscillator model show good agreement with the results of experiments and simulations. For the proposed graphene-integrated Fano-resonant asymmetric metasurface (graphene-FRAM) device, a maximum amplitude modulation depth of up to 227.11% was realized at an optical pump power density of 18.87 mW/cm2. The proposed devices will be very helpful in the development of tunable metasurface devices for use in the THz band, which will have potential applications in fields including optical communications, photoswitching, and ultrasensitive THz optical modulators.
2. Geometric structure and numerical model
Figure 1 shows a graphical representation of the proposed bare metasurface structure, which consists of three main components: a quartz bottom layer, a polyimide (PI) film layer, and a top patterned metal array layer. The PI film with a thickness of 10 µm covers the quartz bottom layer. The top patterned metal array is made from 0.2-µm-thick gold with a conductivity δ = 4.56 × 107 S/m, and is fabricated atop the PI film using conventional photolithography techniques. Normal incidence THz waves interact with the patterned gold structures to achieve the Fano resonance response. As shown schematically in Fig. 1, the bare structure is composed of two vertical bars and two U-shaped structures with the same height L = 78 µm and width w = 9 µm. The distances g and x between the U-shaped structure and the vertical bar are 45 µm and 75 µm, respectively. Furthermore, the periodicities px and py of the unit cell size are 150 µm and 290 µm, respectively. In this paper, perovskite and graphene layers are integrated into patterned metal array layers to fabricate the proposed perovskite and graphene-based ultrasensitive THz devices benefitting from the intrinsic tunable optoelectronic properties of these materials.
Fig. 1. Three-dimensional schematic illustration of the bare metasurface structure with geometrical parameters of L = 78 µm, w = 9 µm, x = 75 µm, y = 60 µm, g = 45 µm, px = 150 µm, and py = 290 µm.
3. Design of perovskite-FRAM device and discussion
As shown in Fig. 2(a), the perovskite-based ultrasensitive THz devices are produced by spin-coating MAPbI3 on top of a metallic asymmetric resonator, which is composed of a quartz bottom layer, a PI film layer, and a top patterned metal array layer with a thickness of 500 µm, 10 µm, and 0.2 µm respectively. Microscope schematic diagrams of the perovskite-FRAM device before and after spin-coating of the MAPbI3 film are shown in Fig. 2(b) and (c), respectively. Figure 2(b) shows that the perovskite is covering the patterned metal array uniformly. In the experiments, the perovskite-FRAM device is characterized by THz time-domain spectroscopy system (THz-TDS), which requires a THz scan time of 40 ps [40]. In this case, green light at a wavelength of 532 nm is directly incident on the device as the optical pump, and its spot size diameter was ∼2 mm [41], as illustrated in Fig. 2(d). Optical pumping can excite free carriers and excitons on perovskite surfaces, thus allowing dynamic manipulation of the surface’s conductivity. As a result, the transmission of the perovskite-FRAM can be modulated by varying the optical pump power. To analyze the mechanism of the proposed device, we performed numerical simulation using a time-domain solver of computer simulation (CST) microwave studio software based on finite integral technology.
Fig. 2. Experimental design of the perovskite-FRAM device. (a) Schematic diagram of the fabrication process of the perovskite-FRAM device. Optical microscope images of the fabricated sample (b) before and (c) after spin-coating of the MAPbI3 film. (d) Artistic illustration of the perovskite-FRAM device under irradiation by THz waves and optical pump.
Figure 3 shows a simulated transmission spectrum for the perovskite-FRAM device when the conductivity of the perovskite is 0 S/m. Because of the asymmetry of the metal resonator in the electric field direction, a destructive interference effect is generated around the resonance frequency of 0.8 THz that can produce a significant Fano resonance response [42,43]. To analyze the generation mechanism of the perovskite-FRAM device, we simulated both the surface charge density and the electric field distribution at 0.8 THz. The results for the surface charge density show that there is an antiparallel current passing between the asymmetric metal resonator elements. The electric field distribution shows quadrupolar mode characteristics. Additionally, the radiative loss of the THz beam is suppressed, which results in the Fano resonance response appearing at 0.8 THz.
Fig. 3. Transmission spectrum, surface charge density, and electric field distribution of the perovskite-FRAM device.
Because of the tunability of the perovskite photoconductivity, the perovskite conductivity undergoes a change that alters the strength of the Fano resonance response. As a result, we can actively modulate the transmission spectrum of the proposed perovskite-FRAM device when it is stimulated by the green light, which acts as an external optical pump source. It can be concluded from Fig. 4(a) and (b) that we observed a prominent Fano resonance response with a conductivity of 0 S/m, as illustrated by the black curve in Fig. 4(a), where the transmission spectra measured without application of the optical pump (Fig. 4(b)) are in good agreement with the simulation results. When the perovskite conductivity and the optical pump fluence increase, the strength of the Fano resonance response is reduced. Figure 4(c) and (d) show that the transmission amplitudes at frequencies f1 and f3 show clear increasing trends from 0.22 to 0.371 and from 0.244 to 0.708, respectively. The transmission amplitude and modulation depth at the f2 frequency are shown in Fig. 4(e) and (f), which can be clearly seen that the transmission amplitude decreased from 0.8 to 0.55 with increasing power density in the experiments. To characterize the modulation behavior of these transmission spectra quantitatively, we defined the amplitude modulation depth (AMD) as $AMD = ({|{\varDelta T} |/{T_b}} )\times 100\%$, where $\varDelta T = {T_o} - {T_b}$, and To and Tb represent the transmission amplitudes with and without optical pump stimulation, respectively. The maximum AMD reaches up to 68.82% and 190.2% at frequencies f1 and f3 for a green light power density of only 5.90 mW/cm2, while the transmission amplitude at frequency f2 gradually decreases with increasing pump power. When the optical pump fluence was increased from 0 to 5.90 mW/cm2, Fig. 4(g) clearly illustrates that the AMD increased monotonically, thus indicating the dynamic tunability of the perovskite-FRAM device under relatively low optical pump powers and meaning that it can serve as a potential candidate for ultrasensitive THz optical modulator.
Fig. 4. (a) Simulated transmission spectra of the proposed perovskite-FRAM device with various conductivity values for the perovskite. (b) Corresponding measured transmission spectra at various optical pump power densities. Variations in the experimental transmission amplitude at the frequencies of (c) f1, f3 and (e) f2 when the pump fluence of the green light is changing. The modulation depth of the proposed perovskite-FRAM device at the (d) f1, f3 and (f) f2 frequencies. (g) Color map showing the modulation depth when the pump fluence was varied over the range from 0 to 5.90 mW/cm2 during the experiment.
To provide a better theoretical description of the modulation behavior of the Fano resonance response in the proposed perovskite-FRAM device, the coupled two-oscillator system model is used to illustrate this physical mechanism analytically as follows [44,45]:
It can be concluded from Fig. 5(a) that the analytically-fitted transmission spectra with the various perovskite photoconductivity values basically agree with the simulation results, thus indicating the feasibility and the effectiveness of dynamic modulation of the proposed perovskite-FRAM device. To examine the damping mechanism involved, Fig. 5(b) presents the values of the corresponding fitting parameters k, ${\gamma _1}$, and ${\gamma _2}$ under various conductivities ranging from 0 to 6000 S/m. The coupling coefficient k, which indicates the strength of the Fano resonance response, is inversely proportional to the perovskite conductivity values. Furthermore, the damping factors ${\gamma _1}$ and ${\gamma _2}$ increase with the increasing perovskite conductivities. Therefore, the number of free carriers on the perovskite surfaces are increased with the increment of the perovskite conductivities, indicating that the value of the perovskite photoconductivity affects the local field at the asymmetric metal resonator, causing the strength of the Fano resonance response to be weakened substantially. It can be seen from Fig. 5(c) that the AMD of theoretically calculated results have a monotonous increasing trend with the increase of perovskite conductivity. Therefore, the effectiveness of the modulation approach in the proposed perovskite-FRAM device was demonstrated further.
Fig. 5. (a) Comparison of simulated and theoretically calculated transmission spectra from the coupled two-oscillator system model in the proposed perovskite-FRAM device, and (b) the values of fitting parameters k, ${\gamma _1}$, and ${\gamma _2}$ when the perovskite conductivity increases from 0 to 6000 S/m. (c) Color map showing the modulation depth of the theoretically calculated results.
4. Design of graphene-FRAM device and discussion
After coating the bare structure of the metallic asymmetric resonator with a PI film, a 1 cm × 1 cm three-layer graphene film grown by copper-catalyzed chemical vapor deposition (CVD) is transferred onto the surface of the sample, which completes fabrication of the proposed graphene-FRAM device, as illustrated in Fig. 6(a). Microscope schematic diagrams of the device with and without graphene integration are shown in Fig. 6(b). In the experiment, the graphene-FRAM device is exposed to a THz beam and an optical pump beam, as shown in Fig. 6(c). The transmission spectra of the proposed graphene-FRAM device were characterized by THz-TDS, and the optical pump beam was used to provide external stimulation. The extraordinary properties of graphene allow the Fano resonance response of the proposed graphene-FRAM device to be modulated through with changes in the Fermi energy of the graphene induced by applying external optical pumps.
Fig. 6. Experimental design of the graphene-FRAM device. (a) Schematic diagram of the fabrication process for the graphene-FRAM device. (b) Optical microscope image of the fabricated sample after transfer of the graphene. (c) Artistic illustration of the graphene-FRAM device under irradiation by THz waves and optical pump. (d) Raman spectra of graphene when measured using a 514 nm excitation laser.
Next, we simulated the Raman spectra of the graphene at three different positions to explain the quality of the graphene. As shown in Fig. 6(d), the Raman spectrum has three main peaks, designated the D peak, the G peak, and the 2D (G’) peak under 514 nm external laser stimulation. Among these peaks, the D peak (∼1348 cm−1) has nearly disappeared. The amplitude ratio of the G peak (∼1578 cm−1) to the 2D peak (∼2697 cm−1) is approximately 1.23, and the full width at half maximum of the 2D peak is approximately 56 cm−1. These features further illustrate the high quality of the graphene used in this work.
In the experiments, the transmission spectra of the proposed graphene-FRAM devices are modulated by applying external optical pump stimulation to excite more carriers in the graphene. Figure 7(a) shows that the transmission spectra are modulated with increasing optical pump power densities from 0 to 18.87 mW/cm2, and a significant Fano resonance response is also produced without the external optical pump, as illustrated by the black curve. In addition, we simulated the transmission spectra with the Fermi energy of 0.01 eV, which shows a relatively high Fano resonance response in the red curve. This indicates that the dynamic modulation of the THz waves can be realized in the graphene-based device by changing external optical pump power that controls the Fermi energy of graphene. Figure 7(b) and (c) show that when the green light power increases, the transmission amplitudes undergo significant increases from 0.186 to 0.344 and from 0.195 to 0.638 at frequencies f1 and f3, respectively. It should also be noted that the maximum AMD values reach up to 85.2% and 227.11% at frequencies f1 and f3, respectively, when the optical pump power is only 18.87 mW/cm2. The transmission amplitude and modulation depth at the f2 frequency are represented in Fig. 7(d) and (e). It can be found that the transmission amplitude decreased gradually with the increasing of optical pump power. When the conductivity increased over the range from 0 to 18.87 mW/cm2, Fig. 7(f) clearly illustrates the monotonicity of the increasing AMD, which further confirms the dynamic tunability of the proposed graphene-FRAM device at a relatively low optical pump power. Based on the results obtained, the maximum modulation depth ranges up to 227.11% for the proposed graphene-FRAM device, which is much better than the corresponding performance of the proposed perovskite-FRAM device.
Fig. 7. (a) Experimental transmission spectrum of graphene-FRAM devices at different optical pump powers and simulation result at 0.01 eV Fermi energy. Variations in the experimental transmission amplitude at the frequencies of (b) f1, f3 and (d) f2 when the pump fluence of the green light is changing. The modulation depth of the proposed graphene-FRAM device at the (c) f1, f3 and (e) f2 frequencies. (f) Color map showing the modulation depth when the pump fluence was varied between 0 and 18.87 mW/cm2 in the experiment.
5. Conclusion
In summary, to take advantage of the unique optoelectronic properties of perovskite and graphene materials, we fabricated two tunable THz metasurface devices by either spin-coating MAPbI3 or transferring graphene layers onto the top of metallic asymmetric resonators. It was demonstrated experimentally that ultrasensitive modulation of the transmission amplitude was achieved by varying the external optical pump power. Our results demonstrated that the maximum AMDs of the two metadevices reached up to 190.2% and 227.11% at optical pump power densities of 5.90 mW/cm2 and 18.87 mW/cm2, respectively. This work has great potential for use in optical modulation of THz waves and provides a new reference for the design of ultrasensitive THz devices, e.g., modulators, sensors, and photoswitching devices.
Funding
National Natural Science Foundation of China (12005108, 62201312); Natural Science Foundation of Shandong Province (ZR2020QF016, ZR2022QF014); Qilu University of Technology (Shandong Academy of Sciences) Science and Education Production Project (2022PT095).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (62201312, 12005108); Natural Science Foundation of Shandong Province (ZR2022QF014, ZR2020QF016); Qilu University of Technology (Shandong Academy of Sciences) Science and Education Production Project (2022PT095).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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