Solution-Processed Lead Iodide for Ultrafast All-Optical Switching of Terahertz Photonic Devices

Color coded metadevices toward programmed terahertz switching

Terahertz modulators play a critical role in high-speed wireless communication, non-destructive imaging, and so on, which have attracted a large amount of research interest. Nevertheless, all-optical terahertz modulation, an ultrafast dynamical control approach, remains to be limited in terms of encoding and multifunction. Here we experimentally demonstrated an optical-programmed terahertz switching realized by combining optical metasurfaces with the terahertz metasurface, resulting in 2-bit dual-channel terahertz encoding. The terahertz metasurface, made up of semiconductor islands and artificial microstructures, enables effective all-optical programming by providing multiple frequency channels with ultrafast modulation at the nanosecond level. Meanwhile, optical metasurfaces covered in terahertz metasurface alter the spatial light field distribution to obtain color code. According to the time-domain coupled mode theory analysis, the energy dissipation modes in terahertz metasurface can be independently controlled by color excitation, which explains the principle of 2-bit encoding well. This work establishes a platform for all-optical programmed terahertz metadevices and may further advance the application of composite metasurface in terahertz manipulation.

A Flexible Terahertz Metamaterial Sensor for Pesticide Sensing and Detection

Terahertz (THz) waves have garnered significant interest across various fields, particularly in high-sensitivity sensing applications. Metamaterials can be employed in THz sensors, specifically for refractive index sensing and pesticide detection due to their high-sensitivity characteristics. In this Article, a dual-band flexible THz metamaterial sensor based on polyimide is proposed for refractive index and pesticide sensing, which is fabricated using ultraviolet (UV) lithography technology and measured by a THz time-domain spectroscope (TDS) system. The resonant frequencies of the sensor are at 0.37 and 1.13 THz, with transmission rates of 2.9% and 0.3%, respectively. With an analyte layer attached to the sensor's surface, the sensitivity of refractive index sensing can be calculated as 0.09 and 0.28 THz/RIU (refractive index unit) at the two resonant frequencies. In order to validate the exceptional pesticide sensing performance of the sensor, chlorpyrifos-methyl acetone solutions with various concentrations are added on it. Furthermore, a monolayer of graphene is coated on the sensor's surface, which is proved capable of improving pesticide sensing sensitivity at low concentrations due to strong π-π stacking interactions with π-electrons in chlorpyrifos-methyl solutions. Therefore, the graphene-coated sensor can be utilized in detecting pesticide solutions with low concentrations, and the sensor without graphene is preferred for high concentration detection. This work provides a novel option for the THz metamaterial sensor with high sensitivity covering a wide pesticide concentration range.

Stretchable, Transparent, and Ultra-Broadband Terahertz Shielding Thin Films Based on Wrinkled MXene Architectures

With the increasing demand for terahertz (THz) technology in security inspection, medical imaging, and flexible electronics, there is a significant need for stretchable and transparent THz electromagnetic interference (EMI) shielding materials. Existing EMI shielding materials, like opaque metals and carbon-based films, face challenges in achieving both high transparency and high shielding efficiency (SE). Here, a wrinkled structure strategy was proposed to construct ultra-thin, stretchable, and transparent terahertz shielding MXene films, which possesses both isotropous wrinkles (height about 50 nm) and periodic wrinkles (height about 500 nm). Compared to flat film, the wrinkled MXene film (8 nm) demonstrates a remarkable 36.5% increase in SE within the THz band. The wrinkled MXene film exhibits an EMI SE of 21.1 dB at the thickness of 100 nm, and an average EMI SE/t of 700 dB μm-1 over the 0.1-10 THz. Theoretical calculations suggest that the wrinkled structure enhances the film's conductivity and surface plasmon resonances, resulting in an improved THz wave absorption. Additionally, the wrinkled structure enhances the MXene films' stretchability and stability. After bending and stretching (at 30% strain) cycles, the average THz transmittance of the wrinkled film is only 0.5% and 2.4%, respectively. The outstanding performances of the wrinkled MXene film make it a promising THz electromagnetic shielding materials for future smart windows and wearable electronics.

A triple Fano resonance Si-graphene metasurface for multi-channel tunable ultra-narrow band sensing

In this work, a dielectric metasurface composed of a silicon nanodisk etched with a square hole is proposed. By introducing C4v symmetry breaking, the symmetry-protected bound states in the continuum (SP-BIC) is transformed into a quasi-BIC (Q-BIC), simultaneously inducing triple Fano resonances in the near-infrared light band corresponding to one dipole and two Q-BIC resonances. The characteristics of Q-BIC resonances are elucidated through multipole decomposition and near-field distribution analysis. Subsequently, monolayer graphene is integrated into the Si metasurface. The light field in the composite metasurface can be flexibly modulated by changing the Fermi level of graphene. This modulation enables optimal transmission with an enhancement of up to 252%, while the confined electromagnetic energy experiences a remarkable increase of about 1020%. Simulation results demonstrate that the Si-graphene composite metasurface exhibits a high refractive index sensitivity of 162 nm RIU-1, accompanied by a figure of merit of 170.526 RIU-1. This composite metasurface holds promise as a high-performance sensor in the near-infrared band and has potential for application in the fields of active tunable optical devices and biochemical sensing.

Insights into synaptic functionality and resistive switching in lead iodide flexible memristor devices

Neuromorphic platforms are gaining popularity due to their superior efficiency, low power consumption, and adaptable parallel signal processing capabilities, overcoming the limitations of traditional von Neumann architecture. We conduct an in-depth investigation into the factors influencing the resistive switching mechanism in memristor devices utilizing lead iodide (PbI2). We establish correlations between device performance and morphological features, unveiling synaptic like behaviour of device making it suitable for range of flexible neuromorphic applications. Notably, a highly reliable unipolar switching mechanism is identified, exhibiting stability even under mechanical strain (with a bending radius of approximately 4 mm) and in high humidity environment (at 75% relative humidity) without the need for encapsulation. The investigation delves into the complex interplay of charge transport, ion migration and the active interface, elucidating the factors contributing to the remarkable resistive switching observed in PbI2-based memristors. The detailed findings highlight synaptic behaviors akin to the modulation of synaptic strengths, with an impressive potentiation and depression of 2 × 104 cycles, emphasizing the role of spike time-dependent plasticity (STDP). The flexible platform demonstrates exceptional performance, achieving a simulated accuracy rate of 95.06% in recognizing modified patterns from the National Institute of Standards and Technology (MNIST) dataset with just 30 training epochs. Ultimately, this research underscores the potential of PbI2-based flexible memristor devices as versatile component for neuromorphic computing. Moreover, it demonstrate the robustness of PbI2 memristors in terms of their resistive switching capabilities, showcasing resilience both mechanically and electrically. This underscores their potential in replicating synaptic functions for advanced information processing systems.

Quasi-bound states in the continuum induced by supercell coupling

In this paper, we propose what we believe to be a novel coupling mechanism for generating quasi-bound states in the continuum (quasi-BIC) in symmetrical metasurface structures. We demonstrate for the first time in theoretical predictions that supercell coupling can induce quasi-BIC(s). We utilize the coupled mode theory (CMT) to analyze the physical mechanism for the generation of quasi-bound states in such symmetrical structures, which result from our investigation of the coupling between sub-cells that are separated from supercells. We verify our theory by using both full-wave simulations and experiments.

Generation of symmetry-protected bound states in the continuum in a graphene plasmonic waveguide system for optical switching

A graphene-involved plasmonic lossy system that allows coupling between surface plasmon polaritons and waveguide modes is proposed. The physical mechanism behind the hybrid resonance modes is investigated carefully through finite element method (FEM) simulations and rigorous coupled wave theory (RCWA). We demonstrate that by introducing an incident angle to break the symmetry of the structure, the bound states in the continuum (BIC) evolve to an observable quasi-BIC with new resonance dips, and the generated signals possess a very high Q-factor. Such transformation is investigated carefully by calculating the band structure of the system and the corresponding Q-factors. The results showed that the calculated results from the band structure are consistent with the simulations. In addition, the hybrid plasmonic system allows for switching modulation due to the tunability of graphene, and the max modulation depth of nearly 100% is reached. The outstanding Q-factor and dynamic tunability of this easy-to-fabricate hybrid structure may be helpful in engineering various plasmonic devices, including tunable optical switches, absorbers, sensors, etc.

A high Q-factor dual-band terahertz metamaterial absorber and its sensing characteristics

In this paper, a dual-band metamaterial absorber in the terahertz frequencies is proposed and its refractive index sensing characteristics is analyzed. The metamaterial structure is designed using a square metal ring with four T-shaped strips loaded outside of the ring, where the metal periodic array is on top of a silicon wafer backed with a metal ground plane. The resonant frequencies of the absorber are at 0.89 and 1.36 THz, whose absorption rates are both over 99% under normal TE and TM polarized incidences. The full widths at half maximum of them are 4.4 and 11.2 GHz, respectively, resulting in high quality factors (Q-factors) for these two frequency bands. The absorption rate of the absorber remains stable as the incident and polarized angles are changed. Several proposed metamaterial absorbers are experimentally fabricated and electron beam lithography (EBL) technology is employed. Good measurement results of the dual-band absorption performance are obtained using a terahertz time-domain spectroscopy system based on photoconductive antennas. Furthermore, the metamaterial absorber also shows sensing properties for analytes with different refractive indices or thicknesses. This work provides a new choice for the design of high-Q dual-band terahertz metamaterial absorbers and their application to refractive index sensing.

Ultrafast Modulation of THz Waves Based on MoTe2-Covered Metasurface

The sixth generation (6G) communication will use the terahertz (THz) frequency band, which requires flexible regulation of THz waves. For the conventional metallic metasurface, its electromagnetic properties are hard to be changed once after being fabricated. To enrich the modulation of THz waves, we report an all-optically controlled reconfigurable electromagnetically induced transparency (EIT) effect in the hybrid metasurface integrated with a 10-nm thick MoTe₂ film. The experimental results demonstrate that under the excitation of the 800 nm femtosecond laser pulse with pump fluence of 3200 μJ/cm², the modulation depth of THz transmission amplitude at the EIT window can reach 77%. Moreover, a group delay variation up to 4.6 ps is observed to indicate an actively tunable slow light behavior. The suppression and recovery of the EIT resonance can be accomplished within sub-nanoseconds, enabling an ultrafast THz photo-switching and providing a promising candidate for the on-chip devices of the upcoming 6G communication.

Reassessing Fano Resonance for Broadband, High-Efficiency, and Ultrafast Terahertz Wave Switching

Miniaturized ultrafast switchable optical components with high efficiency and broadband response are in high demand to the development of optical imaging, sensing, and high-speed communication. Sharp Fano-type resonance switched by active materials is one of the key concepts that underpins the control of light in metaoptics with high sensitivity. However, actuating such metasurfaces exhibits a long-standing trade-off between modulation depth and operational bandwidth. Here, the limitations are circumvented by theoretical analysis, numerical simulation, and experimental realization of an achromatic Fano metasurface so that a high contrast of tunability with ultrafast switching rate over a broad range of frequency is achieved. By developing the physics of inter-mode coupling, the Fano metasurface is designed according to a complete phase diagram derived from coupled mode theory. Unlike conventional Fano metasurfaces, the cross-polarized inter-metaatoms coupling is discovered as a superior ability of high-efficiency broadband achromatic polarization conversion. To prove the ultrasensitive nature, a metadevice is constructed by incorporating a thin amorphous Ge layer with a weak photoconductivity perturbation. Transmission modulation over broadband frequency range from 0.6 to 1.1 THz is thus successfully realized, featuring its merits of modulation depth over 90% and On-Off-On switching cycle less than 10 ps.

Photoactive terahertz metasurfaces for ultrafast switchable sensing of colorectal cells

Metasurfaces with a strongly enhanced local field are envisioned as a powerful platform for ultrasensitive optical sensors to significantly amplify imperceptible differences between compatible bioanalytes. Through the use of phototunable silicon-based terahertz (THz) metasurfaces, we experimentally demonstrate ultrafast switchable sensing functions. It is found that the THz responses of the coupled-resonances in the metasurfaces shift from Lorentz-lattice mode to electromagnetism-induced transparency (EIT) mode under optical pumping within an ultrashort time of 32 ps, enabling an ultrafast sensitive sensor. For the Lorentz-lattice mode, the THz time-domain signal directly shows a highly sensitive response to detect tiny analytes without extra Fourier transformation as the mismatch between the two modes increases. Once the metasurfaces are switched to the EIT mode, the silicon-metal hybrid structure supports frequency-domain sensing ability due to strong field confinement with a sensitivity of 118.4 GHz/RIU. Both of the sensing configurations contribute to more subtle information and guarantee the accuracy of the sensor performance. Combined with the aforementioned advantages, the proposed metasurfaces have successfully identified colorectal cells between normal, adenoma, and cancer states in experiments. This work furnishes a new paradigm of constructing reliable and flexible metasurface sensors and can be extended to other optics applications.

Highly efficient and tunable terahertz polarization converter based on double subwavelength metallic gratings infiltrated with liquid crystal

In this work, a tunable cross-polarized transmission structure at the terahertz frequency was demonstrated, and the polarization state during modulation was investigated. The proposed structure can significantly enhance the polarization conversion performance of nematic liquid crystals by leveraging the formation of a Fabry-Perot-like resonant cavity that consists of two metal gratings. As a result, the incident waves are continuously reflected in the liquid crystal layer to complete the accumulation of polarization angle changes. From the experimental results acquired, it was concluded that the insertion loss of the cross-polarization transmission was less than 3 dB and the extinction ratio was larger than 28 dB in the frequency range of 388-426 GHz. Our work provides useful insights for improving the efficiency of cross-polarization conversion by enhancing the resonance process in a Fabry-Perot-like resonant cavity and, thus, significantly extending the equivalent optical path.

Recent Advances in Tunable Metasurfaces: Materials, Design, and Applications

Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc., and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

Dual control of multi-band resonances with a metal-halide perovskite-integrated terahertz metasurface

The phenomenon of multi-resonant Fano resonances is important for the design of biosensors and communication fields. There are very few studies reporting the multi-band Fano resonance metamaterials with more than three resonance frequencies, or the tunable optical metamaterials to control the multi-band Fano resonance characteristics. Here, we report dual control of multi-band Fano resonances with a metal-halide perovskite-integrated terahertz metasurface by lasers and an electrical field. By tuning the conductivity of the perovskite film on the metasurface, ultrasensitive optoelectronic modulation was achieved. The terahertz transmission amplitude exhibited increasing and decreasing stages. We analyzed the physical phenomena and found that capacitance effects and Fermi-level enhancement had significant roles in the optical- and electronic-modulation experiments. The resonant frequencies in the electronic modulation had broader frequency shifts and a higher and wider tunable modulation depth range. More importantly, the maximum modulation depth was as high as 197%, with a significant fluctuation in the amplitude and more unstable frequency shifts in the transmission spectra.

Tunable high-quality-factor absorption in a graphene monolayer based on quasi-bound states in the continuum

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Active terahertz modulator based on optically controlled organometal halide perovskite MAPbI2Br

In this paper, an active terahertz modulator based on optically controlled organometal halide perovskite MAPbI₂Br is proposed. The terahertz wave time-domain transmission of the MAPbI₂Br/Al₂O₃ sample was measured by a terahertz time-domain spectrometer. Experimental results indicate that the MAPbI₂Br/Al₂O₃ sample showed an obvious optical-power-dependent modulation effect on transmission of the terahertz wave; the maximum modulation depth of the modulator can reach 59.99% at 0.3 THz when the external pump optical power is up to 1500 mW.

Photothermal conversion of Ti2O3 film for tuning terahertz waves

Dynamic tuning of terahertz (THz) wave is vital for the development of next generation THz devices. Utilization of solar energy for tuning THz waves is a promising, eco-friendly, and sustainable way to expand THz application scenarios. Ti₂O₃ with an ultranarrow bandgap of 0.1eV exhibits intriguing thermal-induced metal-insulator transition (MIT), and possesses excellent photothermal conversion efficiency. Herein, Ti₂O₃ film was fabricated by a two-step magnetron sputtering method, and exhibited an excellent photothermal conversion efficiency of 90.45% and demonstrated temperature-dependent THz transmission characteristics with a wideband at 0.1-1 THz. We supposed to combine photothermal conversion characteristics with temperature-dependent THz transmission properties of Ti₂O₃ film, which could introduce solar light as the energy source for tuning THz waves. Our work will provide new sight for investigating MIT characteristics of Ti₂O₃ at THz regime and exhibit huge potential in the application of tuning terahertz waves in outdoor scenarios in the future.

Dual-Stimulus Control for Ultra-Wideband and Multidimensional Modulation in Terahertz Metasurfaces Comprising Graphene and Metal Halide Perovskites

Perovskites and graphene are receiving a meteoric rise in popularity in the field of active photonics because they exhibit excellent optoelectronic properties for dynamic manipulation of light-matter interactions. However, challenges still exist, such as the instability of perovskites under ambient conditions and the low Fermi level of graphene in experiments. These shortcomings limit the scope of applications when they are used alone in advanced optical devices. However, the combination of graphene and perovskites is still a promising route for efficient control of light-matter interactions. Here, we report a dual-optoelectronic metadevice fabricated by integrating terahertz metasurfaces with a sandwich complex composed of graphene, polyimide, and perovskites for ultra-wideband and multidimensional manipulation of higher-order Fano resonances. Owing to the photogenerated carriers and electrostatic doping effect, the dual optoelectronic metadevice showed different manipulation behavior at thermal imbalance (electrostatic doping state of the system). The modulation depth of the transmission amplitude reached 200%, the total resonant frequency shift was 800 GHz, and the tunable range of the resonant frequency was 68.8%. In addition, modulation of the maximum phase reached 346°. This work will inspire a new generation of metasurface-based optical devices that combine two active materials.

Dynamic switching between bound states in the continuum (BIC) and quasi-BIC based on a Dirac semimetal terahertz metasurface

This work presents a Dirac semimetal–metal metasurface for the dynamic switching of terahertz-band BIC to quasi-BIC. The display of clear letters is achieved by arranging the metasurface array and applying different voltages.

Optically Controlled Ultrafast Terahertz Metadevices with Ultralow Pump Threshold

Arming metasurface with active materials furnishes a feasible solution to dynamically control over terahertz (THz) waves, which is extremely significant for the realization of upcoming sixth generation telecommunications. However, the present active materials are mainly limited to single external driving field, hindering the capability of metasurface for flexible manipulation of THz waves. Besides, less attention has been paid to the energy question how to significantly reduce the pump threshold for achieving the desired function. Here, a germanium (Ge) hybrid Fano metasurface under dual-stimulus control is experimentally demonstrated. Photoexcitation of Ge thin film enables 100% modulation depth of Fano resonance and ultrafast switching time within 10 ps. By adding current-bias, the pump threshold to modulate the metasurface is greatly reduced from 1600 to 200 µJ cm^-2 . Different from the optical modulation independent of film thickness, it is found that the current function is in proportion with the thickness of Ge thin film. Moreover, it is demonstrated that compared to the single optical-stimulus, the THz amplitude modulation is increased by 56.3% under dual-stimulus function. This work naturally improves the flexibility and practicality of Ge-based metadevice and inspires more innovations to boost the development of switchable sensing, lasing spacer, and nonlinear systems.

Edge Raman enhancement at layered PbI2platelets induced by laser waveguide effect

As a two-dimensional (2D) layered semiconductor, lead iodide (PbI₂) has been widely used in optoelectronics owing to its unique crystal structure and distinctive optical and electrical properties. A comprehensive understanding of its optical performance is essential for further application and progress. Here, we synthesized regularly shaped PbI₂platelets using the chemical vapor deposition method. Raman scattering spectroscopy of PbI₂platelets was predominantly enhanced when the laser radiated at the edge according to Raman mapping spectroscopy. Combining the outcome of polarized Raman scattering spectroscopy and finite-difference time domain simulation analysis, the Raman enhancement was proven to be the consequence of the enhancement effects inherent to the high refractive index contrast waveguide, which is naturally formed in well-defined PbI₂platelets. Because of the enlarged excited area determined by the increased propagation length of the laser in the PbI₂platelet formed waveguide, the total Raman enhancements are acquired rather than a localized point enhancement. Finally, the Raman enhancement factor is directly related to the thickness of the PbI₂platelet, which further confirms the waveguide-enhanced edge Raman. Our investigation of the optical properties of PbI₂platelets offers reference for potential 2D layered-related optoelectronic applications.

Active Control of Nanodielectric-Induced THz Quasi-BIC in Flexible Metasurfaces: A Platform for Modulation and Sensing

A bound state in the continuum (BIC) is a nonradiating state of light embedded in the continuum of propagating modes providing drastic enhancement of the electromagnetic field and its localization at micro-nanoscale. However, access to such modes in the far-field requires symmetry breaking. Here, it is demonstrated that a nanometric dielectric or semiconductor layer, 1000 times thinner than the resonant wavelength (λ/1000), induces a dynamically controllable quasi-bound state in the continuum (QBIC) with ultrahigh quality factor in a symmetric metallic metasurface at terahertz frequencies. Photoexcitation of nanostrips of germanium activates ultrafast switching of a QBIC resonance with 200% transmission intensity modulation and complete recovery within 7 ps on a low-loss flexible substrate. The nanostrips also form microchannels that provide an opportunity for BIC-based refractive index sensing. An optimization model is presented for (switchable) QBIC resonances of metamaterial arrays of planar symmetric resonators modified with any (active) dielectric for inverse metamaterial design that can serve as an enabling platform for active micro-nanophotonic devices.

Actively Tunable Fano Resonance Based on a Bowtie-Shaped Black Phosphorus Terahertz Sensor

An ultrasensitive Terahertz (THz) sensor consisting of the sub-wavelength bowtie black phosphorus (BP) and an annular gold (Au) ring is proposed. The interference between the resonance generated by the bowtie BP and the Au ring creates a Fano-type resonance and makes ultrasensitive sensing realizable. Firstly, we demonstrate the Fano resonance of the coupled nanostructures by adjusting the geometry dimensions of the Au ring and the Fermi level of BP. Moreover, the Poynting vector distributions of the THz sensor are simulated to analyze the properties of Fano resonance. Importantly, a figure of merit (FOM) value as high as 69.3 is achieved and the proposed Fano resonance sensor shows a maximum sensitivity of 9.3 μm/RIU. Our structure can function as a facile and efficient building block of biochemical nano-sensing application based on Fano resonance at THz frequency.

Active polarization-independent plasmon-induced transparency metasurface with suppressed magnetic attenuation

A tunable polarization-independent plasmon-induced transparency (PIT) metasurface based on connected half-ring and split-ring resonators is proposed to working in the terahertz band. We analyze the PIT effect in metasurfaces comprising of ring resonator and split ring resonator. Due to the magnetic attenuation caused by the reverse current between the two resonators, the relative position of the ring resonator and the split-ring resonator greatly affects the strength of the PIT effect. Magnetic attenuation weakens the dark mode of the split ring resonator. Through simulation and experiment, it is found that connecting the ring resonator and split-ring resonator can avoid magnetic attenuation and achieve a stronger PIT window. Furthermore, the fourfold rotation structure of the connected half-ring and split-ring resonator on silicon substrate achieves an optically controlled polarization-independent PIT effect. The design would provide significant guidance in multifunctional active devices, such as modulators and switches in terahertz communication.

Bifunctional Spatiotemporal Metasurfaces for Incident Angle-Tunable and Ultrafast Optically Switchable Electromagnetically Induced Transparency

Advances in tunable metamaterials/metasurfaces facilitates their utilization in novel optical components, and lead to many breakthroughs in light tailoring by giving birth to diverse spatiotemporal dynamics. In the ascendant field of terahertz (THz) photonics, the ultrafast modulation is the fundamental process of technological advancements in high-speed wireless communications, sensing, and imaging. However, the current research efforts have been mainly devoted to studies of single functionality under the control of one stimulus, which has plateaued in terms of innovative new features. Here, building on the incident angle-induced C₂ symmetry breaking of split ring pairs, we experimentally demonstrate extremely versatile, ultrafast THz switching behaviors at continuously alterable resonant states. The direction-controlled resonance hybridization provides another excellent degree of routing freedom, owing to its robustness, simplicity, and wide tunability. By leveraging such virtues, single LC mode and EIT-like resonance under normal and oblique incidence conditions are both effectively switched-off by means of photon injection. Considering the ultrashort lifetime of free carriers in MoSe₂ crystal, the corresponding transient dynamics show an ultrafast recovery time within 700 ps. The strategy proposed here is a viable pathway for multidimensional THz wave manipulation, which gears up a crucial step for diversified functionalities in deployable metaphotonic devices.

Fano-Resonant Hybrid Metamaterial for Enhanced Nonlinear Tunability and Hysteresis Behavior

Artificial resonant metamaterial with subwavelength localized filed is promising for advanced nonlinear photonic applications. In this article, we demonstrate enhanced nonlinear frequency-agile response and hysteresis tunability in a Fano-resonant hybrid metamaterial. A ceramic cuboid is electromagnetically coupled with metal cut-wire structure to excite the high-Q Fano-resonant mode in the dielectric/metal hybrid metamaterial. It is found that the significant nonlinear response of the ceramic cuboid can be employed for realization of tunable metamaterials by exciting its magnetic mode, and the trapped mode with an asymmetric Fano-like resonance is beneficial to achieve notable nonlinear modulation on the scattering spectrum. The nonlinear tunability of both the ceramic structure and the ceramic/metal hybrid metamaterial is promising to extend the operation band of metamaterials, providing possibility in practical applications with enhanced light-matter interactions.

Pump-Color Selective Control of Ultrafast All-Optical Switching Dynamics in Metaphotonic Devices

Incorporating active materials into metamaterials is expected to yield exciting advancements in the unprecedented versatility of dynamically controlling optical properties, which sheds new light on the future optoelectronics. The exploration of emerging semiconductors into terahertz (THz) meta-atoms potentially allows achieving ultrafast nanodevices driven by various applications, such as biomedical sensing/imaging, ultrawide-band communications and security scanners. However, ultrafast optical switching of THz radiation is currently limited to a single level of tuning speed, which is a main hurdle to achieve multifunctionalities. Here, a hybrid metadevice which can realize the pump-wavelength controlled ultrafast switching response by the functionalization of double photoactive layers is demonstrated experimentally. A whole cycle of electromagnetically induced transparency switching with a half-recovery state changes from 0.78 ns to 8.8 ps as pump wavelength varies from near infrared to near ultraviolet regions. The observed pump-color selective switching speed changing from nanosecond scale to picosecond scale is ascribed to the wavelength-dependent penetration length of Ge and the contrasting defect states between noncrystalline Ge and epitaxial Si layers. It is believed that the schemes regarding pump-color controllable ultrafast switching behavior introduced here can inspire more innovations across the field of ultrafast photonics and can boost the reconfigurable metamaterial applications.

Flexible and Giant Terahertz Modulation Based on Ultra-Strain-Sensitive Conductive Polymer Composites

Dynamic tuning of terahertz (THz) wave has a great potential application as smart THz devices, such as switches, modulators, sensors, and so on. However, the realization of flexible THz modulation with high efficiency is rarely observed, which is nearly absent from the booming development and demands on flexible electronics. Here, we report a flexible THz modulation based on conductive polymer composites composed of thermoplastic polyurethane (TPU) and conductive particles (Ni). By designing the additive content of Ni particles, such a flexible layer exhibits resistivity change of 6-7 orders under tensile strain due to the formation of an electron-transport channel provided by the in situ evolution of the Ni network. It could be used to dynamically control the THz transmission with a giant modulation depth of around 96%, at a high strain operation (up to around 58.5%). Moreover, these characteristics are demonstrated to be available for highly tension sensitive THz spectroscopy and imaging. This work opens up a connection between flexible polymer-based composites and THz dynamic devices. It proposes an unprecedented flexible THz modulation with giant tuning efficiency and provides a scheme for contactless and passive tension sensors.