Excitons in 2D perovskites for ultrafast terahertz photonic devices

Charge Generation Dynamics in Organic Photovoltaic Blends under One-Sun-Equivalent Illumination Detected by Highly Sensitive Terahertz Spectroscopy

Organic photovoltaic (OPV) devices attain high performance with nonfullerene acceptors by utilizing the synergistic dual channels of charge generation that originate from excitations in both the donor and acceptor materials. However, the specific intermediate states that facilitate both channels are subject to debate. To address this issue, we employ time-resolved terahertz spectroscopy with improved sensitivity (ΔE/E < 10-6), enabling direct probing of charge generation dynamics in a prototypical PM6:Y6 bulk heterojunction system under one-sun-equivalent excitation density. Charge generation arising from donor excitations is characterized with a rise time of ∼9 ps, while that from acceptor excitations shows a rise time of ∼18 ps. Temperature-dependent measurements further reveal notably distinct activation energies for these two charge generation pathways. Additionally, the two channels of charge generation can be substantially manipulated by altering the ratio of bulk to interfaces. These findings strongly suggest the presence of two distinct intermediate states: interfacial and intramoiety excitations. These states are crucial in mediating the transfer of electrons and holes, driving charge generation within OPV 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.

Transient Loss-Induced Non-Hermitian Degeneracies for Ultrafast Terahertz Metadevices

Non-Hermitian degeneracies, also known as exceptional points (EPs), have attracted considerable attention due to their unique physical properties. In particular, metasurfaces related to EPs can open the way to unprecedented devices with functionalities such as unidirectional transmission and ultra-sensitive sensing. Herein, an active non-Hermitian metasurface with a loss-induced parity-time symmetry phase transition for ultrafast terahertz metadevices is demonstrated. Specifically, the eigenvalues of the non-Hermitian transmission matrix undergo a phase transition under optical excitation and are degenerate at EPs in parameter space, which is accompanied by the collapse of chiral transmission. Ultrafast EP modulation on a picosecond time scale can be realized through variations in the transient loss at a non-Hermitian metasurface pumped by pulsed excitation. Furthermore, by exploiting the physical characteristics of chiral transmission EPs, a switchable quarter-wave plate based on the photoactive metasurface is designed and experimentally verified and realized the corresponding function of polarization manipulation. This work opens promising possibilities for designing functional terahertz metadevices and fuses EP physics with active metasurfaces.

Ultrafast Carrier Drift Transport Dynamics in CsPbI3 Perovskite Nanocrystalline Thin Films

We study the early time carrier drift dynamics in CsPbI3 nanocrystal thin films with a sub 25 ps time resolution. Prior to trapping, carriers exhibit band-like transport characteristics, which is similar to those of traditional semiconductor solar absorbers including Si and GaAs due to optical phonon and carrier scattering at high temperatures. In contrast to the popular polaron scattering mechanism, the CsPbI3 nanocrystal thin film demonstrates the strongest optical phonon scattering mechanism among other inorganic-organic hybrid perovskites, Si, and GaAs. This ultrafast dynamics study establishes a foundation for understanding the fundamental carrier drift properties that drive perovskite nanocrystal optoelectronics.

Ultrasensitive optical modulation in hybrid metal-perovskite and metal-graphene metasurface THz devices

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.

Impact of the uniaxial strain on terahertz modulation characteristics in flexible epitaxial VO2 film across the phase transition

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO₂) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO₂ film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.

Boosting Charge Transport in a 2D/3D Perovskite Heterostructure by Selecting an Ordered 2D Perovskite as the Passivator

We demonstrate that an ordered 2D perovskite can significantly boost the photoelectric performance of 2D/3D perovskite heterostructures. Using selective fluorination of phenyl-ethyl ammonium (PEA) lead iodide to passivate 3D FA_0.8 Cs_0.2 PbI₃ , we find that the 2D/3D perovskite heterostructures passivated by a higher ordered 2D perovskite have lower Urbach energy, yielding a remarkable increase in photoluminescence (PL) intensity, PL lifetime, charge-carrier mobilities (ϕμ), and carrier diffusion length (L_D ) for a certain 2D perovskite content. High performance with an ultralong PL lifetime of ≈1.3 μs, high ϕμ of ≈18.56 cm²  V^-1  s^-1 , and long L_D of ≈7.85 μm is achieved in the 2D/3D films when passivated by 16.67 % para-fluoro-PEA₂ PbI₄ . This carrier diffusion length is comparable to that of some perovskite single crystals (>5 μm). These findings provide key missing information on how the organic cations of 2D perovskites influence the performance of 2D/3D perovskite heterostructures.

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.

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.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Metamaterials and Metasurfaces: A Review from the Perspectives of Materials, Mechanisms and Advanced Metadevices

Throughout human history, the control of light, electricity and heat has evolved to become the cornerstone of various innovations and developments in electrical and electromagnetic technologies. Wireless communications, laser and computer technologies have all been achieved by altering the way light and other energy forms act naturally and how to manage them in a controlled manner. At the nanoscale, to control light and heat, matured nanostructure fabrication techniques have been developed in the last two decades, and a wide range of groundbreaking processes have been achieved. Photonic crystals, nanolithography, plasmonics phenomena and nanoparticle manipulation are the main areas where these techniques have been applied successfully and led to an emergent material sciences branch known as metamaterials. Metamaterials and functional material development strategies are focused on the structures of the matter itself, which has led to unconventional and unique electromagnetic properties through the manipulation of light-and in a more general picture the electromagnetic waves-in widespread manner. Metamaterial's nanostructures have precise shape, geometry, size, direction and arrangement. Such configurations are impacting the electromagnetic light waves to generate novel properties that are difficult or even impossible to obtain with natural materials. This review discusses these metamaterials and metasurfaces from the perspectives of materials, mechanisms and advanced metadevices in depth, with the aim to serve as a solid reference for future works in this exciting and rapidly emerging topic.

Reconfigurable terahertz metamaterials: From fundamental principles to advanced 6G applications

Terahertz (THz) electromagnetic spectrum ranging from 0.1THz to 10THz has become critical for sixth generation (6G) applications, such as high-speed communication, fingerprint chemical sensing, non-destructive biosensing, and bioimaging. However, the limited response of naturally existing materials THz waves has induced a gap in the electromagnetic spectrum, where a lack of THz functional devices using natural materials has occurred in this gap. Metamaterials, artificially composed structures that can engineer the electromagnetic properties to manipulate the waves, have enabled the development of many THz devices, known as "metadevices". Besides, the tunability of THz metadevices can be achieved by tunable structures using microelectromechanical system (MEMS) technologies, as well as tunable materials including phase change materials (PCMs), electro-optical materials (EOMs), and thermo-optical materials (TOMs). Leveraging various tuning mechanisms together with metamaterials, tremendous research works have demonstrated reconfigurable functional THz devices, playing an important role to fill the THz gap toward the 6G applications. This review introduces reconfigurable metadevices from fundamental principles of metamaterial resonant system to the design mechanisms of functional THz metamaterial devices and their related applications. Moreover, we provide perspectives on the future development of THz photonic devices for state-of-the-art applications.

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.

Highly Tunable Enhancement and Switching of Nonlinear Emission from All-Inorganic Lead Halide Perovskites via Electric Field

Herein, we demonstrate a highly tunable enhancement and switching of nonlinear emission from all-inorganic metal halide perovskites based on an asymmetrically biased metal-insulator-semiconductor (MIS) structure. We achieve 2 orders of magnitude enhancement of the two-photon-pumped photoluminescence (TPL) from CsPbBr₃ microplates with the MIS structure, due to comprehensive effects including localized field effect, trap-filling effect, and collection enhancement. In particular, taking advantage of electric-field-induced passivation/activation of Br vacancies, we realize highly tunable TPL enhancement, ranging from ∼61.2-fold to ∼370.3-fold. Moreover, we demonstrate an efficient modulation of the two-photon-pumped lasing from the MIS structure, which exhibits electric field induced switching with a high on/off ratio of 67:1. This work has opened new avenues for steering carrier transport and nonlinear emission in lead halide perovskites, which shows great promise for realizing high-efficiency and tunable nonlinear nanophotonic devices.

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.

Advances in Flexible Memristors with Hybrid Perovskites

Hybrid organic-inorganic metal halide perovskite (HOIP)-based memristors have captured strong attention not only as an emerging candidate for next-generation high-density information storage technology but also for use in healthcare technology and the Internet of Things (IoT) because of their unique properties: low weight, flexibility, compatibility, stretchability, and low power consumption. In this Perspective, we review the recent advances of various aspects of flexible memristors focusing on the selection of the flexible substrates, materials, interfaces, several resistive switching mechanisms, and different methodologies of perovskite growth. The current state of the art of the memristor as an artificial synapse, light-induced resistive switching, and logic gates is comprehensively and systematically reviewed. Finally, we briefly discuss the stability factors of perovskites and present the conclusion with a broad outlook on the progress and challenges in the field of perovskite-based flexible memristors.

Interfacial Embedding of Laser-Manufactured Fluorinated Gold Clusters Enabling Stable Perovskite Solar Cells with Efficiency Over 24

Tackling the interfacial loss in emerged perovskite-based solar cells (PSCs) to address synchronously the carrier dynamics and the environmental stability, has been of fundamental and viable importance, while technological hurdles remain in not only creating such interfacial mediator, but the subsequent interfacial embedding in the active layer. This article reports a strategy of interfacial embedding of hydrophobic fluorinated-gold-clusters (FGCs) for highly efficient and stable PSCs. The p-type semiconducting feature enables the FGC efficient interfacial mediator to improve the carrier dynamics by reducing the interfacial carrier transfer barrier and boosting the charge extraction at grain boundaries. The hydrophobic tails of the gold clusters and the hydrogen bonding between fluorine groups and perovskite favor the enhancement of environmental stability. Benefiting from these merits, highly efficient formamidinium lead iodide PSCs (champion efficiency up to 24.02%) with enhanced phase stability under varied relative humidity (RH) from 40% to 95%, as well as highly efficient mixed-cation PSCs with moisture stability (RH of 75%) over 10 000 h are achieved. It is thus inspiring to advance the development of highly efficient and stable PSCs via interfacial embedding laser-generated additives for improved charge transfer/extraction and environmental stability.

Ultrafast femtosecond pressure modulation of structure and exciton kinetics in 2D halide perovskites for enhanced light response and stability

The carriers’ transportation between layers of two-dimensional (2D) perovskites is inhibited by dielectric confinement. Here, for the first time, we employ a femtosecond laser to introduce ultrafast shock pressure in the range of 0~15.45 GPa to reduce dielectric confinement by modulating the structure and exciton dynamics in a perovskite single crystal (PSCs), e.g. (F-PEA)₂PbI₄ (4-fluorophenethylammonium, F-PEA). The density functional theory (DFT) simulation and experimental results show that the inorganic framework distortion results in a bandgap reduction. It was found that the exciton-optical phonon coupling and free excitons (FEs) binding energy are minimized at 2.75 GPa shock pressure due to a reduction in dielectric confinement. The stability testing under various harsh light and humid thermal conditions shows that femtosecond laser shocking improves the stability of (F-PEA)2PbI4 PSCs. Femtosecond laser shock processing provides a new approach for regulating the structure and enhancing halide perovskite properties.

Exciton in two-dimensional perovskite is strongly influenced by dielectric confinement of the organic components. Here, the authors employ femtosecond laser to induce ultrashock pressure to investigate how the structural changes and the reduction of dielectric confinement affects exciton behaviour and dynamic.

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.

Engineering the Optical Emission and Robustness of Metal-Halide Layered Perovskites through Ligand Accommodation

The unique combination of organic and inorganic layers in 2D layered perovskites offers promise for the design of a variety of materials for mechatronics, flexoelectrics, energy conversion, and lighting. However, the potential tailoring of their properties through the organic building blocks is not yet well understood. Here, different classes of organoammonium molecules are exploited to engineer the optical emission and robustness of a new set of Ruddlesden-Popper metal-halide layered perovskites. It is shown that the type of molecule regulates the number of hydrogen bonds that it forms with the edge-sharing [PbBr₆ ]^4- octahedra layers, leading to strong differences in the material emission and tunability of the color coordinates, from deep-blue to pure-white. Also, the emission intensity strongly depends on the length of the molecules, thereby providing an additional parameter to optimize their emission efficiency. The combined experimental and computational study provides a detailed understanding of the impact of lattice distortions, compositional defects, and the anisotropic crystal structure on the emission of such layered materials. It is foreseen that this rational design can be extended to other types of organic linkers, providing a yet unexplored path to tailor the optical and mechanical properties of these materials and to unlock new functionalities.

Hybrid Perovskite Terahertz Photoconductive Antenna

Hybrid organic–inorganic perovskites, while well examined for photovoltaic applications, remain almost completely unexplored in the terahertz (THz) range. These low-cost hybrid materials are extremely attractive for THz applications because their optoelectronic properties can be chemically engineered with relative ease. Here, we experimentally demonstrate the first attempt to apply solution-processed polycrystalline films of hybrid perovskites for the development of photoconductive terahertz emitters. By using the widely studied methylammonium-based perovskites MAPbI₃ and MAPbBr₃, we fabricate and characterize large-aperture photoconductive antennas. The work presented here examines polycrystalline perovskite films excited both above and below the bandgap, as well as the scaling of THz emission with the applied bias field and the optical excitation fluence. The combination of ultrafast time-resolved spectroscopy and terahertz emission experiments allows us to determine the still-debated room temperature carrier lifetime and mobility of charge carriers in halide perovskites using an alternative noninvasive method. Our results demonstrate the applicability of hybrid perovskites for the development of scalable THz photoconductive devices, making these materials competitive with conventional semiconductors for THz emission.

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.

Exciton and Carrier Dynamics in Two-Dimensional Perovskites

Two-dimensional Ruddlesden-Popper hybrid lead halide perovskites have become a major topic in perovskite optoelectronics. Here, we aim to unravel the ultrafast dynamics governing the evolution of charge carriers and excitons in these materials. Using a combination of ultrabroadband time-resolved THz (TRTS) and fluorescence upconversion spectroscopies, we find that sequential carrier cooling and exciton formation best explain the observed dynamics, while exciton-exciton interactions play an important role in the form of Auger heating and biexciton formation. We show that the presence of a longer-lived population of carriers is due to the latter processes and not to a Mott transition. Therefore, excitons still dominate at laser excitation densities. We use kinetic modeling to compare the phenethylammonium and butylammonium organic cations while investigating the stability of the resulting films. In addition, we demonstrate the capability of using ultrabroadband TRTS to study excitons in large binding energy semiconductors through spectral analysis at room temperature.

Assembling Nano-Microarchitecture for Electromagnetic Absorbers and Smart Devices

Smart devices, nowadays, are inspiring the infinite vitality and possibilities of intelligent life, such as self-power electromagnetic (EM) nanogenerator and microsensor, smart window, thermally-driven EM absorber, interstellar energy deliverer, and so on. Herein, the latest and most impressive works of 3D nano-micro architectures and their smart EM devices are highly focused on. The most key information, including assembly strategy and mechanism, EM response, and approach-structure-function relationship, is extracted and well-organized with profundity and easy-to-understand approach. The merit and demerit are revealed by comparison. What's more, the brightest and most cutting-edge smart EM devices constructed by 3D nano-micro architectures are reported as highlights, and the device principles are deeply dissected. Finally, a profound and top comment on the fast-growing field as well as challenges are proposed, and the future directions are predicted intelligently.

Air-Stable Bulk Halide Single-Crystal Scintillator Cs3Cu2I5 by Melt Growth: Intrinsic and Tl Doped with High Light Yield

Ternary metal halides with large exciton binding energy have recently gained considerable attention in the optoelectronic field due to their high photoluminescence quantum yield and large Stokes shift. Here, efficient scintillators are designed based on these advantageous properties. For the first time, bulk Cs₃Cu₂I₅ is grown using a melt method other than the intensively reported solution growth, and behaved as an intrinsic scintillator, emitting bright blue (∼450 nm) light under X-ray and γ-ray irradiation. Successful Tl doping at Cs sites tune the emission band over the entire visible range (400-700 nm) due to the synergetic effects of self-trapped excitons (STEs) and Tl centers. Notably, after doping with 1% Tl⁺, the scintillation light yield of Cs₃Cu₂I₅ increases by nearly three times to 51 000 ± 2000 ph/MeV (Cs-137, 662 keV). Cs₃Cu₂I₅:Tl shows a higher energy resolution of 4.5% at 662 keV than that of NaI:Tl and an excellent nonproportionality (<3%) in the γ-ray energy range of 60-1275 keV. A model of energy relaxation in Cs₃Cu₂I₅:Tl scintillators is proposed and discussed. In particular, it is the first Cu-based halide scintillator that has air stability, good stopping power, and the ability to grow large bulk single crystals for practical application. This work provides a strategy for tuning and broadening the spectral range of STE emitters, and bridges the lead-free halide derivatives with scintillators.

Ultrafast modulation of the spectral filtering properties of a THz metasurface

We demonstrate ultrafast tuning of a plasmonic spectral filter at terahertz (THz) frequencies. The device is made of periodically spaced gold crosses deposited on the surface of an undoped silicon wafer in which transient free carriers can be optically injected with a femtosecond resonant pulse. We demonstrate the concept by measuring the transmission spectrum of a notch filter using time-domain THz spectroscopy. Proper synchronization of the THz probe and visible excitation pulses leads to an enhanced transmission at the resonance by more than two orders of magnitude. Finite-difference time-domain simulations, which are in agreement with the experimental results, show that the underlying mechanisms responsible for the resonance blueshift and linewidth broadening can be attributed to the photoinduced change in dielectric properties of the substrate. This is supported by the numerically simulated field distribution and reflection/transmission coefficients. The device can be used in future pulse shaping and ultrafast switching experiments.

Ultrafast charge carrier dynamics in quantum confined 2D perovskite

We studied the charge carrier dynamics in 2D perovskite NBT₂PbI₄ by ultrafast optical pump-THz probe spectroscopy. We observed a few ps long relaxation dynamics that can be ascribed to the band to band carrier recombination, in the absence of any contribution from many-body and trap assisted processes. The transient conductivity spectra show that the polaron dynamics is strongly modulated by the presence of a rich exciton population. The polarization field resulting from the exciton formation acts as the source of a restoring force that localizes polarons. This is revealed by the presence of a negative imaginary conductivity. Our results show that the dynamics of excitons in 2D perovskites at room temperature can be detected by monitoring their effect on the conductivity of the photoinduced polaronic carrier.