2D Materials for Optical Modulation: Challenges and Opportunities

Polymorphic phases in 2D In2Se3: fundamental properties, phase transition modulation methodologies and advanced applications

Two-dimensional (2D) In2Se3, which is a multifunctional semiconductor, exhibits multiple crystallographic phases, each of which possesses distinct electronic, optical, and thermal properties. This inherent phase variability makes it a promising candidate for a wide range of applications, including memory devices, photovoltaics, and photodetectors. This review comprehensively explores the latest progress of various polymorphic phases of 2D In2Se3, emphasizing their unique properties, characterization methods, phase modulation strategies, and practical applications. Commencing with a rigorous examination of the structural attributes inherent in its various phases, we introduce sophisticated techniques for its characterization. Subsequently, modulation strategies, encompassing variations in temperature, application of electric fields, induced stress, and alterations in pressure, are explored, each exerting an influence on the phase transitions in 2D In2Se3. Finally, we highlight recent advancements and applications resulting from these phase transitions, including homoepitaxial heterophase structures, optical modulators, and phase change memory (PCM). By synthesizing insights into phase properties, modulation strategies, and potential applications, this review endeavours to provide a comprehensive understanding of the significance and prospects of In2Se3 in the semiconductor field.

Enhanced Backgate Tunability on Interfacial Carrier Concentration in Ionic Liquid-Gated MoS2 Devices

The periodic spatial modulation potential arising from the zig-zag distribution of ions at large gate voltage in an ionic liquid-gated device may enable functionalities in a similar way as nanopatterning and moiré engineering. However, the inherent coupling between periodic modulation potential and carrier concentration in ionic liquid devices has hindered further exploration. Here, the feasibility of decoupling manipulation on periodic modulation potential and carrier density in an ionic liquid device is demonstrated by using a conventional backgate. The backgate is found to have a tunability on carrier concentration comparable to that of ionic gating, especially at large ionic liquid gate voltage, by activating the bulk channels mediated back tunneling between the trapped bands and interfacial channel.

Non-Volatile Reconfigurable Four-Mode van der Waals Transistors and Transformable Logic Circuits

Emerging applications in data-intensive computing and circuit security demand logic circuits with high functional density, reconfigurability, and energy efficiency. Here, we demonstrate nonvolatile reconfigurable four-mode field-effect transistors (NVR4M-FETs) based on two-dimensional (2D) MoTe2 and CuInP2S6 (CIPS), offering both polarity switching and threshold voltage modulation. The device exploits the ferroelectric polarization of CIPS at the source/drain regions to achieve dynamic control over the transistor polarity, enabling transitions between n-type and p-type states through polarization-induced local electrostatic doping. Additionally, multilayer graphene floating gates are incorporated to modulate the threshold voltage, yielding four distinct nonvolatile operating modes: n-type logic, p-type logic, always-on memory, and always-off memory. Leveraging the four-mode property, the NVR4M-FET can function as a one-transistor-per-bit ternary content-addressable memory (TCAM). In addition, we demonstrate the construction of transformable logic gates with 14 distinct logic functions using two NVR4M-FETs and a reconfigurable half a dder/subtractor using three NVR4M-FETs integrated with load resistors. Furthermore, we show that a 2-input look-up table can be achieved with eight NVR4M-FETs compared to 12 transistors using reconfigurable transistors, highlighting the potential of NVR4M-FETs for high-density logic circuits. These results underscore the potential of NVR4M-FETs as essential building blocks for energy-efficient, in-memory computing, and secure hardware applications.

Regulating transient optical responses in twisted bilayer WS2

The optical response manipulation of two-dimensional materials is crucial for designing and optimizing high-performance optoelectronic devices. Previously, optical modulation in two-dimensional semiconductors primarily relied on adjusting carrier density through optical excitation or charge injection using the energy band-filling effect. Recently, twist angle has been found to tune the optical and optoelectronic properties of van der Waals structure, but its impact on the transient optical response remains unexplored. Herein, we demonstrate that twist angle can effectively regulate carrier behaviors by tracing the evolution of optical responses in twisted bilayer WS2from 0° to 60°. Both Raman and PL spectra consistently show that the optical responses of WS2bilayers are highly dependent on the twist angle. Exciton behavior and phonon modes exhibit similarity at twist angles near 0° and 60°, but significantly change as the angle approaches 30°. Moreover, the impact of the twist angle on the transient optical responses was carefully investigated using a femtosecond pump-probe technique. The results reveal a significant decrease in carrier thermalization/relaxation time and exciton formation/recombination time at the WS2bilayers with twist angle of ∼31.0°, as compared to twist angles of ∼2.9° and ∼58.9°, which can be attributed to the accumulation of intralayer carriers due to weakened interlayer coupling. These results demonstrate that twist angle can effectively modulate the optical response of twisted 2D materials. Our study elucidates the dynamic carrier behavior in twisted bilayer WS2and provides new insights for designing future optoelectronic and photonic devices.

Emerging Trends in Conductive Two-Dimensional Covalent Organic Frameworks for Large-Area Electronic Applications

Two-dimensional covalent organic frameworks (2D COFs) are emerging as promising materials for advanced electronic applications due to their tunable porosity, crystalline order, and π-conjugated structures. These properties enable efficient charge transport and bandgap modulation, making 2D COFs strong candidates for electronic devices such as transistors and memristors. However, the practical application of COFs remains limited by challenges in achieving high-quality thin films with large-area uniformity and improved crystallinity. This review explores recent advancements in the fabrication and application of conductive 2D COFs for electronics. Various synthesis strategies, including direct growth, vapor-assisted conversion, and interfacial methods, are discussed in the context of enhancing film quality and scalability. The integration of COFs into electronic devices is classified based on their operation mechanism─planar and vertical field-effect transistors (FETs), electrochemical transistors (ECTs), and memristors─to highlight their electronic properties and device performance. Looking forward, the challenges of large-scale production, material compatibility, and device integration are outlined, alongside potential solutions through innovative synthesis techniques and collaborative research efforts. By addressing these challenges, 2D COFs are poised to drive breakthroughs in electronic devices by their adoption in next-generation semiconducting technologies.

Layer-dependent Schottky contact at TaX2-BY (X = S, Se, Te; Y = P, As, Sb) van der Waals interfaces

The mechanical, thermal and dynamical stabilities, electronic structure, contact type, and height of the barrier at the interface of TaX2 (X = S, Se, Te) and BY (Y = P, As, Sb) metal-semiconductor (MS) contact are investigated via first principles calculations. Binding energies, mechanical properties, phonon spectra and ab initio molecular dynamics (AIMD) simulations confirm the stabilities of these systems. TaX2-BY (X = S, Se, Te; Y = P, As, Sb) MS van der Waals heterostructures (vdWHs) are found to be metal with a Schottky contact at the interface. Formation of the n-type Schottky contact at the interface of TaX2-BY (X = S, Se, Te; Y = P, As, Sb) MS vdWHs favors electron conduction over hole conduction. Small (higher) effective mass (carrier mobility) make TaS2-BSb, TaSe2-BSb and TaTe2-BSb MS vdWHs, potential candidates for high speed nanoelectronic applications. Bader charge analysis shows that at the interface of TaX2-BY (X = S, Se, Te; Y = P, As, Sb) MS vdWHs, in TaX2 (BP, BAs) the electrons transfer from the TaX2 layer to the BP and BAs layer, while in TaX2 (BSb) the electrons transfer from the BSb layer to TaX2 layer.

Small-molecule organic ice microfibers

Small organic molecules are essential building blocks of our universe, from cosmic dust to planetary surfaces to life. Compared to their well-known gaseous and liquid forms that have been extensively studied, small organic molecules in the form of ice at low temperatures receive much less attention. Here, we show that supercooled small-molecule droplets can be drawn into highly uniform amorphous ice microfibers with lengths up to 5 cm and diameters down to 200 nm. In the experimental test, these fiber-like ices manifest excellent mechanical flexibilities with elastic strain up to 3.3%. Meanwhile, they can guide light with loss down to 0.025 dB/cm that approaches the material absorption limit and offer high optical nonlinearity for low-threshold supercontinuum generation. Notable temperature-dependent Young's modulus and icing-induced refractive-index increase are also found. These results may open a promising category of low-temperature materials for both scientific research and technological applications.

Polymerization Improvement of Graphitic Carbon Nitride Films Derived from Melamine and Thiourea

Deposition of low-cost, efficient, and environmentally friendly graphitic carbon nitride (g-CN) films as photoanodes is a crucial step for constructing photoelectrochemical (PEC) cells and exploring their PEC performance. Currently, the improvement of the photocurrent density of g-CN films is badly needed for their practical applications in PEC water splitting. Enhancing the g-CN crystallinity by optimizing their synthesis conditions only through screening appropriate reactant precursors is insufficient for this purpose. Herein, using melamine and thiourea precursors with mass ratio 5:1, the degree of polymerization of g-CN thin films is successfully improved by a thermal vapor condensation method. The obtained pure g-CN exhibits a remarkably enhanced photocurrent density of 404.4 µA cm-2 at 1.23 V versus reversible hydrogen electrodes. Theoretical calculations reveal that the continuous attachment of small carbodiimide (HN═C═NH) mainly generated by thiourea to the melamine matrix facilitates the formation of large-area conjugated structure, which fundamentally determines better charge carrier separation and transfer thereby enhancing the PEC performance. This work realizes the synthesis of well-polymerized g-CN films with improved PEC activity and offers a computational understanding for the nucleation and growth mechanism of the polycrystalline g-CN.

Programmable nonlinear optical neuromorphic computing with bare 2D material MoS2

Nonlinear optical responses in two-dimensional (2D) materials can build free-space optical neuromorphic computing systems. Ensuring the high performance and the tunability of the system is essential to encode diverse functions. However, common strategies, including the integration of external electrode arrays or photonic structures with 2D materials, and barely patterned 2D materials, exhibit a contradiction between performance and tunability. Because the unique band dispersions of 2D materials can provide hidden paths to boost nonlinear responses independently, here we introduced a new free-space optical computing concept within a bare molybdenum disulfide array. This system can preserve high modulation performance with fast speed, low energy consumption, and high signal-to-noise ratio. Due to the freedom from the restrictions of fixed photonic structures, the tunability is also enhanced through the synergistic encodings of the 2D cells and the excitation pulses. The computing mechanism of transition from two-photon absorption to synergistic excited states absorption intrinsically improved the modulation capability of nonlinear optical responses, revealed from the relative transmittance modulated by a pump-probe-control strategy. Optical artificial neural network (ANN) and digital processing were demonstrated, revealing the feasibility of the free-space optical computing based on bare 2D materials toward neuromorphic applications.

Surface Plasmons in Two-Dimensional MXenes

MXenes, a class of layered two-dimensional transition metal carbides and nitrides, exhibit excellent optoelectronic properties and show promise for fields ranging from photonics and communications to energy storage and catalysis. Some members of the MXene family are metallic and exhibit large in-plane conductivity, making them possibly suited for 2D plasmonics. The highly variable chemical structure of MXenes offers a broad chemical space to tune material properties for plasmonic applications, including plasmon-enhanced catalysis, surface-enhanced Raman spectroscopy (SERS), and electromagnetic shielding. However, this synthetic complexity has also presented several roadblocks in the process of moving MXene plasmonics into applications. For example, in the prototypical MXene Ti3C2Tx, there remains disagreement over the bulk plasmon energy and the assignment of a prominent resonance around 1.7 eV. We discuss fundamental models and theories of plasmon physics and apply these models to MXenes in order to clarify some of these problems. We outline the potential for hyperbolic plasmons in MXenes and propose new avenues for MXene photonics research.

A broadband hyperspectral image sensor with high spatio-temporal resolution

Hyperspectral imaging provides high-dimensional spatial-temporal-spectral information showing intrinsic matter characteristics1-5. Here we report an on-chip computational hyperspectral imaging framework with high spatial and temporal resolution. By integrating different broadband modulation materials on the image sensor chip, the target spectral information is non-uniformly and intrinsically coupled to each pixel with high light throughput. Using intelligent reconstruction algorithms, multi-channel images can be recovered from each frame, realizing real-time hyperspectral imaging. Following this framework, we fabricated a broadband visible-near-infrared (400-1,700 nm) hyperspectral image sensor using photolithography, with an average light throughput of 74.8% and 96 wavelength channels. The demonstrated resolution is 1,024 × 1,024 pixels at 124 fps. We demonstrated its wide applications, including chlorophyll and sugar quantification for intelligent agriculture, blood oxygen and water quality monitoring for human health, textile classification and apple bruise detection for industrial automation, and remote lunar detection for astronomy. The integrated hyperspectral image sensor weighs only tens of grams and can be assembled on various resource-limited platforms or equipped with off-the-shelf optical systems. The technique transforms the challenge of high-dimensional imaging from a high-cost manufacturing and cumbersome system to one that is solvable through on-chip compression and agile computation.

Adsorption of Atomic Hydrogen on Hydrogen Boride Sheets Studied by Photoelectron Spectroscopy

Hydrogen boride (HB) sheets are emerging as a promising two-dimensional (2D) boron material, with potential applications as unique electrodes, substrates, and hydrogen storage materials. The 2D layered structure of HB was successfully synthesized using an ion-exchange method. The chemical bonding and structure of the HB sheets were investigated using Fourier Transform Infrared (FT-IR) spectroscopy and Transmission Electron Microscopy (TEM), respectively. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical states and transformation of the components before and after atomic hydrogen adsorption, thereby elucidating the atomic hydrogen adsorption process on HB sheets. Our results indicate that, upon atomic hydrogen adsorption onto the HB sheets, the B-H-B bonds were broken and converted into B-H bonds. This research highlights and demonstrates the changes in chemical states and component transformations of the boron element on the HB sheets' surface before and after atomic hydrogen adsorption, thus providing a clearer understanding of the unique bonding and structural characteristics of the HB sheets.

A clean transfer approach to prepare centimetre-scale black phosphorus crystalline multilayers on silicon substrates for field-effect transistors

Recently reported direct growth of highly crystalline centimetre-sized black phosphorus (BP) thin films on mica substrates by pulsed laser deposition (PLD) has attracted considerable research interest. However, an effective and general transfer method to incorporate them into (opto-)electronic devices is still missing. Here, we show a wet transfer method utilizing ethylene-vinyl acetate (EVA) and an ethylene glycol (EG) solution to transfer high-crystalline large-area PLD-BP films onto SiO2/Si substrates. The transferred films were used to fabricate BP-based bottom-gate field-effect transistor (FET) arrays exhibiting good uniformity and continuity, with carrier mobility and current switching ratios comparable to those obtained in as-grown BP films on mica substrates. Our work presents a promising transfer strategy for scalable integration of on-substrate grown 2D BP into devices with more complex structures and further investigation of material properties.

2D Reconfigurable Memory Device Enabled by Defect Engineering for Multifunctional Neuromorphic Computing

In this era of artificial intelligence and Internet of Things, emerging new computing paradigms such as in-sensor and in-memory computing call for both structurally simple and multifunctional memory devices. Although emerging two-dimensional (2D) memory devices provide promising solutions, the most reported devices either suffer from single functionalities or structural complexity. Here, this work reports a reconfigurable memory device (RMD) based on MoS2/CuInP2S6 heterostructure, which integrates the defect engineering-enabled interlayer defects and the ferroelectric polarization in CuInP2S6, to realize a simplified structure device for all-in-one sensing, memory and computing. The plasma treatment-induced defect engineering of the CuInP2S6 nanosheet effectively increases the interlayer defect density, which significantly enhances the charge-trapping ability in synergy with ferroelectric properties. The reported device not only can serve as a non-volatile electronic memory device, but also can be reconfigured into optoelectronic memory mode or synaptic mode after controlling the ferroelectric polarization states in CuInP2S6. When operated in optoelectronic memory mode, the all-in-one RMD could diagnose ophthalmic disease by segmenting vasculature within biological retinas. On the other hand, operating as an optoelectronic synapse, this work showcases in-sensor reservoir computing for gesture recognition with high energy efficiency.

Nonlinear Optical Properties of 2D Materials and their Applications

2D materials are a subject of intense research in recent years owing to their exclusive photoelectric properties. With giant nonlinear susceptibility and perfect phase matching, 2D materials have marvelous nonlinear light-matter interactions. The nonlinear optical properties of 2D materials are of great significance to the design and analysis of applied materials and functional devices. Here, the fundamental of nonlinear optics (NLO) for 2D materials is introduced, and the methods for characterizing and measuring second-order and third-order nonlinear susceptibility of 2D materials are reviewed. Furthermore, the theoretical and experimental values of second-order susceptibility χ(2) and third-order susceptibility χ(3) are tabulated. Several applications and possible future research directions of second-harmonic generation (SHG) and third-harmonic generation (THG) for 2D materials are presented.

Emerging 2D Nanomaterials-Integrated Hydrogels: Advancements in Designing Theragenerative Materials for Bone Regeneration and Disease Therapy

This review highlights recent advancements in the synthesis, processing, properties, and applications of 2D-material integrated hydrogels, with a focus on their performance in bone-related applications. Various synthesis methods and types of 2D nanomaterials, including graphene, graphene oxide, transition metal dichalcogenides, black phosphorus, and MXene are discussed, along with strategies for their incorporation into hydrogel matrices. These composite hydrogels exhibit tunable mechanical properties, high surface area, strong near-infrared (NIR) photon absorption and controlled release capabilities, making them suitable for a range of regeneration and therapeutic applications. In cancer therapy, 2D-material-based hydrogels show promise for photothermal and photodynamic therapies, and drug delivery (chemotherapy). The photothermal properties of these materials enable selective tumor ablation upon NIR irradiation, while their high drug-loading capacity facilitates targeted and controlled release of chemotherapeutic agents. Additionally, 2D-materials -infused hydrogels exhibit potent antibacterial activity, making them effective against multidrug-resistant infections and disruption of biofilm generated on implant surface. Moreover, their synergistic therapy approach combines multiple treatment modalities such as photothermal, chemo, and immunotherapy to enhance therapeutic outcomes. In bio-imaging, these materials serve as versatile contrast agents and imaging probes, enabling their real-time monitoring during tumor imaging. Furthermore, in bone regeneration, most 2D-materials incorporated hydrogels promote osteogenesis and tissue regeneration, offering potential solutions for bone defects repair. Overall, the integration of 2D materials into hydrogels presents a promising platform for developing multifunctional theragenerative biomaterials.

Tailoring of the polarization-resolved second harmonic generation in two-dimensional semiconductors

Second harmonic generation is a non-linear optical phenomenon in which coherent radiation with frequency ω interacts with a non-centrosymmetric material and produces coherent radiation at frequency 2ω. Owing to the exciting physical phenomena that take place during the non-linear optical excitation at the nanoscale, there is currently extensive research in the non-linear optical responses of nanomaterials, particularly in low-dimensional materials. Here, we review recent advancements in the polarization-resolved second harmonic generation propertied from atomically thin two-dimensional (2D) crystals and present a unified theoretical framework to account for their nonlinear optical response. Two major classes of 2D materials are particularly investigated, namely metal chalcogenides and perovskites. The first attempts to tune and control the second harmonic generation properties of such materials via the application of specific nanophotonic schemes are additionally demonstrated and discussed. Besides presenting recent advances in the field, this work also delineates existing limitations and highlights emerging possibilities and future prospects in this field.

Voltage-controlled nonlinear optical properties in gold nanofilms via electrothermal effect

Dynamic control of the optical properties of gold nanostructures is crucial for advancing photonics technologies spanning optical signal processing, on-chip light sources and optical computing. Despite recent advances in tunable plasmons in gold nanostructures, most studies are limited to the linear or static regime, leaving the dynamic manipulation of nonlinear optical properties unexplored. This study demonstrates the voltage-controlled Kerr nonlinear optical response of gold nanofilms via the electrothermal effect. By applying relatively low voltages (~10 V), the nonlinear absorption coefficient and refractive index are reduced by 40.4% and 33.1%, respectively, due to the increased damping coefficient of gold nanofilm. Furthermore, a voltage-controlled all-fiber gold nanofilm saturable absorber is fabricated and used in mode-locked fiber lasers, enabling reversible wavelength-tuning and operation regimes switching (e.g., mode-locking-Q-switched mode-locking). These findings advance the understanding of electrically controlled nonlinear optical responses in gold nanofilms and offer a flexible approach for controlling fiber laser operations.

Q-switched Nd:YVO4 laser operating at 1064 nm with NiPS3 nanoflakes onto a silica metasurface as saturable absorbers

In this work, we show that a metasurface can be used to improve the performance of the two-dimensional (2D) material saturable absorber in a Nd:YVO4 solid-state laser. To our knowledge, the hybrid saturable absorber was fabricated by spraying the NiPS3 nanoflakes onto a silica metasurface for the first time. It is shown that the optical absorption, modulation depth, saturation intensity, and ultrafast recovery time of the metasurface-NiPS3 saturable absorber exhibit better performance than the 2D material control device. In a proof-of-concept experiment, the Q-switched pulses with a pulse duration of 20.5 ns, repetition rate of 4.35 MHz, output power of 2.3 W, peak power of 30.61 W, and pulse energy of 0.63 μJ were experimentally demonstrated. These findings suggest that a hybrid saturable absorber is a promising candidate for developing pulsed laser and optical modulators.

Introduction to 2D materials and their applications

Diana Berman, Agnieszka Jastrzebska, Massimiliano Papi, and Andreas Rosenkranz introduce the RSC Advances themed issue on 2D materials and their applications.

Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal

Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe2 and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

Layer-by-layer thinning of two-dimensional materials

Etching technology - one of the representative modern semiconductor device makers - serves as a broad descriptor for the process of removing material from the surfaces of various materials, whether partially or entirely. Meanwhile, thinning technology represents a novel and highly specialized approach within the realm of etching technology. It indicates the importance of achieving an exceptionally sophisticated and precise removal of material, layer-by-layer, at the nanoscale. Notably, thinning technology has gained substantial momentum, particularly in top-down strategies aimed at pushing the frontiers of nano-worlds. This rapid development in thinning technology has generated substantial interest among researchers from diverse backgrounds, including those in the fields of chemistry, physics, and engineering. Precisely and expertly controlling the layer numbers of 2D materials through the thinning procedure has been considered as a crucial step. This is because the thinning processes lead to variations in the electrical and optical characteristics. In this comprehensive review, the strategies for top-down thinning of representative 2D materials (e.g., graphene, black phosphorus, MoS2, h-BN, WS2, MoSe2, and WSe2) based on conventional plasma-assisted thinning, integrated cyclic plasma-assisted thinning, laser-assisted thinning, metal-assisted splitting, and layer-resolved splitting are covered in detail, along with their mechanisms and benefits. Additionally, this review further explores the latest advancements in terms of the potential advantages of semiconductor devices achieved by top-down 2D material thinning procedures.

Mild chemistry synthesis of ultrathin Bi2O2S nanosheets exhibiting 2D-ferroelectricity at room temperature

Modern technology demands miniaturization of electronic components to build small, light, and portable devices. Hence, discovery and synthesis of new non-toxic, low cost, ultra-thin ferroelectric materials having potential applications in various electronic and optoelectronic devices are of paramount importance. However, achieving room-temperature ferroelectricity in two dimensional (2D) ultra-thin systems remains a major challenge as conventional three-dimensional ferroelectric materials lose their ferroelectricity when the thickness is brought down below a critical value owing to the depolarization field. Herein, we report room-temperature ferroelectricity in ultra-thin single-crystalline 2D nanosheets of Bi2O2S synthesized by a simple, rapid, and scalable solution-based soft chemistry method. The ferroelectric ground state of Bi2O2S nanosheets is confirmed by temperature-dependent dielectric measurements as well as piezoelectric force microscopy and spectroscopy. High resolution transmission electron microscopy analysis and density functional theory-based calculations suggest that the ferroelectricity in Bi2O2S nanosheets arises due to the local distortion of Bi2O2 layers, which destroys the local inversion symmetry of Bi2O2S.

Can 2D Semiconductors Be Game-Changers for Nanoelectronics and Photonics?

2D semiconductors have interesting physical and chemical attributes that have led them to become one of the most intensely investigated semiconductor families in recent history. They may play a crucial role in the next technological revolution in electronics as well as optoelectronics or photonics. In this Perspective, we explore the fundamental principles and significant advancements in electronic and photonic devices comprising 2D semiconductors. We focus on strategies aimed at enhancing the performance of conventional devices and exploiting important properties of 2D semiconductors that allow fundamentally interesting device functionalities for future applications. Approaches for the realization of emerging logic transistors and memory devices as well as photovoltaics, photodetectors, electro-optical modulators, and nonlinear optics based on 2D semiconductors are discussed. We also provide a forward-looking perspective on critical remaining challenges and opportunities for basic science and technology level applications of 2D semiconductors.

Investigations on Optical Absorption and the Pyro-phototronic Effect with Selectively Patterned Black Silicon for Advanced Photodetection

A novel property existing in the stain-etching technique that eliminates the need for expensive etchant masks in the texturization process of silicon wafers was identified. Through the combination of grayscale lithography and stain-etching methodologies, selective patterning of silicon with AR-P 3510 T, a positive-photoresist mask, was carried out. The etch area ratio was varied in nine different patterns of various feature sizes ranging from 400 to 1500 μm. The optical characteristics of the patterned substrates were determined from diffuse reflectance spectroscopy analysis, and the results were supported with finite-difference time-domain simulations. Complimenting the improvement in optical properties, the electrical losses in microwell-patterned photodetector devices have been reduced with an electro-optic optimum value of the surface enhancement factor, γ. The photodetecting efficiency of a selectively patterned microwell photodetector device exceeded the planar and black silicon photodetector devices with a considerable improvement in the pyro-phototronic effect. This work suggests an alternative for black silicon optoelectronic devices providing a new route to fabricate selectively patterned substrates with an achieved detectivity 16- and 20-fold higher than black and planar silicon photodetector devices, respectively.

Large second-order susceptibility from a quantized indium tin oxide monolayer

Due to their high optical transparency and electrical conductivity, indium tin oxide thin films are a promising material for photonic circuit design and applications. However, their weak optical nonlinearity has been a substantial barrier to nonlinear signal processing applications. In this study, we show that an atomically thin (~1.5 nm) indium tin oxide film in the form of an air/indium tin oxide/SiO2 quantum well exhibits a second-order susceptibility χ2 of ~1,800 pm V-1. First-principles calculations and quantum electrostatic modelling point to an electronic interband transition resonance in the asymmetric potential energy of the quantum well as the reason for this large χ2 value. As the χ2 value is more than 20 times higher than that of the traditional nonlinear LiNbO3 crystal, our indium tin oxide quantum well design can be an important step towards nonlinear photonic circuit applications.

Two-Dimensional Molybdenum Boride (MBene) Mo4/3B2Tx with Broadband and Termination-Dependent Ultrafast Nonlinear Optical Response

Two-dimensional molybdenum borides (MBenes) comprise a new class of 2D transition metal borides that exhibit potential photonics applications. Recently, the synthesis of individual single-layer Mo4/3B2Tx (T = O, F, OH) MBene sheets has been realized, which attracted considerable attention in optoelectronics. However, there is still a lack of understanding and regulation of the photophysical processes of Mo4/3B2Tx MBene. Here, we demonstrate that Mo4/3B2Tx MBene exhibits a surface termination-dependent electronic structure, carrier dynamics, and nonlinear optical response over a wide wavelength range (500-1550 nm). As prepared 2D Mo4/3B2F2 MBene possesses a semimetal material property that exhibits a shorter intraband scattering process (<100 ps) and a considerable nonlinear optical response at a broadband cover optical communication C band at 1550 nm. These thrilling results are confirmed theoretically and experimentally. The analysis of these results adds to the regulating and understanding of the basic photophysical processes, which is anticipated to be beneficial for the further design of MBene-based photonics and nanoelectronics devices.

Anisotropic Strain-Tailoring Nonlinear Optical Response in van der Waals NbOI2

Two-dimensional (2D) NbOI2 demonstrates significant second-harmonic generation (SHG) with a high conversion efficiency. To unlock its full potential in practical applications, it is desirable to modulate the SHG behavior while utilizing the intrinsic lattice anisotropy. Here, we demonstrate direction-specific modulation of the SHG response in NbOI2 by applying anisotropic strain with respect to the intrinsic lattice orientations, where more than 2-fold enhancement in the SHG intensity is achieved under strain along the polar axis. The strain-driven SHG evolution is attributed to the strengthened built-in piezoelectric field (polar axis) and the enlarged Peierls distortions (nonpolar axis). Moreover, we provide quantifications of the correlation between strain and SHG intensity in terms of the susceptibility tensor. Our results demonstrate the effective coupling of orientation-specific strain to the anisotropic SHG response through the intrinsic polar order in 2D nonlinear optical crystals, opening a new paradigm toward the development of functional devices.

Hybridization of graphene-gold plasmons for active control of mid-infrared radiation

Many applications in environmental and biological sensing, standoff detection, and astronomy rely on devices that operate in the mid-infrared range, where active devices can play a critical role in advancing discovery and innovation. Nanostructured graphene has been proposed for active miniaturized mid-infrared devices via excitation of tunable surface plasmons, but typically present low efficiencies due to weak coupling with free-space radiation and plasmon damping. Here we present a strategy to enhance the light-graphene coupling efficiency, in which graphene plasmons couple with gold localized plasmons, creating novel hybridized plasmonic modes. We demonstrate a metasurface in which hybrid plasmons are excited with transmission modulation rates of 17% under moderate doping (0.35 eV) and in ambient conditions. We also evaluate the metasurface as a mid-infrared modulator, measuring switching speeds of up to 16 kHz. Finally, we propose a scheme in which we can excite strongly coupled gold-graphene gap plasmons in the thermal radiation range, with applications to nonlinear optics, slow light, and sensing.

Third Harmonic Generation in Thin NbOI2 and TaOI2

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI2 and TaOI2. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS2, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Ultrafast optical properties and applications of anisotropic 2D materials

Two-dimensional (2D) layered materials exhibit strong light-matter interactions, remarkable excitonic effects, and ultrafast optical response, making them promising for high-speed on-chip nanophotonics. Recently, significant attention has been directed towards anisotropic 2D materials (A2DMs) with low in-plane crystal symmetry. These materials present unique optical properties dependent on polarization and direction, offering additional degrees of freedom absent in conventional isotropic 2D materials. In this review, we discuss recent progress in understanding the fundamental aspects and ultrafast nanophotonic applications of A2DMs. We cover structural characteristics and anisotropic linear/nonlinear optical properties of A2DMs, including well-studied black phosphorus and rhenium dichalcogenides, as well as emerging quasi-one-dimensional materials. Then, we discuss fundamental ultrafast anisotropic phenomena occurring in A2DMs, such as polarization-dependent ultrafast dynamics of charge carriers and excitons, their direction-dependent spatiotemporal diffusion, photo-induced symmetry switching, and anisotropic coherent acoustic phonons. Furthermore, we review state-of-the-art ultrafast nanophotonic applications based on A2DMs, including polarization-driven active all-optical modulations and ultrafast pulse generations. This review concludes by offering perspectives on the challenges and future prospects of A2DMs in ultrafast nanophotonics.

Optimization of the cavity length and pulse characterization based on germanene as a saturable absorber in an Er-doped fiber laser

In this study, germanene-nanosheets (NSs) were synthesized by liquid-phase exfoliation, followed by an experimental investigation into the nonlinear saturable absorption characteristics and morphological structure of germanene. The germanene-NSs were employed as saturable absorbers, exhibiting saturation intensity and modulation depth values of 22.64M W/c m 2 and 4.48%, respectively. This demonstrated the feasibility of utilizing germanene-NSs passively mode-locked in an erbium-doped fiber laser (EDFL). By optimizing the cavity length, improvements in the output of EDFL characteristics were achieved, resulting in 883 fs pulses with a maximum average output power of 19.74 mW. The aforementioned experimental outcomes underscore the significant potential of germanene in the realms of ultrafast photonics and nonlinear optics.

Strong non-linear optical response of Sb2Se3 nanorods in a liquid suspension based on spatial self-phase modulation and their all-optical photonic device applications

The field of nonlinear optics is constantly expanding and gaining new impetus through the discovery of fresh nonlinear materials. Herein, for the first time, we have performed spatial self-phase modulation (SSPM) experiments with an emerging anisotropic Sb2Se3 layered material in a liquid suspension for an all-optical diode and all-optical switching application. The third-order broadband nonlinear optical susceptibility (χ(3)single layer ∼ 10-9 esu) and nonlinear refractive index (n2 ∼ 10-6 cm2 W-1) of Sb2Se3 have been determined using a 671 nm laser beam. This result could be unambiguously explained by the anisotropic hole mobility of Sb2Se3. The linear relationship of χ(3) and carrier mobility emphasizes the establishment of nonlocal hole coherence, the origin of the diffraction pattern. Consequently, the time evolution of diffraction rings follows the 'Wind-Chime' model. A novel photonic diode based on Sb2Se3/SnS2 has been demonstrated using the nonreciprocal propagation of light. The self-phase modulation (SPM) technique uses laser lights of different wavelengths and intensities to demonstrate the all-optical logic gates, particularly OR logic gates. The exploration of nonlinear optical phenomena in Sb2Se3 opens up a new realm for optical information processing and communication. We strongly believe that this result will help to underpin the area of optical nonlinearities among its various applications.

A multifunctional optoelectronic device based on 2D material with wide bandgap

Low-dimensional materials exhibit unique quantum confinement effects and morphologies as a result of their nanoscale size in one or more dimensions, making them exhibit distinctive physical properties compared to bulk counterparts. Among all low-dimensional materials, due to their atomic level thickness, two-dimensional materials possess extremely large shape anisotropy and consequently are speculated to have large optically anisotropic absorption. In this work, we demonstrate an optoelectronic device based on the combination of two-dimensional material and carbon dot with wide bandgap. High-efficient luminescence of carbon dot and extremely large shape anisotropy (>1500) of two-dimensional material with the wide bandgap of >4 eV cooperatively endow the optoelectronic device with multi-functions of optically anisotropic blue-light emission, visible light modulation, wavelength-dependent ultraviolet-light detection as well as blue fluorescent film assemble. This research opens new avenues for constructing multi-function-integrated optoelectronic devices via the combination of nanomaterials with different dimensions.

Structural and Electronic Properties of Two-Dimensional Materials: A Machine-Learning-Guided Prediction

The growing number of studies and interest in two-dimensional (2D) materials has not yet resulted in a wide range of material applications. This is a result of difficulties in getting the properties, which are often determined through numerical experiments or through first-principles predictions, both of which require lots of time and resources. Here we provide a general machine learning (ML) model that works incredibly well as a predictor for a variety of electronic and structural properties such as band gap, fermi level, work function, total energy and area of unit cell for a wide range of 2D materials derived from the Computational 2D Materials Database (C2DB). Our predicted model for classification of samples works extraordinarily well and gives an accuracy of around 99 %. We are able to successfully decrease the number of studied features by employing a strict permutation-based feature selection method along with the sure independence screening and sparsifying operator (SISSO), which further supports the design recommendations for the identification of novel 2D materials with the desired properties.

Reconfigurable nonlinear losses of nanomaterial covered waveguides

Optical waveguides covered with thin films, which transmittance can be controlled by external action, are widely used in various applications from optical modulators to saturable absorbers. It is natural to suggest that the losses through such a waveguide will be proportional to the absorption coefficient of the covering material. In this letter, we demonstrate that under certain conditions, this simple assumption fails. Instead, we observe that the reduction of the material loss of the film can lead to an increase in the propagation losses through the waveguide. For this, we use a side polished fiber covered with a single-walled carbon nanotube thin film whose absorption can be attenuated either by a short pulse illumination (due to absorption saturation) or with electrochemical gating. For the films thicker than 50 nm, we observe saturable absorption to turn into optical limiting with nonmonotonic dependence on the incident power. With a numerical simulation, we identify that this nontrivial behavior comes from mode reshaping due to changes in the absorption coefficient of the covering film. We demonstrate the applicability of the observed effect by fabricating the device which nonlinear optical response can be controllably switched between saturable absorbing and optical limiting. Finally, we utilize an analytical approach to predict the required parameters and corresponding nontrivial shapes of the nonlinear absorbance curves. These results provide new perspectives for engineering complex reconfigurable nonlinear optical responses and transmittance dependences of nanomaterial covered waveguides.

All optical modulation in vertically coupled indium tin oxide ring resonator employing epsilon near zero state

We present an innovative approach to achieve all-optical modulation within an ITO-based vertically coupled ring resonator. This method leverages the material's enhanced nonlinear response in the near-infrared wavelengths, particularly within the epsilon-near-zero (ENZ) state. To enhance the interaction between light and the material while minimizing scattering losses, our approach employs an ITO-based vertically connected ring resonator. The vertical arrangement eliminates the need for etching fine gaps to separate the ring and bus waveguide. The novel waveguide design addresses the necessity of high sensitivity, non-linear effects and compact size opening the possibilities for all-optical signal processing. This unique resonator structure effectively facilitates the coupling of a high-intensity pump wavelength into the ITO-based micro-ring resonator. Consequently, this optical pumping induces electron heating within the ITO material, leading to a significant increase in its nonlinear optical properties. This, in turn, results in a noteworthy alteration of ITO's refractive index, specifically in the unity order, thereby modifying the complex effective index of the optical beam propagating at 1550 nm. Our experimental findings demonstrate an impressive extinction ratio of 18 dB for a 30 µm long device, which highlights the efficiency of our approach in achieving all-optical modulation through the optical pumping of an ITO-based vertically coupled ring resonator. The proposed all-optical modulator has outperformed as compared to conventional waveguide-based modulators in terms of extinction ratio and footprint. This novel technique holds immense potential for advancing high-speed data communication systems in the future. As the demand for advanced processing capabilities, such as artificial intelligence, continues to grow, all-optical modulation emerges as a groundbreaking technology poised to revolutionize the next generation of computing and communication systems.

Giant enhancement of second harmonic generation from monolayer 2D materials placed on photonic moiré superlattice

We numerically investigate second harmonic generation (SHG) from a monolayer of 2D-material placed on photonic moiré superlattice fabricated by dielectric materials. The greatly enhanced local field at the resonance modes of moiré superlattice can dramatically boost the SHG response in 2D materials. Considering a typical 2D-material MoS2 monolayer placed on a photonic moiré superlattice of a twist angle 9.43°, the maximum SHG conversion efficiency reaches up to 10-1 at a relatively low intensity of fundamental light 1 kW/cm2, which is around 14 orders of magnitude larger than that from the monolayer placed on a flat dielectric slab without moiré superlattices. The SHG conversion efficiency from the monolayer can be further enhanced with the decrease of the twist angles of moiré superlattice due to the even more confinement of local field. The flat bands in the moiré superlattices formed by the small twist angles can particularly ensure the efficiency even under wide-angle illuminations. The results indicate that photonic moiré superlattice which can tightly confine light is a promising platform for efficient nonlinear optics.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Nonlinear Optical Properties from Engineered 2D Materials

Two-dimensional (2D) materials with atomic thickness, tunable light-matter interaction, and significant nonlinear susceptibility are emerging as potential candidates for new-generation optoelectronic devices. In this review, we briefly cover the recent research development of typical nonlinear optic (NLO) processes including second harmonic generation (SHG), third harmonic generation (THG), as well as two-photon photoluminescence (2PPL) of 2D materials. Nonlinear light-matter interaction in atomically thin 2D materials is important for both fundamental research and future optoelectronic devices. The NLO performance of 2D materials can be greatly modulated with methods such as carrier injection tuning, strain tuning, artificially stacking, as well as plasmonic resonant enhancement. This review will discuss various nonlinear optical processes and corresponding tuning methods and propose its potential NLO application of 2D materials.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

The Atomic Drill Bit: Precision Controlled Atomic Fabrication of 2D Materials

The ability to deterministically fabricate nanoscale architectures with atomic precision is the central goal of nanotechnology, whereby highly localized changes in the atomic structure can be exploited to control device properties at their fundamental physical limit. Here, an automated, feedback-controlled atomic fabrication method is reported and the formation of 1D-2D heterostructures in MoS₂ is demonstrated through selective transformations along specific crystallographic orientations. The atomic-scale probe of an aberration-corrected scanning transmission electron microscope (STEM) is used, and the shape and symmetry of the scan pathway relative to the sample orientation are controlled. The focused and shaped electron beam is used to reliably create Mo₆ S₆ nanowire (MoS-NW) terminated metallic-semiconductor 1D-2D edge structures within a pristine MoS₂ monolayer with atomic precision. From these results, it is found that a triangular beam path aligned along the zig-zag sulfur terminated (ZZS) direction forms stable MoS-NW edge structures with the highest degree of fidelity without resulting in disordering of the surrounding MoS₂ monolayer. Density functional theory (DFT) calculations and ab initio molecular dynamic simulations (AIMD) are used to calculate the energetic barriers for the most stable atomic edge structures and atomic transformation pathways. These discoveries provide an automated method to improve understanding of atomic-scale transformations while opening a pathway toward more precise atomic-scale engineering of materials.

Low-Threshold, Multiple High-Order Harmonics Fiber Laser Employing Cr2Si2Te6 Saturable Absorber

Abundant research findings have proved the value of two-dimensional (2D) materials in the study of nonlinear optics in fiber lasers. However, there remains two problems: how to reduce the start-up threshold, and how to improve the damage threshold, of fiber lasers based on 2D materials. A 15.1 mW low-threshold mode-locked fiber laser, based on a Cr₂Si₂Te₆ saturable absorber (SA) prepared by the liquid-phase exfoliation method, is demonstrated successfully in this work. This provides a useful and economical method to produce SAs with low insertion loss and low saturation intensity. Besides, multiple high-order harmonics, from the fundamental frequency (12.6 MHz) to the 49th-order harmonic (617.6 MHz), mode-locked operations are recorded. The experimental results indicate the excellent potential of Cr₂Si₂Te₆ as an optical modulator in exploring the soliton dynamics, harmonic mode locking, and other nonlinear effects in fiber lasers.

A New, Thorough Look on Unusual and Neglected Group III-VI Compounds Toward Novel Perusals

The attention on group III-VI compounds in the last decades has been centered on the optoelectronic properties of indium and gallium chalcogenides. These outstanding properties are leading to novel advancements in terms of fundamental and applied science. One of the advantages of these compounds is to present laminated structures, which can be exfoliated down to monolayers. Despite the large knowledge gathered toward indium and gallium chalcogenides, the family of the group III-VI compounds embraces several other noncommon compounds formed by the other group III elements. These compounds present various crystal lattices, among which a great deal is offered from layered structures. Studies on aluminium chalcogenides show interesting potential as anodes in batteries and as semiconductors. Thallium (Tl), which is commonly present in the +1 oxidation state, is one of the key components in ternary chalcogenides. However, binary Tl-Q (Q = S, Se, Te) systems and derived films are still studied for their semiconducting and thermoelectric properties. This review aims to summarize the biggest features of these unusual materials and to shed some new light on them with the perspective that in the future, novel studies can revive these compounds in order to give rise to a new generation of technology.

Polarization-sensitive multi-frequency switches and high-performance slow light based on quadruple plasmon-induced transparency in a patterned graphene-based terahertz metamaterial

A periodic patterned graphene-based terahertz metamaterial comprising three transverse graphene strips and one longitudinal continuous graphene ribbon is proposed to achieve a dynamically tunable quadruple plasmon-induced transparency (PIT) effect. Further analysis of the magnetic field distribution along the x-direction shows that the quadruple-PIT window can be produced by the strong destructive interference between the bright mode and the dark mode. The spectral response characteristics of the quadruple-PIT effect are numerically and theoretically investigated, and the results obtained by the finite-difference time-domain (FDTD) simulation fit well with that by the coupled mode theory (CMT) calculation. In addition, two hepta-frequency asynchronous switches are achieved by tuning the Fermi energy of the graphene, and their maximum modulation depths are 98.9% and 99.7%, corresponding to the insertion losses of 0.173 dB and 0.334 dB, respectively. Further studies show that polarization light has a significant impact on the quadruple-PIT, resulting in a polarization-sensitive switch being realized with a maximum modulation depth of 99.7% and a minimum insertion loss of 0.048 dB. In addition, when the Fermi energy is equal to 1.2 eV, the maximum time delay and group refractive index of the quadruple-PIT can be respectively as high as 1.065 ps and 3194, and the maximum delay-bandwidth product reaches 1.098, which means that excellent optical storage is achieved. Thus, our proposed quadruple-PIT system can be used to design a terahertz multi-channel switch and optical storage.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

Multi-Pulse Bound Soliton Fiber Laser Based on MoTe2 Saturable Absorber

Bound solitons have become a hot topic in the field of nonlinear optics due to their potential applications in optical communication, information processing and radar systems. However, the trapping of the cascaded bound soliton is still a major challenge up to now. Here, we propose and experimentally demonstrate a multi-pulse bound soliton fiber laser based on MoTe₂ saturable absorber. In the experiment, MoTe₂ nanosheets were synthesized by chemical vapor deposition and transferred to the fiber taper by optical deposition. Then, by inserting the MoTe₂ saturable absorber into a ring cavity laser, the two-pulse, three-pulse and four-pulse bound solitons can be stably generated by properly adjusting the pump strength and polarization state. These cascaded bound solitons are expected to be applied to all-optical communication and bring new ideas to the study of soliton lasers.

The Road for 2D Semiconductors in the Silicon Age

Continued reduction in transistor size can improve the performance of silicon integrated circuits (ICs). However, as Moore's law approaches physical limits, high-performance growth in silicon ICs becomes unsustainable, due to challenges of scaling, energy efficiency, and memory limitations. The ultrathin layers, diverse band structures, unique electronic properties, and silicon-compatible processes of 2D materials create the potential to consistently drive advanced performance in ICs. Here, the potential of fusing 2D materials with silicon ICs to minimize the challenges in silicon ICs, and to create technologies beyond the von Neumann architecture, is presented, and the killer applications for 2D materials in logic and memory devices to ease scaling, energy efficiency bottlenecks, and memory dilemmas encountered in silicon ICs are discussed. The fusion of 2D materials allows the creation of all-in-one perception, memory, and computation technologies beyond the von Neumann architecture to enhance system efficiency and remove computing power bottlenecks. Progress on the 2D ICs demonstration is summarized, as well as the technical hurdles it faces in terms of wafer-scale heterostructure growth, transfer, and compatible integration with silicon ICs. Finally, the promising pathways and obstacles to the technological advances in ICs due to the integration of 2D materials with silicon are presented.