Multimodal Nonlinear Optical Imaging of MoS₂ and MoS₂-Based van der Waals Heterostructures

Precise Crystal Orientation Identification and Twist-Induced Giant Modulation of Optical Anisotropy in 1T'-ReS2

The ability to precisely identify crystal orientation as well as to nondestructively modulate optical anisotropy in atomically thin rhenium dichalcogenides is critical for the future development of polarization programmable optoelectronic devices, which remains challenging. Here, we report a modified polarized optical imaging (POI) method capable of simultaneously identifying in-plane (Re chain) and out-of-plane (c-axis) crystal orientations of the monolayer to few-layer ReS2, meanwhile, propose a nondestructive approach to modulate the optical anisotropy in ReS2 via twist stacking. The results show that parallel and near-cross POI are effective to independently identify the in-plane and out-of-plane crystal orientations, respectively, while regulating the twist angle allows for giant modulation of in-plane optical anisotropy from highly intrinsic anisotropy to complete optical isotropy in the stacked ReS2 bilayer (with either the same or opposite c-axes), as well modeled by linear electromagnetic theory. Overall, this study not only develops a simple optical method for precise crystal orientation identification but also offers an efficient light polarization control strategy, which is a big step toward the practical application of anisotropic van der Waals materials in the design of nanophotonic and optoelectronic devices.

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.

Enabling Waveguide Optics in Rhombohedral-Stacked Transition Metal Dichalcogenides with Laser-Patterned Grating Couplers

Waveguides play a key role in the implementation of on-chip optical elements and, therefore, lie at the heart of integrated photonics. To add the functionalities of layered materials to existing technologies, dedicated fabrication protocols are required. Here, we build on laser writing to pattern grating structures into bulk noncentrosymmetric transition metal dichalcogenides with grooves as sharp as 250 nm. Using thin flakes of 3R-MoS2 that act as waveguides for near-infrared light, we demonstrate the functionality of the grating couplers with two complementary experiments: first, nano-optical imaging is used to visualize transverse electric and magnetic modes, whose directional outcoupling is captured by finite element simulations. Second, waveguide second-harmonic generation is demonstrated by grating-coupling femtosecond pulses into the slabs in which the radiation partially undergoes frequency doubling throughout the propagation. Our work provides a straightforward strategy for laser patterning of van der Waals crystals, demonstrates the feasibility of compact frequency converters, and examines the tuning knobs that enable optimized coupling into layered waveguides.

Dephasing Dynamics Accessed by High Harmonic Generation: Determination of Electron-Hole Decoherence of Dirac Fermions

We reveal the critical effect of ultrashort dephasing on the polarization of high harmonic generation in Dirac fermions. As the elliptically polarized laser pulse falls in or slightly beyond the multiphoton regime, the elliptically polarized high harmonic generation is produced and exhibits a characteristic polarimetry of the polarization ellipse, which is found to depend on the decoherence time T2. T2 could then be determined to be a few femtoseconds directly from the experimentally observed polarimetry of high harmonics. This shows a sharp contrast with the semimetal regime of higher pump intensity, where the polarimetry is irrelevant to T2. An access to the dephasing dynamics would extend the prospect of high harmonic generation into the metrology of a femtosecond dynamic process in the coherent quantum control.

Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness, which circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such heterosystems remains unexplored. In this study, we investigate exciton-sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as made evident by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how the interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures.

Probing the Twist-Controlled Interlayer Coupling in Artificially Stacked Transition Metal Dichalcogenide Bilayers by Second-Harmonic Generation

Interlayer coupling plays a critical role in the electronic band structures and optoelectronic properties of van der Waals (vdW) materials and heterostructures. Here, we utilize optical second-harmonic generation (SHG) measurements to probe the twist-controlled interlayer coupling in artificially stacked WSe2/WSe2 homobilayers and WSe2/WS2 and WSe2/MoS2 heterobilayers with a postannealing procedure. In the large angle twisted WSe2/WSe2 and WSe2/WS2, the angular dependence of the SHG intensity follows the interference relations up to angles above 10°. For lower angles, the SHG is significantly suppressed. Furthermore, for the twisted WSe2/MoS2 the SHG intensity largely deviates from the coherent superposition model and shows consistent quenching for all the stacking angles. The suppressed SHG in twisted transition metal dichalcogenide (TMDC) bilayers is predominantly attributed to the interlayer coupling between the two adjacent monolayers. The evolution of the interlayer Raman mode in WSe2 demonstrates that the interlayer coupling in the twisted WSe2/WSe2 and WSe2/WS2 is highly angle-dependent. Alternatively, the interlayer coupling generally exists in the twisted WSe2/MoS2, regardless of the different angles. The interlayer coupling is further confirmed by the quenching and red-shift of the photoluminescence of WSe2 in the twisted TMDC bilayers. Combined with density functional theory calculations, we reveal that the stacking-angle-modulated interlayer coupling originates from the variation of the interlayer spacing and the binding energy in the twisted TMDC bilayers.

Simultaneous 3D Construction and Imaging of Plant Cells Using Plasmonic Nanoprobe-Assisted Multimodal Nonlinear Optical Microscopy

Nonlinear optical (NLO) imaging has emerged as a promising plant cell imaging technique due to its large optical penetration, inherent 3D spatial resolution, and reduced photodamage; exogenous nanoprobes are usually needed for nonsignal target cell analysis. Here, we report in vivo, simultaneous 3D labeling and imaging of potato cell structures using plasmonic nanoprobe-assisted multimodal NLO microscopy. Experimental results show that the complete cell structure can be imaged via the combination of second-harmonic generation (SHG) and two-photon luminescence (TPL) when noble metal silver or gold ions are added. In contrast, without the noble metal ion solution, no NLO signals from the cell wall were acquired. The mechanism can be attributed to noble metal nanoprobes with strong nonlinear optical responses formed along the cell walls via a femtosecond laser scan. During the SHG-TPL imaging process, noble metal ions that crossed the cell wall were rapidly reduced to plasmonic nanoparticles with the fs laser and selectively anchored onto both sides of the cell wall, thereby leading to simultaneous 3D labeling and imaging of the potato cells. Compared with the traditional labeling technique that needs in vitro nanoprobe fabrication and cell labeling, our approach allows for one-step, in vivo labeling of plant cells, thus providing a rapid, cost-effective method for cellular structure construction and imaging.

Noninvasive and Contactless Characterization of Electronic Properties at the Semiconductor/Dielectric Interface Using Optical Second-Harmonic Generation

Optical second-harmonic generation (SHG) is a reliable technique for probing material surface and interface characteristics. Here, we have demonstrated a non-destructive, contactless SHG-based semiconductor/dielectric interface characterization method to measure the conduction band offset and quantitatively evaluate charge densities at the interface in oxide and at the oxide surface. This technique extracts the interface-trapped charge type (donor/acceptor) and qualitatively analyzes the process-induced variation in interface states (Dit), oxide, and oxide surface state density. These qualitative and quantitative analyses provide us with a glimpse into the band bending. The metrology method is validated through a detailed characterization of the Si/HfO2 interface. An optical setup has been developed to monitor the time-dependent second-harmonic generation (TDSHG) from the semiconductor/oxide interface. The temporal characteristics of TDSHG are explained with its relationship to the filling of Dit and spatio-temporal trapping of photoexcited charge in oxide and at the oxide surface. A numerical solver, based on plausible carrier dynamics, is used to model the experimental data and to extract the electronic properties at the Si/HfO2 interface.

Second Harmonic Generation in Janus Transition Metal Chalcogenide Oxide Monolayers: A First-Principles Investigation

Due to the unique optical responses induced by vertical atomic asymmetry inside a monolayer, two-dimensional Janus structures have been conceived as promising building blocks for nanoscale optical devices. In this paper, second harmonic generation (SHG) in Janus transition metal chalcogenide oxide monolayers is systematically investigated by the first-principles calculations. Second-order nonlinear susceptibilities are theoretically determined for Janus MXO (M = Mo/W, X = S/Se/Te) monolayers. The calculated values are comparable in magnitude with Janus MoSSe monolayer. X-M-O symmetry breaking leads to non-zero components in vertical direction, compared with the non-Janus structure. Focusing on the SHG induced by incident light at 1064 nm, polarization-dependent responses of six Janus MXO monolayers are demonstrated. The symmetry of p-polarization changes from six-fold to three-fold with acute incidence angle. Moreover, the effects of biaxial strain on band structures and SHG are further investigated, taking MoSO as an exemplary case. We expect these results to bring in recipes for designing nonlinear optical devices based on Janus transition metal chalcogenide oxide monolayers.

Ultrafast Nanoimaging of Electronic Coherence of Monolayer WSe2

Transition-metal dichalcogenides (TMDs) have demonstrated a wide range of novel photonic, optoelectronic, and correlated electron phenomena for more than a decade. However, the coherent dynamics of their excitons, including possibly long dephasing times and their sensitivity to spatial heterogeneities, are still poorly understood. Here we implement adiabatic plasmonic nanofocused four-wave mixing (FWM) to image the coherent electron dynamics in monolayer WSe₂. We observe nanoscale heterogeneities at room temperature with dephasing ranging from T₂ ≲ 5 to T₂ ≳ 60 fs on length scales of 50-100 nm. We further observe a counterintuitive anticorrelation between FWM intensity and T₂, with the weakest FWM emission at locations of longest coherence. We interpret this behavior as a nonlocal nano-optical interplay between spatial coherence of the nonlinear polarization and disorder-induced scattering. The results highlight the challenges associated with heterogeneities in TMDs limiting their photophysical properties, yet also the potential of their novel nonlinear optical phenomena.

Ferroelectric Domain Control of Nonlinear Light Polarization in MoS2 via PbZr0.2 Ti0.8 O3 Thin Films and Free-Standing Membranes

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS₂ exhibit exceptionally strong nonlinear optical responses, while nanoscale control of the amplitude, polar orientation, and phase of the nonlinear light in TMDCs remains challenging. In this work, by interfacing monolayer MoS₂ with epitaxial PbZr_0.2 Ti_0.8 O₃ (PZT) thin films and free-standing PZT membranes, the amplitude and polarization of the second harmonic generation (SHG) signal are modulated via ferroelectric domain patterning, which demonstrates that PZT membranes can lead to in-operando programming of nonlinear light polarization. The interfacial coupling of the MoS₂ polar axis with either the out-of-plane polar domains of PZT or the in-plane polarization of domain walls tailors the SHG light polarization into different patterns with distinct symmetries, which are modeled via nonlinear electromagnetic theory. This study provides a new material platform that enables reconfigurable design of light polarization at the nanoscale, paving the path for developing novel optical information processing, smart light modulators, and integrated photonic circuits.

Ultrafast Nonlinear Absorption and Second Harmonic Generation in Cu0.33In1.30P2S6 van der Waals Layered Crystals

The advancement of ultrafast photonics and optoelectronic devices necessitates the exploration of new materials with optical and chemical stability to implement practical applications. Layered quaternary metal-thio/selenophosphate has attracted much interest over the past few years. Ferroelectric CuInP₂S₆ (CIPS) is an emerging material that belongs to this family. When synthesized with Cu deficiencies, CIPS forms self-assembled in-plane heterostructures, which in turn exhibit properties that are both compositionally and thermally dependent. These characteristics can be explored for applications in nonlinear optoelectronic and photonic devices. Herein, we study the second and third order nonlinear optical behavior of Cu_0.33In_1.30P₂S₆ bulk heterostructure. We observed large two photon induced nonlinear absorptions and self-defocusing at 1032 nm pulsed laser excitation using the Z-scan technique. Furthermore, we identified a polarization-dependent second harmonic signal and determined the laser-induced optical damage threshold. Our observations allow for the designing of optoelectronic and ultrafast photonic devices based on these materials.

Interfacial Coupling and Modulation of van der Waals Heterostructures for Nanodevices

In recent years, van der Waals heterostructures (vdWHs) of two-dimensional (2D) materials have attracted extensive research interest. By stacking various 2D materials together to form vdWHs, it is interesting to see that new and fascinating properties are formed beyond single 2D materials; thus, 2D heterostructures-based nanodevices, especially for potential optoelectronic applications, were successfully constructed in the past few decades. With the dramatically increased demand for well-controlled heterostructures for nanodevices with desired performance in recent years, various interfacial modulation methods have been carried out to regulate the interfacial coupling of such heterostructures. Here, the research progress in the study of interfacial coupling of vdWHs (investigated by Photoluminescence, Raman, and Pump-probe spectroscopies as well as other techniques), the modulation of interfacial coupling by applying various external fields (including electrical, optical, mechanical fields), as well as the related applications for future electrics and optoelectronics, have been briefly reviewed. By summarizing the recent progress, discussing the recent advances, and looking forward to future trends and existing challenges, this review is aimed at providing an overall picture of the importance of interfacial modulation in vdWHs for possible strategies to optimize the device's performance.

Electronic, transport, magnetic and optical properties of graphene nanoribbons review

Low dimensional materials have attracted great research interest from both theoretical and experimental point of view. These materials exhibit novel physical and chemical properties due to the confinement effect in low dimensions. The experimental observations of graphene open a new platform to study the physical properties of materials restricted to two dimensions. This featured article provides a review on the novel properties of quasi one-dimensional (1D) material known as graphene nanoribbon. Graphene nanoribbons can be obtained by unzipping carbon nanotubes (CNTs) or cutting the graphene sheet. Alternatively, it is also called the finite termination of graphene edges. It gives rise different edge geometries namely zigzag and armchair among others. There are various physical and chemical techniques to realize these materials. Depending on the edge type termination, these are called the zigzag and armchair graphene nanoribbons (ZGNR and AGNR). These edges play an important role in controlling the properties of graphene nanoribbons. The present review article provides an overview of the electronic, transport, optical and magnetic properties of graphene nanoribbons. However, there are different ways to tune these properties for device applications. Here, some of them are highlighted such as external perturbations and chemical modifications. Few applications of graphene nanoribbon have and chemical modifications. Few applications of graphene nanoribbon have also been briefly discussed.

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.

Electrically Tunable Second Harmonic Generation in Atomically Thin ReS2

Electrical tuning of second-order nonlinearity in optical materials is attractive to strengthen and expand the functionalities of nonlinear optical technologies, though its implementation remains elusive. Here, we report the electrically tunable second-order nonlinearity in atomically thin ReS₂ flakes benefiting from their distorted 1T crystal structure and interlayer charge transfer. Enabled by the efficient electrostatic control of the few-atomic-layer ReS₂, we show that second harmonic generation (SHG) can be induced in odd-number-layered ReS₂ flakes which are centrosymmetric and thus without intrinsic SHG. Moreover, the SHG can be precisely modulated by the electric field, reversibly switching from almost zero to an amplitude more than 1 order of magnitude stronger than that of the monolayer MoS₂. For the even-number-layered ReS₂ flakes with the intrinsic SHG, the external electric field could be leveraged to enhance the SHG. We further perform the first-principles calculations which suggest that the modification of in-plane second-order hyperpolarizability by the redistributed interlayer-transferring charges in the distorted 1T crystal structure underlies the electrically tunable SHG in ReS₂. With its active SHG tunability while using the facile electrostatic control, our work may further expand the nonlinear optoelectronic functions of two-dimensional materials for developing electrically controllable nonlinear optoelectronic devices.

Anisotropic Second-Harmonic Generation Induced by Reduction of In-Plane Symmetry in 2D Materials with Strain Engineering

Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS₂, GaSe, and In₂Se₃ monolayers. The uniaxial strain can break the in-plane symmetry of 2D materials, leading to both trade-off breaking of the nonlinear coefficient and new emergent nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent SHG pattern is transformed into a distorted sixfold SHG pattern under the strain. Due to the lattice symmetry breaking and the uneven charge density distribution in strained 2D materials, the SHG patterns also depend on the excitation photon energy. The results could give a guide for the SHG pattern analysis in experiments, suggesting strain engineering on 2D materials for the tunable anisotropy in polarized and flexible nonlinear optical devices.

In Situ Preparation of MXenes in Ambient-Temperature Organic Ionic Liquid Aluminum Batteries with Ultrastable Cycle Performance

A fluorine-free and water-free electrochemical preparation of MXenes is achieved in Lewis acidic molten salts at ambient temperature. In addition, the anode reaction of the MAX phase V₂AlC is studied in the organic ionic liquid aluminum battery and the extraction voltages of the metal atoms Al and V in the MAX phase V₂AlC are determined. This points out the direction for the constant-voltage electrochemical preparation of MXenes. Furthermore, the electrochemical performance of the etched V₂AlC (E-V₂AlC) in an aluminum battery is studied. The one-stop preparation-application process prevents the MXenes from contacting water and air, and the MXenes etched in the aluminum battery are more conducive to the intercalation/deintercalation of Al³⁺. Therefore, E-V₂AlC exhibits excellent electrochemical performance in an aluminum battery. Under the conditions of a voltage window of 0.01-2.3 V (V vs Al/Al³⁺) and a current density of 500 mA g^-1, the specific discharge capacity is about 100 mAh g^-1 after 6500 cycles. In addition, the energy storage mechanism and Faraday energy storage method of E-V₂AlC in an aluminum battery are studied. The diffusion coefficient D of Al³⁺ is determined by a galvanostatic intermittent titration technique. The reasons for its excellent electrochemical performance are clarified from the perspective of kinetics.

Concurrent and Selective Determination of Dopamine and Serotonin with Flexible WS2 /Graphene/Polyimide Electrode Using Cold Plasma

Makers of point-of-care devices and wearable diagnostics prefer flexible electrodes over conventional electrodes. In this study, a flexible electrode platform is introduced with a WS₂ /graphene heterostructure on polyimide (WGP) for the concurrent and selective determination of dopamine and serotonin. The WGP is fabricated directly via plasma-enhanced chemical vapor deposition (PECVD) at 150 °C on a flexible polyimide substrate. Owing to the limitations of existing fabrication methods from physical transfer or hydrothermal methods, many studies are not conducted despite excellent graphene-based heterostructures. The PECVD synthesis method can provide an innovative WS₂ /graphene heterostructure of uniform quality and sufficient size (4 in.). This unique heterostructure affords excellent electrical conductivity in graphene and numerous electrochemically active sites in WS₂ . A large number of uniform qualities of WGP electrodes show reproducible and highly sensitive electrochemical results. The synergistic effect enabled well-separated voltammetric signals for dopamine and serotonin with a potential gap of 188 mV. Moreover, the practical application of the flexible sensor is successfully evaluated by using artificial cerebrospinal fluid.

Broadband Plasmon-Enhanced Four-Wave Mixing in Monolayer MoS2

Two-dimensional transition-metal dichalcogenide monolayers have remarkably large optical nonlinearity. However, the nonlinear optical conversion efficiency in monolayer transition-metal dichalcogenides is typically low due to small light-matter interaction length at the atomic thickness, which significantly obstructs their applications. Here, for the first time, we report broadband (up to ∼150 nm) enhancement of optical nonlinearity in monolayer MoS₂ with plasmonic structures. Substantial enhancement of four-wave mixing is demonstrated with the enhancement factor up to three orders of magnitude for broadband frequency conversion, covering the major visible spectral region. The equivalent third-order nonlinearity of the hybrid MoS₂-plasmonic structure is in the order of 10^-17 m²/V², far superior (∼10-100-times larger) to the widely used conventional bulk materials (e.g., LiNbO₃, BBO) and nanomaterials (e.g., gold nanofilms). Such a considerable and broadband enhancement arises from the strongly confined electric field in the plasmonic structure, promising for numerous nonlinear photonic applications of two-dimensional materials.

Dispersion Property and Evolution of Second Harmonic Generation Pattern in Type-I and Type-II van der Waals Heterostructures

Dispersion property and second harmonic generation (SHG) pattern of novel two-dimensional (2D) van der Waals heterostructures (vdWHs) is of great significance not only for the characterization of material symmetry but also for understanding nonlinear photophysical phenomena. Herein, we demonstrate the SHG response of 2D type-I (MoTe₂/WSe₂) and type-II (MoSe₂/WSe₂) band alignment of vdWHs. In the dispersion relation of the second-order nonlinear coefficient, the pronounced peaks of the d₁₆ element for both vdWHs are mainly contributed by resonance in the interband transition processes, whereas other elements are derived from the intraband transition processes because of the highly efficient charge transfer from WSe₂ to MoTe₂ in type-I vdWHs and the ultrafast charge separation between WSe₂ and MoSe₂ in type-II vdWHs, respectively. Besides, more nonzero nonlinear coefficient elements can participate in a nonlinear response at the oblique incidence, to which special attention needs paid. The polarization angle α-dependent SHG patterns display a rotational fourfold symmetry, whereas the azimuthal angle ϕ-dependent SHG patterns show sixfold symmetry for both type-I and type-II vdWHs at any wavelength under normal incidence. Under oblique incidence, the α-dependent (ϕ-dependent) SHG patterns will reduce to twofold (threefold) symmetry for both vdWHs. The results highlight the potential to deterministically engineer novel nonlinear optical properties for tunable anisotropic applications of nonlinear optoelectronic devices based on vdWHs.

Two-Dimensional Boron Phosphide/MoGe2N4 van der Waals Heterostructure: A Promising Tunable Optoelectronic Material

A van der Waals (VDW) heterostructure offers an effective strategy to create designer physical properties in vertically stacked two-dimensional (2D) materials, and offers a new paradigm in designing novel 2D heterostructure devices. In this work, we investigate the structural and electronic features of the BP/MoGe₂N₄ heterostructure. We show that the BP/MoGe₂N₄ heterostructure exists in a multiple structurally stable stacking configuration, thus revealing the experimental feasibility of fabricating such heterostructures. Electronically, the BP/MoGe₂N₄ heterostructure is a direct band gap semiconductor exhibiting type-II band alignment, which is highly beneficial for the spatial separation of electrons and holes. Upon forming the BP/MoGe₂N₄ heterostructure, the band gap of the constituent BP and MoGe₂N₄ monolayers are substantially reduced, thus allowing the easier creation of an electron-hole pair at a lower excitation energy. Interestingly, both the band gap and band alignment of the BP/MoGe₂N₄ heterostructure can be modulated by an external electric field and a vertical strain. The optical absorption of the BP/MoGe₂N₄ heterostructure is enhanced in both the visible-light and ultraviolet regions, thus suggesting a strong potential for solar cell application. Our findings reveal the promising potential of the BP/MoGe₂N₄ vdW heterostructure in high-performance optoelectronic device applications.

Nonlinear Optical Response of a WS2 Monolayer at Room Temperature upon Multicolor Laser Excitation

Currently, the nonlinear optical properties of 2D materials are attracting the attention of an ever-increasing number of research groups due to their large potential for applications in a broad range of scientific disciplines. Here, we investigate the interplay between nonlinear photoluminescence (PL) and several degenerate and nondegenerate nonlinear optical processes of a WS2 monolayer at room temperature. We illuminate the sample using two femtosecond laser pulses at frequencies ω1 and ω2 with photon energies below the optical bandgap. As a result, the sample emits light that shows characteristic spectral peaks of the second-harmonic generation, sum-frequency generation, and four-wave mixing. In addition, we find that both resonant and off-resonant nonlinear excitation via frequency mixing contributes to the (nonlinear) PL emission at the A-exciton frequency. The PL exhibits a clear correlation with the observed nonlinear effects, which we attribute to the generation of excitons via degenerate and nondegenerate multiphoton absorption. Our work illustrates a further step toward understanding the fundamental relation between parametric and nonparametric nondegenerate optical mechanisms in transition-metal dichalcogenides. In turn, such understanding has great potential to expand the range of applicability of nonlinear optical processes of 2D materials in different fields of science and technology, where nonlinear mechanisms are typically limited to degenerate processes.

Second-harmonic generation enhancement in monolayer transition-metal dichalcogenides by using an epsilon-near-zero substrate

Monolayer transition-metal dichalcogenides (TMDCs) present high second-order optical nonlinearity, which is extremely desirable for, e.g., frequency conversion in nonlinear photonic devices. On the other hand, the atomic thickness of 2D materials naturally leads to low frequency converted intensities, highlighting the importance of designing structures that enhance the nonlinear response for practical applications. A number of methods to increase the pump electric field at 2D materials have been reported, relying on complex plasmonic and/or metasurface structures. Here, we take advantage of the fact that unstructured substrates with a low refractive index naturally maximize the pump field at a dielectric interface, offering a simple means to promote enhanced nonlinear optical effects. In particular, we measured second harmonic generation (SHG) in MoS2 and WS2 on fluorine tin oxide (FTO), which presents an epsilon-near zero point near our 1550 nm pump wavelength. Polarized SHG measurements reveal an SHG intensity that is one order of magnitude higher on FTO than on a glass substrate.

Nonlinear Optical Characterization of 2D Materials

Characterizing the physical and chemical properties of two-dimensional (2D) materials is of great significance for performance analysis and functional device applications. As a powerful characterization method, nonlinear optics (NLO) spectroscopy has been widely used in the characterization of 2D materials. Here, we summarize the research progress of NLO in 2D materials characterization. First, we introduce the principles of NLO and common detection methods. Second, we introduce the recent research progress on the NLO characterization of several important properties of 2D materials, including the number of layers, crystal orientation, crystal phase, defects, chemical specificity, strain, chemical dynamics, and ultrafast dynamics of excitons and phonons, aiming to provide a comprehensive review on laser-based characterization for exploring 2D material properties. Finally, the future development trends, challenges of advanced equipment construction, and issues of signal modulation are discussed. In particular, we also discuss the machine learning and stimulated Raman scattering (SRS) technologies which are expected to provide promising opportunities for 2D material characterization.

Leaving the Limits of Linearity for Light Microscopy

Nonlinear microscopy has enabled additional modalities for chemical contrast, deep penetration into biological tissues, and the ability to collect dynamics on ultrafast timescales across heterogenous samples. The additional light fields introduced to a sample offer seemingly endless possibilities for variation to optimize and customize experimentation and the extraction of physical insight. This perspective highlights three areas of growth in this diverse field: the collection of information across multiple timescales, the selective imaging of interfacial chemistry, and the exploitation of quantum behavior for future imaging directions. Future innovations will leverage the work of the studies reviewed here as well as address the current challenges presented.

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS₂) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS₂ relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

Realizing Saturable Absorption and Reverse Saturable Absorption in a PEDOT:PSS Film via Electrical Modulation

Electrical tuning of the nonlinear absorption of materials has promising application potential, while studies remain rare. In this work, we show that the third-order nonlinear absorption of poly(3,4-ethylenedioxythiophene) chemically doped with poly(styrene sulfonic acid) [PEDOT:PSS] can be effectively modulated by external voltage. The nonlinear absorption of the film can be varied between reverse saturable absorption (RSA) and saturable absorption (SA) via voltage control with laser excitation at 800 nm, and the corresponding nonlinear absorption coefficient can be tuned in the range -1606 ± 73 to 521 ± 9 cm GW^-1. The doping level and energy structure of PEDOT are modulated with different voltages. The undoped film affords two-photon absorption and accordingly the RSA response. A moderately doped sample has two polaron levels, and Pauli blocking associated with these two polaron levels results in SA. The bipolaron level in heavily doped PEDOT leads to excited-state absorption and therefore RSA behavior. The approach reported here can be applied to other semiconductors and is a convenient, effective, and promising method for the electrical tuning of the optical nonlinearity.

Difference frequency generation in monolayer MoS2

Difference frequency generation has long been employed for numerous applications, such as coherent light generation, sensing and imaging. Here, we demonstrate difference frequency generation down to atomic thickness in monolayer molybdenum disulfide. By mixing femtosecond optical pulses at wavelength of 406 nm with tunable pulses in the spectral range of 1300-1520 nm, we generate tunable pulses across the spectral range of 550-590 nm with frequency conversion efficiency up to ∼2 × 10^-4. The second-order nonlinear optical susceptibility of monolayer molybdenum disulfide, χ, is calculated as ∼1.8 × 10^-8 m V^-1, comparable to the previous results demonstrated with second harmonic generation. Such a highly efficient down-conversion nonlinear optical process in two-dimensional layered materials may open new ways to their nonlinear optical applications, such as coherent light generation and amplification.

Nonlinear Optical Imaging, Precise Layer Thinning, and Phase Engineering in MoTe2 with Femtosecond Laser

The control of layer thickness and phase structure in two-dimensional transition metal dichalcogenides (2D TMDCs) like MoTe₂ has recently gained much attention due to their broad applications in nanoelectronics and nanophotonics. Continuous-wave laser-based thermal treatment has been demonstrated to realize layer thinning and phase engineering in MoTe₂, but requires long heating time and is largely influenced by the thermal dissipation of the substrate. The ultrafast laser produces a different response but is yet to be explored. In this work, we report the nonlinear optical interactions between MoTe₂ crystals and femtosecond (fs) laser, where we have realized the nonlinear optical characterization, precise layer thinning, and phase transition in MoTe₂ using a single fs laser platform. By using the fs laser with a low fluence as an excitation light source, we observe the strong nonlinear optical signals of second-harmonic generation and four-wave mixing in MoTe₂, which can be used to identify the odd-even layers and layer numbers, respectively. With increasing the laser fluence to the ablation threshold (F_th), we achieve layer-by-layer removal of MoTe₂, while 2H-to-1T' phase transition occurs with a higher laser fluence (2F_th to 3F_th). Moreover, we obtain highly ordered subwavelength nanoripples on both the thick and few-layer MoTe₂ with a controlled fluence, which can be attributed to the fs laser-induced reorganization of the molten plasma. Our study provides a simple and efficient ultrafast laser-based approach capable of characterizing the structures and modifying the physical properties of 2D TMDCs.

Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMD) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMD monolayers and heterostructures in consideration of the material's structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS2flakes grown on a SiO2/ Si(001) substrate are presented.

Electrical Control of Interband Resonant Nonlinear Optics in Monolayer MoS2

Monolayer transition-metal dichalcogenides show strong optical nonlinearity with great potential for various emerging applications. Here we demonstrate the gate-tunable interband resonant four-wave mixing and sum-frequency generation in monolayer MoS₂. Up to 80% modulation depth in four-wave mixing is achieved when the generated signal is resonant with the A exciton at room temperature, corresponding to an effective third-order optical nonlinearity |χ⁽³⁾_eff| tuning from (∼12.0 to 5.45) × 10^-18 m²/V². The tunability of the effective second-order optical nonlinearity |χ⁽²⁾_eff| at 440 nm C-exciton resonance wavelength is also demonstrated from (∼11.6 to 7.40) × 10^-9 m/V with sum-frequency generation. Such a large tunability in optical nonlinearities arises from the strong excitonic charging effect in monolayer transition-metal dichalcogenides, which allows for the electrical control of the interband excitonic transitions and thus nonlinear optical responses for future on-chip nonlinear optoelectronics.

Direct Observation of Incommensurate-Commensurate Transition in Graphene-hBN Heterostructures via Optical Second Harmonic Generation

Commensurability effects play a crucial role in the formation of electronic properties of novel layered heterostructures. The interest in these moiré superstructures has increased tremendously since the recent observation of a superconducting state (Nature 2018, 556, 43-50) and metal-insulator transition (Nature 2018, 556, 80-84) in twisted bilayer graphene. In this regard, a straightforward and efficient experimental technique for detection of the alignment of layered materials is desired. In this work, we use optical second harmonic generation, which is sensitive to the inversion symmetry breaking, to investigate the alignment of graphene/hexagonal boron nitride heterostructures. To achieve that, we activate a commensurate-incommensurate phase transition by a thermal annealing of the sample. We find that this structural change in the system can be directly observed via a strong modification of a nonlinear optical signal. Unambiguous interpretation of obtained results reveals the potential of a second harmonic generation technique for probing of structural changes in layered systems.

Dual Resonant Sum Frequency Generations from Two-Dimensional Materials

We propose dual resonant optical sum frequency generation (SFG), where the two most singular resonances could be selected, and report for the monolayer (1L-) WSe₂ when one (ω₁) of two excitation pulses is resonant to A exciton and their sum frequency (ω₁ + ω₂) to D exciton. The dual resonant SFG confirms that, under an irradiation of ω₁ and ω₂ pulses with the same fluence of ∼1.4 × 10¹⁰ W/m², its signal intensity could be enhanced about 20 times higher than the resonant SHG (i.e., 2ω₁ to the D excitonic absorption). Further, the dual resonant SFG intensity of 1L-WSe₂ is found to be 1 order of magnitude higher than the single resonant SFG intensity of 1L-WS₂ under the same condition of two-pulse irradiation. Finally, observations of the dual resonant SFG are thoroughly examined using real-time time-dependent density functional theory (rt-TDDFT), and the relevant nonlinear optical characteristics are scrutinized using the Greenwood-Kubo formalism.

2D Material Optoelectronics for Information Functional Device Applications: Status and Challenges

Graphene and the following derivative 2D materials have been demonstrated to exhibit rich distinct optoelectronic properties, such as broadband optical response, strong and tunable light-mater interactions, and fast relaxations in the flexible nanoscale. Combining with optical platforms like fibers, waveguides, grating, and resonators, these materials has spurred a variety of active and passive applications recently. Herein, the optical and electrical properties of graphene, transition metal dichalcogenides, black phosphorus, MXene, and their derivative van der Waals heterostructures are comprehensively reviewed, followed by the design and fabrication of these 2D material-based optical structures in implementation. Next, distinct devices, ranging from lasers to light emitters, frequency convertors, modulators, detectors, plasmonic generators, and sensors, are introduced. Finally, the state-of-art investigation progress of 2D material-based optoelectronics offers a promising way to realize new conceptual and high-performance applications for information science and nanotechnology. The outlook on the development trends and important research directions are also put forward.

Vertically-oriented MoS2 nanosheets for nonlinear optical devices

Transition metal dichalcogenides such as MoS₂ represent promising candidates for building blocks of ultra-thin nanophotonic devices. For such applications, vertically-oriented MoS₂ (v-MoS₂) nanosheets could be advantageous as compared to conventional horizontal MoS₂ (h-MoS₂) given that their inherent broken symmetry would favor an enhanced nonlinear response. However, the current lack of a controllable and reproducible fabrication strategy for v-MoS₂ limits the exploration of this potential. Here we present a systematic study of the growth of v-MoS₂ nanosheets based on the sulfurization of a pre-deposited Mo-metal seed layer. We demonstrate that the sulfurization process at high temperatures is driven by the diffusion of sulfur from the vapor-solid interface to the Mo seed layer. Furthermore, we verify an enhanced nonlinear response in the resulting v-MoS₂ nanostructures as compared to their horizontal counterparts. Our results represent a stepping stone towards the fabrication of low-dimensional TMD-based nanostructures for versatile nonlinear nanophotonic devices.

Polar coupling enabled nonlinear optical filtering at MoS2/ferroelectric heterointerfaces

Complex oxide heterointerfaces and van der Waals heterostructures present two versatile but intrinsically different platforms for exploring emergent quantum phenomena and designing new functionalities. The rich opportunity offered by the synergy between these two classes of materials, however, is yet to be charted. Here, we report an unconventional nonlinear optical filtering effect resulting from the interfacial polar alignment between monolayer MoS₂ and a neighboring ferroelectric oxide thin film. The second harmonic generation response at the heterointerface is either substantially enhanced or almost entirely quenched by an underlying ferroelectric domain wall depending on its chirality, and can be further tailored by the polar domains. Unlike the extensively studied coupling mechanisms driven by charge, spin, and lattice, the interfacial tailoring effect is solely mediated by the polar symmetry, as well explained via our density functional theory calculations, pointing to a new material strategy for the functional design of nanoscale reconfigurable optical applications.

Photocarrier-Induced Active Control of Second-Order Optical Nonlinearity in Monolayer MoS2

Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light-TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second-harmonic generation (SHG) from a monolayer MoS₂ crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low-dimensional materials for all-optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second-order dielectric susceptibility χ⁽²⁾ and the density of photoexcited carriers in MoS₂ . Indeed, the depopulation of the conduction band electrons, at the vicinity of the high-symmetry K/K' points of MoS₂ , suppresses the contribution of interband electronic transitions in the effective χ⁽²⁾ of the monolayer crystal, enabling the all-optical modulation of the SHG signal. The strong dependence of the second-order optical response on the density of photocarriers reveals the promise of time-resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.

Continuous Wave Sum Frequency Generation and Imaging of Monolayer and Heterobilayer Two-Dimensional Semiconductors

We report continuous-wave second harmonic and sum frequency generation from two-dimensional transition metal dichalcogenide monolayers and their heterostructures with pump irradiances several orders of magnitude lower than those of conventional pulsed experiments. The high nonlinear efficiency originates from above-gap excitons in the band nesting regions, as revealed by wavelength-dependent second order optical susceptibilities quantified in four common monolayer transition metal dichalcogenides. Using sum frequency excitation spectroscopy and imaging, we identify and distinguish one- and two-photon resonances in both monolayers and heterobilayers. Data for heterostructures reveal responses from constituent layers accompanied by nonlinear signal correlated with interlayer transitions. We demonstrate spatial mapping of heterogeneous interlayer coupling by sum frequency and second harmonic confocal microscopy on heterobilayer MoSe₂/WSe₂.

Second harmonic generation in Janus MoSSe a monolayer and stacked bulk with vertical asymmetry

A recently synchronized Janus TMD material with broken out-of-plane symmetry offers a vertical dipole to enhance nonlinear optical behavior. Here, by comparing the second harmonic generation properties of MoS₂ and MoSSe monolayers, we investigated the nonzero out-of-plane SHG susceptibilities of a Janus MoSSe 2D material. A three-fold enhancement of out-of-plane SHG susceptibilities exists in three stacked bulks of Janus MoSSe compared to that in the monolayer. A sensitivity to their stack pattern is also found. The broken out-of-plane symmetry, vertical dipole, and intrinsic tunable electronic properties of Janus two-dimensional materials make MoSSe a promising nanomaterial for nonlinear optical devices.

Vertical Stacking of Copper Sulfide Nanoparticles and Molybdenum Sulfide Nanosheets for Enhanced Nonlinear Absorption

The construction of p-n junctions is necessitated by applications which require effective charge separation. Here, a novel heterostructure (HS) of molybdenum sulfide (MoS₂) and copper sulfide (Cu_2-xS) was synthesized by chemical vapor deposition, with Cu_2-xS nanoparticles vertically stacked on a MoS₂ nanosheet. A well-defined epitaxial relationship between MoS₂ and Cu_2-xS is established, although the corresponding lattice mismatch is as large as 20%. The band-edge alignment is experimentally determined, indicating that the MoS₂-Cu_2-xS HS is a type II heterojunction. Photoluminescence quenching indicates effective charge separation in HS. The resultant HS shows enhanced nonlinear absorption in comparison with single-component MoS₂ nanosheets and Cu_2-xS nanoparticles.

Robust Piezo-Phototronic Effect in Multilayer γ-InSe for High-Performance Self-Powered Flexible Photodetectors

The piezo-phototronic effect has been promising as an effective means to improve the performance of two-dimensional (2D) semiconductor based optoelectronic devices. However, the current reported monolayer 2D semiconductors are not regarded as suitable for actual flexible piezotronic photodetectors due to their insufficient optical absorption and mechanical durability, although they possess strong piezoelectricity. In this work, we demonstrate that, unlike 2H-phase transition-metal dichalcogenides, γ-phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers and has a strong second-harmonic generation effect. Moreover, driven by the piezo-phototronic effect, a flexible self-powered photodetector based on multilayer γ-InSe, which can work without any energy supply, is proposed. The device exhibited ultrahigh photon responsivity of 824 mA/W under light illuminations of 400 nm (0.368 mW/cm²). Moreover, the responsivity and response speed of this photodetector were enhanced further by as much as 696% and 1010%, respectively, when a 0.62% uniaxial tensile strain was applied. Our devices exhibit high reliability and stability during a 6 month test time. These significant findings offer a promising pathway to construct high-performance flexible piezo-phototronic photodetectors based on multilayer 2D semiconductors.

Anisotropic Enhancement of Second-Harmonic Generation in Monolayer and Bilayer MoS2 by Integrating with TiO2 Nanowires

The ability to design and enhance the nonlinear optical responses in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is both of fundamental interest and highly desirable for developing TMDC-based nonlinear optical applications, such as nonlinear convertors and optical modulators. Here, we report for the first time a strong anisotropic enhancement of optical second-harmonic generation (SHG) in monolayer molybdenum disulfide (MoS₂) by integrating with one-dimensional (1D) titanium dioxide nanowires (NWs). The SHG signal from the MoS₂/NW hybrid structures is over 2 orders of magnitude stronger than that in the bare monolayer MoS₂. Polarized SHG measurements revealed a giant anisotropy in SHG response of the MoS₂/NW hybrid. The pattern of the anisotropic SHG depends highly on the stacking angle between the nanowire direction and the MoS₂ crystal orientation, which is attributed to the 1D NW-induced directional strain fields in the layered MoS₂. A similar effect has also been observed in bilayer MoS₂/NW hybrid structure, further proving the proposed scenario. This work provides an effective approach to selectively and directionally designing the nonlinear optical response of layered TMDCs, paving the way for developing high-performance, anisotropic nonlinear photonic nanodevices.

Nonlinear Optical Response in Graphene/WX2 (X = S, Se, and Te) van der Waals Heterostructures

Light-frequency conversion based on two-dimensional (2D) materials is of great importance for modern nano- and integrated photonics. Herein, we report both the intrinsic (from the pure WX₂ (X = S, Se, and Te)) and extrinsic (from the interface of graphene/WX₂) second-order nonlinear coefficient tensor from graphene/WX₂ van der Waals (vdW) heterostructures by first-principles calculations. The prominent peaks in the dispersion relation of the intrinsic second-order nonlinear coefficient in monolayer WX₂ are due to the Van Hove singularity in the high-symmetry point or along the high-symmetry line with high joint density of states. The enhanced nonlinear optical response in the infrared band can be achieved in graphene/WS₂ vdW heterostructures, resulting from the interlayer charge transfer between graphene and WS₂. The value of the intrinsic second-order nonlinear coefficients of graphene/WSe₂ vdW heterostructures is 1.5 times larger than that of pure monolayer WSe₂ at the band gap energy of monolayer WSe₂ because of the enhanced carrier generation after the heterostructure formation. Different from pure monolayer WX₂, azimuthal angle-dependent second harmonic generation from graphene/WX₂ vdW heterostructures exhibits extraordinary rotational symmetry at different photon energies, which can be used to deduce the extrinsic second-order nonlinear coefficient. These results pave the way for the nonlinear optical coefficient design based on 2D heterostructures for nonlinear nanophotonics and integrated devices.

Phase Identification and Strong Second Harmonic Generation in Pure ε-InSe and Its Alloys

Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), ε phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the as-prepared pure InSe, alloyed InSe_{1- x}Te _x and InSe_{1- x}S _x ( x = 0.1 and 0.2) are ε phase structures and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the noncentrosymmetric crystal structure of the ε-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multilayer pure ε-InSe and alloyed InSe_0.9Te_0.1 and InSe_{1- x}S _x ( x = 0.1 and 0.2) are around 1-2 orders of magnitude higher than that of the monolayer TMD systems and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility χ⁽²⁾ of ε-InSe is larger than that of ε-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of ε-InSe and indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.