Giant All-Optical Modulation of Second-Harmonic Generation Mediated by Dark Excitons

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, lattice temperature, electron doping, and all-optical tuning of third-harmonic generation are combined in a hexagonal boron nitride-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. The work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors, and frequency converters.

Magnetic Switching of Second-Harmonic Generation from Single Cerium-Based Coordination Polymer Microcrystals

Conventional access and modulation of second-harmonic generation (SHG) require precise control of crystal orientation, which faces great mechanical challenges in the case of micro/nanocrystals. Here, we demonstrate the magnetic-field-tunable SHG performance of lanthanide coordination polymer (Ce-BTC CP) microcrystals through field-aligned orientations. The coordination of Ce ions and organic ligands constructs a noncentrosymmetric structure, which not only contributes to a favorable powder SHG efficiency 3.2 times larger than that of the benchmark KH2PO4 (KDP) but also endows the microcrystals with strong magnetic anisotropy. The SHG efficiency (∼0 to 10 × KDP) depends on the orientation of the crystallographic c-axis, whereas magnetic anisotropy always aligns the c-axis with the magnetic field at a specific angle. Accordingly, the SHG can be magnetically switched by field-induced alignments. The adsorption of dyes by Ce-BTC CPs further facilitates the magnetic switching of multicolor fluorescence that can be excited by the SHG. Our work provides a new pathway for achieving SHG modulation at the microscopic level.

Optical Second Harmonic Generation of Low-Dimensional Semiconductor Materials

In recent years, the phenomenon of optical second harmonic generation (SHG) has attracted significant attention as a pivotal nonlinear optical effect in research. Notably, in low-dimensional materials (LDMs), SHG detection has become an instrumental tool for elucidating nonlinear optical properties due to their pronounced second-order susceptibility and distinct electronic structure. This review offers an exhaustive overview of the generation process and experimental configurations for SHG in such materials. It underscores the latest advancements in harnessing SHG as a sensitive probe for investigating the nonlinear optical attributes of these materials, with a particular focus on its pivotal role in unveiling electronic structures, bandgap characteristics, and crystal symmetry. By analyzing SHG signals, researchers can glean invaluable insights into the microscopic properties of these materials. Furthermore, this paper delves into the applications of optical SHG in imaging and time-resolved experiments. Finally, future directions and challenges toward the improvement in the NLO in LDMs are discussed to provide an outlook in this rapidly developing field, offering crucial perspectives for the design and optimization of pertinent devices.

All-optical generation of static electric field in a single metal-semiconductor nanoantenna

Electric field is a powerful instrument in nanoscale engineering, providing wide functionalities for control in various optical and solid-state nanodevices. The development of a single optically resonant nanostructure operating with a charge-induced electrical field is challenging, but it could be extremely useful for novel nanophotonic horizons. Here, we show a resonant metal-semiconductor nanostructure with a static electric field created at the interface between its components by charge carriers generated via femtosecond laser irradiation. We study this field experimentally, probing it by second-harmonic generation signal, which, in our system, is time-dependent and has a non-quadratic signal/excitation power dependence. The developed numerical models reveal the influence of the optically induced static electric field on the second harmonic generation signal. We also show how metal work function and silicon surface defect density for different charge carrier concentrations affect the formation of this field. We estimate the value of optically-generated static electric field in this nanoantenna to achieve ≈108V/m. These findings pave the way for the creation of nanoantenna-based optical memory, programmable logic and neuromorphic devices.

All-Optical Reconfigurable Excitonic Charge States in Monolayer MoS2

Excitons are quasi-particles composed of electron-hole pairs through Coulomb interaction. Due to the atomic-thin thickness, they are tightly bound in monolayer transition metal dichalcogenides (TMDs) and dominate their optical properties. The capability to manipulate the excitonic behavior can significantly influence the photon emission or carrier transport performance of TMD-based devices. However, on-demand and region-selective manipulation of the excitonic states in a reversible manner remains challenging so far. Herein, harnessing the coordinated effect of femtosecond-laser-driven atomic defect generation, interfacial electron transfer, and surface molecular desorption/adsorption, we develop an all-optical approach to manipulate the charge states of excitons in monolayer molybdenum disulfide (MoS₂). Through steering the laser beam, we demonstrate reconfigurable optical encoding of the excitonic charge states (between neutral and negative states) on a single MoS₂ flake. Our technique can be extended to other TMDs materials, which will guide the design of all-optical and reconfigurable TMD-based optoelectronic and nanophotonic devices.

Optical Control of High-Harmonic Generation at the Atomic Thickness

High-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light-matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.

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.

Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies

Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS₂ , such as multiple excitonic Rydberg states and free-particle energy bands are probed with a high relative contrast of up to ≥200 via broadband (from ≈1.79 to 3.10 eV) static third-harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding ≈94% at ≈2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier-exciton and carrier-phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.