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

Controlled growth of 3R phase niobium diselenide and its properties

Inversion symmetry broken 3R phase transition metal dichalcogenides (TMDs) show fascinating prospects in spintronics, valleytronics, and nonlinear optics. However, the controlled synthesis of 3R phase TMDs is still a great challenge. In this work, two-dimensional 3R-NbSe2 single crystals up to 0.2 mm were synthesized for the first time through chemical vapor deposition method by designing a space-confined system. The crystal size and morphology can be controlled by the location of the stacked substrates and the amount of the Nb2O5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe2 shows obvious second harmonic generation signal which intensity intensified as thickness increases. Density functional theory calculations and optical absorption demonstrate the coexistence of metallic and semiconducting optical properties of 3R-NbSe2. We designed a NbSe2/WS2/NbSe2 photodetector utilizing the metallicity of 3R-NbSe2, which shows good performance especially an ultrafast response (6-7 μs, 0.5 ms - 7.9 s for Au electrodes in literature). The proposed strategy and findings are of great significance for the growth of many other 3R-TMDs and applications of nonlinear optical and ultrafast devices.

2D Material Infrared Photonics and Plasmonics

Two-dimensional (2D) materials including graphene, transition metal dichalcogenides, black phosphorus, MXenes, and semimetals have attracted extensive and widespread interest over the past years for their many intriguing properties and phenomena, underlying physics, and great potential for applications. The vast library of 2D materials and their heterostructures provides a diverse range of electrical, photonic, mechanical, and chemical properties with boundless opportunities for photonics and plasmonic devices. The infrared (IR) regime, with wavelengths across 0.78 μm to 1000 μm, has particular technological significance in industrial, military, commercial, and medical settings while facing challenges especially in the limit of materials. Here, we present a comprehensive review of the varied approaches taken to leverage the properties of the 2D materials for IR applications in photodetection and sensing, light emission and modulation, surface plasmon and phonon polaritons, non-linear optics, and Smith-Purcell radiation, among others. The strategies examined include the growth and processing of 2D materials, the use of various 2D materials like semiconductors, semimetals, Weyl-semimetals and 2D heterostructures or mixed-dimensional hybrid structures, and the engineering of light-matter interactions through nanophotonics, metasurfaces, and 2D polaritons. Finally, we give an outlook on the challenges in realizing high-performance and ambient-stable devices and the prospects for future research and large-scale commercial applications.

Giant Tunability of Charge Transport in 2D Inorganic Molecular Crystals by Pressure Engineering

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb₂ O₃ molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb₂ O₃ flake shows a sharp drop by 5 orders of magnitude in 0-3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

Extraordinary Nonlinear Optical Interaction from Strained Nanostructures in van der Waals CuInP2S6

Local strain engineering and structural modification of 2D materials furnish benevolent control over their optoelectronic properties and provide an exciting approach to tune light-matter interaction in layered materials. Application of strain at the nanoscale is typically obtained through permanently deformed nanostructures such as nanowrinkles, which yield large band gap modulation, photoluminescence enhancement, and surface potential. Ultrathin transition metal dichalcogenides (TMDs) have been greatly analyzed for such purposes. Herein, we extend strain-induced nanoengineering to an emerging 2D material, CuInP₂S₆ (CIPS), and visualize extraordinary control over nonlinear light-matter interaction. Wrinkle nanostructures exhibit ∼160-fold enhancement in second harmonic generation (SHG) compared to unstrained regions, which is additionally influenced by a change in the dielectric environment. The SHG enhancement was significantly modulated by the percentage of applied strain which was numerically estimated. Furthermore, polarization-dependent SHG revealed quenching and enhancement in the parallel and perpendicular directions, respectively, due to the direction of the compressive vector. Our work provides an important advancement in controlling optoelectronic properties beyond TMDs for imminent applications in flexible electronics.

Nonlinear Optical Activities in Two-Dimensional Gallium Sulfide: A Comprehensive Study

The nonlinear optical (NLO) properties of two-dimensional (2D) materials are fascinating for fundamental physics and optoelectronic device development. However, relatively few investigations have been conducted to establish the combined NLO activities of a 2D material. Herein, a study of numerous NLO properties of 2D gallium sulfide (GaS), including second-harmonic generation (SHG), two-photon excited fluorescence (TPEF), and NLO absorption are presented. The layer-dependent SHG response of 2D GaS identifies the noncentrosymmetric nature of the odd layers, and the second-order susceptibility (χ²) value of 47.98 pm/V (three-layers of GaS) indicates the superior efficiency of the SHG signal. In addition, structural deformation induces the symmetry breaking and facilitates the SHG in the bulk samples, whereas a possible efficient symmetry breaking in the liquid-phase exfoliated samples results in an enhancement of the SHG signal, providing prospective fields of investigation for researchers. The generation of TPEF from 800 to 860 nm depicts the two-photon absorption characteristics of 2D GaS material. Moreover, the saturable absorption characteristics of 2D GaS are realized from the largest nonlinear absorption coefficient (β) of -9.3 × 10³, -91.0 × 10³, and -6.05 × 10³ cm/GW and giant modulation depths (T_s) of 24.4%, 35.3%, and 29.1% at three different wavelengths of 800, 1066, and 1560 nm, respectively. Hence, such NLO activities indicate that 2D GaS material can facilitate in the technical advancements of future nonlinear optoelectronic devices.