Photothermionic Effect-Assisted Ultrafast Charge Transfer in NbS2/MoS2 Heterostructure.

Two-dimensional (2D) van der Waals heterostructures (vdW HSs) composed of transition metal dichalcogenides (TMDCs) have emerged as frontrunners in the optoelectronics field, owing to their exceptional optical and electrical properties. Recent research on the intrinsic interlayer charge transfer mechanism has been primarily focused on the Type II HSs, while metal-semiconductor (MS) vertical HSs, promising for advancing photodetector technology, have received comparatively less attention. Here, we reveal the first experimental observation of photothermionic effect-assisted ultrafast interlayer charge transfer in the NbS2/MoS2 heterostructure using femtosecond transient absorption technology and first-principles calculations, effectively ignoring the Schottky barrier height. We demonstrate that within 500 fs, charge transfer occurs from NbS2 to MoS2 in the heterostructure, resulting in supplementary carrier generation in the visible spectrum when excited with infrared light below the MoS2 bandgap, at wavelengths of 1030 and 1500 nm. Such promising characteristics of 2D NbS2-semiconductor heterostructures offer a potential platform for synergistically combining low contact resistance with broadband photocarrier generation, marking a significant advancement in optoelectronics and light harvesting.

Layer-dependent nonlinear optical properties of two-dimensional InSe and its applications in waveguide lasers.

The thickness-dependent third-order nonlinear optical properties of two-dimensional ß-InSe and its potential applications as a saturable absorber in pulsed laser generation are investigated. InSe sheets with different layers are prepared by the chemical vapor deposition. Using open-aperture femtosecond Z-scan technique at 1030 nm, the modulation depth and nonlinear absorption coefficient are obtained to be 36% and -1.6 × 104 cm·GW-1, respectively. The intrinsic mechanism of the layer-dependent energy band structure evolution is analyzed based on density functional theory, and the theoretical analysis is consistent with the experimental results. Based on a waveguide cavity, a Q-switched mode-locked laser at 1 µm with a repetition frequency of 8.51 GHz and a pulse duration of 28 ps is achieved by utilizing the layered InSe as a saturable absorber. This work provides an in-depth understanding of layer-dependent properties of InSe and extends its applications in laser technology for compact light devices.