Enhanced Nonlinear Optical Response of Rectangular MoS₂ and MoS₂/TiO₂ in Dispersion and Film

The nonlinear response of the two-dimensional materials could be measured by the spatial self-phase modulation (SSPM) and Z scan. Most of these materials are irregular shapes. In this paper, we studied the nonlinear optical properties of rectangular layered MoS₂ and the corresponding MoS₂/TiO₂ composite in dispersion and the film, respectively. For dispersion measured by SSPM, the nonlinear refraction index of rectangular layered MoS₂ dispersion is larger than that of the MoS₂/TiO₂ composite dispersion. For film measured by Z scan, the nonlinear refractive index of the MoS₂/TiO₂ composite thin film is larger than that of the rectangular layered MoS₂ thin film. This research confirms the possibility of the nonlocal electron coherence established among the alignment in SSPM and provides two different paths for the enhancement of the nonlinear optical properties of MoS₂

Air-Induced High-Quality CH₃NH₃PbI₃ Thin Film for Efficient Planar Heterojunction Perovskite Solar Cells

Efficient planar heterojunction perovskite solar cells (PHJ–PSCs) with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al were fabricated using air-induced high-quality CH₃NH₃PbI₃ perovskite thin films, in which the air-inducing process was controlled with a humidity of ∼40%. The air exposure of CH₃NH₃PbI₃ thin films could dramatically improve their properties with large grains and smooth surface, as well as uniform morphology, resulting in an impressive enhancement in carrier lifetime. The ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy results proved that the CH₃NH₃PbI₃ film was <i>n</i>-doped by the absorption of H₂O on the surface but was very stable without obvious degradation for 10 days’ exposure in air. The power conversion efficiency (PCE) of PHJ–PSCs with an air exposure process showed a significant increase up to 16.21% as compared to reference PHJ–PSCs with a PCE of 12.02%. The research work demonstrated that an air-exposure process with a suitable humidity could produce high-quality perovskite thin film for efficient PHJ–PSCs, which may pave a boulevard for fabricating high-efficiency PHJ–PSCs in atmospheric environment

Graphene–Bi₂Te₃ Heterostructure as Saturable Absorber for Short Pulse Generation

Rapid progresses have been achieved in the photonic applications of two-dimensional materials such as graphene, transition metal dichalcogenides, and topological insulators. The strong light–matter interactions and large optical nonlinearities in these atomically thin layered materials make them promising saturable absorbers for pulsed laser applications. Either Q-switching or mode-locking pulses with particular output characteristics can be achieved by using different saturable absorbers. However, it remains still very challenging to produce saturable absorbers with tunable optical properties, in particular, carrier dynamics, saturation intensity as well as modulation depth, to suit for self-starting, high energy or ultrafast pulse laser generation. Here we report a new type of saturable absorber which is a van der Waals heterostructure consisting of graphene and Bi₂Te₃. The synergetic integration of these two materials by epitaxial growth affords tunable optical properties, that is, both the photocarrier dynamics and the nonlinear optical modulation are variable by tuning the coverage of Bi₂Te₃ on graphene. We further fabricated graphene–Bi₂Te₃ saturable absorbers and incorporated them into a 1.5 μm fiber laser to demonstrate both Q-switching and mode-locking pulse generation. This work provides a new insight for tailoring two-dimensional heterostructures so as to develop desired photonic applications

Highly Efficient and Air-Stable Infrared Photodetector Based on 2D Layered Graphene–Black Phosphorus Heterostructure

The presence of a direct band gap and high carrier mobility in few-layer black phosphorus (BP) offers opportunities for using this material for infrared (IR) light detection. However, the poor air stability of BP and its large contact resistance with metals pose significant challenges to the fabrication of highly efficient IR photodetectors with long lifetimes. In this work, we demonstrate a graphene–BP heterostructure photodetector with ultrahigh responsivity and long-term stability at IR wavelengths. In our device architecture, the top layer of graphene functions not only as an encapsulation layer but also as a highly efficient transport layer. Under illumination, photoexcited electron–hole pairs generated in BP are separated and injected into graphene, significantly reducing the Schottky barrier between BP and the metal electrodes and leading to efficient photocurrent extraction. The graphene–BP heterostructure phototransistor exhibits a long-term photoresponse at near-infrared wavelength (1550 nm) with an ultrahigh photoresponsivity (up to 3.3 × 10³ A W^–1), a photoconductive gain (up to 1.13 × 10⁹), and a rise time of about 4 ms. Considering the thickness-dependent band gap in BP, this material represents a powerful photodetection platform that is able to sustain high performance in the IR wavelength regime with potential applications in remote sensing, biological imaging, and environmental monitoring

Ultrathin 2D Transition Metal Carbides for Ultrafast Pulsed Fiber Lasers

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, and black phosphorus, have attracted intense interest for applications in ultrafast pulsed laser generation, owing to their strong light–matter interactions and large optical nonlinearities. However, due to the mismatch of the bandgap, many of these 2D materials are not suitable for applications at near-infrared (NIR) waveband. Here, we report nonlinear optical properties of 2D α-Mo₂C crystals and the usage of 2D α-Mo₂C as a new broadband saturable absorber for pulsed laser generation. It was found that 2D α-Mo₂C crystals have excellent saturable absorption properties in terms of largely tunable modulation depth and very low saturation intensity. In addition, ultrafast carrier dynamic results of 2D α-Mo₂C reveal an ultrashort intraband carrier recovery time of 0.48 ps at 1.55 μm. By incorporating 2D α-Mo₂C saturable absorber into either an Er-doped or Yb-doped fiber laser, we are able to generate ultrashort pulses with very stable operation at central wavelengths of 1602.6 and 1061.8 nm, respectively. Our experimental results demonstrate that 2D α-Mo₂C can be a promising broadband nonlinear optical media for ultrafast optical applications