No abstract

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.

Phonon polaritons (PhPs) have attracted significant interest in the nano-optics communities because of their nanoscale confinement and long lifetimes. Although PhP modification by changing the local dielectric environment has been reported, controlled manipulation of PhPs by direct modification of the polaritonic material itself has remained elusive. Here, chemical switching of PhPs in α-MoO 3 is achieved by engineering the α-MoO 3 crystal through hydrogen intercalation. The intercalation process is non-volatile and recoverable, allowing reversible switching of PhPs while maintaining the long lifetimes. Precise control of the intercalation parameters enables analysis of the intermediate states, in which the needle-like hydrogenated nanostructures functioning as in-plane antennas effectively reflect and launch PhPs and form well-aligned cavities. We further achieve spatially controlled switching of PhPs in selective regions, leading to in-plane heterostructures with various geometries. The intercalation strategy introduced here opens a relatively non-destructive avenue connecting infrared nanophotonics, reconfigurable flat metasurfaces and van der Waals crystals.

Recent advances in the development of 2D-layered materials have witnessed the use of these materials as intriguing building blocks for various optoelectronic applications. The versatility of 2D material systems makes them particularly attractive for photodetection with fast response and high sensitivity over a broad spectral coverage, ranging from ultraviolet, visible to infrared. However, due to the atomically thin nature and inherent electronic structure, light that is harvested by monolayer 2D materials is extremely low and the photodetector devices often operate as Schottky junctions, which significantly limit the efficiency for photocurrent generation. Here, recent progress on the exploration of 2D material-based heterostructures and the engineering of the band structures for energy-efficient optoelectronic applications is reviewed. First, the strategies to introduce a bandgap in graphene are reviewed and discussed. This is followed by a discussion on the engineering of electronic structures in 2D transition metal dichalcogenides by localized chemical doping, dual gating, liquid gating, thickness modulation, and constructing heterojunctions. It is concluded by a summary and perspective on the challenges and future directions.

Bulk germanium as a group‐IV photonic material has been widely studied due to its relatively large refractive index and broadband and low propagation loss from near‐infrared to mid‐infrared. Inspired by the research of graphene, the 2D counterpart of bulk germanium, germanene, has been discovered and the characteristics of Dirac electrons have been observed. However, the optical properties of germanene still remain elusive. In this work, several layers of germanene are prepared with Dirac electronic characteristics and its morphology, band structure, carrier dynamics, and nonlinear optical properties are systematically investigated. It is surprisingly found that germanene has a fast carrier‐relaxation time comparable to that of graphene and a relatively large nonlinear absorption coefficient, which is an order of magnitude higher than that of graphene in the near‐infrared wavelength range. Based on these findings, germanene is applied as a new saturable absorber to construct an ultrafast mode‐locked laser, and sub‐picosecond pulse generation in the telecommunication band is realized. The results suggest that germanene can be used as a new type of group‐IV material for various nonlinear optics and photonic applications.