ABSTRACT
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.
ABSTRACT
Bismuthene, a monoelemental two-dimensional material, has shown promise in the biomedical, electronic, and energy fields due to its high carrier mobility and stability at room temperature. However, its use in biosensing applications is restricted due to its undefined quenching mechanism for dye molecules. Herein, we developed a novel ultrathin bismuthene-based sensing platform for microRNA (miRNA)-specific detection that even discriminates single-base mismatches. The detection limit can reach 60 pM. Excitingly, with the fluorescence quenching mechanism of bismuthene, ground state weakly fluorescent charge transfer is determined via femtosecond pump-probe spectroscopy. This finding provides a proof-of-concept platform to (i) fundamentally explore the quenching mechanism of bismuthene and (ii) sensitively detect miRNA molecules for early cancer.
