Germanium Nanosheets with Dirac Characteristics as a Saturable Absorber for Ultrafast Pulse Generation

• Published in: Advanced materials (Weinheim) 2021-08, Vol.33 (32), p.e2101042-n/a
• Main Authors: Mu, Haoran, Liu, Yani, Bongu, Sudhakara Reddy, Bao, Xiaozhi, Li, Lei, Xiao, Si, Zhuang, Jincheng, Liu, Chen, Huang, Yamin, Dong, Yemin, Helmerson, Kristian, Wang, Jiaou, Liu, Guanyu, Du, Yi, Bao, Qiaoliang
• Format: Article
• Language: English
• Subjects: Absorbers; Absorptivity; Broadband; Dynamic structural analysis; Germanium; germanium nanosheets; Graphene; mode‐locking; Morphology; Nanosheets; Nonlinear dynamics; Nonlinear optics; Optical properties; Photonics; Refractivity; Relaxation time; Two dimensional materials; ultrafast carrier dynamics
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202101042

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. Ge nanosheets with √3 × √3 germanene as the top layer are successfully grown on a Ag (111) substrate. Dirac characteristics, ultrafast carrier dynamics, and large nonlinear absorption coefficient are experimentally observed from the Ge nanosheets, which are further applied as new saturable absorbers to construct an ultrafast mode‐locked laser, and sub‐picosecond pulse generation in the telecommunication band is realized.