Controllable Synthesis of Narrow-Gap van der Waals Semiconductor Nb2GeTe4 with Asymmetric Architecture for Ultrafast Photonics

Ultrafast photonics has become an interdisciplinary topic of great consequence due to the spectacular progress of compact and efficient ultrafast pulse generation. Wide spectrum bandwidth is the key element for ultrafast pulse generation due to the Fourier transform limitation. Herein, monoclinic Nb2GeTe4, an emerging class of ternary narrow-gap semiconductors, was used as a real saturable absorber (SA), which manifests superior wide-range optical absorption. The crystallization form and growth mechanism of Nb2GeTe4 were revealed by a thermodynamic phase diagram. Furthermore, the Nb2GeTe4-SA showed reliable saturation intensity and larger modulation depth, ascribed to a built-in electric field driven by the asymmetric crystal architecture confirmed via X-ray diffraction, polarized Raman spectra, and scanning transmission electron microscopy. Based on the Nb2GeTe4-SA, femtosecond mode-locked operation with good overall performance was achieved by a properly designed ring cavity. These results suggest that Nb2GeTe4 shows great promise for ultrafast photonic applications and arouse interests in exploring the intriguing properties of the ternary van der Waals material family.