Thickness-dependent ultrafast nonlinear optical response of germanium selenide nanosheets

Germanium selenide (GeSe), a mono-sulfide compound of group VI elements, has a crystal and semiconductor structure similar to black phosphorus, which means that it is very promising for applications in nanophotonic devices. In this paper, we prepared 5 mg ml⁻¹ GeSe nanosheet suspensions by liquid-phase exfoliation technique and centrifuged the suspensions at different speeds to obtain GeSe nanosheets with different layer numbers to investigate the effect of nanosheet layer numbers on the third-order nonlinear optical properties of GeSe nanosheets. AFM microscopy is used to show that the number of layers in a nanosheet can be controlled using differential centrifugation techniques. The structure and defects of the exfoliated nanosheets were analyzed based on the results of Raman spectroscopy and X-ray energy spectrometry. The open-aperture Z-scan technique revealed that the nonlinear absorption effect of the nanosheet changed from saturation absorption to reverse saturation absorption when the number of nanosheet layers was gradually reduced. The closed-aperture Z-scan technique reveals that the nonlinear refraction effect enhanced when the number of nanosheet layers is gradually reduced.