Size-dependent saturable absorption and mode-locking of dispersed black phosphorus nanosheets

Abstract

Size dependence of the saturable absorption (SA) property for black phosphorus (BP) was investigated. Three types of BP dispersions with different sizes were prepared using the liquid phase exfoliation method. By fitting the Z-scan data with the analytical solution of the propagation equation, the SA coefficients 𝛼NL and the imaginary part of the third order optical susceptibility Im𝜒(3) are compared at two specific wavelengths, one above the bandgap of the BP monolayer at 515 nm and another between the bandgaps of the monolayer and bulk at 1030 nm. A figure of merit (FOM, |Imχ(3)/α0|, α0 linear absorption) was employed to assess the SA performance of the three different dispersions. The FOM, about (2.1 ± 0.3) × 10−14 esu cm at 515 nm, hardly depends on the size and hence the layer number of the dispersed BP nanosheets, whereas that at 1030 nm decreases from (1.14 ± 0.20) × 10−14 to (0.50 ± 0.02) × 10−14 esu cm when the size of the nanosheets becomes smaller.