Fig. 1. Crystal structure diagram of black phosphorus. (a) Side view of monolayer black phosphorus; (b) top view of monolayer black phosphorus; (c) side view of trilayer black phosphorus

Fig. 2. Energy band structure of black phosphorus and variation in band gap width with number of layers. (a) Monolayer black phosphorus; (b) bilayer black phosphorus; (c) trilayer black phosphorus; (d) variation in band gap width with number of layers

Fig. 3. Two-photon absorption spectra of monolayer, bilayer, and trilayer black phosphorus. (a) Incident light is polarized along x direction; (b) incident light is polarized along y direction

Fig. 4. Two-photon absorption spectra of black phosphorus with different polarization directions. (a) Trilayer black phosphorus; (b) bilayer black phosphorus; (c) monolayer black phosphorus

Fig. 5. Variation in two-photon absorption peak with polarization direction