Fig. 1. (a) Crystal structure of black phosphorus; (b) atomic displacements of phonon modes in black phosphorus; (c) phonon dispersion, vibration density of states (VDOS) and schematic diagram of first Brillouin zone of bulk black phosphorus. Raman modes at the Brillouin zone center are labeled^[9]

Fig. 2. (a) Experimental configuration. The polarization direction of Raman signal is fixed ( ). The angle between polarization direction of incident light and x axis is , which can be changed by rotating a half-wave plate; (b) Raman spectra in the range of , , , and modes, under the VV( ) and HV( ) configurations; (c) the -dependent Raman intensity excited by different wavelengths. The solid lines indicate fitting results.

Fig. 3. (a) Raman spectra of black phosphorus excited by six excitation wavelengths between 473–671 nm. Raman spectra at and are given for each excitation. Three main first-order Raman peaks and eleven high-order Raman peaks (P₁–P₁₁) are marked by vertical dotted lines; (b) the fitting result of P₄–P₁₁ for Raman spectra by six excitations at .

Fig. 4. Polarization-dependent Raman intensity of P₁–P₁₁ Raman modes, excited by 488, 532 and 633 nm lasers.

Table 1. Assignments of high order Raman peaks of BP.