Resonant Multi-phonon Raman scattering of black phosphorus

Black phosphorus (BP) has been attracting intense interest due to its unique anisotropic properties. The investigations on phonon dispersion and electronic band structure could expand the understanding of the properties of BP and promote its application on next generation nano-electronic devices. As the fingerprint of materials, Raman spectroscopy can provide the information of their phonon dispersion and electronic band structure. According to the Raman selection rule, Raman process involving multiple (two or more) phonons can be used to probe the phonon density of states within the whole Brillouin zone. However, the intensity of high-order Raman modes is much lower than that of the first-order Raman mode. To break through the limit of low intensity, here, we measured the resonant Raman spectroscopy of BP excited by several wavelength lasers and observed rich information about high-order Raman modes in the spectral range of 680–930 cm^–1. To further investigate high-order Raman modes and avoid the birefringence effects from optical anisotropy on Raman intensity, we employ a special polarization configuration to obtain resonant Raman spectra and Raman intensity as a function of excitation wavelength. All the observed high-order Raman modes are certainly assigned, according to the phonon dispersion and symmetry analysis of related phonons. This indicates the great contribution of phonons within the Brillouin zone to the second- and third-order Raman scattering. This work proposes a general and systematical method to investigate high-order Raman modes, and paves ways for the researches of phonon dispersion and resonance Raman spectroscopy in other anisotropic materials.