Infrared Optical Absorption Due to One and Two Phonon Processes in Black Phosphorus

Using a single crystal of black phosphorus, anisotropic absorption spectra due to the two-phonon summation process have been found in the wavenumber region from 300 to 900 cm -1 . By an oblique incidence method, the dipole moment of an infrared active mode at 470 cm -1 has been confirmed to be in the z-direction of the crystal as predicted by Kaneta, Yoshida and Morita. Frequencies of unobserved A u and B 1u modes are estimated to be about 420 and 250 cm -1 from the absorption spectra of the two-phonon process.