Low-frequency vibrational excitations in a niobium–phosphate glass for Raman gain applications

Raman gain for all-optical amplification depends significantly on the cross section for spontaneous Raman scattering. We present measurements of the Raman spectrum of binary niobium–phosphate glasses in the frequency range from 5 to 1300 cm−1. The spectra of these glasses show a low-frequency enhancement of the vibrational density of states (‘Boson peak’) suggesting a significant gain profile for application in Raman amplifiers as well as providing an additional mechanism for Raman gain. To separate the effects of density of states from the light-vibration coupling we carried out an inelastic neutron scattering investigation of 40Nb2O5–60NaPO3 using the time-of-flight spectrometer (TOFTOF) at the FRM II neutron source in Munich. The Boson peak appears between 40 and 70 cm−1 as an increase of the reduced density of states over the flat Debye approximation. Possible mechanisms contributing to the low-frequency excitations such as disorder-induced irregular vibrational states are discussed.