  The nature of bosonic excitations in disordered materials has remained
elusive due to the difficulties in defining key concepts such as
quasi-particles in the presence of disorder. We report on the experimental
observation of phonon-polaritons in glasses, including a boson peak (BP), i.e.
excess of THz modes over the Debye law. A theoretical framework based on the
concept of diffusons is developed to model the broadening linewidth of the
polariton due to disorder-induced scattering. It is shown that the scaling of
the BP frequency with the diffusion constant of the linewidth strongly
correlates with that of the Ioffe-Regel (IR) crossover frequency of the
polariton. This result demonstrates the universality of the BP in the
low-energy spectra of collective bosonic-like excitations in glasses, well
beyond the traditional case of acoustic phonons, and establishes the IR
crossover as the fundamental physical mechanism behind the BP.
