Dispersion of bulk phonon polaritons inside reststrahlen bands

When pure spatial damping is taken into account, i.e. when the wave vector is treated as complex and the frequency as a real quantity, dispersion curves of bulk phonon polaritons exhibit turnabouts caused by limiting momenta for the wave vectors which lead to branches within the frequency range of reststrahlen bands. The evaluation of the dispersion curves from reflection experiments for fixed frequency and swept angle of incidence confirms these features. Depending on the experimental arrangement dispersion curves inside of reststrahlen bands are investigated for α‐quartz. The measurements turn out to be a sensitive aid for the determination of damping parameters.