  Coherent phonon-polaritons have attracted a considerable amount of interest
owing to their relevance to nonlinear optics and terahertz (THz)-wave
emissions. Therefore, it is important to analyze the THz electric-fields of
phonon-polaritons. However, in the majority of previous measurements, only a
single component of the THz electric field was detected. In this paper, we
demonstrate that pump-probe electro-optical imaging measurements using the
Stokes parameters of probe polarization enable the phase-resolved selective
detection of THz electric-field components that are associated with the
phonon-polariton. We experimentally distinguish the mode profiles of ordinary
and extraordinary phonon-polaritons, and clarify the excitation mechanism as
optical rectification. These results are explained by numerical calculations of
Maxwell equations for the THz electric field. The technique of selectively
observing the THz electric field components may be useful for designing
efficient THz-wave emitters.
