Photon-phonon resonance of PbB4O7 crystal

We observed a photon-phonon resonance or polariton arises in PbB
₄
O
₇
crystal. We identified this resonance state is a phonon, nu
_TO
= 1.56 THz and q = 1930 cm
^-1
, because the transmissivity T(nu
_p
) is almost zero, the dielectric function epsiv(nu) satisfies the Lyddane-Sachs-Teller (LST) relation, and a split appears at 1.56 THz in nu(k) curve.