Nonlinear polariton effects in the phenanthrene crystal

Two-photon fluorescence excitation (TPE) and second harmonic generation (SHG) spectra of the origin region of the 3500 Å system of the phenanthrene crystal at 2 K are presented and compared for both melt-grown and sublimed crystals. The dependence of the upper Davydov component position on the photon polarization and propagation direction reported previously is confirmed for both TPE and SHG- This behavior is considered to be a manifestation of phase matching to the upper branch of the b-polariton dispersion curve and is intrinsic to the two-photon process for transitions which are allowed in both one- and two-photon absorption. Both TPE and SHG are understood as resulting from the fundamental process of polariton fusion. The appearance of the dipole-forbidden lower Davydov component in these spectra is commented on in terms of misalignment and excitation of a longitudinal exciton.