Two-photon absorption and nonlinear polariton effects in organic crystals
Two-photon excitation (TPE) and second harmonic generation (SHG) have been studied in phenanthrene crystals at low temperatures (2 to 6K) in order to investigate mechanisms of two-photon absorption (TPA), the relationship between TPE and SHG, and polariton effects. The transition studied is of special interest because it is both one- and two-photon allowed in the dipole approximation. Consequently, polariton states and SHG play a role in the two-photon process. In the polariton model, both TPA and SHG are understood as resulting from the process of polariton fusion;Polarized TPE spectra of the 350 nm ('1)A(,1) (<---) ('1)A(,1) absorption system in phenanthrene crystals, recorded previously, are supplemented by further results obtained using a nitrogen laser-pumped dye laser. Analysis of vibronic structure reveals that the dominant intermediate states for TPA are those of B(,2) (L axis) symmetry. TPE spectra principally associated with intermediate states of A(,1) (M) symmetry exhibit strong intramolecular vibronic coupling by totally symmetric vibrations. Polarized TPE spectra of mixed crystals of phenanthrene in fluorene confirm the predominance of the long-axis mechanism in TPA and the participation of states of A(,1) symmetry by vibronic coupling;The position of the upper Davydov component of the origin depends dramatically on photon propagation direction and polarization in both TPE and SHG. The appearance of the dipole-forbidden lower Davydov component in these spectra is discussed in terms of misalignment and excitation of a longitudinal exciton. The similarity of TPE and SHG spectra supports the conclusion that both result from the fundamental process of polariton fusion. The calculated polariton dispersion curve accounts for the shifts in energy of the upper component in TPE and SHG. The requirement of phase matching on the a*b face leads to creation of a polariton on the upper polariton branch, above the one-photon absorption frequency, for a*- and b-polarized light. This interpretation is further confirmed by the observation of the second harmonic generated from two beans focused on the crystal. As the angle between the beams was varied, a portion of the upper branch of the polariton dispersion curve was mapped out. The lower component polariton was reached by excitation from two beans of perpendicular polarization;('(DAG))USDOE Report IS-T-962. This work was performed under Contract W-7405-eng-82 with the Department of Energy.