Knowledge Bank

Evidence of strong exciton-photon coupling associated with the polariton model has been found in naphthalene and phenanthrene crystals by two-photon excitation and resonant second harmonic generation. The theory of polariton $fusion,1$ which describes two-photon transitions to a state strongly coupled to the radiation field, accounts for the wave-vector and temperature dependence of the two-photon excitation and second harmonic generation in these crystals. Evidence of polariton phase matching has been observed in phenanthrene. In naphthalene, we have shown the temperature dependence of the ratio of second harmonic generation to two-photon excitation intensities. The results suggest that in a perfect crystal no two-photon absorption occurs, only second harmonic generation.