Nonlinear polariton effects in naphthalene

Resonant second harmonic generation (SHG) and two-photon excited emission (TPE) were studied in pure, strain-free crystals of naphthalene at frequencies near that of the (0,0) a-exciton in order to probe the relationship between the two signals and to investigate the effect of polariton states on second order nonlinearities in molec- ular crystals. The results are presented, and the strong coupling of the 31475 cm(\u27-1) exciton in naphthalene to the photon field is demon- strated to dictate the second-order nonlinear behavior of naphtha- lene crystals at frequencies near half-resonance. The dynamics of polaritons produced coherently via nonlinear interactions which are dependent on phase matching is shown to deviate in a controllable way from the dynamics of the one-photon polaritons produced in a linear experiment. The nature of the excitation remains principally that of an exciton;The necessity of using a strong coupling model to explain orienta- tional dispersion and intensity and lineshape behavior is established. The experimental angular frequency dispersion of the SHG and TPE signals are fit to theoretical polariton dispersion curves. The orienta- tion of the naphthalene optical indicatrix at 31475 cm(\u27-1) is shown to be very nearly the same as that reported for visible light. The temper- ature dependences of the SHG and TPE signal intensities are successfully predicted from the polariton fusion model by inclusion of temporal damping in the fusion rate expression. The simple expressions which result describe the two signals as competing for intensity, with the TPE:SHG branching ratio approaching infinity at temperatures (GREATERTHEQ) (for most cases) 20 K. The shapes of the SHG and TPE profiles are compared to shapes predicted from the semi- classical theory. The theoretical peak shapes match those of the observed signals well, with the low-temperature profiles dominated by the phase matching function. At near-normal angles of incidence, the experimental and theoretical SHG profiles display multiple fringing maxima ( Maker fringes ) as a result of the behavior of;(DELTA)k near resonance, including a pronounced inflection point at the frequency of the transverse exciton resonance; *USDOE Report IS-T-1136. This work was performed under Contract No. W-7405-eng-82 with the Department of Energy