Electron correlation theory and experimental measurements of the third-order nonlinear optical properties of conjugated linear chains

James Randolph Heflin, University of Pennsylvania

Abstract

Comprehensive theoretical and experimental studies of the magnitude, sign, dispersion, and length dependence of the third order molecular susceptibility $\gamma\sb{ijkl}$(-$\omega\sb4$;$\omega\sb1$, $\omega\sb2$, $\omega\sb3$) demonstrate that the microscopic origin of the nonresonant third order nonlinear optical properties of conjugated linear chains is determined by the effects of electron correlation due to electron-electron repulsion. Multiple-excited configuration interaction calculations of $\gamma\sb{ijkl}$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$, $\omega\sb3$) for the archetypal class of quasi-one dimensional conjugated structures known as polyenes reveal for the first time the principal role of strongly correlated, energetically high-lying, two photon $\sp1$A$\sb{\rm g}$ virtual states in the largest of the two dominant, competing virtual excitation processes that determine $\gamma\sb{ijkl}$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$,$\omega\sb3$). It is also found in studies of the effects of conformation on $\gamma\sb{ijkl}$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$,$\omega\sb3$) that the origin of the third order optical properties remains basically the same for the all-trans and cis-transoid polyenes, and the results for the two conformations are unified by a common power law dependence of the dominant tensor component $\gamma\sb{xxxx}$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$,$\omega\sb3$) on the physical end-to-end length L of the chain with an exponent of 3.5. Calculations for a noncentrosymmetric conjugated chain demonstrate that virtual excitation processes involving diagonal transition moments that are forbidden in centrosymmetric structures lead to a more than an order of magnitude enhancement in $\gamma\sb{xxxx}$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$,$\omega\sb3$) compared to the analog centrosymmetric structure. Experimental measurements of the dispersion in the isotropically averaged dc-induced second harmonic susceptibility $\langle\gamma$(-2$\omega;\omega,\omega$,0)$\rangle$ and third harmonic susceptibility $\langle\gamma$(-3$\omega$;$\omega$,$\omega$,$\omega$)$\rangle$ in two important polyene structures confirm the electron correlation description of $\gamma\sb{ijkl}$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$,$\omega\sb3$). The measured values of $\langle\gamma$(-2$\omega$;$\omega$,$\omega$,0)$\rangle$ and $\langle\gamma$(-3$\omega$;$\omega$,$\omega$,$\omega$)$\rangle$ at several near infrared fundamental wavelengths for hexatriene, the N = 6 carbon site polyene chain, are in excellent agreement with the calculated magnitude, sign, and dispersion. Corresponding measurements for $\beta$-carotene, a substituted N = 22 polyene chain, together with the results for hexatriene, also verify the calculated power law dependence of $\langle\gamma$(-$\omega\sb4$;$\omega\sb1$,$\omega\sb2$,$\omega\sb3$)$\rangle$ on L. It was further found in the course of the experimental measurements that the common reference standard for the macroscopic susceptibility $\chi\sp{(3)}$(-3$\omega$;$\omega$,$\omega$,$\omega$) is too large by a factor of 2.0, and an improved standard value is proposed.

Subject Area

Condensation|Optics|Molecules

Recommended Citation

Heflin, James Randolph, "Electron correlation theory and experimental measurements of the third-order nonlinear optical properties of conjugated linear chains" (1990). Dissertations available from ProQuest. AAI9112573.
https://repository.upenn.edu/dissertations/AAI9112573

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