Abstract
We show that a very large enhancement of third-order optical nonlinearity $(\ensuremath{\gamma})$ of $\ensuremath{\pi}$-conjugated molecules can be realized without a major redshift of the absorption spectrum that disturbs optical transparency in the visible region. By changing the number $(n)$ of C=C bonds of $\ensuremath{\beta}$ carotene $(n=11)$ from 7 to 15, a remarkable 3.4-fold increase in the $\ensuremath{\gamma}$ value was observed when $n=15$ relative to that of $\ensuremath{\beta}$ carotene. This enhancement of $\ensuremath{\gamma}$ mainly originates from three-photon resonance of a lowest optically allowed excited state. The controversial higher-lying essential state is not important for generating the large value of $\ensuremath{\gamma}$.
Published Version
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