Dispersion of Third-Harmonic-Generation Optical Susceptibility inC70Thin Films

We report wave dispersed optical third harmonic generation (THG) studied in the fundamental wavelength range of 0.8-1.9 μm in thin films of ${\mathrm{C}}_{70}$. The THG spectrum shows a broad resonance enhancement in cubic susceptibility ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ with a maximum value of ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}(-3\mathrm{}\omega ;\omega ,\omega ,\omega )=(0.9\mathrm{}\pm 0.1)\times {10}^{-10\mathrm{}}$ esu located at 1.42 μm, and a dramatic decrease at higher energies. We interpret the data in terms of a three-level model consisting of a totally symmetric ground state, along with two excited states: a one-photon state and a two-photon state. We predict the presence of a low-lying dipole-forbidden electronic level in close proximity to a dipole-allowed level.