Abstract

Endohedral fullerenes are an intriguing elass of materials in which the spheroidal fullerene molecular structure is employed to encapsulate a small number of atoms (one to four) internal to the caged ¹ Until recently, however, the difficult separation process of endohedral fullerenes had limited their availability to submilligram levels. Electron paramagnetic resonance measurements have shown the interesting property that for the case of La@C₈₂, for example, the La atom transfers three electrons to the fullerene cage and resides in the +3 oxidation state,² Metal-to-cage transfer appears to be a common feature of transition metal-containing endohedral metallo fullerenes. Meanwhile, nonlinear optical studies of empty- cage C₆₀ and C₇₀ have demonstrated that these fullerenes possess both large third-order susceptibilities χ⁽³⁾ (ω₄; ω₁, ω_2, ω₃)³ (~10⁻¹¹ esu) and strong optical limiting behavior.^4,5 We report here the first nonlinear optical measurements of an endohedral metallofullcrene and find a dramatic enhancement in the third-order nonlinear optical response. Degenerate four- wave-mixing (DPWM) experiments on solutions of the endohedral metallofullerene Er₂@C₈₂ (Fig. 1) show that the metal-to-cage charge transfer provides a mechanism for increasing χ⁽³⁾ (ω₄; ω₁, ω_2, ω₃) by orders of magnitude relative to empty-cage fullerenes.

© 1997 Optical Society of America