A fullerene-doped nonlinear optical material is a dilute solution of a fullerene in a liquid, a liquid crystal, or a solid matrix. We rule out any direct

chemical interaction between the fullerene and the matrix and restrict analysis to solutions in inert molecular matrices. In what follows, fullerene means either C60 or its derivatives. The energy spectrum of impurity states in a dilute molecular solution is generally represented as the single-molecule spectrum of the impurity embedded in the band gap of the host material, with a weak shift and a broadening due to interactions both between impurity molecules and between impurity and matrix molecules. This representation is suggested by the empirical observation that most impurities have molecular type spectra. However, the spectra of fullerenes in solution (for example, their absorption spectra in toluene, xylene, hexane, and other conventional solvents) are much more complex than a mere combination of the single-molecule spectrum of a fullerene and the spectrum of the solvent. To sort out these spectra, we œrst recall the general structure of the excitation spectrum of a doped molecular crystal.