Ultrafast carrier relaxation in solid buckminsterfullerene

Abstract

The time dependence of the carrier relaxation in undoped solid C$\sb{60}$ thin films has been studied by monitoring changes in the optical transmission upon photoexcitation using sub picosecond laser pulses at 2 eV. Optical excitation induces an increase in the optical absorption of the sample. The relaxation process is observed to closely follows a Kohlrausch-Williams-Watts decay, exp($\lbrack-{t\over\tau}\rbrack\sp\beta)$ with $\beta$ = 0.42, and $\tau$ = 43 ps. This functional form is typical of carrier relaxation associated with states of localized rather than extended character. The dependence of the relaxation dynamics on temperature and excitation intensity are reported. Since the stretched exponential form of relaxation indicates a large range of relaxation times, an experimental technique for measuring optically induced relaxation processes over nearly seven orders of magnitude in time with subpicosecond resolution has been developed. This technique is applied to the measurement of the Kohlrausch-William-Watts relaxation in C$\sb{60}$ films. The Kohlrausch-William-Watts function is found to fit the observed decay dynamics over the range 0.1 to 5 $\times$ 10$\sp5$ ps. These results are discussed in the context of phenomena producing localization in solids. In order to extend the wavelength range accessible to the pump-probe experiments, we have developed a scheme for broad bandwidth second harmonic generation of ultrashort pulse laser sources. This system will also be discussed.