Abstract

Recent advances in the optical detection and spectroscopy of single impurity centers in crystals has provided the possibility of obtaining information about defect-solid interactions on a truly local level. For the system composed of pentacene impurity molecules in the crystal p-terphenyl, single molecules have been studied using both absorption¹ and fluorescence excitation² techniques at liquid helium temperatures. The superior signal-to-noise of the latter technique has led to direct observations of the lifetime-limited Lorentzian homogeneous profile of a single pentacene impurity³ as well as the surprising observation of spontaneous spectral diffusion⁴. Spectral diffusion, or changes in the resonance frequency of an impurity molecule with time as a result of low-energy excitations (two-level system transitions (TLS)),⁵ is generally expected in amorphous hosts. Indeed, the presence of TLSs in amorphous solids is intimately connected with nonphotochemical mechanisms for persistent spectral hole-burning (PSHB) in amorphous materials^6-8. Using perylene impurity molecules in poly(ethylene), we have observed the optical spectra of single molecules in a polymeric host for the first time. At 1.5K, individual perylene molecules show the expected spectral diffusion; moreover, we observe light-induced changes in resonance frequency, i.e., persistent spectral hole-burning, which allows one to envision optical storage on the single-molecule level.

© 1991 Optical Society of America