Experimental studies of excitons at high densities

Abstract

We have given a discussion of some of the types of experiments that can be conducted on HDE systems. We have discussed, in detail, reflectivity of intense light beams and optical gain measurements on highly excited CuCl. We find that the reflectivity spectra are modified when a high intensity probe is used, and that this modification can be phenomenologically explained in terms of an exciton damping parameter which varies linearly with incident light intensity. We are not able, at this time, to give a physical explanation of the phenomenon. From gain measurements in CuCl we find that the observed kinetics of the optical gain are not consistent with the predictions based on a simple 3-level model of an excitonic molecule. The origin of this discrepancy is not clear, but it points up the need for a definitive investigation of the existence of excitonic molecules in CuCl.

Such an investigation would require new experimental approaches to studying HDE systems. Careful reflectivity, absorption and ellipsiometric measurements on such systems could provide important information. Further infrared absorption, or possibly Raman scattering experiments could obtain information about internal structure of the molecular state (if such a state exists in CuCl).

The author is indebted to R. E. Nahory and R. F. Leheny for helpful discussions and for their collaboration in some of the work discussed here. W. F. Brinkman has made many useful comments and suggestions. The author also wishes to thank Prof. C. Benoît à la Guillaume for his helpful comments on the 3-level model discussed here.