1087example, we have not determined yet whether in the process of observing phonon-assisted laser operation macroscopic occupation of a phonon mode occurs and whether we are operating a phonon oscillator. High-pressure measurements, among others, may help resolve these questions. In previous high-pressure measurements on QWHs we (N.H.) with H. Drickamer and our students have been able to probe, because of easily resolved confined-particle exciton states, the "inside" of an energy band and not just the band edges.21 High-Q heat sinking of a QWH and photopumping present an opportunity to observe confined-particle transitions (recombination radiation) and phonon sidebands (not just bulk-(21) Kirchoefer, S. W.; Holonyak, Jr., N.; Hess, K.; Gulino, D. A.; Drickamer, H. G.; Coleman, J. J.; Dapkus, P. D. Appl. Phys. Lett 1982, 40, 821.crystal band-edge processes) and see how they "tune" with pressure. These are experiments worth pursuing. We wish many more vigorous years for Harry Drickamer and the opportunity to do more high-pressure studies with him, including high-pressure versions of the experiments described here.
Acknowledgment.We are grateful to John Bardeen for helpful discussions (N.H.) on macroscopic phonon occupation in photopumped QWHs. For technical assistance we thank B. L. Marshall, R. T. Gladin, and B. L. Payne.