Professor Klein has long used Raman spectroscopy and other optical techniques to study the vibrational and electronic excitations in semiconductors and semiconductor superlattices. He has used the same techniques to study vibrational, electronic, and magnetic excitations in metals, especially superconductors. His group was the first to measure the superconducting gap using Raman spectroscopy, and the resulting experimental and theoretical work from the group has laid the foundation for most of the Raman work on high-temperature superconductors. He received the Frank Isakson Prize of the American Physical Society and was a Sloan Foundation fellow and a University of Illinois Scholar. He is a member of the National Academy of Sciences and a fellow of the American Physical Society, the American Association for the Advancement of Science, and the American Academy of Arts and Sciences. His primary scientific activity consists of refereeing papers for several journals.

Professor Klein died peacefully on Thursday, September 15, 2022.