New directions in modulation spectroscopy

Since its inception in 1964, modulation spectroscopy has achieved a high level of maturity as applied to the study of interband critical point transitions in “perfect” crystals. Several papers at this conference describe applications of the physical principles and experimental techniques of modulation spectroscopy in ways that were completely unforeseen eight years ago. In this paper we review and discuss additional new directions not explicitly covered elsewhere in this volume, with emphasis on electric field modulation of optical properties in systems where the initial and/or final states are not Bloch (band) states. These include molecular spectroscopy (determination of excited state dipole moments by electric field modulated luminescence; electroabsorption in charge transfer complexes), impurity and defect spectroscopy (Stark effect of impurities; piezospectroscopy of deep defect levels), and applications in materials science (phase separation in III–V alloy thin films, and surface reactions on IV–VI epitaxial films).