Professor Hellwarth's research is focused on understanding and developing materials for nonlinear optical devices. His research is particularly concentrated on polymers, suspensions, and photorefractive crystals, physics of optical glasses (especially ac oustic attenuation in glass), nonlinear imaging devices, wavefront reversal, and nonlinear optics for systolic-architecture computers. The ultimate usefulness of nonlinear optics in many applications depends on the maximum nonlinearity possible. Professor Hellwarth has shown that for nominally transparent nonlinear materials, the minimum power required to perform a single logic function can never be below the average thermal energy per excitation divided by the optical-wave period. This value is well below that commonly used in practice. The significance of this result is not only to allow comparison among projected logic elements, bu t also to underscore the surprisingly low power thresholds for nonlinear effects that may be expected to be found at microwave and longer wavelengths. The first experimental evidence has been obtained for existence of such thresholds in specially construc ted materials.