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.
Professor Hellwarth made the first observation of the hyperfine resonance absorption of a radioactive isotope. He was co-developer of the widely used "precessing-vector" model of two-level atoms. He witnessed the making of the first laser at the Hughes R esearch Laboratories in 1960, and became an early and continuing contributor to the new optics spawned by this development. He invented the technique of "Q-switching," by which the peak optical power output of many common lasers may be made to rival the peak electrical power output of Hoover Dam. He was co-discoverer of a new kind of laser action, to which he gave its theoretical basis and the name, "stimulated scattering."
At USC, Professor Hellwarth developed a new, and now widely employed, method for reversing the lightwave pattern of an optical image, a process often called "optical beam phase conjugation." He also invented widely used laser-spectroscopic techniques, wh ich he named Raman-induced Kerr effect and Raman-induced phase-conjugation.