Arthur Leonard Schawlow developed methods of laser spectroscopy that resolve features hidden by Doppler broadening. His lecture explains saturation spectroscopy with a strong pump beam and a weaker counterpropagating probe beam. At the common resonance, both beams address the same velocity class of chemical atoms or molecules; modulating the pump reveals a narrow feature in probe transmission. Such measurements distinguish intrinsic spectroscopic structure from the broadening caused by thermal motion.[ref]
Spectroscopy in a New Light by Arthur Leonard Schawlow. 1981 authored lecture, counterpropagating pump/probe arrangement and Doppler-free resonances
Portrait of Arthur Leonard Schawlow
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Arthur Leonard Schawlow (1921-2019) was an American physicist who is best known for his contributions to the field of laser technology. He was awarded the Nobel Prize in Physics in 1981, along with Nicolaas Bloembergen and Kai Siegbahn, for their work in the development of laser spectroscopy, a technique that uses lasers to analyze the properties of materials.