Alfred Kastler won the 1966 Nobel Prize in Physics for the discovery and development of optical methods for studying Hertzian resonances in atoms.
Kastler’s work included optical pumping, a technique in which light changes the distribution of atoms among quantum energy or magnetic-sublevel states. This made it possible to investigate atomic resonance with great precision.
The method became important in magnetic-resonance studies and influenced the development of masers and lasers. Kastler carried out much of his research in France and trained several physicists who later made major contributions to atomic physics.
The question is sometimes confused with laser-related Nobel Prizes. Optical pumping is a method for preparing and probing atoms; it is not itself the same thing as stimulated emission, the physical process underlying lasers.