An analogy would be the striking of a tuning fork: When it is struck gently, you hear a pure tone; but when it is struck hard, you hear the harmonics. Similarly, when matter is struck with an intense enough laser beam, you get a light harmonic, which is a nonlinear optical effect.
His major contribution to the development of the laser was the creation of a three-level pumping system, which made it much easier to pump atoms from their ground state to a higher energy state, allowing the device to operate continuously.
The pumping scheme was originally designed for the laser’s predecessor, the maser, which amplified microwaves instead of light. It offered a much more practical and easier way of making lasers.
“He was one of the major intellectual forces in the explosion of science and applications related to the laser,” said John Armstrong, a retired IBM research director who worked as a postdoctoral student in Dr. Bloembergen’s lab in the 1960s. “There are a thousand applications of lasers, not only in surgery but in all forms of manufacturing and all forms of diagnostics for material properties.”
Before his major advancements in nonlinear optics and laser development, Dr. Bloembergen found early success as a pioneer in nuclear magnetic resonance spectroscopy, a method of detecting the faint magnetism of the atomic nucleus, which is used to study molecular structures and measure magnetic fields.
His doctoral thesis, “Nuclear Magnetic Relaxation,” explored what controlled the shape of spectral lines, which can occur when atoms in their excited state emit radiation. It was used to produce a paper published in 1948 with his Harvard colleagues Edward M. Purcell and Robert V. Pound that became one of the most cited works of physics and was turned into a widely read book in the field.