By 1960 while at Harvard, he experimented with microwave spectroscopy. Bloembergen had modified the maser of Charles Townes, and in 1956, Bloembergen developed a crystal maser, which was more powerful than the standard gaseous version.
With the advent of the laser, he participated in the development of the field of laser spectroscopy, which allows precise observations of atomic structure using lasers. Following the development of second-harmonic generation by Peter Franken and others in 1961, Bloembergen studied how a new structure of matter is revealed, when one bombards matter with a focused and high-intensity beam of photons. This he termed the study of nonlinear optics. In reflection to his work in a Dutch newspaper in 1990, Bloembergen said: "We took a standard textbook on optics and for each section we asked ourselves what would happen if the intensity was to become very high. We were almost certain that we were bound to encounter an entirely new type of physics within that domain".
From this theoretical work, Bloembergen found ways to combine two or more laser sources consisting of photons in the visible light frequency range to generate a single laser source with photons of different frequencies in the infrared and ultraviolet ranges, which extends the amount of atomic detail that can be gathered from laser spectroscopy.