Optical spectroscopists strive to further our understanding of matter by studying how it absorbs and emits light. Starting from Isaac Newton’s use of a prism to observe the spectrum of sunlight, spectroscopic techniques remained fundamentally unchanged for centuries. Light was dispersed—for example, by a prism or diffraction grating—and the intensity measured as a function of wavelength. Variation in the spectral intensity is either inherent in the source or due to the light passing through a medium that absorbed at specific wavelengths. The frequency of an optical absorption or emission was eventually understood as the energy difference between electronic or vibrational quantum states.
At first, spectroscopic measurements were all done in the linear regime, in which the measured signals are proportional to the intensity of the incident light. A new era of optical spectroscopy emerged with the advent of the laser, which can produce light that is no longer weak compared...
