High Sensitivity Frequency Modulation Spectroscopy and the Path to Single Molecule Detection

Advances in optical spectroscopy rely on improved detection techniques that reduce noise to fundamental detection limits. When properly applied, frequency modulation laser spectroscopy (FM spectroscopy or FMS) approaches the quantum limit for detection. Having demonstrated that double modulation FMS could achieve such sensitivity in absorption spectroscopy, researchers raced to the goal of measuring the absorption of an individual molecule. In 1989, Moerner and Kador performed the first optical detection and probing of single dopant molecules in solids, spawning the field of single-molecule spectroscopy. In this Perspective, we trace the development of FMS from its initial demonstration in 1979 to Moerner's remarkable achievement with an emphasis on the double modulation methods needed to achieve high sensitivity measurement of very small absorptions.