Frequency-modulation (FM) spectroscopy permits high-resolution, high-sensitivity, easily calibrated absorption measurements of atomic and molecular species and narrow spectral features in solids. This paper reviews some important developments in laser FM spectroscopy, from ...

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.

We demonstrate high-quality FM spectra with nanosecond laser pulses. Transform-limited pulses with FM sidebands are produced by pulsed amplification of a phase-modulated cw laser. The pulses can be shifted to the UV by nonlinear mixing. We report both initial experiments on I(2) and what is to our knowledge the first observation of a far-UV transition by FM spectroscopy, at 214.5 nm. Major advantages of this method include (1) spectral resolution of the order of 0.001 cm(-1), (2) better-defined optical phase, and (3) a much smaller and more easily detected modulation frequency, ~500 MHz. The absorption sensitivity is ~10(-4), and considerable further improvement is expected.