Origin of giant difference between fluorescence, resonance, and nonresonance Raman scattering enhancement by surface plasmons

Fluorescence, resonance, and off-resonance Raman spectroscopy are all precise and versatile techniques for indentifying small quantities of chemical and biological substances. One way to improve the sensitivity and specificity of these measurement techniques is to use enhancement of optical fields in the vicinity of metal nanoparticles. The degree of enhancement, however, is drastically different as Raman enhancement of 10 orders of magnitude or more has been consistently measured in experiment, while the enhancement of the seemingly similar process of fluorescence is typically far more modest. While resonance Raman scattering has the advantages of higher sensitivity and specificity when compared with the ordinary, nonresonant Raman process, its plasmon enhancement is far less spectacular. In fact, both fluorescence and resonance Raman measurements are subject to quenching when the molecule is placed too close to the metal surface. Such an effect, however, is completely absent from the normal nonresonant Raman process. In this work, we present an analytical model that reveals the physics behind the strikingly different orders of magnitude in enhancement that have been observed, provide a fundamental explanation for the quenching effect observed in fluorescence and resonance Raman but not in normal Raman, establish limits for attainable enhancement, and outline the path to optimization of all three processes.