ABSTRACT
In comparison to absorption and (laser-induced) fluorescence, the Raman scattering process is notoriously weak. Since its inception, strenuous efforts have been made to improve on its signal response. In this context, the basic equation describing the Raman signal is revisited. The discussion leads to the conclusion that the Raman signal can be increased by (1) a larger number of participating molecules; (2) increasing the excitation laser intensity; and (3) augmenting the transition probability. All three approaches are addressed in this chapter, with a few practical solutions described in detail. For increasing the number of participating molecules, the only viable approach is to increase the interaction length. This is elegantly accomplished by waveguide; common implementations based on, e.g., hollow-core metal-lined or photonic crystal fibers (HC-MLF/HC-PCF) are discussed. For an increase in local laser intensity, cavity-enhanced Raman spectroscopy is the method of choice (building up high laser power inside a resonant, external cavity). Finally, the probability for Raman scattering can be increased by tuning the excitation laser close to an electronic transition frequency (resonance Raman scattering), which yields huge increases in transition probability. Representative experimental examples are given for any of the highlighted approaches.
