Fig. 2
Illustration of single-photon and two-photon fluorescence, Rayleigh and Raman scattering and CARS. Fluorescence (a and b): Absorption of light at a frequency ωi excites the molecule to a higher electronic energy level. The molecule can revert to the electronic ground state by non-radiative transitions and the emission of fluorescence at frequency ωf, which is lower than ωi (red-shifted). In two-photon fluorescence (b) the molecule is simultaneously excited by two photons of approximately half the energy necessary for one-photon excitation (ωi/2). The resulting two-photon fluorescence signal is blue-shifted compared to the incident light. Light scattering (c): Light at a frequency ωi excites a molecule to a virtual state. The molecule can revert to the ground state by elastic light scattering (Rayleigh scattering) or inelastic scattering with an energy loss at frequency ωs (Stokes Raman scattering) or energy gain at frequency ωas (anti-Stokes Raman scattering). CARS (d): The CARS process is driven by three photons from at least two different laser sources. A pump beam at frequency ωp excites a molecule from the ground state to a virtual state, which subsequently is depopulated by a Stokes beam at frequency ωs. The last photon from the probe beam ωpr excites the molecule to a higher virtual state. In our setup the pump and probe photon are provided by the same laser at ωp. The resulting signal at frequency ωas is blue-shifted compared to the incident laser light and – if the conditions are met – coherently amplified [31]