Figure 1.

Jablonski diagram showing the interaction of multiple infrared photons with the electronic and vibrational energy levels of a molecule. (a) In two-photon excitation fluorescence (2PEF), the molecule absorbs two infrared photons that promote it to the excited electronic state. After relaxation to a lower vibrational level, the molecule emits a lower energy (red-shifted) photon. (b) In second harmonic generation (SHG), two infrared photons are instantaneously upconverted to a single photon of twice the energy. (c) In third harmonic generation (THG), three infrared photons are instantaneously upconverted to a single photon of thrice the energy. (d) In Coherent anti-Stokes Raman Scattering (CARS), two photons with energies hν _p and hν _s coherently excite the vibrational level with energy hΩ = hν _p − hν _s. An additional photon, hν _p, interacts with the vibrationally excited molecule emitting a photon with energy given by the original incident photon energy plus the vibrational energy, hν _CARS = hν _p + hΩ, leaving the molecule in its original ground state. (Note that photon energy is given by Planck's constant, h, multiplied by the frequency of the photon ν.)