Optical Spectroscopy for Noninvasive Monitoring of Stem Cell Differentiation

Figure 2

Schematic energy level diagrams for Raman, CARS, and FTIR processes. In Raman scattering one illuminating laser photon is absorbed (at pump frequency 𝜈_𝑃), and another is radiated (at Stokes frequency 𝜈_𝑆)—the difference in frequencies being equal to a vibrational frequency, 𝜈_VIB. In CARS three photons are absorbed—two at frequency 𝜈_𝑃 and one at 𝜈_𝑆—and one is emitted at the anti-Stokes frequency 𝜈_AS. FTIR relies solely on the absorption of infrared radiation at 𝜈_VIB.