ABSTRACT
Optical imaging, both traditional and emerging, plays a signicant and widespread role in medicine and surgery, as noted by the ubiquitous use of microscopes, endoscopes, and video-based imaging systems. e ultimate goal is to rapidly quantify the local concentration of the biomolecule of interest on the microscopic scale, preferably without external labeling. is is intrinsically dicult for common techniques such as confocal microscopy, infrared microscopy, (spontaneous) Raman microspectroscopy, optical coherence tomography (OCT), x-ray uorescence microscopy, and conventional incoherent or coherent multiphoton microscopy including two-photon uorescence, second-order harmonic generation, and third-order harmonic generation. In this respect, coherent anti-Stokes Raman scattering (CARS) has attracted a growing interest since 1999 [1] due to three prominent advantages. First, the multiphoton process of CARS permits high-resolution subcellular imaging in three dimensions, termed as nonlinear sectioning capability. Second, fast imaging can be conducted on unstained (unperturbed) samples with chemically specic vibration (Raman) contrast (Figure 12.1). is label-free molecular imaging is enabled by the coherent process of CARS that constructively amplies the otherwise weak spontaneous signal of Raman microspectroscopy. ird, the optical frequency upconverted CARS signal can be easily separated from the excitation, spontaneous Raman photons, and uorescence background. Note that the coherence process of CARS requires a phase-matching condition to maintain momentum conservation (Figure 12.1), which typically leads to geometrically complicated noncollinear imaging [2]. Fortunately, in the scenario of CARS microscopy with tight focusing conditions, collinear excitation beams at the focus come from a solid angle, which is suciently large to cover the phase-matching condition [1].
