Highly efficient coherent anti-Stokes Raman scattering (CARS)-microscopy

Coherent Anti-Stokes Raman Scattering (CARS)-microscopy has in recent years developed as a promising microscopical technique for label-free microscopy of living cells. We propose a new concept, spectral focusing, for highly efficient coherent anti-Stokes Raman scattering (CARS) microscopy. It allows optimal use of the excitation energy of ferntosecond laser pulses in terms of generated CARS signal against a low background. This is accomplished by introducing a linear chirp in the excitation pulses. The temporal delaying of the excitation pulses can be used to record vibrational spectra of a sample. Despite the inherently broad spectral width of the excitation pulses, the technique enables resolution of spectral features 60 times narrower than the bandwidth of the probe light. First applications of this technique are exemplified with CARS of micron sized crystallites of sodium nitroprusside, a commonly used hypotensive agent.