Focused field symmetries for background-free coherent anti-Stokes Raman spectroscopy
Résumé
Spatial symmetries inherent to tight focus optical beams are applied to control the field phase generated at interfaces in nonlinear optical processes. We use these symmetries to obtain a background-free pure Raman spectrum in coherent anti-Stokes Raman scattering spectroscopy from a resonant medium forming a transverse interface with a nonresonant medium. We demonstrate that the nonresonant medium acts as a local oscillator with which the Raman spectrum is measured in heterodyne detection. This method is relevant to remove the nonresonant background in other third-order resonant processes excited with focused beams.