Interface-specific χ(4) coherent Raman spectroscopy in the frequency domain

We demonstrate interface-specific fourth-order (chi((4))) coherent Raman spectroscopy in the frequency domain for the first time. Because the chi((4)) Raman spectroscopy uses only visible (vis) or near-IR light, it is expected to be a potential alternative to the widely utilized IR-vis sum frequency generation spectroscopy that cannot be applied to interfaces buried in thick IR absorbers such as water. We present the vibrational vertical bar chi((4))vertical bar(2) spectrum of rhodamine 800 at the air/water interface in a wide spectral range 100-3600 cm(-1). Comparison of the vertical bar chi((4))vertical bar(2) spectrum with the vertical bar chi((3))vertical bar(2) spectrum leads us to conclude that the present chi((4)) spectroscopy successfully probes the interface distinguished clearly from the bulk.