Probing resonant nonlinearities in organic materials using photonic-crystal fiber frequency converters

Specially designed photonic-crystal fiber frequency converters of ultrashort laser pulses are shown to be ideally suited for probing resonant nonlinearities in organic materials. Photonic-crystal fibers used in our experiments provide an efficient frequency upshifting of femtosecond pulses of 1.25-μm Cr: forsterite laser radiation, generating broadband radiation within the 570-700-nm wavelength range. This radiation is then employed to probe the quadratic nonlinearity of a poled azopolymer film through noncollinear sum-frequency generation.