Photonic-crystal fiber sources for nonlinear spectroscopy

Photonic-crystal fibers allow the creation of novel compact and convenient light sources for nonlinear spectroscopy. In experiments discussed in this paper, photonic-crystal fibers provide efficient nonlinear-optical transformations of femtosecond Cr:forsterite laser pulses, delivering linearly chirped frequency-shifted broadband light pulses with central wavelengths ranging from 400 to 900 nm. These pulses were cross-correlated with the femtosecond second-harmonic output of the Cr:forsterite laser in toluene solution, used as a test object, in boxcars geometry to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene molecules (XFROG CARS). The blue-shifted chirped-pulse output of a photonic-crystal fiber with a spectrum stretching from 530 to 680 nm is shown to be ideally suited for time-resolved nonlinear absorption spectroscopy of exciton bands in molecular aggregates excited by femtosecond laser pulses.