Near-UV supercontinua generated in photonic crystal fibers for femtosecond spectroscopy

We demonstrate the use of broad super-continua generated in photonic crystal fibers (PCFs) for a novel femtosecond absorption pump-probe experiment that records time- and wavelength resolved data. While such experiments relied up to now on amplified sources for the supercontinuum generation, the non-linearities in the microstructured fibers can be induced with a Ti:sapphire oscillator only. We test the performances of such a compact femtosecond spectrometer for the study of molecules in the liquid phase. The aim is to cover the 400-1100 nm spectral range with a single pulse shorter than 200 fs. A commercial Ti:Sapphire oscillator (KM Labs) cavity, delivering 40 fs pulses, has been extended so as to lower the repetition rate (27 MHz) and to increase the pulse energy. Up to 6 nJ are focussed into 8-mm long pieces of commercial (NL-710, Blazephotonics) and home-made PCFs using a microscope objective. The fiber output spectra are measured from 300-1100 nm with a Peltier-cooled CCD (Spec10, Roper Scientific). Most recently, using the fibers produced at XLIM, we have been able to generate a supercontinuum spectrum extending down to 380 nm. While a full characterization of the temporal properties of the home-built fibers is still in progress, we find a single pulse output for the commercial fibers, with negligible chirp for λ= 600-750 nm. Obviously in these short fiber lengths, soliton fission does not take place. With this system the photo-induced dynamics of malachite green have been studied with a noise-floor of 10-4 relative absorption change.

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