Abstract
The bandwidth of ultrafast pulses in the UV is limited by the finite acceptance bandwidth of the nonlinear crystals used for their generation. For fundamental laser pulses it is well established that spectral broadening can be used to overcome intrinsic bandwidth limits. We show that self-phase modulation of UV pulses in bulk materials leads to large spectral broadening and allows for a significant reduction of the pulse duration. We find that for pulse energies in the range of a few μJ, a thin crystal is favorable due to the strong dispersion in the UV and the limitations set by self-focusing. In contrast to spectral broadening in gaseous media, the self-focus has to lie outside the crystal to avoid beam break up. We focus UV pulses into a 1 mm thick CaF2 crystal. For moderately short input pulses, a shortening factor up to 2.4 is achieved: the 120 fs long third harmonic output of a Ti:sapphire amplifier is compressed down to 50 fs FWHM. For a central wavelength of 315 nm, we generate pulses as short as 14.9 fs after compression with an UV pulse shaper. In both cases the resulting beam shape is close to Gaussian and fully usable for spectroscopic experiments. We use the pulses in a collinear 2D-UV experiment and clearly resolve vibronic off-diagonal peaks of the S2 1B2u vibronic progression of pyrene.
Highlights
For nonlinear optics and time resolved spectroscopic studies of atoms, molecules and biological processes there is a rising need for ultrashort light pulses in the UV and deep UV spectral domain.Straightforward frequency upconversion of short visible pulses in non-linear crystals already allows for the generation of sub-20 fs UV pulses [1] and 30 fs pulses down to 189 nm [2]
With the insights gained by the simulations, we experimentally tested the broadening of UV pulses by focusing them into a CaF2 crystal and subsequently compensate the phase of the broadened spectra
First we tuned the chirp of the input pulse and investigated the influence of the peak intensity on the self-phase modulation (SPM) process
Summary
For nonlinear optics and time resolved spectroscopic studies of atoms, molecules and biological processes there is a rising need for ultrashort light pulses in the UV and deep UV spectral domain.Straightforward frequency upconversion of short visible pulses in non-linear crystals already allows for the generation of sub-20 fs UV pulses [1] and 30 fs pulses down to 189 nm [2]. The phase-matching bandwidth can be increased by advanced angular dispersion methods leading to sub-10 fs pulses [3,4]. Other important techniques capable of generating UV pulses are four wave mixing in a gas filled hollow wave guide providing sub-10 fs pulses at 270 nm [5]. Third harmonic generation of the fundamental pulses at around 800 nm provide sub-30 fs pulses in air [6] or even 3-fs pulses in a Neon filled gas cell [7]. These techniques are characterized by a low conversion efficiency and require an elaborate experimental implementation
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