Multimodal hollow-core fibers: a new route for few-cycle visible pulse generation

Ultrashort high-energy visible pulses have enabled unprecedented opportunities in temporally resolving ultrafast dynamics in physics, chemistry, and biology. Until now, high-energy sub-10 fs visible pulses have been mostly obtained through complex non-collinear optical parametric amplification setups, followed by some pulse post-compression technique. Here, we present an alternative approach, which relies on considering the typically-undesired multimode nature of large-core hollow-core capillary fibers (HCFs) as an essential asset. In our experiments, 1 mJ 175-fs-long pulses centered at 1035 nm, emitted by an Yb:KGW (Pharos – Light Conversion) laser, were coupled into a 3-m-long HCF (few-cycle Inc.) filled with Argon gas. At a selected pressure of 2.9 bar, a fast energy transfer from the laser broadened via self-phase modulation towards the arising visible light was observed starting at around 0.8 mJ laser pulse energy. At the maximum pulse energy of 0.94 mJ, a continuous spectrum of visible light between 800 nm and 400 nm was measured, with an overall energy of approximately 30 µJ. To understand this process, we implemented 3D carrier-resolved pulse propagation simulations based on the guided mode theory. The simulations predict the direct formation of a pulse of about 5 fs right at the exit of the fiber, considering the visible spectrum in the range 525 - 750 nm. We found that the presence of higher-order modes is crucial to generate such visible pulses and that the Kerr effect is the dominant nonlinearity enabling the modal energy transfer. Experimentally, we characterized the visible pulses by means of a transient-grating frequency-resolved optical gating setup (TG-FROG). At 0.94 mJ and 2.9 bar, a visible pulse duration of 4.6 fs was measured. We also implemented a cross-correlation TG-XFROG, using the separately-compressed laser light and the visible pulses, which demonstrates the possibility of directly implementing high-energy NIR-pump VIS-probe measurements on a sub-10-fs scale.