Spatially confined self-compressed blue-shifted few-cycle pulses from gas-filled capillary fibers.

We demonstrate blue-shifted few-cycle pulse generation in a short Krypton-filled capillary fiber operated near the self-focusing threshold. Using a 25 cm capillary, the propagation remains sufficiently short to limit the buildup of accumulated nonlinear effects and to access the early self-focusing-driven dynamics. Experimentally, the onset of weak self-focusing is accompanied by stable sideband generation, spatial confinement of the blue-shifted frequencies, and progressive temporal self-compression as the spectrum broadens. Multimode simulations support a short-distance self-action picture in which energy is redistributed from the fundamental mode into low-order radially symmetric modes, followed by rapid, blue-shifted spectral broadening. These results demonstrate that near-threshold self-focusing in weakly confined gas-filled capillary fibers can be used as a controllable route to energetic few-cycle visible/ultraviolet pulses and as a platform for studying multimode nonlinear dynamics.