Optimization of pulse self-compression in hollow capillary fibers using decreasing pressure gradients

The improvement of techniques for the generation of near-infrared (NIR) few-cycle pulses is paving the way for new scenarios in time-resolved spectroscopy and the generation of ultrashort extreme-ultraviolet pulses through high-harmonic generation. In this work, we numerically study how to optimize the self-compression of NIR pulses using decreasing pressure gradients in hollow capillary fibers (HCFs). We identify a moderate nonlinear regime in which sub-cycle pulses are obtained with very good temporal quality from an input 30 fs pulse centered at a 800 nm wavelength and coupled as the fundamental mode of an argon-filled HCF fully evacuated at the output end. Surprisingly, we observe that there is a relatively broad region of parameters for which the optimum self-compression takes place, defined by a simple relation between the input pulse energy and the initial gas pressure.