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

Abstract

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.

Pulse self-compression down to the sub-cycle regime in hollow capillary fibers with decreasing pressure gradients

“…2(a) reached a sub-cycle full width at half maximum (FWHM) duration of 1.1 fs, corresponding to a peak power of 10.9 GW. Similar results were obtained at other energies and gas pressures lying in the optimal region [7].…”

“…Furthermore, we have obtained a simple relation between the input pressure and the pulse energy based on the B-integral which allows to place the optimal region for self-compression in the 2D parameter space [7]. One of the output pulses along this curve is shown in Fig.…”

“…In this work, we explore further advantages of the negative pressure gradient technique, showing that it can be applied to the self-compression of standard near-infrared (NIR) pulses in the fundamental mode of typical HCFs to generate sub-cycle pulses with shorter durations, higher peak powers and a better temporal structure when compared to those obtained in the equivalent constant pressure situations [7], defined by matching the accumulated nonlinear phase shift, also known as B-integral, along the propagation [8]. * e-mail: marinafergal@usal.es…”

Tools for numerical modelling of nonlinear propagation in hollow capillary fibres and their application

“…To select the right parameters for the self-compression effect, we have to take into account that the linear dispersion experimented by the laser pulse inside the HCF depends on the gas dispersion and the HCF contributions. Moreover, the nonlinear response depends not only on the gas properties and pressure configuration [24], but also on the mode confinement. For instance, since both linear and nonlinear propagation properties depend on the spatial profile of the beam inside the capillary (see section 2.3 for an explanation of the HCF spatial modes), the use of high order hybrid modes [25][26][27] or structured beams with different number of lobes [28,29] have been proposed to obtain self-compression in HCFs.…”

Robust Isolated Attosecond Pulse Generation with Self-Compressed Subcycle Drivers from Hollow Capillary Fibers

“…These simple fibers are routinely used for ultrashort pulse compression and allow for significant energy scaling and nonlinearity and dispersion tuning by modifying the pressure of the filling gas . In particular, if the latter is chosen so that an input multicycle pulse propagates in the HCF with anomalous dispersion, the simultaneous nonlinear spectral broadening by self-phase modulation (SPM) and phase compensation arising from the negative group-velocity dispersion (GVD) can lead to soliton self-compression well down into the subcycle regime. , Recent studies have demonstrated that this extreme pulse compression can be further enhanced by pumping the fiber with a decreasing pressure gradient and that broadly similar high-quality subcycle IR fields can be generated in different HCF scenarios . In addition, the use of decreasing pressure could be of great interest for HHG experiments, as it allows for the direct delivery to vacuum of the self-compressed pulses free of distortions from transmission optics. , Altogether, the combination of HHG beamlines with HCFs delivering intense self-compressed IR transients opens a very promising scenario to develop compact and versatile scientific tools for the generation of high-frequency IAPs, but theoretical investigations and design guidelines are still missing to make it a feasible technique.…”