Bright extreme-ultraviolet high-order-harmonic radiation from optimized pulse compression in short hollow waveguides

Abstract

Multimodal nonlinear propagation dominates the evolution of intense laser pulses propagating in high pressure gas-filled capillaries used for high harmonic generation. A fully multimodal nonlinear propagation model is used to predict pulse evolution along such a capillary, and the length and pressure distribution optimised to produce the shortest pulses at the capillary output. This optimisation is shown theoretically to result in self compression of the pulse from ∼53 fs to ∼7 fs, and shown experimentally to increase the flux of high harmonic radiation from the capillary by an order of magnitude over comparable capillary and gas jet designs.PACS numbers: 42.65. Ky, 42.65.Re At optical intensities above > 10 13 W cm −2 the onset of tunnel ionisation allows for the observation of intense high-order harmonic generation (HHG) within atomic [1] and molecular gases [2]. Thanks to their small experimental footprint, high achievable photon energies and high coherence, lab-scale HHG-based sources have an array of potential applications if high photon flux in the extreme ultraviolet (XUV) and soft x-ray wavelength ranges can be generated.High photon flux HHG requires many different criteria to be fulfilled. At relatively low intensities and gas densities, the fraction of the target gas which is ionised is low, and the plasma generated as a result of ionisation has a low density. Thus, propagation of the pump pulse is close to linear, and any effect of depletion of the ground state of the gas atoms can be neglected. Under these conditions, elegant solutions have been proposed for phase matching in gas jets using the Gouy shift[3] and in gas-filled capillaries using modal dispersion [4]. In capillaries, quasi-phasematching via linear propagation based techniques such as variation in capillary geometry [5] and mode-beating [6] have been demonstrated.As laser intensities and gas pressures are increased, both the ionisation fraction and the plasma density can become large. In this case, ground state depletion and nonlinear propagation both act to make efficient generation harder to achieve. Generation in gas jets using very short (<10 fs) pulses has been shown to improve harmonic yield at high intensities [7], by reducing ground state depletion at the peak of the pulse. The effect of nonlinear propagation on phase matching is also reduced, because the leading edge of the pulse sees much lower plasma density. Within a capillary waveguide, the generation of large densities of plasma causes significant nonlinear coupling between capillary modes, which redistributes energy between modes as a function of propagation distance. Thus phase matching based on the excitation of a single mode or combination of modes along the entire capillary will be severely degraded.Nonlinear propagation effects can, however, be useful within the capillary waveguide. At low gas pressures (<10 mbar) within the capillary, self compression of propagating pulses by a factor of ∼2.3 has previously been observed [8] under conditions of high ionisation fra...

High-energy laser-pulse self-compression in short gas-filled fibers

“…The fused-silica fiber design used here was 50 mm in length with a hollow core diameter of 150 µm and was assumed to be homogeneously filled with 100 mbar of argon. These parameters are consistent with those used in our experimental work [10,22].…”

“…4(c)). It has been suggested that by analysing the high harmonic spectrum in the far-field these could be verified experimentally [33], and in fact we have recently demonstrated a similar approach in our experiments [10]. , (e) and (f) highlight these properties as a function of the input pulse energy.…”

“…In such a geometry self-compression of 30 fs pulses with multi-mJ energies has been demonstrated experimentally after just 25 mm of propagation [8]. An advantage of this geometry is that it allows optimized high field experiments to be performed in-situ [9,10]; this property is becoming invaluable as the high field community continues its shift towards the MIR [11][12][13][14] and VIS/UV [15,16] where ultra broadband optics have not been developed.…”

“…The authors also compare these solutions to experiments where 2.2 mJ 30 fs pulses were compressed to 13 fs after a 25 mm fiber filled with ≃ 5 mbar of argon. Recently, Skobelev et al [20] and Butcher et al [10] predict self-compression into the few-cycle regime within gas filled fibers and capillaries. Butcher et al also begin to explore this experimentally by examining the high harmonic output of such pulse compressors.…”

Spectral broadening and temporal compression of ∼100 fs pulses in air-filled hollow core capillary fibers

“…These simulations are based on a multimode generalized nonlinear Schrödinger equation [17] that accounts for mode dispersion, nonlinear Kerr and Raman effects, self-steepening, ionization, plasma refractive index, and nonlinear mode coupling, which has been tested previously for high-power pulse propagation in short capillaries [18,19]. The lowest 15 transverse modes with rotational symmetry were included in the simulations (for details of mode calculations and sample dispersion curves see Sec.…”