Abstract
High-order harmonic generation (HHG) provides a promising tabletop source of coherent short wavelength radiation. However, the low generation efficiency limits the mean photon flux of HHG sources when driven with low average power, kHz repetition rate lasers. The HHG flux could be increased by employing MHz repetition rate driving lasers. However, the low pulse energy necessitates tight focusing, which limits the effective interaction volume. Generating harmonics in a gas-filled photonic crystal fiber (PCF) mitigates this problem, but for both free focus and PCF targets, the HHG conversion efficiency is optimized at technologically challenging multibar gas pressures. Here, we perform HHG with μJ-level driving pulses in a hollow-core PCF. We observe dramatic 60-fold enhancement of the HHG flux through a time-multiplexed multimodal quasi-phase-matching technique at low gas pressures. We also observe high harmonic photon energies up to 61.6 eV with continuous spectral tunability made possible by controlled ionization-induced blue shifting of the driving laser.
Highlights
Compact, bright sources of coherent extreme ultraviolet (XUV) radiation open new opportunities for applications that require a high average photon flux such as lithography and coherent imaging
To set more controllable experimental framework conditions for the test of MM-QPM in our negativecurvature PCF (NC-PCF), we developed an innovative concept to induce mode beating between pairs of waveguide modes: We use intermodal delay dispersion to temporally separate the modes excited by a pair of femtosecond pulses; this allows selected, isolated modes to be temporally overlapped at the exit of the NC-PCF
Our model shows that a uniformly filled NC-PCF would enable intermodal delay-controlled QPM (IDC-QPM) at significantly lower input pressures while at the same time resulting in an additional order of magnitude enhancement (i.e., a 600-fold increase compared to the single pulse driver case) of the high harmonic generation (HHG) yield
Summary
Bright sources of coherent extreme ultraviolet (XUV) radiation open new opportunities for applications that require a high average photon flux such as lithography and coherent imaging. An alternative approach is quasi-phase-matching (QPM) in which destructive interference of harmonic radiation emitted at different locations is inhibited by suppressing HHG periodically along the propagation axis This can be achieved, for instance, through longitudinal intensity modulation of the driving laser with a period LQPM 2πj∕jΔkj, where j is the order of QPM. To set more controllable experimental framework conditions for the test of MM-QPM in our NC-PCF, we developed an innovative concept to induce mode beating between pairs of waveguide modes: We use intermodal delay dispersion to temporally separate the modes excited by a pair of femtosecond pulses; this allows selected, isolated modes to be temporally overlapped at the exit of the NC-PCF The concept of this intermodal delay-controlled QPM (IDC-QPM) is illustrated in Fig. 3(a) together with the results of a numerical unidirectional pulse propagation simulation [24] in Fig. 3(b), limited to the five lowest order LP0n modes of the NC-PCF used in the experiment. In the overlap region the carrier waves of both pulses interfere and cause amplitude modulation (AM) of the electric field, shown in the inset of Fig. 3(b)
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