Quasi-phase-matched high-harmonic generation in gas-filled hollow-core photonic crystal fibre

Thesis

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 multi-bar gas pressures. In this thesis, HHG is performed with µJ-level driving pulses in hollow-core PCFs. Dramatic 160-fold enhancement of the HHG flux through a novel, time-multiplexed multi-modal quasi-phasematching technique is observed at low gas-pressures. Non-phase-matched generation of 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 is presented and fully phase-matched HHG in an argon-filled PCF is explored.

Synthesized, multi-colour ultrashort pulses of light open the door to a new class of interesting experiments in atomic, molecular and optical science. Characterization of these pulses is not possible using established pulse characterization techniques, if the spectrum of the synthesized pulse exhibits extended intensity nulls, since the relative spectral phase of different spectral regions cannot be determined. In this thesis, a spectral-gap-immune pulse characterization technique is demonstrated. Two mutually coherent ultrafast pulses are derived from a Ti:Sapphire laser. An extended spectral gap is introduced into the spectrum of one pulse using an amplitude-only pulse shaper. The pulse is then spectrally sheared and subsequently interfered with the other uncharacterised pulse. By using the MICE technique, the spectral intensities and phases of the two fields are retrieved from a series of interferograms. The experimental results presented in this thesis show that SPICE is capable of reconstructing the spectral phase across an extended spectral gap enabling temporal characterization of multi-colour light pulses.

Authors: Wiegandt, AFCL

• Alternative title: High-harmonic generation in gas-filled hollow-core photonic crystal fibres
• DOI: 10.5287/ora-qmvo8yax2
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: HHG; Photonic-Crystal-Fibre
• Subjects: Attoscience; Optics; Physics; Lasers