One contribution of 15 to a theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays' .High harmonic generation (HHG) of an intense laser pulse is a highly nonlinear optical phenomenon that provides the only proven source of tabletop attosecond pulses, and it is the key technology in attosecond science. Recent developments in highintensity infrared lasers have extended HHG beyond its traditional domain of the XUV spectral range (10-150 eV) into the soft X-ray regime (150 eV to 3 keV), allowing the compactness, stability and subfemtosecond duration of HHG to be combined with the atomic site specificity and electronic/structural sensitivity of X-ray spectroscopy. HHG in the soft X-ray spectral region has significant differences from HHG in the XUV, which necessitate new approaches to generating and characterizing attosecond pulses. Here, we examine the challenges and opportunities of soft X-ray HHG, and we use simulations to examine the optimal generating conditions for the development of high-flux, attosecond-duration pulses in the soft X-ray spectral range.This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.
We experimentally study the interaction between intense infrared few-cycle laser pulses and an ultrathin (∼2 µm) flat liquid sheet of isopropanol running in vacuum. We observe a rapid decline in transmission above a critical peak intensity of 50 TW/cm2 of the initially transparent liquid sheet, and the emission of a plume of material. We find both events are due to the creation of a surface plasma and are similar to processes observed in dielectric solids. After calculating the electron density for different laser peak intensities, we find an electron scattering rate of 0.3 fs-1 in liquid isopropanol to be consistent with our data. We study the dynamics of the plasma plume to find the expansion velocity of the plume front.
The observation of non-perturbative harmonic emission in solids from ultrashort laser pulses [1] sparked a wave of studies [2,3] as a probe of charge carrier dynamics in solids under strong fields and a route to extreme ultraviolet (XUV) attosecond photonic devices [4]. High harmonic generation (HHG) in liquids [5,6] is far less explored, despite their relevance to biological media, and the mechanism is hotly debated. Using few-cycle pulses below the breakdown threshold, we demonstrate HHG in alcohol with data showing carrierenvelope-phase-dependent XUV spectra extending to 50 eV from isopropanol. We study the mechanism of the harmonic emission through its dependence on the driving field and find it to be consistent with a strong-field recombination mechanism. This maps emitted photon energy to the electron trajectories. We explore the role of the liquid environment in scattering the trajectories and find evidence that information on electron scattering from neighbouring molecules is encoded in the harmonic spectra. Using simulations we exploit this to estimate the scattering cross section and we confirm that the cross-section in liquid isopropanol is significantly reduced compared to vapour. Our findings suggest an in situ measurement strategy for retrieving accurate values of scattering cross sections in liquids, and also a pathway to liquid-based attosecond XUV devices.Bright coherent high harmonic generation is a universal phenomenon observed when a strong laser field interacts with a phase of matter, e.g. plasmas, gases, solids and liquids. In gases this method is widely used for producing attosecond XUV pulses [7,8] for studying the structure and dynamics of matter at electronic timescales [9][10][11]. In a real-space recombination picture, a strong laser field distorts the atomic/molecular potential or band-structure of the material [12]. Electron wavepackets, formed by tunnelling through the distorted potentials, are accelerated and returned by the oscillating field to the hole state. The accrued kinetic energy is converted into photons ranging from XUV to soft Xray energies. In atomic gases the highest achievable photon energy, hω max , can be estimated with the classical cutoff law: hω max = I pot + 3.17U ponder where I pot and U ponder are the ionisation potential and pondermotive energy respectively. This manifests in a cutoff energy which is linear with intensity. In solids a momentum-space picture is used to describe strong field driven electron dynamics with a recombination component occurring between electronic bands (inter-band), and a non-linear intra-band component. Both processes give cutoff energies which are linear with the electric field amplitude.In contrast the mechanism behind HHG in the liquid phase is less well understood. This is partly due to the challenges of detecting XUV from thick and absorbing cylindrical jets or spherical droplets. Nonperturbative harmonics [5,6] generated in the liquid phase alone were made possible with new technology for optically flat and thin sheets of liq...
We present a Ho:YLF Chirped-Pulse Amplification laser for pumping a longwave infrared Optical Parametric Chirped Pulse Amplifier at a 1 kHz repetition rate. By utilizing a Ti:Sapphire laser as a frontend, 5-μJ seed pulses at 2051 nm laser pulse are generated in a Dual-Chirp Optical Parametric Amplifier, which are amplified to 28 mJ pulses with a pulse duration of 5.6 ps. The scheme offers a potential driver for two-color (800 nm and 8 μm) high harmonic generation with an increased keV x-ray photon flux.
We present a multi-pass Chirped Pulse Amplification system using Ho:YLF that can produce 32 mJ pules at 2.05 pm and 1 kHz repetition rate. The seed is generated from DC-OPA driven by a Ti:Sapphire laser.
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