Single-pass high harmonic generation at high repetition rate and photon flux

Abstract

Sources of short wavelength radiation with femtosecond to attosecond pulse durations, such as synchrotrons or free electron lasers, have already made possible numerous, and will facilitate more, seminal studies aimed at understanding atomic and molecular processes on fundamental length and time scales. Table-top sources of coherent extreme ultraviolet to soft x-ray radiation enabled by high harmonic generation (HHG) of ultrashort pulse lasers have also gained significant attention in the last few years due to their enormous potential for addressing a plethora of applications, therefore constituting a complementary source to large-scale facilities (synchrotrons and free electron lasers). Ti:sapphire based laser systems have been the workhorses for HHG for decades, but are limited in repetition rate and average power. On the other hand, it has been widely recognized that fostering applications in fields such as photoelectron spectroscopy and microscopy, coincidence detection, coherent diffractive imaging and frequency metrology requires a high repetition rate and high photon flux HHG sources. In this article we will review recent developments in realizing the demanding requirement of producing a high photon flux and repetition rate at the same time. Particular emphasis will be put on suitable ultrashort pulse and high average power lasers, which directly drive harmonic generation without the need for external enhancement cavities. To this end we describe two complementary schemes that have been successfully employed for high power fiber lasers, i.e. optical parametric chirped pulse amplifiers and nonlinear pulse compression. Moreover, the issue of phase-matching in tight focusing geometries will be discussed and connected to recent experiments. We will highlight the latest results in fiber laser driven high harmonic generation that currently produce the highest photon flux of all existing sources. In addition, we demonstrate the first promising applications and discuss the future direction and challenges of this new type of HHG source.

High Average Power Near‐Infrared Few‐Cycle Lasers

“…However, the conversion efficiencies are typically very low (<10 −6 ). Few‐cycle pulses are beneficial for driving HHG since they allow the process to be driven at higher intensities without exceeding the critical ionization fraction which is crucial for phase‐matching . Thus, few‐cycle pulses allow for the highest cutoff energy (see Figure a) and the highest efficiency with a particular laser wavelength .…”

Phase-retrieval algorithm for the characterization of broadband single attosecond pulses

“…High-order harmonics in the "water window" with photon energy ranging from 280 to 530 eV have been reported in a number of laboratories in recent years [7][8][9][10][11][12][13][14][15]. Such soft X-ray harmonics are needed to excite core-level transitions in materials.…”

“…Various techniques have been developed to generate broadband extreme ultraviolet (XUV) light that has available continuum bandwidth capable of supporting pulse duration of a few tens to hundreds of attoseconds, such as amplitude gating [2], ionization gating [3], polarization gating [5], double optical gating [4], and wavefront rotation method [6]. Similarly, using few-cycle phase-stabilized mid-infrared wavelength lasers, ultrabroadband super-continuum harmonics up to the waterwindow and beyond have been reported [7][8][9][10][11][12][13][14][15]. If these pulses are transform-limited, the resulting durations of the attosecond pulses are expected to range from 30 attoseconds down to even a few attoseconds.…”

X-ray Pump–Probe Investigation of Charge and Dissociation Dynamics in Methyl Iodine Molecule

“…Tabletop optical lasers in near infrared, VUV, and XUV regimes have been widely applied in work regarding the evolution of nuclear wavepackets and electronic wavepackets, typically with a pump-probe scheme [4,5]. However, pump-probe experiments in the X-ray regime with tabletop lasers are still a challenge, due to the low photon output in the X-ray photon range, e.g., from high harmonic generation [6]. With the recent developments in free electron lasers (FELs), short wavelength pump-probe experiments have become practical, due to the high flux of FELs and have been realized with various X-ray split and delay (XRSD) designs [7,8].…”