Probing temporal aspects of high-order harmonic pulses via multi-colour, multi-photon ionization processes

High-order harmonics generated through the interaction of atoms and strong laser fields are a versatile, laboratory-scale source of extreme ultraviolet (XUV) radiation on a femtosecond or even attosecond time-scale. In order to be a useful experimental tool, however, this radiation has to be well characterized, both temporally and spectrally. In this paper we discuss how multi-photon, multi-colour ionization processes can be used to completely characterize either individual harmonics or attosecond pulse trains. In particular, we discuss the influence of the intensity and duration of the probe laser, and how these parameters effect the accuracy of the XUV characterization

VUV frequency combs from below-threshold harmonics

Recent demonstrations of high-harmonic generation (HHG) at very high repetition frequencies (~100 MHz) may allow for the revolutionary transfer of frequency combs to the vacuum ultraviolet (VUV). This advance necessitates unifying optical frequency comb technology with strong-field atomic physics. While strong-field studies of HHG have often focused on above-threshold harmonic generation (photon energy above the ionization potential), for VUV frequency combs an understanding of below-threshold harmonic orders and their generation process is crucial. Here we present a new and quantitative study of the harmonics 7-13 generated below and near the ionization threshold in xenon gas. We show multiple generation pathways for these harmonics that are manifested as on-axis interference in the harmonic yield. This discovery provides a new understanding of the strong-field, below-threshold dynamics under the influence of an atomic potential and allows us to quantitatively assess the achievable coherence of a VUV frequency comb generated through below threshold harmonics. We find that under reasonable experimental conditions temporal coherence is maintained. As evidence we present the first explicit VUV frequency comb structure beyond the 3rd harmonic.Comment: 16 pages, 4 figures, 1 tabl

Polarization control of isolated high-harmonic pulses

High-harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, thus far, the shortest isolated attosecond pulses have only been produced with linear polarization, which limits the range of physics that can be explored. Here, we demonstrate robust polarization control of isolated extreme-ultraviolet pulses by exploiting non-collinear high-harmonic generation driven by two counter-rotating few-cycle laser beams. The circularly polarized supercontinuum is produced at a central photon energy of 33 eV with a transform limit of 190 as and a predicted linear chirp of 330 as. By adjusting the ellipticity of the two counter-rotating driving pulses simultaneously, we control the polarization state of isolated extreme-ultraviolet pulses—from circular through elliptical to linear polarization—without sacrificing conversion efficiency. Access to the purely circularly polarized supercontinuum, combined with full helicity and ellipticity control, paves the way towards attosecond metrology of circular dichroism.The experimental work was carried out at National Tsing Hua University, Institute of Photonics Technologies, supported by the Ministry of Science and Technology, Taiwan (grants 105-2112-M-007-030-MY3, 105-2112-M-001-030 and 104-2112-M-007-012-MY3). The concept of isolated circularly polarized attosecond pulses was developed by C.H.-G., D.D.H., M.M.M., C.G.D., H.C.K., A.B. and A.J.-B.. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under Research Executive Agency grant agreement no. 328334. C.H.-G. and L.P. acknowledge support from Junta de Castilla y León (SA046U16) and the Ministerio de Economía y Competitividad (FIS2013-44174-P, FIS2016-75652-P). C.H.-G. acknowledges support from a 2017 Leonardo Grant for Researchers and Cultural Creators (BBVA Foundation). M.M.M. and H.C.K. acknowledge support from the Department of Energy Basic Energy Sciences (award no. DE-FG02-99ER14982) for the concepts and experimental set-up. For part of the theory, A.B., A.J.-B., C.G.D., M.M.M. and H.C.K. acknowledge support from a Multidisciplinary University Research Initiatives grant from the Air Force Office of Scientific Research (award no. FA9550-16-1-0121). A.J.-B. also acknowledges support from the US National Science Foundation (grant no. PHY-1734006). This work utilized the Janus supercomputer, which is supported by the US National Science Foundation (grant no. CNS-0821794) and the University of Colorado, Boulder. This research made use of the high-performance computing resources of the Castilla y León Supercomputing Center (SCAYLE, www.scayle.es), financed by the European Regional Development Fund (ERDF). J.L.E. acknowledges support from the National Science Foundation Graduate Research Fellowship (DGE-1144083). L.R. acknowledges support from the Ministerio de Educación, Cultura y Deporte (FPU16/02591)

Macroscopic studies of short-pulse high-order harmonic generation using the time-dependent Schrodinger equation

We consider high harmonic generation by ultrashort (27-108 fs) laser pulses and calculate the macroscopic response of a collection of atoms to such a short pulse. We show how the harmonic spectrum after propagation through the medium is significantly different from the single-atom spectrum. We use single-atom data calculated by integration of the time-dependent Schrodinger equation and propose a method, based on an adiabatic approximation;:to extract the data necessary to perform a propagation calculation

Generation of attosecond pulses in macroscopic media

We describe theoretically the generation of ultrashort (subfemtosecond) pulses using high-order harmonics of a laser pulse with a time-dependent degree of ellipticity. The single-atom response is calculated by using a low-frequency strong-field approximation. Propagation effects are taken into account using a method going beyond the slowly varying envelope approximation. Propagation modifies significantly the results obtained in the single-atom response and, in certain conditions, makes the generation of one attosecond pulse possible. We discuss prospects for the observation of these ultrashort pulses. [S1050-2947(97)09411-0]

Attosecond transient absorption probing of electronic superpositions of bound states in neon: Detection of quantum beats

Electronic wavepackets composed of multiple bound excited states of atomic neon lying between 19.6 and 21.5 eV are launched using an isolated attosecond pulse. Individual quantum beats of the wavepacket are detected by perturbing the induced polarization of the medium with a time-delayed few-femtosecond near-infrared (NIR) pulse via coupling the individual states to multiple neighboring levels. All of the initially excited states are monitored simultaneously in the attosecond transient absorption spectrum, revealing Lorentzian to Fano lineshape spectral changes as well as quantum beats. The most prominent beating of the several that were observed was in the spin-orbit split 3d absorption features, which has a 40 femtosecond period that corresponds to the spin-orbit splitting of 0.1 eV. The few-level models and multilevel calculations confirm that the observed magnitude of oscillation depends strongly on the spectral bandwidth and tuning of the NIR pulse and on the location of possible coupling states

Yield and temporal characterization of high-order harmonics from intense midinfrared excitation of a cesium vapor

Cesium vapor produced high harmonic radiation in the visible/UV spectral range when interacting with a tightly focused, intense midinfrared source. It was found that the measured yields of harmonic orders 9-17 yielded quantitative agreement with numerical simulations. Using an autocorrelation method, the 5th-9th harmonic orders were generated with sufficient pulse energies for direct temporal measurements. The cause of the spectral broadening was found to be a blueshift connected to a strong time-dependent ionization

Attosecond transient absorption probing of electronic superpositions of bound states in neon: Detection of quantum beats

Electronic wavepackets composed of multiple bound excited states of atomic neon lying between 19.6 and 21.5 eV are launched using an isolated attosecond pulse. Individual quantum beats of the wavepacket are detected by perturbing the induced polarization of the medium with a time-delayed few-femtosecond near-infrared (NIR) pulse via coupling the individual states to multiple neighboring levels. All of the initially excited states are monitored simultaneously in the attosecond transient absorption spectrum, revealing Lorentzian to Fano lineshape spectral changes as well as quantum beats. The most prominent beating of the several that were observed was in the spin-orbit split 3d absorption features, which has a 40 femtosecond period that corresponds to the spin-orbit splitting of 0.1 eV. The few-level models and multilevel calculations confirm that the observed magnitude of oscillation depends strongly on the spectral bandwidth and tuning of the NIR pulse and on the location of possible coupling states

Spatially and spectrally resolved quantum path interference with chirped driving pulses.

We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short (${\alpha }_{{\rm{s}}}$) and long (${\alpha }_{{\rm{l}}}$) trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure