High Harmonic Generation In Gases Research Articles

Experimental and numerical demonstration of driver pulse spectral width and phase dependence in near-single-cycle pulse post-compression generation

For many years, light-matter interaction in the strong-field regime has benefited from continuous improvement of femtosecond lasers, in terms of peak power or repetition rate. One of the most current major challenges is the achievement of high-energy, near single-cycle pulses. Such performances are of primary interest in attosecond science for producing intense isolated bursts of extreme ultraviolet light through high-harmonic generation in gases or solids. We present here a detailed experimental and numerical study on a helium filled hollow-core fiber-based post-compression stage. Our measurements highlight the importance of the width and phase of the input spectrum on the spectral broadening, and on the resulting post-compressed pulse. Near Fourier-transform-limited pulses as short as 3.5 fs, carrying a 2.5 mJ energy centered at 750 nm at 1 kHz repetition rate, and leading to a compression factor greater than seven, are demonstrated. The numerical results are in good agreement with the experimental data. Here, spectral broadening is governed by the Kerr effect and the self-steepening on the trailing edge of the guided pulse.

Spectrally tunable ultrashort monochromatized extreme ultraviolet pulses at 100 kHz

We present the experimental realization of spectrally tunable, ultrashort, quasi-monochromatic extreme ultraviolet (XUV) pulses generated at 100 kHz repetition rate in a user-oriented gas high harmonic generation beamline of the Extreme Light Infrastructure—Attosecond Light Pulse Source facility. Versatile spectral and temporal shaping of the XUV pulses is accomplished with a double-grating, time-delay compensated monochromator accommodating the two composing stages in a novel, asymmetrical geometry. This configuration supports the achievement of high monochromatic XUV flux (2.8 ± 0.9 × 1010 photons/s at 39.7 eV selected with 700 meV full width at half maximum bandwidth) combined with ultrashort pulse duration (4.0 ± 0.2 fs using 12.1 ± 0.6 fs driving pulses) and small spot size (sub-100 µm). Focusability, spectral bandwidth, and overall photon flux of the produced radiation were investigated, covering a wide range of instrumental configurations. Moreover, complete temporal (intensity and phase) characterization of the few-femtosecond monochromatic XUV pulses—a goal that is difficult to achieve by conventional reconstruction techniques—has been realized using a ptychographic algorithm on experimentally recorded XUV-infrared pump–probe traces. The presented results contribute to in situ, time-resolved experiments, accessing direct information on the electronic structure dynamics of novel target materials.

Angular Momentum Conservation for High-Harmonic Generation in Gases

Angular Momentum Conservation for High-Harmonic Generation in Gases

Intense infrared lasers for strong-field science

The advent of chirped-pulse amplification in the 1980s and femtosecond Ti:sapphire lasers in the 1990s enabled transformative advances in intense laser–matter interaction physics. Whereas most of experiments have been conducted in the limited near-infrared range of 0.8–1 μm, theories predict that many physical phenomena such as high harmonic generation in gases favor long laser wavelengths in terms of extending the high-energy cutoff. Significant progress has been made in developing few-cycle, carrier-envelope phase-stabilized, high-peak-power lasers in the 1.6–2 μm range that has laid the foundation for attosecond X ray sources in the water window. Even longer wavelength lasers are becoming available that are suitable to study light filamentation, high harmonic generation, and laser–plasma interaction in the relativistic regime. Long-wavelength lasers are suitable for sub-bandgap strong-field excitation of a wide range of solid materials, including semiconductors. In the strong-field limit, bulk crystals also produce high-order harmonics. In this review, we first introduce several important wavelength scaling laws in strong-field physics, then describe recent breakthroughs in short- (1.4–3 μm), mid- (3–8 μm), and long-wave (8–15 μm) infrared laser technology, and finally provide examples of strong-field applications of these novel lasers. Some of the broadband ultrafast infrared lasers will have profound effects on medicine, environmental protection, and national defense, because their wavelengths cover the water absorption band, the molecular fingerprint region, as well as the atmospheric infrared transparent window.

Lithium fluoride detectors for high spatial resolution imaging of tabletop XUV from high harmonic generation in gases

As high harmonic generation (HHG) sources have proliferated, the need for high resolution, affordable extreme ultraviolet (XUV) detectors has become ubiquitous. We studied lithium fluoride (LiF) crystals, traditionally used for x rays, as a detector for a tabletop XUV source from high harmonic generation in gases. The LiF response curve showed a dynamic range of 10 3 (not saturated), with a minimum threshold fluence of 66 µ J / c m 2 . Imaging tests were performed revealing sub-micrometer spatial resolution. We successfully recorded a scan of the focal plane of a Fresnel zone plate with high dynamic range. With this study, we showed that LiF can be used as a detector for HHG XUV, with a high potential to do near field imaging of complex objects such as the focus of structured light.

Generation of necklace-shaped high harmonics in a two-color vortex field.

We numerically studied gas high-harmonic generation in a two-color vortex laser field using the non-adiabatic Lewenstein model. Macroscopic responses were calculated by numerically solving the three-dimensional propagation equation in cylindrical coordinates. It was confirmed that unique high-harmonic signals with necklace-like shapes exhibit orbital angular momentum (OAM). The azimuthally distributed necklace harmonics exhibit periodic modulation as a function of laser frequency and topological charges of the driving field. Phase investigation showed that the OAM of the necklace harmonics is attributable to the tuning of the relative intensity of the two driving pulses. These findings provide a new dimension for high-harmonic manipulation in the vortex field. The two-color vortex field is the first scheme proposed for manipulating the intensity profile of high harmonics.

High numerical aperture Hartmann wave front sensor for extreme ultraviolet spectral range.

We present a novel, to the best of our knowledge, Hartmann wave front sensor for extreme ultraviolet (EUV) spectral range with a numerical aperture (NA) of 0.15. The sensor has been calibrated using an EUV radiation source based on gas high harmonic generation. The calibration, together with simulation results, shows an accuracy beyond λ/39 root mean square (rms) at λ=32nm. The sensor is suitable for wave front measurement in the 10 nm to 45 nm spectral regime. This compact wave front sensor is high-vacuum compatible and designed for in situ operations, allowing wide applications for up-to-date EUV sources or high-NA EUV optics.

Beam optimization in a 25 TW femtosecond laser system for high harmonic generation

It has been demonstrated in the past that high fluxes of extreme ultraviolet (XUV) light could be obtained by driving high harmonic generation (HHG) with high energy lasers. However, the peak intensity at the focal point of a femtosecond laser with more than 100 mJ can be too high for phase-matched HHG in gases. We propose a method to optimize the spatial profile at off-focus locations of the high energy driving laser to avoid fully ionizing the target atoms. The beam profile before or after the focal point depends on the wavefront quality of the laser beam and the near field intensity distribution. The beam spot diameter reaches three times that at the focus by adding customized wavefront terms. An XUV pulse energy of 5.6 μJ was obtained from HHG when a larger off-focus spot of a 500 mJ Ti: sapphire laser beam was applied in an argon gas cell.

High-flux source of coherent XUV pulses for user applications.

We present experimental results obtained at a user-oriented XUV beamline implemented at the ELI Beamlines facility. The coherent XUV radiation is produced via high harmonic generation in gases in a loose focusing geometry. The beamline is designed to be driven by 1 kHz, 100 mJ, 20 fs pulses centered at a wavelength of 830 nm. Results such as XUV spectra, beam wavefront and pulse energy obtained during the beamline commissioning with a commercial 1 kHz, 5 mJ, 40 fs laser system are presented. A unique XUV spectrometer for source characterization designed to reach a very high sensitivity is described in detail, and we demonstrate a novel technique for single-shot and every-shot XUV pulse energy measurement.

Computation of intensity and polarization state of diffracted fields from reflective gratings in conical geometry.

Conical diffraction geometry has interesting properties for monochromatization of short pulse sources of extreme-UV or soft X-ray radiation, like high harmonic generation in gases. It enables high diffraction efficiency at the cost of a reduced angular dispersion. Computation of grating efficiency is complex when the incidence plane is not perpendicular to the grating lines because S and P polarizations are coupled. We have therefore developed a special code, named COROX, which implements a rigorous coupled-wave analysis algorithm. The code is able to compute the complex diffracted field from any incidence direction and polarization on any kind of grating (thick lamellar and blazed gratings with single or multilayer coating). COROX is developed with C++ and uses an eigen linear algebra library. This paper outlines the method of computation, and the results are given for a 150 l/mm blazed grating in a conical Littrow configuration.

Tailored semiconductors for high-harmonic optoelectronics.

The advent of high-harmonic generation in gases 30 years ago set the foundation for attosecond science and facilitated ultrafast spectroscopy in atoms, molecules, and solids. We explore high-harmonic generation in the solid state by means of nanostructured and ion-implanted semiconductors. We use wavelength-selective microscopic imaging to map enhanced harmonic emission and show that the generation medium and the driving field can be locally tailored in solids by modifying the chemical composition and morphology. This enables the control of high-harmonic technology within precisely engineered solid targets. We demonstrate customized high-harmonic wave fields with wavelengths down to 225 nanometers (ninth-harmonic order of 2-micrometer laser pulses) and present an integrated Fresnel zone plate target in silicon, which leads to diffraction-limited self-focusing of the generated harmonics down to 1-micrometer spot sizes.

Spatial properties of odd and even low order harmonics generated in gas.

High harmonic generation in gases is developing rapidly as a soft X-ray femtosecond light-source for applications. This requires control over all the harmonics characteristics and in particular, spatial properties have to be kept very good. In previous literature, measurements have always included several harmonics contrary to applications, especially spectroscopic applications, which usually require a single harmonic. To fill this gap, we present here for the first time a detailed study of completely isolated harmonics. The contribution of the surrounding harmonics has been totally suppressed using interferential filtering which is available for low harmonic orders. In addition, this allows to clearly identify behaviors of standard odd orders from even orders obtained by frequency-mixing of a fundamental laser and of its second harmonic. Comparisons of the spatial intensity profiles, of the spatial coherence and of the wavefront aberration level of 5ω at 160 nm and 6ω at 135 nm have then been performed. We have established that the fundamental laser beam aberrations can cause the appearance of a non-homogenous donut-shape in the 6ω spatial intensity distribution. This undesirable effect can be easily controlled. We finally conclude that the spatial quality of an even harmonic can be as excellent as in standard generation.

Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources

We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 μW and μJ per harmonic using the respective generation mechanisms.

Application of high intensity THz pulses for gas high harmonic generation

AbstractThe main effects of an intense THz pulse on gas high harmonic generation are studied via trajectory analysis on the single atom level. Spectral and temporal modifications to the generated radiation are highlighted.

A flexible apparatus for attosecond photoelectron spectroscopy of solids and surfaces

We describe an apparatus for attosecond photoelectron spectroscopy of solids and surfaces, which combines the generation of isolated attosecond extreme-ultraviolet (XUV) laser pulses by high harmonic generation in gases with time-resolved photoelectron detection and surface science techniques in an ultrahigh vacuum environment. This versatile setup provides isolated attosecond pulses with photon energies of up to 140 eV and few-cycle near infrared pulses for studying ultrafast electron dynamics in a large variety of surfaces and interfaces. The samples can be prepared and characterized on an atomic scale in a dedicated flexible surface science end station. The extensive possibilities offered by this apparatus are demonstrated by applying attosecond XUV pulses with a central photon energy of ∼125 eV in an attosecond streaking experiment of a xenon multilayer grown on a Re(0001) substrate.

Polarization-resolved pump–probe spectroscopy with high harmonics

High harmonic generation in gases can be used as a probe of the electronic structure of the emitting medium, with attosecond temporal resolution and angström spatial resolution. The prospect of measuring molecular dynamics by pump–probe spectroscopy with such precision is attracting a lot of interest. An important issue in pump–probe spectroscopy lies in the ability to detect small signals: the detected signal can be easily dominated by the contributions from non-excited molecules or from a carrier gas. In this paper, we demonstrate that polarization-resolved pump–probe spectroscopy can be used to overcome this issue. We study high harmonic generation from rotationally excited molecules. We show that by measuring the harmonic field that is generated orthogonally to the driving laser field, the contrast in the detection of alignment revivals in nitrogen can be increased by a factor 4. We use this configuration to measure alignment revivals in an argon–nitrogen mixture, in which the total harmonic signal is dominated by the contributions from argon.