Agnieszka Jaron-Becker

Bright coherent ultrahigh harmonics in the keV x-ray regime from mid-infrared femtosecond lasers.

From Long to Short When you play a string instrument, you raise the frequency, or pitch, of the note by shortening the vibrating portion of the string: Drop the length in half, and you hear a harmonic at double the frequency. It is possible to do essentially the same thing with light waves by using selective excitation and relaxation processes of the electrons in crystals or high-pressure gases through which the beam of light is directed to produce light harmonics. Over the past decade, researchers have been optimizing the conversion of red light to the far edge of the ultraviolet, which corresponds to tens of harmonics. Popmintchev et al. (p. 1287) now show that mid-infrared light can undergo a process in high-pressure gas to generate ultrahigh harmonics up to orders greater than 5000 in the x-ray regime. An electron excitation process in a high-pressure gas converts infrared light into a well-confined beam of x-rays. High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo–electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

Bright circularly polarized soft x-ray high harmonics for x-ray magnetic circular dichroism

Significance The new ability to generate circularly polarized coherent (laser-like) beams of short wavelength high harmonics in a tabletop-scale setup is attracting intense interest worldwide. Although predicted in 1995, this capability was demonstrated experimentally only in 2014. However, all work to date (both theory and experiment) studied circularly polarized harmonics only in the extreme UV (EUV) region of the spectrum at wavelengths >18 nm. In this new work done in a broad international collaboration, we demonstrate the first soft X-ray high harmonics with circular polarization to wavelengths λ < 8 nm and the first tabletop soft X-ray magnetic circular dichroism (XMCD) measurements, and also uncover new X-ray light science that will inspire many more studies of circular high-harmonic generation (HHG). We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 µm, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N4,5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

Significance Attosecond pulses driven by femtosecond lasers make it possible to capture the fastest electron dynamics in molecules and materials. To date, attosecond pulses driven by widely available 800-nm lasers were limited to the extreme UV region of the spectrum, which restricted the range of materials, liquid, and molecular systems that could be explored because of the limited penetrating power. Our recent work showed that longer-wavelength midinfrared driving lasers at wavelengths from 1 to 4 µm are optimal for generating shorter-wavelength, bright, soft X-ray beams. Here we show that longer-pulse-duration midinfrared lasers are also optimal for generating shorter-pulse-duration, attosecond, soft X-rays. This is an unexpected and beautiful convergence of physics: bright, soft X-ray high harmonics naturally emerge as isolated attosecond bursts. High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10−18 s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

Short wavelengths birth shorter ones The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev et al. used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. Science, this issue p. 1225 Ultraviolet pulses show unexpected efficiency in generating the higher-frequency emission underlying attosecond spectroscopy. High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching—the constructive addition of x-ray waves from a large number of atoms—favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth–limited pulse trains of ~100 attoseconds.

Tomographic reconstruction of circularly polarized high-harmonic fields: 3D attosecond metrology

Circularly polarized attosecond pulse trains in the EUV region were reconstructed using 3D attosecond metrology. Bright, circularly polarized, extreme ultraviolet (EUV) and soft x-ray high-harmonic beams can now be produced using counter-rotating circularly polarized driving laser fields. Although the resulting circularly polarized harmonics consist of relatively simple pairs of peaks in the spectral domain, in the time domain, the field is predicted to emerge as a complex series of rotating linearly polarized bursts, varying rapidly in amplitude, frequency, and polarization. We extend attosecond metrology techniques to circularly polarized light by simultaneously irradiating a copper surface with circularly polarized high-harmonic and linearly polarized infrared laser fields. The resulting temporal modulation of the photoelectron spectra carries essential phase information about the EUV field. Utilizing the polarization selectivity of the solid surface and by rotating the circularly polarized EUV field in space, we fully retrieve the amplitude and phase of the circularly polarized harmonics, allowing us to reconstruct one of the most complex coherent light fields produced to date.

Visualizing electron rearrangement in space and time during the transition from a molecule to atoms

Imaging and controlling reactions in molecules and materials at the level of electrons is a grand challenge in science, relevant to our understanding of charge transfer processes in chemistry, physics, and biology, as well as material dynamics. Direct access to the dynamic electron density as electrons are shared or transferred between atoms in a chemical bond would greatly improve our understanding of molecular bonding and structure. Using reaction microscope techniques, we show that we can capture how the entire valence shell electron density in a molecule rearranges, from molecular-like to atomic-like, as a bond breaks. An intense ultrashort laser pulse is used to ionize a bromine molecule at different times during dissociation, and we measure the total ionization signal and the angular distribution of the ionization yield. Using this technique, we can observe density changes over a surprisingly long time and distance, allowing us to see that the electrons do not localize onto the individual Br atoms until the fragments are far apart (∼5.5 Å), in a region where the potential energy curves for the dissociation are nearly degenerate. Our observations agree well with calculations of the strong-field ionization rates of the bromine molecule.

Dependence of strong-field photoelectron angular distributions on molecular orientation

We have analysed angular distributions of the photoelectron yields arising from strong-field ionization of diatomic and polyatomic linear molecules using a leading-order intense-field S-matrix theory. For molecules with active π electrons the distribution is found to strongly depend on the degree of molecular alignment, showing a nodal minimum along the laser polarization direction as a characteristic signature.

Wavelength Dependence of the Suppressed Ionization of Molecules in Strong Laser Fields

We study ionization of molecules by an intense laser field over a broad wavelength regime, ranging from 0.8 to 1.5 μm experimentally and from 0.6 to 10 μm theoretically. A reaction microscope is combined with an optical parametric amplifier to achieve ionization yields in the near-infrared wavelength regime. Calculations are done using the strong-field S-matrix theory and agreement is found between experiment and theory, showing that ionization of many molecules is suppressed compared to the ionization of atoms with identical ionization potentials at near-infrared wavelengths at around 0.8 μm, but not at longest wavelengths (10 μm). This is due to interference effects in the electron emission that are effective at low photoelectron energies but tend to average out at higher energies. We observe the transition between suppression and nonsuppression of molecular ionization in the near-infrared wavelength regime (1-5 μm).

Group velocity matching in high-order harmonic generation driven by mid-infrared lasers

Author(s): Hernandez-Garcia, C; Popmintchev, T; Murnane, MM; Kapteyn, HC; Plaja, L; Becker, A; Jaron-Becker, A | Abstract:

Numerical simulation of time delays in light-induced ionization

(Received 23 November 2012; published 27 March 2013)We apply a fundamental definition of time delay, as the difference between the time a particle spends withina finite region of a potential and the time a free particle spends in the same region, to determine results forphotoionization of an electron by an extreme ultraviolet laser field using numerical simulations on a grid. Ournumerical results are in good agreement with those of the Wigner-Smith time delay, obtained as the derivative ofthe phase shift of the scattering wave packet with respect to its energy, for the short-range Yukawa potential. Inthe case of the Coulomb potential we obtain time delays for any finite region, while—as expected—the results donot converge as the size of the region increases towards infinity. The impact of an ultrashort near-infrared probepulse on the time delay introduced here is analyzed for both the Yukawa and the Coulomb potential and is foundto be small for intensities below 10