Session S58: Ultrafast Spectroscopies and Coherent Phenomena in the X-ray Domain I.

Theory and implementation of nonlinear X-ray spectroscopies: application to time-resolved diffraction, X-ray chirality and transient grating experiments

Thanks to the progresses of X-ray light sources, ultrafast non-linear X-ray experiments are now feasible. We discuss how such spectroscopies can be theoretically described in terms of the minimal coupling Hamiltonian, allowing a unified description of diffraction-detected and resonant techniques. Coherent X-ray sources can access new material quantities such as transition charge densities thanks to their atomic resolution. We present how X-ray experiments can make use of their element selectivity to probe matter chirality with additional structural information. Finally, we present hybrid visible pump - X-ray probe transient grating measurements conducted on Bi4Ge3O12 (BGO). This 3rd order technique allows to probe excited carrier dynamics and propagation at unprecedented time and length scale.   

Quantum control of high-energy harmonic generation by pulse shaping

High harmonic generation (HHG) is the most extreme high-order nonlinear optical process, which can be selectively enhanced at lower photon energies around ~40eV by using temporally shaped laser pulses [1]. Here we demonstrate the ability for enhancing the HHG flux and spectrum at higher photon energies around ~100eV, relevant for imaging and magnetic spectroscopies. By changing the spectral phase of the 800nm driving laser pulses using a genetic algorithm, we can enhance the total HHG flux, or selectively enhance individual HHG orders, or spectrally shift the HHG peaks. These control capabilities can be explained by intra-atomic phase matching on attosecond timescales [2]. Here the phase of the driving laser is optimized so that HHG bursts from each half cycle of the laser pulse interfere coherently, leading to enhancement or suppression in HHG spectrum. This demonstrates that we can extend these quantum control capabilities to higher photon energies, to design and tailor HHG sources for applications in ultrafast spectroscopy, resonant magnetic scattering, and hyperspectral imaging.
[1] Bartels et al., Nature 406, 164 (2000)
[2] Christov et al., Phys. Rev. Lett. 86, 5458 (2001)   

Attosecond soft-X-ray spectroscopy in the gas and liquid phases

Attosecond spectroscopy has the potential to address fundamental questions in chemical sciences. A promising approach is offered by the element- and site-sensitivity of X-ray spectroscopy. We have recently demonstrated the potential of table-top X-ray absorption spectroscopy with a water-window high-harmonic source, observing the temporal evolution of unoccupied molecular orbitals and molecular shape resonances during chemical reactions [1]. Compressing the mid-infrared driving pulses to less than 2 optical cycles, we have demonstrated the extension of this light source to fully cover the oxygen K-edge [2]. Using the same technique, we have also demonstrated the generation of isolated attosecond pulses, which have established a new record of the shortest light pulses ever measured (43 attoseconds) [3].
Since the vast majority of chemical processes takes place in the liquid phase, the extension of attosecond spectroscopy to liquids is desirable. I will discuss the first observation of extreme-ultraviolet high-harmomic generation from liquids, achieved through the application of ultrathin (0.6-2 μm) flat microjets [4]. I will also present the extension of attosecond time-resolved spectroscopy from molecules [5] to liquids [6]. The time delays between photoemission from gaseous and liquid water range from 50-70 attoseconds and are shown to mainly originate from the solvation of water molecules, with liquid-phase electron scattering playing a minor role. These developments set the stage for attosecond time-resolved studies of molecular systems of chemical complexity.

1. Y. Pertot et al., Science 355, 264 (2017)
2. C. Schmidt et al., Opt. Exp. 26, 11834 (2018)
3. T. Gaumnitz et al., Opt. Exp. 25, 27506 (2017)
4. T. T. Luu et al., Nature Communications 9, 3723 (2018)
5. M. Huppert et al., Phys. Rev. Lett. 117, 093001 (2016)
6. I. Jordan et al.,submitted (2019)   

Probing Photo-Physical Processes in Solid-state Organic Semiconductors Using Optical Pump - X-ray Probe Spectroscopy

Fundamental understanding of photo-induced charge generation and charge transfer in organic semiconductors is of crucial importance for understanding the functionality of organic optoelectronics. Transient X-ray absorption spectroscopy (Tr-XAS) offers the opportunity for tracking photo-physical processes at the atomic and molecular scale. Here, using Tr-XAS combined with transient absorption spectroscopy and transient microwave conductivity, we investigate the local structure and time evolution of photoinduced charge-separated species in thin films of molecularly dispersed octabutoxy-Zinc(II) phthalocyanine (OButx-ZnPc) in a matrix of the well-known conjugated polymer poly(3-hexylthiophene) (P3HT). We probed a transient signal associated with OButx-ZnPc charged-species in the host polymer (P3HT). Our findings reveal that the XANES spectra of OButx-ZnPc and the transient signal are largely affected by the degree of order of the host material, which is tuned by the regioregularity of P3HT side chains (an amorphous regiorandom P3HT vs a semicrystalline regioregular P3HT.   

Enhancement of X-ray image brightness through transient ionic resonances

Most of our high-resolution imaging methods compromise between temporal or spatial resolutions akin to a pinhole camera. This still limits our capabilities to observe fast processes at the nanoscale, especially as the required brightness often damages the sample. Examples of such processes include chemical and catalytic reactions, nucleation dynamics and growth of nanoparticles, as well as other fragile/intermediate states of matter. One idea to overcome this obstacle is to use Free Electron Lasers (FELs), such as the LCLS at SLAC Nat. Lab., which are capable of producing very bright bursts of coherent X-rays within a few femtoseconds. X-ray FELs help to visualize transient processes in “frozen” time steps via single shot coherent X-ray diffractive imaging (CDI). Currently, the resolution of CDI images is close to tens of nanometers, which is limited by the brightness of the images [1-3]. Our recent study at LCLS suggests that transient ionic resonances (TIR) above an absorption edge can enhance diffraction efficiency before significant structural damage can occur. This is surprising as TIRs are usually regarded as a signature of increased X-ray absorption/damage and thus, detrimental to image quality [4-6]. We recorded a large data set of single exposure diffraction patterns with different FEL pulse durations from Xe nanoparticles by scanning the FEL energy in the vicinity of the 3d absorption edge. TIRs seem to increase the scattering efficiency above the edge even for sub-fs pulses. Our experimental results and a theoretical simulation suggest, that TIRs may provide a pathway to increase the quality of CDI with soft and hard X-ray FELs.

[1] Neutze, R., et al. Nature 406, 752–757 (2000)
[2] Aquila, A., et al. Struct. Dyn. 2.4 (2015): 041701.
[3] Gorkhover, T, et al. Nat. Phot. 12.3 (2018): 150.
[4] Kanter, E. P., et al. Phys. Rev. Lett. 107.23 (2011): 233001.
[5] Rudek, B, et al. Nat. Phot. 6.12 (2012): 858.
[6] Bostedt, Ch, et al. Phys. Rev. Lett. 108.9 (2012): 093401.   

Ultrafast X-ray Absorption and Time-Resolved Resonant X-ray Scattering in a Mott Insulator

The time-evolution of collective excitations encodes information on photoinduced transient states in correlated systems. The frozen charge degrees of freedom in a Mott insulator can be activated by photodoping, which provides information about the interplay between various intertwined instabilities. Here, we simulate both ultrafast x-ray absorption and time-resolved resonant inelastic x-ray scattering (tr-RIXS) for a Hubbard model under the influence of a strong, transient drive. While absorption primarily provides information on transient photodoping, tr-RIXS allows us to study the evolution of bimagnon, Mott-gap, doublon, and single-particle in-gap excitations, as well as anti-Stokes relaxation following an ultrafast excitation of the system. This work provides a theoretical foundation for existing and future tr-RIXS experiments.   

Ultrafast Dynamics and Imaging at the Nanoscale using Tabletop Coherent X-ray Sources

Laser-like beams at very short wavelengths (1-50nm) can be routinely generated using high harmonic up-conversion (HHG) of tabletop femtosecond lasers.[1,2] This new quantum light source is both advancing rapidly in fundamental capabilities, and is providing a powerful new tool to understand nanoscale material properties and function. This talk will discuss new capabilities for controlling the polarization, spin and orbital angular momentum of HHG beams by sculpting the driving laser beams, [3,4] and will discuss applications in nanoscale magnetic and thermal dynamics using the short wavelength and element specificity of the EUV light-matter interaction. Highlights include the discovery of a new regime of collective nanoscale heat transport,[5] and work highlighting our current incomplete understanding of dynamic mechanisms in magnetic materials.[6]
References:
[1] A. Rundquist et al., Science 280, 1412 (1998).
[2] T. Popmintchev et al., Science 336, 1287 (2012).
[3] L. Rego et al., Science 364, eaaw9486 (2019).
[4] K. Dorney et al., Nature Photonics 13, 123 (2019).
[5] KM Hoogeboom-Pot et al, PNAS 112(16), 4846-4851 (2015).
[6] P Tengdin, et al, Science Advances 4(3), 8 (2018).