Emily Sistrunk

Imaging ultrafast molecular dynamics with laser-induced electron diffraction

Establishing the structure of molecules and solids has always had an essential role in physics, chemistry and biology. The methods of choice are X-ray and electron diffraction, which are routinely used to determine atomic positions with sub-ångström spatial resolution. Although both methods are currently limited to probing dynamics on timescales longer than a picosecond, the recent development of femtosecond sources of X-ray pulses and electron beams suggests that they might soon be capable of taking ultrafast snapshots of biological molecules and condensed-phase systems undergoing structural changes. The past decade has also witnessed the emergence of an alternative imaging approach based on laser-ionized bursts of coherent electron wave packets that self-interrogate the parent molecular structure. Here we show that this phenomenon can indeed be exploited for laser-induced electron diffraction (LIED), to image molecular structures with sub-ångström precision and exposure times of a few femtoseconds. We apply the method to oxygen and nitrogen molecules, which on strong-field ionization at three mid-infrared wavelengths (1.7, 2.0 and 2.3 μm) emit photoelectrons with a momentum distribution from which we extract diffraction patterns. The long wavelength is essential for achieving atomic-scale spatial resolution, and the wavelength variation is equivalent to taking snapshots at different times. We show that the method has the sensitivity to measure a 0.1 Å displacement in the oxygen bond length occurring in a time interval of ∼5 fs, which establishes LIED as a promising approach for the imaging of gas-phase molecules with unprecedented spatio-temporal resolution.

Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 μm from an optical filament

We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 microJ 55 fs pulses at 2 microm produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 microm carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 microJ pulse energy. The self-locked phase offset of the 2 microm difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths (lambda>0.8 microm).

Strong-field and attosecond physics in solids

We review the status of strong-field and attosecond processes in bulk transparent solids near the Keldysh tunneling limit. For high enough fields and low-frequency excitations, the optical and electronic properties of dielectrics can be transiently and reversibly modified within the applied pulse. In Ghimire et al (2011 Phys. Rev. Lett. 107 167407) non-parabolic band effects were seen in photon-assisted tunneling experiments in ZnO crystals in a strong mid-infrared field. Using the same ZnO crystals, Ghimire et al (2011 Nat. Phys. 7 138–41) reported the first observation of non-pertubative high harmonics, extending well above the bandgap into the vacuum ultraviolet. Recent experiments by Schubert et al (2014 Nat. Photonics 8 119–23) showed a carrier envelope phase dependence in the harmonic spectrum in strong-field 30 THz driven GaSe crystals which is the most direct evidence yet of the role of sub-cycle electron dynamics in solid-state harmonic generation. The harmonic generation mechanism is different from the gas phase owing to the high density and periodicity of the crystal. For example, this results in a linear dependence of the high-energy cutoff with the applied field in contrast to the quadratic dependence in the gas phase. Sub-100 attosecond pulses could become possible if the harmonic spectrum can be extended into the extreme ultraviolet (XUV). Here we report harmonics generated in bulk MgO crystals, extending to ∼26 eV when driven by ∼35 fs, 800 nm pulses focused to a ∼1VA −1 peak field. The fundamental strong-field and attosecond response also leads to Wannier–Stark localization and reversible semimetallization as seen in the sub-optical cycle behavior of XUV absorption and photocurrent experiments on fused silica by Schiffrin et al (2013 Nature 493 70–4) and Schultze et al (2013 Nature 493 75–8). These studies are advancing our understanding of fundamental strong-field and attosecond physics in solids with potential applications for compact coherent short-wavelength sources and ultra-high speed optoelectronics.

Strong field physics with long wavelength lasers

The generation of short, intense, mid-infrared laser pulses allows for the exploration of atom–laser interactions deep in the tunnelling regime as well as providing the ability to explore scaled interactions. In this paper we present recent experimental and theoretical results for this largely unexplored parameter space.

Mid-infrared strong field ionization angular distributions

We present a study of the photoelectrons energy distribution from ionization of Argon by a linearly polarized, intense, mid-infrared laser field with special attention to the recently discovered Low Energy Structure (LES) [1] whose origin is not yet fully understood. In this paper we will go deeper in the analysis of the LES by studying its angular distribution and examine its behavior in circularly polarized light.

An investigation of harmonic generation in liquid media with a mid-infrared laser

We present a harmonic generation experiment using liquid H(2)O and D(2)O interrogated by a mid-infrared, 3.66 mum, laser at a maximum intensity of 8x10(13) W/cm(2). The unique aspects of the experiment include the long wavelength and short (9 cycle-110 fs) pulse duration of the laser as well as the near-resonant excitation of the H(2)O and D(2)O vibrational modes. We observe up to the 13th harmonic order in H(2)O and intensity scaling is consistent with a direct perturbative process up to the 9th harmonic order. Phase matching and resonant absorption are unable to account for the observed differences in harmonic yields between samples.

Broadband extreme ultraviolet probing of transient gratings in vanadium dioxide.

Nonlinear spectroscopy in the extreme ultraviolet (EUV) and soft x-ray spectral range offers the opportunity for element selective probing of ultrafast dynamics using core-valence transitions (Mukamel et al., Acc. Chem. Res. 42, 553 (2009)). We demonstrate a step on this path showing core-valence sensitivity in transient grating spectroscopy with EUV probing. We study the optically induced insulator-to-metal transition (IMT) of a VO(2) film with EUV diffraction from the optically excited sample. The VO(2) exhibits a change in the 3p-3d resonance of V accompanied by an acoustic response. Due to the broadband probing we are able to separate the two features.

Inelastic scattering of broadband electron wave packets driven by an intense midinfrared laser field.

Intense, 100 fs laser pulses at 3.2 and 3.6 μm are used to generate, by multiphoton ionization, broadband wave packets with up to 400 eV of kinetic energy and charge states up to Xe(+6). The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distribution for (e, ne) electron impact ionization. The analysis also suggests a contribution from a 4d core excitation, or giant resonance, in xenon.

A Beamline for Time-Resolved Extreme Ultraviolet and Soft X-Ray Spectroscopy

High harmonic generation is a convenient way to obtain extreme ultraviolet light from table-top laser systems and the experimental tools to exploit this simple and powerful light source for time-resolved spectroscopy are being developed by several groups. For these applications, brightness and stability of the high harmonic generation is a key feature. This article focuses on practical aspects in the generation of extreme ultraviolet pulses with ultrafast commercial lasers, namely generation parameters and online monitoring as well as analysis of generation yield and stability.

Scaling of High-Order Harmonic Generation in the Long Wavelength Limit of a Strong Laser Field

The development of intense, ultrashort, table-top lasers operating in the mid-infrared spectral region, offers many new avenues for strong-field physics. Atoms submitted to such radiation allow photoelectrons to acquire huge quiver energies well over an order of magnitude larger than the binding energy of the neutral. Consequently, many interesting phenomena arise. First, wavelength offers a convenient experimental knob to tune the ionization regime by controlling the Keldysh parameter. Second, high harmonic generation depends directly on the quiver energy and can, therefore, be pushed to unprecedented limits. Third, wavelength controls the spectral phase of harmonics, and hence the possibility to improve the generation of pulses in the attosecond regime. The use of long wavelength lasers is critical to studying high-order harmonic generation in condensed phase systems, because they facilitate harmonic generation within the transmission window of the material and increase the damage threshold. We review some of the recent discoveries in long wavelength driven high-order harmonic generation in the case of isolated atoms, bulk crystals, and liquid.