Session D05: Strong Field Processes and Dynamics in Extended Media

Controlling High-order Harmonic Generation in Solids

The observation of high-order harmonic generation from bulk ZnO crystals in 2011 opened a new playground for strong-field physics. Since this first observation, experiments and calculations have shown that the structure and morphology of solids, as well as the properties of the driving laser field, play a critical role in determining the features of the high-order harmonic spectrum. In this talk, I will present recent results of high-order harmonic generation from different solid-state media, which demonstrate the important roles played by crystalline order and symmetry in addition to the laser pulse parameters.   

Transient electric-field-driven dynamics in condensed matter: from Terahertz to Petahertz

In the 1980s, femtosecond ultrafast lasers enabled optical generation of electric fields at terahertz frequencies. With the recent progress in few-optical-cycle femtosecond and attosecond pulse generation with full electric field control, the frequency regime can be extended into the petahertz. The electron motion under the influence of such a high-frequency electric field ultimately determines the material limit for high-speed device performance. We have started to explore materials in a regime where the quiver energy (or ponderomotive energy Up ) of the electrons in such an oscillating electrical field becomes comparable to the photon energy of the driving laser. The system transitions from a more classical (field driven) regime to a more quantum- mechanical (photon driven) regime and we explored these regimes in diamond [Science 353, 916, 2016] and GaAs [Nature Physics, in press]. We also explored how long it takes for an excited electron in a metal to “feel” its effective mass [Optica 4, 1492, 2017]. The long-term goal is to explore such electric-field-driven dynamics in strongly correlated materials - such as high-T$_{c}$ superconductors for example - where we have even less physical understanding today.   

High-harmonic generation in gases and solids

Attosecond and high-harmonic pulses are generated by electrons that are extracted from a quantum system by an intense light pulse and travel through the continuum under the influence of the electric field of the light. Portions of each electron wave packet are forced to re-collide with its parent ion after the field reverses direction. Upon re-collision, the electron and ion can recombine, emitting soft X-ray radiation that can be in the form of attosecond pulses. This highly nonlinear process occurs in atoms, molecules and solids and offers unique measurement opportunities – for measuring the attosecond pulse itself; the orbital(s) from which it emerged; and the band structure of material in which the wave packets moved.   

Capturing the Fastest Charge and Spin Dynamics in Nanosystems using Tabletop High Harmonic Beams

High harmonic generation (HHG) is a unique quantum light source with fundamentally new capabilities – producing fully spatially and temporally coherent beams with linear or circular polarization throughout the extreme ultraviolet (EUV) and soft X-ray region, all on a tabletop. This talk will introduce and review recent developments in HHG sources, as well as exciting advances in spectroscopy of materials. In recent work we showed that HHG spectroscopies can uncover several new excited-states of quantum materials that traditional spectroscopies are simply blind to.[1-3] We showed that the electron-spin system in a laser-heated ferromagnet can be driven into a previously unknown super-excited state, where the magnetic phase transition is launched on timescales 10 times faster than previously realized, within a fleeting 20 fs. We also measured the shortest lifetime of any state to date, at 212±30 attoseconds, corresponding to an excited state in the band structure of a material. More recently, using a new technique called attosecond-ARPES (angle resolved photoemission) we measured the fastest electron dynamics intrinsic to materials, making it possible to distinguish sub-femtosecond electron-electron scattering and screening for the first time.\\ \\ 1. Tengdin et al., “Critical Behavior within 20fs Drives the Out-of-Equilibrium Laser-induced Magnetic Phase Transition in Nickel,” Science Advances 4, 9744 (2018).\\ 2. Chen et al., “Distinguishing Attosecond Electron-Electron Scattering and Screening in Transition Metals,” PNAS 114, E5300(2017).\\ 3. Tao et al., “Influence of the Attosecond Final-state Lifetime on Photoemission from a Transition Metal”, Science 353, 62 (2016).