In our ultrafast materials laboratory in Giauque Hall on the Berkeley campus we perform cutting-edge spectroscopy experiments focusing on condensed phase quantum dynamics using state-of-the-art laser sources ranging from THz to the soft X-ray range. We employ employ a range of different types of spectroscopy such as transient absorption/reflection spectroscopy and transient X-ray scattering as methods to study chemical and material processes in real-time. In addition to our laboratory experiments, we perform experiments at facilities such as free-electron lasers to study interfacial material dynamics and dynamics in symmetry-broken states using nonlinear X-ray spectroscopies.
With an optical parametric amplification (OPA) system and hollow-core fiber compressor following an 800 nm Ti:Sapphire laser, we generate few-femtosecond, carrier-envelope phase (CEP) stabilized laser pulses throughout the visible and infrared regions in order to subsequently make high-harmonic generated (HHG) attosecond x-ray pulses spanning 20-600 eV. Combinations of such pulses are employed to perform core-level pump-probe experiments with high spatio-temporal resolution and atom-specificity where samples can remain under ultrahigh vacuum and ultracold temperature conditions. In our approach we combine methods of attosecond spectroscopy with coherent diffraction imaging. The use of XUV and X-ray photon energies enables us to study materials with element-specificity and we typically reach a time resolution better than ~2 fs using pump-probe, which enables us to study light-matter interaction in realtime, faster than typically relaxation processes set in. The short wavelength of XUV/X-rays enables studying spatial properties of materials and superlattices with a resolution if better than 30 nm.
We are specifically interested in studying and controlling charge carrier and spin dynamics in novel superlattice structures. In that context we explore different types of superlattices, for example, formed by twisting atomically thin van der waals materials forming so-called moiré lattices or superlattices formed by anisotropies in layered ferroelectrica. These platforms are of high interest because they provide a highly-ordered, tunable, and periodic platform for localized excitons, spins, and charged particles. Such materials are of interest for quantum information, spintronics, and optoelectronics applications, as well as for the study of canonical models of phase transitions, symmetry-breaking, and electron correlations in condensed matter and many-body physics. Together with colleagues in the College of Chemistry and at the Lawrence Berkeley National Laboratory we further explore the possibilities of using these superlattices as scaffold for novel types of 2D magnets and spin-qubits.
We actively collaborate with materials scientists, theorists, and FEL facilities around the world to answer questions like, what is the influence of the superlattice on the band structure and optoelectronic properties? Can we manipulate the spatiotemporal dynamics of quasiparticles in the superlattice structures in real time? What kinds of couplings exist between the spin, orbit, charge, and lattice degrees of freedom? Can we control how long these couplings persist before decoherence sets in?
References on time-resolved X-ray/attosecond spectroscopy and diffraction imaging
 B. Buades, et al., “Attosecond state-resolved carrier motion in quantum materials probed by soft X-ray XANES”, Applied Physics Reviews 8, 011408 (2021) // AIP Feature Article.– doi: https://doi.org/10.1063/5.0020649
 S. K. Cushing, et al., “Differentiating Photoexcited Carrier and Phonon Dynamics in the Δ, L, and Γ Valleys of Si(100) with Transient Extreme Ultraviolet Spectroscopy”, Journal of Physical Chemistry C 123, 3343–3352 (2019)– doi: https://doi.org/10.1021/acs.jpcc.8b10887
 P. M. Kraus, M. Zürch, S. K. Cushing, D. M. Neumark, S. R. Leone, “The Ultrafast X-ray Spectrocopic Revolution in Chemical Dynamics” Nature Reviews Chemistry 2, 82-94 (2018)– doi: https://doi.org/10.1038/s41570-018-0008-8
 T. Helk, M. Zürch , and C. Spielmann, “Towards single shot timeresolved microscopy using short wavelength table-top light sources”, Structural Dynamics 6, 010902 (2019)– doi: https://doi.org/10.1063/1.5082686
 M. Zürch, et al., “Spatial Coherence Limited Coherence Diffraction Imaging using a Molybdenum Soft X-ray Laser Pumped at Moderate Pump Energies”, Nature Scientific Reports 7:5314, 1-10 (2017)– doi: https://doi.org/10.1038/s41598-017-05789-w
 M. Zürch, et al., “Direct and Simultaneous Observation of Ultrafast Electron and Hole Dynamics in Germanium”, Nature Communications 8:15734, 1-11 (2017)– doi: https://doi.org/10.1038/ncomms15734
 M. Zürch, et al., “Carrier Thermalization and Trapping in Silicon-Germanium Alloy Probed by Attosecond XUV Transient Absorption Spectroscopy”, Structural Dynamics 4 (4), 044029 (2017)– doi: https://doi.org/10.1063/1.4985056
 M. Zürch, et al., “Real-time and Sub-wavelength Ultrafast Coherent Diffraction Imaging in the Extreme Ultraviolet”, Nature Scientific Reports 4 (7356), 1-5 (2014)– doi: https://doi.org/10.1038/srep07356
The panel above shows the scheme of solid-state high harmonic generation (sHHG). In sHHG an electron wave packet is accelerated by a slowly oscillating mid-infrared laser field while it is bound to the quantum states permitted in the condensed-phase system allowing the emitted sHHG radiation to use as a proxy for material properties. The propagation control due to polarization of the driving laser allows mapping out symmetry axis in a crystal and provide insights into the electronic structure as shown her for a monolayer MoS2 (arXiv:2112.13032 (2021)).
Solids can be used as a medium to generate harmonics of an intense laser pulse. While gas-phase higher harmonic generation (HHG) is widely used in generating attosecond pulses, recently, solid state higher harmonic generation (sHHG) has gained attention. Compared to HHG, sHHG has richer dynamics due to the periodic structure of the dense medium. Rather than the well-established semi-classical tunnel ionization description that explains HHG (c-d), sHHG process involves tunnel ionization followed by interactions of the charge carriers with the periodic lattice potentials. Therefore, sHHG contains valuable information about the band structure of solids (a-b) and is sensitive to additional material properties such as symmetry.
Uniquely equipped with a CEP stabilized tunable source and synchronized OPA’s, we are aiming to elucidate quantum dynamics and phase transitions in solids using sHHG emission as a proxy. In our experimental endstation we can investigate samples at cryogenic temperatures. A broadband THz pulse can be combined with the ultrafast optical probes in the same experiment. Using these capabilities, we are interested in studying material dynamics such as phase transitions and charge transport in several classes of materials such as semiconductors, atomically-thin materials, heterostructures, and intercalated layered materials.
 R. Hollinger, et al., “The role of free carrier interaction in strong field excitations in semiconductors”, Physical Review B 104, 035203 (2021)- doi: https://doi.org/10.1103/PhysRevB.104.035203
 G. Zograf, et al., “High-harmonic generation from metasurfaces empowered by bound states in the continuum”, ACS Photonics 9, 567 (2022) – https://doi.org/10.1021/acsphotonics.1c01511
 R. Hollinger, et al., “Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO”, Nanomaterials 11, 4 (2021)- doi: https://doi.org/10.3390/nano11010004
 Lun Yue, Richard Hollinger, Can B. Uzundal, Bailey Nebgen, Ziyang Gan, Emad Najafidehaghani, Antony George, Christian Spielmann, Daniil Kartashov, Andrey Turchanin, Diana Y. Qiu, Mette B. Gaarde, Michael Zuerch, “Signatures of multi-band effects in high-harmonic generation in monolayer MoS2”. arXiv:2112.13032 (2021)
Free electron lasers (FELs) generate high intensity, ultrafast and coherent X-Ray pulses that are tunable to the elemental edges of interest. We develop and use nonlinear X-Ray techniques at FELs to investigate ultrafast dynamics in solids. Specifically, X-Ray Second Harmonic Generation (XUV-SHG) has been our main spectroscopic tool. In principle, XUV-SHG is particularly attractive as it unites the core-level specificity of X-Ray spectroscopies with the unique properties of second harmonic generation (SHG). As second-order nonlinear process, the SHG is only permitted in media without inversion symmetry. This makes SHG sensitive specific to surfaces and interfaces in cases when the bulk of the material is version symmetric. In combination with X-rays, that have a high penetration depth, this is especially useful to study the electronic sturcture of buried interfaces as seen from one atomic species of the lattice. In addition, SHG can be used to study symmetry-broken media, where then SHG stems from the bulk. In this case, the atomic specificity of X-rays enable to study how a certain atomic species in a lattice contributes to a symmetry-broken state. The XUV-SHG spectra obtained are spectra of the second-order nonlinear susceptibility, which is ultimately a second-rank tensor, i.e., containing also directional information of the dielectric environment.
In our recent experiments, we have studied bulk non-centrosymmetric crystals using XUV-SHG [1,2]. In these crystals due to ion displacements in the unit cell, emergent order such as ferroelectricity appears which makes them an ideal candidate for XUV-SHG studies. In the case of the polar ferroelectric LiOsO3, we demonstrated that the polar phase establishes as a result of Li ion displacements, while by investigating the polarization of XUV-SHG emitted from a well-studied ferroelectric, LiNbO3, we established that the long-accepted theories of angular anisotropy in optical SHG is applicable to XUV-SHG. Along with these studies, we also contributed to the investigation of tabletop sources that achieve XUV-SHG  and demonstrated the capability of XUV-SHG to investigate challenging buried interfaces .
In future experiments we plan to expand XUV-SHG to conduct time-resolved measurements investigating symmetry changes of materials in real-time. We are also exploring the possibility to conduct soft X-ray SHG on liquid jets to learn more about molecular level interactions at the interface of liquids.
 E. Berger, S. Jamnuch, C. Uzundal, C. Woodahl, H. Padmanabhan, P. Manset, Y. Hirata,I. Matsuda, V. Goplan, Y. Kubota,et al., “Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal”, Nano Letters 21, 6095–6101 (2021)- doi: https://doi.org/10.1021/acs.nanolett.1c01502
 C. B. Uzundal, S. Jamnuch, E. Berger, C. Woodahl, P. Manset, Y. Hirata, T. Sumi, A. Amado, H. Akai, Y. Kubota, et al., “Polarization-Resolved Extreme Ultraviolet Second Harmonic Generation from LiNbO3” Phys. Rev. Lett. 127, 237402 (2021).
 T. Helk, E. Berger, S. Jamnuch, L. Hoffmann, A. Kabacinski, J. Gautier, F. Tissandier, J. P. Goddet, H.-T. Chang, J. Oh, C. D. Pemmaraju, T. A. Pascal, S. Sebban, C. Spielmann, M. Zuerch. “Table-top extreme ultraviolet second harmonic generation”, Science Advances 7, eabe2265 (2021)
 C. P. Schwartz, S. L. Raj, S. Jamnuch, C. J. Hull, P. Miotti, K. Lam, D. Nordlund, C. B.Uzundal, C. D. Pemmaraju, L. Foglia,et al., “Angstrom-resolved Interfacial Structure in Organic-Inorganic Junctions,” Phys. Rev. Lett. 127, 096801 (2021)
 Lars Hoffmann, Sasawat Jamnuch, Craig P. Schwartz, Tobias Helk, Sumana L. Raj, Hikaru Mizuno, Riccardo Mincigrucci, Laura Foglia, Emiliano Principi, Richard J. Saykally, Walter S. Drisdell, Shervin Fatehi, Tod A. Pascal, Michael Zuerch. “Saturable absorption of free-electron laser radiation by graphite near the carbon K-edge”. Arxiv:2112.12585 (2021)