Education & Certifications

  • Ph.D, University of Alberta, Photonics and Plasmas (2014)
  • Bachelor of Science, South China University of Technology (2007)

All Publications

  • Twist-Angle-Dependent Ultrafast Charge Transfer in MoS2-Graphene van der Waals Heterostructures. Nano letters Luo, D., Tang, J., Shen, X., Ji, F., Yang, J., Weathersby, S., Kozina, M. E., Chen, Z., Xiao, J., Ye, Y., Cao, T., Zhang, G., Wang, X., Lindenberg, A. M. 2021


    Vertically stacked transition metal dichalcogenide-graphene heterostructures provide a platform for novel optoelectronic applications with high photoresponse speeds. Photoinduced nonequilibrium carrier and lattice dynamics in such heterostructures underlie these applications but have not been understood. In particular, the dependence of these photoresponses on the twist angle, a key tuning parameter, remains elusive. Here, using ultrafast electron diffraction, we report the simultaneous visualization of charge transfer and electron-phonon coupling in MoS2-graphene heterostructures with different stacking configurations. We find that the charge transfer timescale from MoS2 to graphene varies strongly with twist angle, becoming faster for smaller twist angles, and show that the relaxation timescale is significantly shorter in a heterostructure as compared to a monolayer. These findings illustrate that twist angle constitutes an additional tuning knob for interlayer charge transfer in heterobilayers and deepen our understanding of fundamental photophysical processes in heterostructures, of importance for future applications in optoelectronics and light harvesting.

    View details for DOI 10.1021/acs.nanolett.1c02356

    View details for PubMedID 34529439

  • Observation of a highly conductive warm dense state of water with ultrafast pump-probe free-electron-laser measurements MATTER AND RADIATION AT EXTREMES Chen, Z., Na, X., Curry, C. B., Liang, S., French, M., Descamps, A., DePonte, D. P., Koralek, J. D., Kim, J. B., Lebovitz, S., Nakatsutsumi, M., Ofori-Okai, B. K., Redmer, R., Roedel, C., Schorner, M., Skruszewicz, S., Sperling, P., Toleikis, S., Mo, M. Z., Glenzer, S. H. 2021; 6 (5)

    View details for DOI 10.1063/5.0043726

    View details for Web of Science ID 000681018600001

  • Electron Kinetics Induced by Ultrafast Photoexcitation of Warm Dense Matter in a 30-nm-Thick Foil PHYSICAL REVIEW LETTERS Chen, Z., Tsui, Y. Y., Mo, M. Z., Fedosejevs, R., Ozaki, T., Recoules, V., Sterne, P. A., Ng, A. 2021; 127 (9): 097403


    We report on the study of electron kinetics induced by intense femtosecond (fs) laser excitation of electrons in the 5d band of Au. Changes in the electron system are observed from the temporal evolution of ac conductivity and conduction electron density. The results reveal an increase of electron thermalization time with excitation energy density, contrary to the Fermi-liquid behavior of the decrease of thermalization time associated with the heating of conduction electrons. This is attributed to the severe mitigation of photoexcitation by Auger decay. The study also uncovers the shortening of 5d hole lifetime with the increase of photoexcitation rates. These unique findings provide valuable insights for understanding electron kinetics under extreme nonequilibrium conditions.

    View details for DOI 10.1103/PhysRevLett.127.097403

    View details for Web of Science ID 000692200100021

    View details for PubMedID 34506197

  • Direct observation of ultrafast hydrogen bond strengthening in liquid water. Nature Yang, J., Dettori, R., Nunes, J. P., List, N. H., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Deponte, D. P., Heinz, T. F., Kozina, M. E., Ledbetter, K., Lin, M., Lindenberg, A. M., Mo, M., Nilsson, A., Shen, X., Wolf, T. J., Donadio, D., Gaffney, K. J., Martinez, T. J., Wang, X. 2021; 596 (7873): 531-535


    Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2-7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04A on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch.

    View details for DOI 10.1038/s41586-021-03793-9

    View details for PubMedID 34433948

  • Ultrafast multi-cycle terahertz measurements of the electrical conductivity in strongly excited solids. Nature communications Chen, Z., Curry, C. B., Zhang, R., Treffert, F., Stojanovic, N., Toleikis, S., Pan, R., Gauthier, M., Zapolnova, E., Seipp, L. E., Weinmann, A., Mo, M. Z., Kim, J. B., Witte, B. B., Bajt, S., Usenko, S., Soufli, R., Pardini, T., Hau-Riege, S., Burcklen, C., Schein, J., Redmer, R., Tsui, Y. Y., Ofori-Okai, B. K., Glenzer, S. H. 2021; 12 (1): 1638


    Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.

    View details for DOI 10.1038/s41467-021-21756-6

    View details for PubMedID 33712576

  • Structure retrieval in liquid-phase electron scattering. Physical chemistry chemical physics : PCCP Yang, J., Nunes, J. P., Ledbetter, K., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Crissman, C., Deponte, D. P., Glenzer, S. H., Lin, M., Mo, M., Rankine, C. D., Shen, X., Wolf, T. J., Wang, X. 2020


    Electron scattering on liquid samples has been enabled recently by the development of ultrathin liquid sheet technologies. The data treatment of liquid-phase electron scattering has been mostly reliant on methodologies developed for gas electron diffraction, in which theoretical inputs and empirical fittings are often needed to account for the atomic form factor and remove the inelastic scattering background. In this work, we present an alternative data treatment method that is able to retrieve the radial distribution of all the charged particle pairs without the need of either theoretical inputs or empirical fittings. The merits of this new method are illustrated through the retrieval of real-space molecular structure from experimental electron scattering patterns of liquid water, carbon tetrachloride, chloroform, and dichloromethane.

    View details for DOI 10.1039/d0cp06045c

    View details for PubMedID 33367391

  • Visualization of ultrafast melting initiated from radiation-driven defects in solids SCIENCE ADVANCES Mo, M., Murphy, S., Chen, Z., Fossati, P., Li, R., Wang, Y., Wang, X., Glenzer, S. 2019; 5 (5): eaaw0392


    Materials exposed to extreme radiation environments such as fusion reactors or deep spaces accumulate substantial defect populations that alter their properties and subsequently the melting behavior. The quantitative characterization requires visualization with femtosecond temporal resolution on the atomic-scale length through measurements of the pair correlation function. Here, we demonstrate experimentally that electron diffraction at relativistic energies opens a new approach for studies of melting kinetics. Our measurements in radiation-damaged tungsten show that the tungsten target subjected to 10 displacements per atom of damage undergoes a melting transition below the melting temperature. Two-temperature molecular dynamics simulations reveal the crucial role of defect clusters, particularly nanovoids, in driving the ultrafast melting process observed on the time scale of less than 10 ps. These results provide new atomic-level insights into the ultrafast melting processes of materials in extreme environments.

    View details for DOI 10.1126/sciadv.aaw0392

    View details for Web of Science ID 000470125000088

    View details for PubMedID 31139748

    View details for PubMedCentralID PMC6534394

  • Author Correction: Generation and characterization of ultrathin free-flowing liquid sheets. Nature communications Koralek, J. D., Kim, J. B., Bruza, P., Curry, C. B., Chen, Z., Bechtel, H. A., Cordones, A. A., Sperling, P., Toleikis, S., Kern, J. F., Moeller, S. P., Glenzer, S. H., DePonte, D. P. 2019; 10 (1): 1615


    The original version of this Article contained an error in Eq. (1). This has been corrected in both the PDF and HTML versions of the Article.

    View details for DOI 10.1038/s41467-019-09457-7

    View details for PubMedID 30944301

  • Characterization of defect clusters in ion-irradiated tungsten by X-Ray diffuse scattering JOURNAL OF NUCLEAR MATERIALS Sun, P., Wang, Y., Frost, M., Schoenwaelder, C., Levitan, A. L., Mo, M., Chen, Z., Hastings, J. B., Tynan, G. R., Glenzer, S. H., Heimann, P. 2018; 510: 322–30
  • Toward quasi-DC conductivity of warm dense matter measured by single-shot terahertz spectroscopy Ofori-Okai, B. K., Descamps, A., Lu, J., Seipp, L. E., Weinmann, A., Glenzer, S. H., Chen, Z. AMER INST PHYSICS. 2018: 10D109


    We present an experimental setup capable of measuring the near DC conductivity of laser generated warm dense matter using single-shot terahertz time-domain spectroscopy. The setup uses a reflective echelon and balanced detection to record THz waveforms with a minimum detectable signal of 0.2% in a single laser pulse. We describe details of the experimental setup and the data analysis procedure and present single-shot terahertz transmission data on aluminum that has been laser heated to an electron temperature of 0.5 eV.

    View details for DOI 10.1063/1.5038944

    View details for Web of Science ID 000449144500044

    View details for PubMedID 30399773

  • Determination of the electron-lattice coupling strength of copper with ultrafast MeV electron diffraction Mo, M. Z., Becker, V., Ofori-Okai, B. K., Shen, X., Chen, Z., Witte, B., Redmer, R., Li, R. K., Dunning, M., Weathersby, S. P., Wang, X. J., Glenzer, S. H. AMER INST PHYSICS. 2018: 10C108


    Electron-lattice coupling strength governs the energy transfer between electrons and the lattice and is important for understanding the material behavior under highly non-equilibrium conditions. Here we report the results of employing time-resolved electron diffraction at MeV energies to directly study the electron-lattice coupling strength in 40-nm-thick polycrystalline copper excited by femtosecond optical lasers. The temporal evolution of lattice temperature at various pump fluence conditions were obtained from the measurements of the Debye-Waller decay of multiple diffraction peaks. We observed the temperature dependence of the electron-lattice relaxation time which is a result of the temperature dependence of electron heat capacity. Comparison with two-temperature model simulations reveals an electron-lattice coupling strength of (0.9 ± 0.1) × 1017 W/m3/K for copper.

    View details for DOI 10.1063/1.5035368

    View details for Web of Science ID 000449144500023

    View details for PubMedID 30399817

  • Interatomic Potential in the Nonequilibrium Warm Dense Matter Regime PHYSICAL REVIEW LETTERS Chen, Z., Mo, M., Soulard, L., Recoules, V., Hering, P., Tsui, Y. Y., Glenzer, S. H., Ng, A. 2018; 121 (7)
  • Dynamics of Electron-Phonon Coupling in Bicontinuous Nanoporous Gold JOURNAL OF PHYSICAL CHEMISTRY C Zheng, Q., Shen, X., Sokolowski-Tinten, K., Li, R. K., Chen, Z., Mo, M. Z., Wang, Z. L., Weathersby, S. P., Yang, J., Chen, M. W., Wang, X. J. 2018; 122 (28): 16368–73
  • Heterogeneous to homogeneous melting transition visualized with ultrafast electron diffraction. Science (New York, N.Y.) Mo, M. Z., Chen, Z., Li, R. K., Dunning, M., Witte, B. B., Baldwin, J. K., Fletcher, L. B., Kim, J. B., Ng, A., Redmer, R., Reid, A. H., Shekhar, P., Shen, X. Z., Shen, M., Sokolowski-Tinten, K., Tsui, Y. Y., Wang, Y. Q., Zheng, Q., Wang, X. J., Glenzer, S. H. 2018; 360 (6396): 1451–55


    The ultrafast laser excitation of matters leads to nonequilibrium states with complex solid-liquid phase-transition dynamics. We used electron diffraction at mega-electron volt energies to visualize the ultrafast melting of gold on the atomic scale length. For energy densities approaching the irreversible melting regime, we first observed heterogeneous melting on time scales of 100 to 1000 picoseconds, transitioning to homogeneous melting that occurs catastrophically within 10 to 20 picoseconds at higher energy densities. We showed evidence for the heterogeneous coexistence of solid and liquid. We determined the ion and electron temperature evolution and found superheated conditions. Our results constrain the electron-ion coupling rate, determine the Debye temperature, and reveal the melting sensitivity to nucleation seeds.

    View details for DOI 10.1126/science.aar2058

    View details for PubMedID 29954977

  • Generation and characterization of ultrathin free-flowing liquid sheets NATURE COMMUNICATIONS Koralek, J. D., Kim, J. B., Bruza, P., Curry, C. B., Chen, Z., Bechtel, H. A., Cordones, A. A., Sperling, P., Toleikis, S., Kern, J. F., Moeller, S. P., Glenzer, S. H., DePonte, D. P. 2018; 9: 1353


    The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas-dynamic forces to generate free-flowing, sub-micron, liquid sheets which are two orders of magnitude thinner than anything previously reported. Optical, infrared, and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 μm  down to less than 20 nm, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources.

    View details for DOI 10.1038/s41467-018-03696-w

    View details for Web of Science ID 000429521200005

    View details for PubMedID 29636445

    View details for PubMedCentralID PMC5893585

  • Self-referenced single-shot THz detection OPTICS EXPRESS Russell, B. K., Ofori-Okai, B. K., Chen, Z., Hoffmann, M. C., Tsui, Y. Y., Glenzer, S. H. 2017; 25 (14): 16140–50


    We demonstrate a self-referencing method to reduce noise in a single-shot terahertz detection scheme. By splitting a single terahertz pulse and using a reflective echelon, both the signal and reference terahertz time-domain waveforms were measured using one laser pulse. Simultaneous acquisition of these waveforms significantly reduces noise originating from shot-to-shot fluctuations. We show that correlation function based referencing, which is not limited to polarization dependent measurements, can achieve a noise floor that is comparable to state-of-the-art polarization-gated balanced detection. Lastly, we extract the DC conductivity of a 30 nm free-standing gold film using a single THz pulse. The measured value of σ0 = 1.3 ± 0.4 × 107 S m-1 is in good agreement with the value measured by four-point probe, indicating the viability of this method for measuring dynamical changes and small signals.

    View details for DOI 10.1364/OE.25.016140

    View details for Web of Science ID 000407815100049

    View details for PubMedID 28789123

  • High resolution x-ray Thomson scattering measurements from cryogenic hydrogen jets using the linac coherent light source REVIEW OF SCIENTIFIC INSTRUMENTS Fletcher, L. B., Zastrau, U., Galtier, E., Gamboa, E. J., Goede, S., Schumaker, W., Ravasio, A., Gauthier, M., MacDonald, M. J., Chen, Z., Granados, E., Lee, H. J., FRY, A., Kim, J. B., Roedel, C., Mishra, R., PELKA, A., Kraus, D., Barbrel, B., Doppner, T., Glenzer, S. H. 2016; 87 (11)


    We present the first spectrally resolved measurements of x-rays scattered from cryogenic hydrogen jets in the single photon counting limit. The 120 Hz capabilities of the LCLS, together with a novel hydrogen jet design [J. B. Kim et al., Rev. Sci. Instrum. (these proceedings)], allow for the ability to record a near background free spectrum. Such high-dynamic-range x-ray scattering measurements enable a platform to study ultra-fast, laser-driven, heating dynamics of hydrogen plasmas. This measurement has been achieved using two highly annealed pyrolytic graphite crystal spectrometers to spectrally resolve 5.5 keV x-rays elastically and inelastically scattered from cryogenic hydrogen and focused on Cornell-SLAC pixel array detectors [S. Herrmann et al., Nucl. Instrum. Methods Phys. Res., Sect. A 718, 550 (2013)].

    View details for DOI 10.1063/1.4959792

    View details for Web of Science ID 000390242300239

    View details for PubMedID 27910564

  • Single-shot mega-electronvolt ultrafast electron diffraction for structure dynamic studies of warm dense matter REVIEW OF SCIENTIFIC INSTRUMENTS Mo, M. Z., Shen, X., Chen, Z., Li, R. K., Dunning, M., Sokolowski-Tinten, K., Zheng, Q., WEATHERSBY, S. P., Reid, A. H., Coffee, R., Makasyuk, I., Edstrom, S., McCormick, D., Jobe, K., Hast, C., Glenzer, S. H., Wang, X. 2016; 87 (11)


    We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime.

    View details for DOI 10.1063/1.4960070

    View details for Web of Science ID 000390242300094

    View details for PubMedID 27910490

  • A single-shot spatial chirp method for measuring initial AC conductivity evolution of femtosecond laser pulse excited warm dense matter REVIEW OF SCIENTIFIC INSTRUMENTS Chen, Z., Hering, P., Brown, S. B., Curry, C., Tsui, Y. Y., Glenzer, S. H. 2016; 87 (11)


    To study the rapid evolution of AC conductivity from ultrafast laser excited warm dense matter (WDM), a spatial chirp single-shot method is developed utilizing a crossing angle pump-probe configuration. The pump beam is shaped individually in two spatial dimensions so that it can provide both sufficient laser intensity to excite the material to warm dense matter state and a uniform time window of up to 1 ps with sub-100 fs FWHM temporal resolution. Temporal evolution of AC conductivity in laser excited warm dense gold was also measured.

    View details for DOI 10.1063/1.4962057

    View details for Web of Science ID 000390242300263

    View details for PubMedID 27910393

  • dc conductivity of two-temperature warm dense gold PHYSICAL REVIEW E Ng, A., Sterne, P., Hansen, S., Recoules, V., Chen, Z., Tsui, Y. Y., Wilson, B. 2016; 94 (3): 033213


    Using recently obtained ac conductivity data we have derived dc conductivity together with free electron density and electron momentum relaxation time in two-temperature warm dense gold with energy density up to 4.1 MJ/kg (0.8×10^{11}J/m^{3}). The derivation is based on a Drude interpretation of the dielectric function that takes into account contributions of intraband and interband transitions as well as atomic polarizability. The results provide valuable benchmarks for assessing the extended Ziman formula for electrical resistivity and an accompanying average atom model.

    View details for DOI 10.1103/PhysRevE.94.033213

    View details for Web of Science ID 000385968400012

    View details for PubMedID 27739824

  • Matter under extreme conditions experiments at the Linac Coherent Light Source JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Glenzer, S. H., Fletcher, L. B., Galtier, E., Nagler, B., Alonso-Mori, R., Barbrel, B., Brown, S. B., Chapman, D. A., Chen, Z., Curry, C. B., Fiuza, F., Gamboa, E., Gauthier, M., Gericke, D. O., Gleason, A., Goede, S., Granados, E., Heimann, P., Kim, J., Kraus, D., MacDonald, M. J., MacKinnon, A. J., Mishra, R., Ravasio, A., Roedel, C., Sperling, P., Schumaker, W., Tsui, Y. Y., Vorberger, J., Zastrau, U., FRY, A., White, W. E., Hasting, J. B., Lee, H. J. 2016; 49 (9)
  • New experimental platform to study high density laser-compressed matter REVIEW OF SCIENTIFIC INSTRUMENTS Gauthier, M., Fletcher, L. B., Ravasio, A., Galtier, E., Gamboa, E. J., Granados, E., Hastings, J. B., Heimann, P., Lee, H. J., Nagler, B., Schropp, A., Gleason, A., Doeppner, T., Lepape, S., Ma, T., Pak, A., MacDonald, M. J., Ali, S., Barbrel, B., Falcone, R., Kraus, D., Chen, Z., Mo, M., Wei, M., Glenzer, S. H. 2014; 85 (11)


    We have developed a new experimental platform at the Linac Coherent Light Source (LCLS) which combines simultaneous angularly and spectrally resolved x-ray scattering measurements. This technique offers a new insights on the structural and thermodynamic properties of warm dense matter. The < 50 fs temporal duration of the x-ray pulse provides near instantaneous snapshots of the dynamics of the compression. We present a proof of principle experiment for this platform to characterize a shock-compressed plastic foil. We observe the disappearance of the plastic semi-crystal structure and the formation of a compressed liquid ion-ion correlation peak. The plasma parameters of shock-compressed plastic can be measured as well, but requires an averaging over a few tens of shots.

    View details for DOI 10.1063/1.4896175

    View details for Web of Science ID 000345646000203