Honors & Awards

  • Carl Ramsauer Award, Physikalische Gesellschaft zu Berlin (2016)
  • Dissertation prize of the DPG, Deutsche Physikalische Gesellschaft (2018)

Professional Education

  • Diplom, Ludwig Maximilian Universitat Munchen (2011)
  • Doctor of Philosophy, Freie Universitat Berlin (2016)
  • Vordiplom, Ruprecht Karl Universitat Heidelberg (2007)

Stanford Advisors

All Publications

  • Terahertz Compression of Electron Pulses at a Planar Mirror Membrane PHYSICAL REVIEW APPLIED Ehberger, D., Mohler, K. J., Vasileiadis, T., Ernstorfer, R., Waldecker, L., Baum, P. 2019; 11 (2)
  • Efficient generation of neutral and charged biexcitons in encapsulated WSe2 monolayers NATURE COMMUNICATIONS Ye, Z., Waldecker, L., Ma, E., Rhodes, D., Antony, A., Kim, B., Zhang, X., Deng, M., Jiang, Y., Lu, Z., Smirnov, D., Watanabe, K., Taniguchi, T., Hone, J., Heinz, T. F. 2018; 9
  • Efficient generation of neutral and charged biexcitons in encapsulated WSe2 monolayers. Nature communications Ye, Z., Waldecker, L., Ma, E. Y., Rhodes, D., Antony, A., Kim, B., Zhang, X., Deng, M., Jiang, Y., Lu, Z., Smirnov, D., Watanabe, K., Taniguchi, T., Hone, J., Heinz, T. F. 2018; 9 (1): 3718


    Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and therefore high excitation levels. Here, we report the emergence of two biexcitons species, one neutral and one charged, in monolayer tungsten diselenide under moderate continuous-wave excitation. The efficient formation of biexcitons is facilitated by the long lifetime of the dark exciton state associated with a spin-forbidden transition, as well as improved sample quality from encapsulation between hexagonal boron nitride layers. From studies of the polarization and magnetic field dependence of the neutral biexciton, we conclude that this species is composed of a bright and a dark excitons residing in opposite valleys in momentum space. Our observations demonstrate that the distinctive features associated with biexciton states can be accessed at low light intensities and excitation densities.

    View details for PubMedID 30214026

  • Ultrafast Heat Flow in Heterostructures of Au Nanoclusters on Thin Films: Atomic Disorder Induced by Hot Electrons. ACS nano Vasileiadis, T., Waldecker, L., Foster, D., Da Silva, A., Zahn, D., Bertoni, R., Palmer, R. E., Ernstorfer, R. 2018


    We study the ultrafast structural dynamics, in response to electronic excitations, in heterostructures composed of size-selected Au nanoclusters on thin-film substrates with the use of femtosecond electron diffraction. Various forms of atomic motion, such as thermal vibrations, thermal expansion, and lattice disordering, manifest as distinct and quantifiable reciprocal-space observables. In photoexcited supported nanoclusters, thermal equilibration proceeds through intrinsic heat flow between their electrons and their lattice and extrinsic heat flow between the nanoclusters and their substrate. For an in-depth understanding of this process, we have extended the two-temperature model to the case of 0D/2D heterostructures and used it to describe energy flow among the various subsystems, to quantify interfacial coupling constants and to elucidate the role of the optical and thermal substrate properties. When lattice heating of Au nanoclusters is dominated by intrinsic heat flow, a reversible disordering of atomic positions occurs, which is absent when heat is injected as hot substrate phonons. The present analysis indicates that hot electrons can distort the lattice of nanoclusters, even if the lattice temperature is below the equilibrium threshold for surface premelting. Based on simple considerations, the effect is interpreted as activation of surface diffusion due to modifications of the potential energy surface at high electronic temperatures. We discuss the implications of such a process in structural changes during surface chemical reactions.

    View details for PubMedID 29995378

  • Coherent and incoherent structural dynamics in laser-excited antimony PHYSICAL REVIEW B Waldecker, L., Vasileiadis, T., Bertoni, R., Ernstorfer, R., Zier, T., Valencia, F. H., Garcia, M. E., Zijlstra, E. S. 2017; 95 (5)
  • Time-domain separation of optical properties from structural transitions in resonantly bonded materials NATURE MATERIALS Waldecker, L., Miller, T. A., Rude, M., Bertoni, R., Osmond, J., Pruneri, V., Simpson, R. E., Ernstorfer, R., Wall, S. 2015; 14 (10): 991-+


    The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

    View details for PubMedID 26213898