About

David A. Reis is a Professor of Photon Science and of Applied Physics at Stanford University. His research interests include ultrafast processes in the solid state and fundamental light-matter interactions. His group investigates nonequilibrium dynamics in solids with atomic level spatial and temporal resolution, focusing on ultrafast dynamics such as electron-phonon and phonon-phonon coupling, coherent phonon dynamics, photo-induced phase transitions, thermal transport, and strong-field induced attosecond electron dynamics. The tools employed in his research include ultrafast optical laser and x-ray sources, as well as ultrafast x-ray lasers like the Linac Coherent Light Source x-ray free-electron laser at SLAC. His group also studies thermal transport across interfaces of dissimilar materials using ultrafast laser and x-ray probes, primarily on the nanoscale involving electron and phonon propagation through thin films and interfaces.

Research topics

• Condensed matter physics
• Optics
• Physics
• Quantum mechanics
• Materials science
• Optoelectronics
• Geometry

Selected publications

• A textured polar phase in strained SrTiO3

  arXiv (Cornell University) · 2026-03-12

  preprintOpen access

  Quantum materials can harbour hidden phases whose microscopic structures differ from conventional ordered states while reproducing their macroscopic signatures, making them easy to miss. Strontium titanate is a longstanding puzzle of this kind: on cooling it shows every hallmark of an incipient ferroelectric, yet never orders, and is usually described as a quantum paraelectric in which fluctuations suppress ferroelectricity. Here we combine uniaxial strain, single-cycle terahertz excitation and femtosecond x-ray scattering to measure the polar collective modes of strontium titanate as a function of momentum and strain. Under modest tensile strain, we observe a new vibrational mode that emerges not at the Brillouin zone centre, as a ferroelectric transition would require, but at finite wavevector, identifying the ordered state as a polar texture on nanometre length scales rather than a uniform ferroelectric. Unstrained quantum paraelectric strontium titanate is then naturally understood as the disordered precursor of this textured phase, offering a resolution to a decades-old puzzle and illustrating how finite-momentum collective excitations can unmask hidden phases in quantum materials.

  PublisherDOI

• A textured polar phase in strained SrTiO3

  ArXiv.org · 2026-03-12

  articleOpen access

  Hidden phases of quantum materials are collective states that exist outside the equilibrium phase diagram and can host exotic properties with transformative potential. However, because they can often mimic known states, identifying them remains challenging. Strontium titanate (SrTiO3) epitomizes this challenge: upon cooling, it displays signatures of ferroelectricity yet never develops this order. We combined mechanical strain with ultrafast laser pulses and x-ray scattering to discover a new polar state in SrTiO3 that is distinct from ferroelectricity. Its signature are distinctive polar vibrations with nanometer wavelengths. This reveals that strain stabilizes a hidden state characterized by a nanoscale polarization modulation rather than conventional homogeneous ferroelectricity. Our findings may offer an alternative explanation for quantum paraelectricity and demonstrate that probing collective excitations at finite momentum is essential for identifying hidden phases in quantum materials.

  PublisherOA PDF

• Nonresonant Raman Control of Ferroelectric Polarization

  Advanced Materials · 2025-08-25 · 3 citations

  article

  Important advances is recently made in the search for materials with complex multi-phase landscapes that host photoinduced metastable collective states with exotic functionalities. In almost all cases so far, the desired phases are accessed by exploiting light-matter interactions via the imaginary part of the dielectric function through above-bandgap or resonant mode excitation. Nonresonant Raman excitation of coherent modes is experimentally observed and proposed for dynamic material control, but the resulting atomic excursion is limited to perturbative levels. Here, this challenge is overcome by employing nonresonant ultrashort pulses with low photon energies well below the bandgap. Using mid-infrared pulses, ferroelectric reversal is induced in lithium niobate, and the large-amplitude mode displacements are characterized through femtosecond stimulated Raman scattering and second harmonic generation. This approach, validated by first-principle calculations, defines a novel method for synthesizing hidden phases with unique functional properties and manipulating complex energy landscapes at reduced energy consumption and ultrafast speeds.

  PublisherDOI

• Observation of polarization density waves in SrTiO3

  Nature Physics · 2025-04-07 · 7 citations

  articleOpen access

  The nature of the incipient ferroelectric transition in SrTiO3 has been a long-standing puzzle in condensed matter physics. One explanation involves the competition between ferroelectricity and an instability characterized by the mesoscopic modulation of the polarization. These polarization density waves, which should intensify near the quantum critical point, break local inversion symmetry and are difficult to characterize with conventional X-ray scattering methods. Here we probe inversion symmetry breaking at finite momenta and visualize the instability of the polarization at the nanometre scale in SrTiO3 by combining a femtosecond X-ray free-electron laser with terahertz coherent control methods. We found polar-acoustic collective modes that are soft, particularly at the tens of nanometre scale. These precursor collective excitations provide evidence for the conjectured mesoscopic-modulated phase in SrTiO3. Despite exhibiting ferroelectric features, SrTiO3 fails to display long-range polar order at low temperatures due to quantum fluctuations. An ultrafast X-ray diffraction experiment now probes polar dynamics of this material at the nanometre scale.

  PublisherDOI

• Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

  Research Square · 2025-07-29

  preprintOpen access
  PublisherOA PDFDOI

• Impulsive excitation of squeezed phonons in single crystal germanium by an x-ray laser

  Applied Physics Letters · 2025-06-02

  article

  In this Letter, we present the experimental observation of squeezed phonon generation in semiconductor germanium (Ge) induced by x-ray excitation. Prior x-ray pump, x-ray probe studies reported coherent longitudinal acoustic phonon generation in insulating oxides like strontium titanate and potassium tantalate. In contrast, such signals were not observed in semiconductors likely due to limited signal-to-noise ratio. Now, with an improved experimental setup, we observe a phonon response in single-crystal germanium. Utilizing x-ray split-delay optics with enhanced stability, we extract the phonon dispersion relation, which shows strong agreement with the calculated transverse acoustic phonon mode. Our results reveal that responses to x-ray excitations in semiconductors are of a similar nature to optical excitations. This suggests that the initial response to x-ray core–hole excitations rapidly diffuses to a non-local excitation, similar to what is observed with optical laser valence excitation on a femtosecond timescale.

  PublisherDOI

• Dynamical Scaling Reveals Topological Defects and Anomalous Evolution of a Photoinduced Phase Transition

  Physical Review X · 2025-08-28 · 1 citations

  articleOpen access

  Nonequilibrium states of quantum materials can exhibit exotic properties and enable unprecedented functionality and applications. These transient states are inherently inhomogeneous, characterized by the formation of topologically protected structures, requiring nanometer spatial resolution on femtosecond timescales to resolve their evolution. Using ultrafast total x-ray scattering at a free electron laser and a sophisticated scaling analysis, we gain unique access to the dynamics on the relevant mesoscopic length scales. Our results provide direct evidence that ultrafast excitation of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>LaTe</a:mi></a:mrow><a:mrow><a:mn>3</a:mn></a:mrow></a:msub></a:mrow></a:math> leads to formation of topological vortex strings of the charge density wave. These dislocations of the charge density wave exhibit anomalous, subdiffusive dynamics, slowing the equilibration process, providing rare insight into the nonequilibrium mesoscopic response in a quantum material. Our findings establish a general framework to investigate properties of topological defects, which are expected to be ubiquitous in nonequilibrium phase transitions and may arrest equilibration and enhance competing orders.

  PublisherOA PDFDOI

• Prospects for the production and detection of Breit-Wheeler tunneling positrons in Experiment 320 at the FACET–II accelerator

  Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment · 2025-11-08

  articleOpen access

  International audience

  PublisherDOI

• A high-flux electron detector system to measure non-linear Compton scattering at LUXE

  Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment · 2025-07-01

  articleOpen access

  Recently, advancements in high-intensity laser technology have enabled the exploration of non-perturbative Quantum Electrodynamics (QED) in strong-field regimes. Notable aspects include non-linear Compton scattering and Breit–Wheeler pair production, observable when colliding high-intensity laser pulses and relativistic electron beams. The LUXE experiment at DESY and the E-320 experiment at SLAC aim to study these phenomena by measuring the created high-flux Compton electrons and photons. We propose a novel detector system featuring a segmented gas-filled Cherenkov detector with a scintillator screen and camera setup, designed to efficiently detect high-flux Compton electrons. Preliminary results from E-320 measurement campaigns demonstrate methods for reconstructing electron energy spectra, aiming to reveal crucial features of non-perturbative QED.

  PublisherDOI

• Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

  ArXiv.org · 2025-06-03 · 1 citations

  preprintOpen access

  Understanding and controlling microscopic dynamics across spatial and temporal scales has driven major progress in science and technology over the past several decades. While ultrafast laser-based techniques have enabled probing nanoscale dynamics at their intrinsic temporal scales down to femto- and attoseconds, the long wavelengths of optical lasers have prevented the interrogation and manipulation of such dynamics with nanoscale spatial specificity. With advances in hard X-ray free electron lasers (FELs), significant progress has been made developing X-ray transient grating (XTG) spectroscopy, aiming at the coherent control of elementary excitations with nanoscale X-ray standing waves. So far, XTGs have been probed only at optical wavelengths, thus intrinsically limiting the achievable periodicities to several hundreds of nm. By achieving sub-femtosecond synchronization of two hard X-ray pulses at a controlled crossing angle, we demonstrate the generation of an XTG with spatial periods of 10 nm. The XTG excitation drives a thermal grating that drives coherent monochromatic longitudinal acoustic phonons in the cubic perovskite, SrTiO3 (STO). With a third X-ray pulse with the same photon energy, time-and-momentum resolved measurement of the XTG-induced scattering intensity modulation provides evidence of ballistic thermal transport at nanometer scale in STO. These results highlight the great potential of XTG for studying high-wave-vector excitations and nanoscale transport in condensed matter, and establish XTG as a powerful platform for the coherent control and study of nanoscale dynamics.

  PublisherOA PDFDOI

Frequent coauthors

• Mariano Trigo

  150 shared

• P. H. Bucksbaum

  109 shared

• Shambhu Ghimire

  99 shared

• Matthieu Chollet

  SLAC National Accelerator Laboratory

  92 shared

• Stephen Fahy

  University College Cork

  92 shared

• David Fritz

  G & A Technical Software (United States)

  87 shared

• Aaron M. Lindenberg

  SLAC National Accelerator Laboratory

  87 shared

• Diling Zhu

  78 shared

Labs

• Stanford PULSE InstitutePI

  Ultrafast science research

Education

• Ph.D., Applied Physics

  Stanford University

  1990

• M.S., Applied Physics

  Stanford University

  1986

• B.S., Physics

  University of California, Berkeley

  1982