32 research outputs found

    Ultrafast manipulation of mirror domain walls in a charge density wave

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    Domain walls (DWs) are singularities in an ordered medium that often host exotic phenomena such as charge ordering, insulator-metal transition, or superconductivity. The ability to locally write and erase DWs is highly desirable, as it allows one to design material functionality by patterning DWs in specific configurations. We demonstrate such capability at room temperature in a charge density wave (CDW), a macroscopic condensate of electrons and phonons, in ultrathin 1T-TaS2_2. A single femtosecond light pulse is shown to locally inject or remove mirror DWs in the CDW condensate, with probabilities tunable by pulse energy and temperature. Using time-resolved electron diffraction, we are able to simultaneously track anti-synchronized CDW amplitude oscillations from both the lattice and the condensate, where photo-injected DWs lead to a red-shifted frequency. Our demonstration of reversible DW manipulation may pave new ways for engineering correlated material systems with light

    Dynamical slowing down in an ultrafast photo-induced phase transition

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    Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photo-induced transition of a model charge-density-wave (CDW) compound, LaTe3_3. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be quantitatively captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems

    Evidence for topological defects in a photoinduced phase transition

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    Upon excitation with an intense ultrafast laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways dissimilar from those in thermal equilibrium. Determining the mechanism underlying these photo-induced phase transitions (PIPTs) has been a long-standing issue in the study of condensed matter systems. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) material, LaTe3_3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a quick (∼ \sim\,1 \,ps) recovery of the CDW amplitude is followed by a slower reestablishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order (LRO) is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other PIPTs by identifying the generation of defects as a governing mechanism

    Time-resolved XUV ARPES with tunable 24–33 eV laser pulses at 30 meV resolution

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    High harmonic generation of ultrafast laser pulses can be used to perform angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure of materials with femtosecond time resolution. However, currently it is difficult to reach high momenta with narrow energy resolution. Here, we combine a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight electron analyzer to develop XUV-based time-resolved ARPES. Our technique can produce tunable photon energy between 24–33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 fs. This technique enables time-, energy- and momentum-resolved investigation of the nonequilibrium dynamics of electrons in materials with a full access to their first Brillouin zone. We evaluate the performance of this setup through exemplary measurements on various quantum materials, including WTe2, WSe2, TiSe2, and Bi2Sr2CaCu2O8+δ.Gordon and Betty Moore Foundation. Emergent Phenomena in Quantum Systems (EpiQS) initiative (Grant GBMF4540

    Compact Multi-Layered THz-driven Photogun

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    All-optical terahertz (THz) based compact accelerators promise to enable new science due to unique features such as reduced space charge broadening of the generated electron bunches and reduced timing-jitter. While prototypes of THz based devices have been demonstrated and showed exceptional capabilities to accelerate and manipulate electron beams on sub-ps timescales, development of practical THz-powered photoguns, however, has lagged due to challenges associated with the physical miniaturization and the larger THz energies required for the driving pulses. Here, we present on the progress of developing a modular all-optically driven setup that allows to power multi-layered electron photo-guns with the potential to reach several hundreds of keV. The modular concept enables convenient exchange of accelerating structures allowing rapid prototyping. As a first prototype, a 3-layered structure designed to reach 200 keV electron energy is presented. Such miniaturized electron sources are promising for seeding compact THz driven accelerators or to deliver ultrafast bunches in electron diffraction experiments
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