Session L10: DLS Focus Session

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The unique properties of graphene due to its gapless linear electronic dispersion and high mobility of charge carriers make it very promising for applications in high speed optoelectronic and photonic devices. To this extent it is essential to understand the relaxation dynamics and cooling mechanisms of photogenerated carriers in graphene. Here we report on carrier dynamics in a gate-tunable graphene device using THz pump-THz probe spectroscopy. At low temperatures we observe that the relaxation dynamics is characterized by a double exponential decay that evolves into a single exponential decay at higher temperatures. Time-resolved THz conductivity measurements at different carrier densities indicate the presence of two components with opposite contributions to the photoconductivity. The two components evolve on distinct timescales consistent with the relaxation dynamics. This suggests that the THz-induced current in graphene is carried by two distinct modes at low temperatures.   

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Light-matter coupling involving classical and quantum light offers a wide range of possibilities to tune the electronic properties of correlated quantum materials. Two paradigmatic results are the dynamical localization of electrons and the ultrafast control of spin dynamics, which have been discussed within classical Floquet engineering and in the deep quantum regime where vacuum fluctuations modify the properties of materials. Here we discuss how these two extreme limits are interpolated by a cavitydriven to the excited states. In particular, this is achieved by formulating a Schrieffer-Wolff transformation for the cavity-coupled system. Some of the extraordinary results of Floquet-engineering, such as the sign reversal of the exchange interaction or electronic tunneling, which are not obtained by coupling to a dark cavity, can already be realized with a single-photon state (no coherent states are needed). The analytic results are verified and extended with numerical simulations on a two-site Hubbard model coupled to a driven cavity mode. Our results generalize the well-established Floquet-engineering of correlated electrons to the regime of quantum light. -- Phys. Rev. Research 2, 033033 (2020)   

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In quantum materials, the crystal lattice shapes the interactions between electrons and governs, in large part, their emergent electronic and magnetic phases. Hence, a powerful approach for controlling functional properties and stabilizing hidden ground states is based on altering a material’s structure through, for instance, strain or interfacial engineering. Here, we show that a desired non-equilibrium crystal structure can be engineered with light to achieve macroscopic behavior beyond that achievable statically. In particular, we generate a highly polarized ferrimagnetic phase in the prototypical antiferromagnet CoF₂ by optically driving atomic motions with resonant THz pulses. By exploiting lattice anharmonicities, a symmetry-breaking structural distortion is created on the picosecond time scale, leading to a site-selective modulation of the Co crystal field and associated magnetic moment. This effect resembles the static piezomagnetic effect in CoF₂; however, the ultrafast magnetization is almost two orders of magnitude larger than that achievable statically. The realization of a targeted light-induced structure and functionality establishes the potential of nonlinear phononics and expands the ultrafast control of quantum materials towards the level of rational design.   

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Recent theory results [Walldorf et al., Phys. Rev. B 100 121110 (R) (2019)], obtained in a one loop non-interacting magnon theory, demonstrate a dynamical phase transition in the antiferromagnetic phase of the 2D Hubbard model upon laser driving. The transition is characterized by a qualitative change in the magnon distribution function as the drive strength is varied. Here we investigate the effects of magnon-magnon interactions using an interacting spin-wave theory in a large spin expansion and a Boltzmann formalism. The scattering leads to qualitative changes with respect to the noninteracting results, in particular to steady states that can be characterized by a generalized Bose-Einstein distribution with an effective drive-dependent chemical potential. Implications for the dynamical phase transition and the Mermin-Wagner theorem for nonthermal states are discussed.   

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Optical quenching has been shown to induce phase transitions and reveal exotic “hidden” phases in quantum materials such as TaS₂ and TaSe₂, exposing potential for ultrafast switching applications and rendering it one of the forefront techniques for manipulation of such materials.

We investigated 1T’-TaTe₂ - a member of the 2D TaX₂ family with unique atomic ordering of Ta atoms including forming trimer superstructures below T_c ~ 170 K. We optically induced phase transition in 1T’-TaTe₂ at 10 K and used the ultrafast electron diffraction facility at Lawrence Berkeley National Laboratory (HiRES [1]) to sensitively probe its structural response [2]. The system was found to switch on picosecond timescale between the trimer superstructure and the corresponding melted phase via a charge transfer mechanism according to density functional theory (DFT) calculations, opening up pathways to manipulate atomic order in this material.

In this contribution, we discuss the observed dynamics in light of dynamical diffraction simulations and DFT and provide an outlook for future studies of this intriguing system.

[1]. K. M. Siddiqui and D. B. Durham et al., arXiv:2009.02891 (2020).
[2]. F. Ji et al.,Commun Phys 2, 54, (2019).   

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We perform angle-resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of Type-II Weyl semimetal TaIrTe₄ under photo excitation. The dynamical relaxation of photoexcited carriers exhibits three exponential decay components relating to optical/acoustic phonon cooling and subsequent heat transfer to the substrate. The angle resolved measurement reveals that the anisotropy of reflectivity is reduced in the pump induced quasi-equilibrium state, suggesting a reduction of the anisotropy in dynamical conductivity in hot carrier dominated regime. These results are indispensable in designing high field, angle sensitive electronic, optoelectronic and remote sensing devices exploiting the dynamical electronic anisotropy with TaIrTe₄.   

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Ultrafast laser excitation can produce states of matter that are thermally inaccessible, enabling the exploration of new properties as well as providing insight into the microscopic interactions. In this work, we use time- and angle-resolved photoemission spectroscopy to study the electronic structure and electron-phonon couplings throughout a newly-uncovered laser-enriched phase diagram in the charge density wave (CDW) material 1T-TaSe₂. First, we drive the material into new metastable intermediate CDW states by finely tuning the laser fluence, to achieve mode-selective electron-phonon coupling. As a result, the transient heat capacity is substantially reduced, which supports an energy-efficient phase transformation route. Moreover, the data suggest a switching of the dominant coupling mechanism between the coherent amplitude mode and electrons. Second, we detect a transient inverted CDW state by strongly exciting the material, to coherently over-drive the periodic lattice distortion. The dynamic electronic band structures show signatures of novel high metallicity, and an inversion of the momentum dependence of the coupling between the amplitude mode and Ta 5d band. These results demonstrate an ultrafast and coherent route to steer and control quantum materials using light.   

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We demonstrate via time-resolved x-ray diffraction that SnSe has a photo-induced lattice instability associated with an orthorhombic distortion of the rock-salt structure. This lattice instability is distinct from the one associated with the high-temperature Cmcm phase. Our findings show that photoexcitation can transiently induce lattice instability inaccessible on the equilibrium phase diagram. This lattice instability is signaled by the drastic softening behavior of the soft phonon while its displacement direction is away from the thermally existent higher symmetry phase. The resonant bonding theory that explains the lattice instability of many rock-salt structure type semiconductor, and predicts the Pnma-Cmcm lattice instability in SnSe, may not be relevant under photoexcitation. The nonthermal distribution of photoexcited carriers in this semiconductor do not impact the lattice like merely raising electron temperatures. The results point to the need for investigating electron phonon coupling in the nonequilibrium region, as well as implications for manipulating matter with light.   

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There has been recent interest in the development of ultrafast imaging techniques that can temporally resolve transient scenes at upwards of 100 Mfps [1]. To allow the capture of events that are non-periodic or sporadic, the full scene dynamics must be captured in single measurements rather than a series of repeated measurements. Here, we present a method for acquiring a sequence of time-resolved images in a single shot, termed Single-Shot Non-Synchronous Array Photography (SNAP). In SNAP, a scene is probed by a train of femtosecond laser pulses over a range of incident angles. These angled incident pulses are then distributed onto a single camera sensor with the aid of a microlens array, producing an array of distinct images that are recorded simultaneously in a manner similar to light field imaging. In our prototype concept, SNAP will be able to acquire 25 images with a field of view of about 2mm and a framerate of 4 Tfps, providing a unique tool for the study of sub-picosecond event.

[1] Jinyang Liang and Lihong V. Wang, "Single-shot ultrafast optical imaging," Optica 5, 1113-1127 (2018)   

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The impressive progress in high-resolution and multi-dimensional angle-resolved photoemission (ARPES) allows insights into the nature of the quantum states in the solid itself. We will discuss how topological properties are manifest in circular dichroism in ARPES. Based on state-of-the-art calculations, we demonstrate how momentum-resolved Berry curvature can be mapped out for prototypical two-dimensional materials.
Furthermore, topological properties can be induced by tailored light. However, realizing the induced Floquet-Chern insulator state and tracing clear experimental manifestions has been a challenge. We tackle this gap between theory and experiment by employing microscopic nonequilibrium Green’s functions (NEGF) calculations including realistic electron-electron and electron-phonon scattering. Combining our nonequilibrium calculations with an accurate one-step theory of photoemission allows us to establish a direct link between the build-up of the topological state and the dichroic pump-probe photoemission signal.   

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Probing ultrafast dynamics and coherence with nm-fs spatio-temporal resolution is of crucial importance for understanding quantum coherence at the nanoscale and advance nonlinear optical applications. However, ultrafast nanoimaging has long been experimentally challenging. Our combination of grating coupled plasmonic nanofocusing with scanning probe techniques enables coherent nonlinear optical imaging with nanometer spatial and femtosecond temporal resolution. We will discuss how spatial nano-confinement and near-field momenta provide new pathways for enhancing nonlinear frequency conversion where, e.g., plasmonic gradient-field effects amplify dipole-forbidden nonlinear responses. We further demonstrate non-local enhancement in four-wave mixing in graphene associated with Doppler broadening due to the broad distribution of near-field momenta. By implementing both spatio-spectral and spatio-temporal nanoimaging we can resolve the few-fs dynamics of electron coherence in graphene. We discuss the extension of these studies to other layered materials such as transition metal dichalcogenides, providing insight into the heterogeneity of nanoscale electronic properties, enhanced frequency conversion, and roles of associated band structures, defects and grain boundaries.   

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THz laser technology has developed in the last decade. Intense THz light enables us to observe nonlinear optical phenomena in magnetic insulators. Among magnetic phases, quantum spin liquids (QSLs) are less-visible ones. Most thermodynamic quantities in QSLs are featureless, and their specific nature can often be found in the dynamical quantities. Motivated by these backgrounds, we theoretically study THz-laser responses of the QSL phase in the Kitaev model which is a frustrated spin-1/2 system on honeycomb lattice with three-type Ising interactions. The excitations on the Kitaev QSL are described by Majorana fermions and its candidate materials such as α-RuCl₃ have been intensively investigated. We focus on THz-laser driven high harmonic generation (HHG) of the Kitaev model with magneto-striction (MS) interaction, through which the AC electric field is coupled to the Ising terms. Applying quantum master equation, we numerically show that the AC electric field induces HHG via the MS coupling, and the HHG spectra exhibit characteristic dependencies of laser frequency, laser intensity, and external DC fields. We report essential aspects of these spectra.   

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We theoretically study transition metal decalcogenide quantum dot high harmonic generation in the field of ultrashort optical pulse. The electron dynamics in such strong pulse is irreversible and after the pulse the electron system resides in highly excited states with a finite dipole moment. Highly nonlinear electron dynamics during the pulse generates the high harmonics of large intensity. We analyze numerically how the intensities of such high harmonics depend on the amplitude of the pulse and the size of the quantum dot.   

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Over the past few decades, ultrashort pulses of light have been widely employed to control the behavior of matter in its different phases. This is a particularly interesting challenge in magnetism, where the speed, dissipation and routes for ultimately fast switching of the spin orientation often lead to proposals for novel approaches in information processing and data recording.^[1]
In this work we show that light-driven lattice vibrations can be utilized to coherently manipulate macroscopic magnetic states. We demonstrate that mid-infrared electric field pulses, tuned in resonance with a vibrational mode of the prototypical antiferromagnetic DyFeO₃, can induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. As the magnetic exchange defines the stability of the macroscopic magnetic state, this provides control over the magnetic phases. For sufficiently strong excitation, we demonstrate a coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders on the picosecond timescale.


[1] P. Němec et al., Nature Physics 14, 229 (2018).   

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The excitonic insulator (EI) is an interesting phase of matter where electron-hole pairs form a Bose-Einstein-like condensate. We report the low energy dynamics of Ta₂NiSe₅, a candidate EI material with T_c = 328 K. This is accomplished using ultrafast near-infrared pump-broadband THz probe spectroscopy. Our broadband probe enables simultaneous measurement of electron and phonon dynamics relevant to the EI state. We will compare the dynamics in with its sister compound Ta₂NiS₅, an isoelectric semiconductor, with a view towards disentangling dynamics associated with conventional semiconducting behavior (and the monoiclinic-orthorhombic structural transition) from pure EI order parameter dynamics.