Research

Femtosecond science on-chip

Quantum materials exhibit remarkable phenomena when driven by the strong fields in femtosecond pulses of light. Examples range from emergent hidden phases in complex oxides¹^–² and dichalcogenides³, to signatures of light-induced superconductivity in cuprates⁴^–⁵ and fullerides⁶, to preliminary demonstrations of ‘Floquet-engineering’ in topological insulators⁷^–⁸ and graphene⁹. Up until this point, our understanding of these effects is based almost entirely on spectroscopic probes. The potential functionalities, however, ultimately manifest in the form of electrical transport. This all-important observable has been difficult to access in such experiments due to the limited bandwidth of conventional electronics, and has consequently remained largely unexplored to date.

Our research group fills this void by providing direct access to ultrafast transport phenomena in quantum materials. We do so using an ultrafast optoelectronic circuitry concept based on photoconductive switches¹⁰. The circuitry is highly flexible in its implementation and devices can be fabricated with a wide range of capabilities. For example, circuits can be designed to probe the ultrafast flow of currents in materials after photoexcitation⁹^,¹¹^–¹², or launch and study the propagation of plasmon-polariton pulses¹³, or even perform near-field time domain terahertz spectroscopy under conditions that are inaccessible to free-space optics¹⁴. In conjunction with spintronic techniques, it may even be possible to directly probe the time-resolved flow of spin in optically-driven materials. Using these newfound experimental capabilities, our goal is to explore ultrafast transport phenomena in quantum materials and take the first steps toward turning novel non-equilibrium effects into functionalities.

Below is an overview of some of the research projects we are working on now.

Floquet-engineered topological transport in Dirac materials

Coherent light-matter interaction has been proposed as a means to engineer topological properties in topologically trivial systems. One proposal for such a ‘Floquet topological insulator’ is based on breaking time-reversal symmetry in graphene through a coherent interaction with circularly polarized light¹⁵. In this theory, the light field drives electrons in circular trajectories through the band structure. Close to the Dirac point, these states acquire a non-adiabatic Berry phase with each optical cycle, where the contributions are equal and opposite for the upper and lower bands. This time-averaged extra phase accumulation amounts to an energy shift that lifts the degeneracy of the Dirac point, opening a topological gap in the effective Floquet band structure.

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The non-trivial topology of the Floquet-Bloch bands forming this gap arises from their non-zero Berry curvature distribution, which integrated over the Brillouin zone defines a topological invariant, called the Chern number¹⁶. Topologically protected transport is predicted to develop if the Fermi level lies inside the gap, exhibiting an anomalous Hall effect carried by edge states in the absence of an applied magnetic field.

Using ultrafast transport techniques, we recently observed a light-induced anomalous Hall effect in graphene due to the realization of these topological Floquet-Bloch bands⁹. However, important open questions remain, in particular surrounding the formation of photon-dressed edge states and their topological protection¹⁷. We are investigating this exciting possibility now using a combination of ultrafast transport and on-chip terahertz techniques.

Optical control of magnetism in correlated insulators

Optical fields can also couple to the spin degree of freedom in solids. For example, magnetic (Zeeman) and magnetoelectric (e.g. Dzyaloshinskii—Moriya) interactions directly couple light to spins and have long been pursued as control mechanisms for potential opto-spintronic applications¹⁸.

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There are also more subtle coupling mechanisms that exist in materials where the magnetic properties are determined by electron correlations, such as in Mott insulators. In these systems, light can affect spin exchange interactions by coherently renormalizing how electrons tunnel between lattice sites, all in the absence of explicit magnetic or magnetoelectric coupling mechanisms¹⁹. Exotic forms of ‘Floquet magnetism’ have been proposed along these lines that have no equilibrium counterpart²⁰^–²¹, some of which have been experimentally realized in quantum simulation settings²².

We are beginning to explore this new class of magnetism in quantum materials using a range of ultrafast optoelectronic techniques. We are particularly interested in studying how spin information is transported in these driven systems, which we are pursuing by combining our ultrafast circuitry with spintronic techniques.

Emergent phenomena in van der Waals heterostructures

Van der Waals heterostructures²³, assembled by mechanically stacking thin flakes of quantum materials, can be designed to host a wide range of electronic phases that can be tuned with an unprecedented level of control. Following the discovery of superconductivity in twisted bilayer graphene heterostructures²⁴, multiple platforms have been established exhibiting correlated insulating, metallic, superconducting and topological phases with various types of magnetic order²⁵^–³⁵. Owing to their outstanding electrical tunability and design flexibility, these systems offer a fresh bottom-up approach to creating, controlling and understanding emergent phenomena in strongly correlated condensed matter.

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While these heterostructures show remarkable effects that are important to understand, they are difficult to probe experimentally due to their small size and the low temperatures required. This has limited the study of these systems to primarily DC transport and scanning tunneling probes, which provide valuable but nevertheless limited information about the complex physics of these systems.

We are investigating the near-field spectroscopic and electrical transport properties of these van der Waals heterostructures at low temperatures and terahertz frequencies. The bandwidth of our circuitry encompasses the natural energy scale of emergent phenomena in these systems (1 THz ~ 4 meV). This allows us to resonantly probe important microscopic interaction parameters, such as the magnitudes and possible symmetries of correlation-induced energy gaps, which will help elucidate the nature of these newly discovered quantum phases.