CM Seminar: James McIver– Columbia University
Title: Ultrafast optoelectronic circuits

Abstract: Ultrafast optoelectronic circuits offer new opportunities for investigating and controlling the electrical responses of microstructured quantum materials and heterostructures at femtosecond timescales and THz frequencies. Based on metal waveguides and laser-triggered photoconductive switches, these chip-scale circuits can be interfaced to quantum materials to directly probe the ultrafast flow of electrical currents or perform near-field THz spectroscopy on length scales orders of magnitude smaller than the diffraction limit.

In this talk, I will present on my group’s activities using these circuits to study the electrical transport properties of quantum materials driven out of equilibrium by femtosecond laser pulses [1]. In monolayer graphene, we observe an anomalous Hall effect induced by circularly polarized light in the absence of an applied magnetic field [2]. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect the formation of a photon-dressed, or “Floquet-engineered”, topological band structure. The results are a critical first step towards realizing and controlling light-induced topological edge states. I will also discuss our recent results on the Weyl semimetal T_d-MoTe₂, where we observe rectified photocurrents that scale linearly with the applied laser field. This scaling violates the perturbative description of nonlinear optics/transport, but can be explained by the formation of a photon-dressed magnetic Weyl semimetal state.

In the second part of my talk, I will present on my group’s efforts in using femtosecond voltage pulses generated on-chip to probe and manipulate gate-tunable van der Waals heterostructures embedded in plasmonic cavities. We perform near-field time-domain THz spectroscopy on these cavities to study the light-matter hybridization. We observe coherent plasmon cavity modes that can be tuned from the weak to ultrastrong coupling regimes with electrostatic gating and cavity geometry. These techniques, which we are extending to mK temperatures and strong magnetic fields, could be used to investigate and control a wide range of topological and strongly correlated phenomena in microstructured quantum materials and heterostructures that fall on the THz/meV energy scale.