Visualizing a Terahertz Superfluid Plasmon in a Two-Dimensional Superconductor

Many fundamental excitations in solids, such as lattice vibrations, collective electron motion, and spin dynamics, naturally evolve on picosecond timescales and meV energy scales. Terahertz (THz) spectroscopy, which directly matches these intrinsic scales, has therefore become a powerful tool to probe their dynamics. However, its poor spatial resolution, limited by the ~100 μm THz wavelength, constrains its application to the rapidly growing field of low-dimensional materials.

We overcome this limitation by placing the sample in direct contact with a microscopically small spintronic THz emitter. The emitter is driven by a tightly focused ultrashort near-infrared laser pulse, which defines the sub-wavelength spatial extent of the THz near field. In this configuration, the confined THz radiation interacts with the sample before diverging into the far field.

Using this approach, we study how tailoring the THz light–matter interaction through the sample’s geometry and dielectric environment gives rise to distinct resonances that confine electromagnetic waves to deeply sub-wavelength scales. We observe clear spectroscopic evidence of a below-gap, two-dimensional superfluid plasmon in few-layer Bi₂Sr₂CaCu₂O₈₊ₓ and spatially resolve its sub-diffractive electrodynamics. This resonance, absent in bulk and emerging only below the superconducting transition (Tc = 87 K), is visualized by raster scanning the confined THz source across the sample. These measurements directly capture the momentum- and frequency-dependent evolution of the superconducting transition in two dimensions and open a pathway to deterministically probe and control THz light–matter coupling in quantum materials, bridging the domains of cavity quantum electrodynamics and low-dimensional condensed matter.