Direct Near-Field Spacetime Imaging of Phase and Group Velocities of Terahertz Surface Plasmon Polaritons in Graphene

Surface polaritons, light-matter hybrid excitations at material interfaces, are extensively studied for their strong field confinement and high propagation velocities, which are ideal for nanoscale waveguide and sensor applications [1], [2]. While near-field microscopy is known to map polariton propagation mostly in the frequency domain, real-space field-resolved detection of broadband-pulse-launched polaritons (spacetime imaging) can offer new insights into their properties [3], [4]. Here, we investigate terahertz surface plasmon polaritons (SPPs) in graphene using spacetime imaging and manipulate their dynamics by ultrafast photoexcitation [5]. A terahertz pulse coupled into the sharp metallic tip of a near-field microscope launches surface polaritons (Fig. 1a). Using electro-optic detection, we extract the electric field profile of SPPs that reflect off graphene edges. By analyzing these waveforms at different distances to a graphene edge, we can directly trace the spatiotemporal evolution of the SPP wave packet, providing insights into the group and phase velocities of the polariton wave packet, as well as damping (Fig. 1 b). Thus, the position and gradient of the polariton dispersion within the probed spectral range can be inferred (Fig. 1c), even for strongly damped polariton modes. Additionally, our straight-forward analysis can be expanded to full 2D images, mapping distinct propagation directions at different edges (Fig. 1 d). Access to the time domain also enables us to probe nonequilibrium dynamics with femtosecond accuracy. Photoexciting graphene with ultrafast visible light pulses transiently changes its conductivity. This allows us to control the SPP amplitude on subcycle timescales (Fig. 1 e) - much faster than electrostatic gating - offering a potential new avenue for ultrafast plasmonic logic devices. Beyond graphene, our approach can be readily extended to a wide range of materials including anisotropic polaritons in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha-\text{MoO}_{3}$</tex> and black phosphorus, or topological insulator surface modes, and other systems where electro-optic sampling of the scattered electric field can be employed.