Polaritons, arising from the coupling between electromagnetic fields and material excitations, provide a powerful platform for manipulating light beyond the diffraction limit. Their propagation and interaction with surrounding materials depend on the local dielectric environment. Direct visualization and control of polaritonic waves are therefore important for understanding light–matter interactions in nanoscale systems. In this seminar, I will present our recent studies on the real-space ultrafast imaging of two different types of polaritons: surface plasmon polaritons (SPPs) at metal–dielectric interfaces and hyperbolic phonon polaritons (HPhPs) in van der Waals heterostructures.
First, we developed an optical imaging method for propagating SPPs using semiconductor quantum dots (QDs) as photofunctional sensitizers [1]. Two-photon photoluminescence from an ultrathin QD layer reveals interference fringes associated with SPP propagation, even at interfaces buried beneath thick dielectric films (> 100 nm). Analysis of these fringe patterns yields quantitative information on SPP dispersion and velocity, while pump–probe imaging captures the motion of locally excited SPP wave packets in real-time and space.
Second, we demonstrate ultrafast nano-imaging and optical control of HPhPs in hBN/WS₂ heterostructures using infrared s-SNOM with grating-filtration of broadband pulse [2]. Photoexcited carriers in WS₂ transiently modify its dielectric response, altering both the amplitude and wavelength of HPhPs in hBN.
Together, these studies illustrate how photofunctional nanomaterials can be used both to visualize and dynamically control propagating polaritonic waves across nanoscale spatial and femtosecond temporal scales.
 
[1] K. Kamada et al., Nano Lett. 26, 7808—7815 (2026).
[2] K. Kamada et al., Nano Lett. 26, 10446—10454 (2026).