Dynamic collinear intermode interference in terahertz polaritons

Double-slit experiments, wherein light passes through a thin subwavelength or near-wavelength double slit, have confirmed the wave-particle duality of many quantum entities. Recently, it has been reported that the double-slit time diffraction of light upon twice gated in time produces the frequency diffraction. Here, we present a collinear intermode interference of light in a dual-mode waveguide, where the two terahertz (THz) waves could not only produce a spatial interference pattern extending along the propagation axis, but also cause frequency shift during the interference of broadband THz pulses. We demonstrate an observation of the dynamic interference and the resulted frequency-shift propagation of the THz polaritons, signifying advancements toward power-independent frequency engineering within the THz spectrum. The spatiotemporal interference can be actively tuned by generating the THz waves within various waveguides. Furthermore, the spatiotemporal interference of THz waves leads to a periodic polariton flow, paving the way for resonant/non-resonant material control, periodic ultrafast switching of material functionality, and also time-grating-based time-varying physics in THz physics and polaritonics.