We demonstrate all-optical control of terahertz (THz) wavemode coupling in a silicon-filled parallel plate waveguide. Using an asymmetric photoexcitation of charge carriers on the surface of the silicon slab within the waveguide, the symmetry is broken, and THz light is partially coupled from TEM to higher-order TM modes. The resulting interference between these modes and the residual TEM mode leads to a strong frequency-dependent transmission modulation. This frequency-selective modulation is widely tunable by adjusting the relative modal phases by translating the excitation along the propagation direction. The experimental observations are well described by a numerical and analytic model of modal interference.
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