Bias-free and compact mode-matched excitation of THz coaxial waveguides

The large mode mismatch between free-space linearly-polarized beams and radially-polarized modes supported in coaxial waveguides leads to a very poor coupling efficiency that have hindered the widespread use of THz coaxial waveguides. We show here a straightforward coupling mechanism that promises 100% efficiency. We exploit the THz pulse generation process from photoexcited charge density gradients to create a radially-polarized THz pulse with the profile matching the fundamental mode profile of the coaxial waveguide. Single mode propagation is demonstrated at the output of the coaxial waveguide.