Propagation of ultrawideband short pulses of terahertz radiation through submillimeter-diameter circular waveguides

We report experimental investigations in which quasi-optical methods were used to efficiently couple freely propagating, optoelectronically generated, subpicosecond pulses of terahertz (THz) radiation into submillimeter circular metal tubes (waveguides) and, consequently, to measure the transmitted pulses from these waveguides. We observe very dispersive low-loss propagation over the frequency band from 0.65 to 3.5 THz, with frequency-dependent group velocities $v_g$ ranging from $c/4$ to $c$ and phase velocities $v_p$ from $4c$ to $c$, where $v_g{v}_p=c^2$. The linearly polarized incoming THz pulses couple significantly only into the ${\mathrm{TE}}_{11}$, ${\mathrm{TM}}_{11}$, and ${\mathrm{TE}}_{12}$ modes of the utilized 24- and 4-mm-long stainless-steel tubes, with inside diameters of 240 and $280 \mathit{\mu }\mathrm{m}$, respectively.