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unta or: ja; acc D 85 guid rtz r GW. ate w paga low polycarbonate waveguide. To reduce the loss, it is concentration of 25 wt. % PS, and a viscosity of October 15, 2007 / Vol. 32, No. 20 / OPTICS LETTERS 2945necessary to deposit a dielectric coating over the me-tallic film as has been done in the IR region, where, for example, an AgI film was deposited over an Ag metal film [12]. We have chosen to deposit polysty-rene (PS) films over Ag films, as PS has a low loss at THz frequencies. A significant challenge in extending metal/ dielectric, hollow waveguide technology to THz fre-quencies is that a much thicker dielectric layer is re-quired for low loss. This may be seen from Eq. (1), which shows that for the HE11 mode the optimum di-electric layer thickness, dd, is proportional to the de-sign wavelength, o: dd =