Time-resolved terahertz spectroscopy in a parallel-plate waveguide

The parallel plate waveguide (PPWG), formed by two conducting parallel plates separated by a distance on the order of the wavelength of the propagating light, has shown itself to be a near ideal terahertz interconnect exhibiting low loss and dispersionless propagation.[1] It is also a useful tool for enhancing the interaction of THz radiation with thin lms to the point where spectroscopy on 20 nm-thick layers of water have been performed.[2] A natural extension ofthis technique is to examine photoexcited samples inside the waveguide, with the addition of a second femtosecond excitation pulse. Here we demonstrate such a technique for time-resolved optical pump-THz probe measurements of materials embedded within a parallel plate waveguide. This is made possible by replacing one of the plates with a transparent conducting oxide, which provides sufcient electrical conductivity to guide the THz pulse, yet allows optical wavelength light to pass through it and excite an embedded sample. We benchmark the performance of the technique by performing time-resolved spectroscopy of photoexcited carriers in silicon embedded inside the waveguide, and show that the time-resolution of the technique is limited by the spatial distribution of injected charge carriers. We further discuss the possibility of using patterned excitation to control THz propagation inside the waveguide, such as creating a 2D mirror with a line excitation.