Novel concepts for terahertz waveguide spectroscopy: [invited]

In the recent years there has been a tremendous interest in various waveguides for the THz range. A waveguide offers strong confinement of the field as well as low-loss propagation over significant distances, properties which are important for sensitive spectroscopy. The confinement of the field leads to high sensitivity, and the long propagation distance further allows high spectral resolution. In this presentation we will review our recent work on metallic parallel-plate waveguides (PPWGs). The PPWG is an appealing waveguide geometry for THz spectroscopic applications due to its low loss and low dispersion [1]. In contrast to previous work on PPWG we have applied transparent metallization to the waveguide. The transparent metallization allows optical access to the dielectric-filled waveguide, and hence spectroscopy of photoinduced processes in the PPWG becomes possible. With such waveguides we demonstrate that it is possible to perform quantitative spectroscopy on very small volumes of sample material inside the PPWG. Using continuous-wave as well as femtosecond excitation we inject carriers into semiconductor material in the transparent PPWG, and perform static as well as transient spectroscopy of the optically injected charges. Ongoing work in our laboratory investigates the lower limits to the amount of sample material required for quantitative spectroscopy. Whereas sensing of extremely small quantities of material is possible with resonant and thus narrow-band THz waveguide techniques, broadband spectroscopy is ultimately less sensitive, but potentially offers more information. We will discuss the connection between spectroscopic bandwidth and sample quantity. We will also discuss recent progress on the fabrication of polymer photonic crystal fibers designed for the THz frequency range. Using advanced fiber drawing technology we demonstrate and characterize low-loss propagation of broadband THz pulses through bendable fibers. The unique measurement techniques offered with THz time-domain techniques allows a highly detailed characterization of the modal profile of the propagating field inside the fiber as well as a full characterization of loss and dispersion of the fibers.