Low Dispersion Propagation of Broad-Band THz Pulses in a Two-Wire Waveguide

Abstract

Terahertz (THz) waveguides are the subject of an intensive research aimed at transferring THz radiation over long distances while preserving the pulse’s amplitude and phase. Standard, optical guided waves approach are not suitable since most of the THz generation techniques deliver extremely large bandwidth pulses, corresponding to single-cycle electric wave packets, which will strongly disperse in dielectric waveguides. This issue is even more important considering that one of the main applications of THz radiation is time-domain spectroscopy, where the temporal coherence of the probing pulse is a must. On the other hand, dispersion-less waveguides have been known at longer wavelengths, e.g. microwave and radio-signals, for several years (Collin, Foundations for microwave engineering. Wiley, New York, 2001). As previously proposed also by Mbonye and coworkers for gigahertz signals (Mbonye et al. Appl Phys Lett 95(23):233506, 2009), we have investigated a two-wire configuration properly scaled for the transmission of THz signals. Here, we demonstrate the characterization of a two-wire waveguide for the dispersion-less delivery of broadband THz pulses, due to a quasi-TEM mode excitation (Tannouri et al. Chin Opt Lett 9(11):110013, 2011). We present our experimental results indicating that low dispersion could be achieved for a broad THz bandwidth over 20 cm of propagation by employing Copper wires separated by 300 μm gap in the air. The waveguide is supported by an Aluminum base plate which acts as its backbone. Two dielectric slabs are then attached to either ends of the base plate. The dielectric slabs have a 800 μm diameter hole drilled through their centres (Fig. 57.1). As shown in the figure, the two copper wires of 250 μm diameter each pass through the centre of these holes on either side of the base plate. As the diameter of the hole is 800 μm, a space of 300 μm is left between the two wires. In order to maintain a uniform separation between the wires, they are kept under tension. Tension is applied by wrapping the four free ends of the wires around screws. A THz TDS setup was used to test the waveguide (80 MHz rep. rate, 125 fs, 800 nm). The broadband THz radiation is generated by an 80 μm gap-size photo-conductive antenna. One of the major issues related to the investigated two-wire configuration is the efficient coupling between the quasi-TEM00 mode of the waveguide and the generated light. We addressed this issue by placing the emitter in close proximity to the input of the waveguide. Figure 57.2 shows the measured THz signal (normalized) for both (a) the signal injected into the waveguide and (b) the one transmitted by the two-wire waveguide and measured via electro-optical sampling. In Fig. 57.2c the normalized amplitude spectra are shown for both the input and the transmitted pulse. The recorded data clearly demonstrate that the single-cycle character of the input pulse is preserved after 20 cm of propagation, with negligible modulations in the power spectral density, on a more than 2 THz bandwidth. Remarkably, the two-wire waveguide supports quasi-TEM mode confined on very small area in between the wires, opening intriguing perspectives e.g. for high-spatial resolution imaging.