Many investigators around the globe are exploring terahertz radiation for applications in remote sensing and imaging, and a good low-loss waveguide in this spectral range likely would facilitate many of these applications. Several approaches are being pursued, one of the more promising of which is hollow waveguides. These are particularly attractive for analyzing air or other gases for contaminants because the sample can be placed inside the waveguide where the terahertz fields are strongest. Recently, James A. Harrington and his colleagues at Rutgers University in Piscataway, and at Bell Laboratories in Murray Hill, both in New Jersey, developed what they believe are the lowest-loss terahertz waveguides. With a propagation loss of 0.95 dB/m, the waveguides are apparently the first to break the 1-dB/m barrier.The investigators previously reported fabricating a hollow glass waveguide for terahertz radiation by depositing copper or silver on the inside of hollow glass tubes. Although these waveguides were functional, their loss — 3.9 dB/m — was too high for many practical applications. The recently achieved 0.95-dB/m result came from adding a dielectric overcoat to the metal coating inside the waveguides.The physics behind the improved result with the dielectric overcoat lies in the reflectance of electromagnetic radiation from a metal surface. The reflectance of metals at near-glancing angles of incidence is high for the s-polarization but low for the p-polarization. Thus, the transverse-electric (TE01) mode — whose electric field is parallel to the walls of the waveguide (i.e., always in the s-polarization at the walls) — experiences relatively low loss in a plain-metal hollow waveguide, even without the dielectric overcoat. But it’s difficult to convert the output of a linearly polarized (or unpolarized) laser into an azimuthally polarized TE01 mode in a hollow waveguide. What’s easy to excite with a linearly polarized input beam is the linearly polarized HE11 hollow-waveguide mode. Figure 1. Propagation loss in the hollow glass waveguide decreased with increasing thickness of the dielectric overcoat on the metal film on the inner surface of the waveguide. These experimental data (points) and theoretical values (solid lines) are for three core diameters: 1.6, 1.7 and 2.2 mm. Images reprinted with permission of Optics Letters.The addition of a dielectric overcoat creates an interference effect that significantly increases the reflectivity of the p-polarization and, therefore, reduces the loss experienced by the HE11 mode. In their experiment, the scientists added a polystyrene film over the silver coating. They propagated radiation from a Coherent Deos terahertz laser into the waveguide and calibrated its loss by measuring the power transmitted through the guide, repeatedly cutting the waveguide to a shorter length and remeasuring the transmitted power. They found that the loss diminished fairly rapidly with increasing thickness of the dielectric overcoat, up to a thickness of about 5 μm (Figure 1). Although the experimental losses are higher than predicted by theory, they nonetheless show the predicted dependence on dielectric thickness. The scientists believe that the lowest measured loss, 0.95 dB/m, is lower than has been measured previously in a hollow glass waveguide.Figure 2. These photos show the mode pattern that emerged from the hollow glass waveguides under various conditions. The numbers under each image are the waveguide’s core diameter, the thickness of the dielectric overcoat and the polarization of the radiation imaged, respectively. HP = horizontal polarization; VP = vertical polarization; NP = no polarizer used.They also used a Spiricon pyroelectric camera to record the mode profiles of the terahertz (119 μm) radiation emerging from the end of a 90-cm waveguide with a 1.6-mm core (Figure 2). The Deos laser launched horizontally polarized radiation into the fiber, evenly divided between TE and TM modes at the input. The scientists used wire-grid polarizers to image the vertical and horizontal components of the transmitted radiation separately.In the top row of Figure 2 (showing no dielectric overcoat on the waveguide), light emerged in a doughnut shape, characteristic of the TE mode favored in a plain-metal waveguide. When a 10-μm-thick dielectric overcoat was added to the waveguide (second row), the light that emerged was a combination of TE and TM modes because the TM mode experienced lower loss as a result of the dielectric overcoat.In a larger-core waveguide (bottom two rows), higher-order modes are clearly visible, although the TE prevailed in the absence of a dielectric overcoat, and the mixed TE and TM modes when the overcoat was added were still present.Optics Letters, Oct. 15, 2007, pp. 2945-2947.