Terahertz Waveguides Research

To expand the range of THz applications it is essential to develop waveguides for Terahertz waves. Most applications will require minimal transmission loss and minimal dispersion in the waveguide link. These requirements are difficult to fulfill at THz frequencies because dielectric materials exhibit large absorption and metals suffer from high Ohmic losses. Therefore several waveguide designs, including photonic crystal fibers and parallel-plate metallic guides, are currently researched. A few years ago THz waveguides with losses below 1 dB/m were finally demonstrated (Opt. Lett. 32, 2945 (2007)). This level of signal attenuation was realized in dielectric-lined cylindrical metallic waveguides, where the dominant mode exhibits remarkably low interaction with absorbing waveguide walls.

Analysis of Modes in THz Waveguides

Identification of waveguide modes is essential for studies of the waveguide transmission properties. It helps to optimize the waveguide design and reduce losses, which present a major challenge for THz waveguide research. We applied time-resolved THz near-field microscopy to determine and investigate propagating modes in hollow metallic waveguides.

The dominant mode in a hollow metallic waveguide has the electric field terminating abruptly at the waveguide wall, while the dominant mode in a similar waveguide with a thin dielectric coating, has the electric field vanishing near the wall. As a result, Ohmic transmission losses are reduced substantially in the dielectric-lined waveguides. The near-field images of the waveguide output clearly show the difference in the mode structure (left and right panels).

A space-time map (bottom panel) measured with the near-field probe also allows to determine whether higher order modes are present. These modes propagate with lower group velocities and have distinctive electric field spatial patterns.

Applied Physics Letters 94, 171104 (2009) PDF AIP link

Graphic: Analysis of waveguide modes

Mode-dependent Transmission Losses in Hollow Metallic Waveguides

Transmission losses at THz frequencies can be substantially reduced in dielectric-lined hollow metallic waveguides compared to simple metallic waveguides.

This work describes cutback loss measurements for two types of metallic waveguides, in which either the TE01 or the HE11 mode has the lowest transmission loss. These waveguides differ only in the thickness of the inner dielectric layer. The classical TE01 mode is the dominant mode for waveguides with ~1-2 micron thick layers (similar to the case of metal-only waveguides). However the HE11 mode becomes the dominant mode if the dielectric layer thickness is increased to ~ 10 microns. The transmission loss in this case falls below 1 dB/m at 2.5 THz.

THz waveguides supporting the HE11 mode can become important for practical applications, where the low-loss transmission properties, good mode confinement and efficient coupling to free-space propagating beams is necessary.

Applied Physics Letters 93, 181104 (2008), PDF AIP link

Transverse and Hybrid modes in hollow THz waveguides

Research was done in collaboration with
J.A. Harrington, B. Bowden and P. Mark (Rutgers University)