Terahertz Waveguides Research

To expand the range of THz applications it is essential to develop waveguides with minimal transmission loss and minimal dispersion. The first requirement is particularly difficult to fulfill because dielectrics and metals exhibit large absorption at THz frequencies. As a result, dielectric guides (widely used for optical waves) and hollow metallic waveguides (used for millimeter waves) exhibit transmission losses of ~5-100 dB/m at THz frequencies.

What is the best waveguide to transmit THz waves securely and without significant losses? Several waveguide designs, including photonic crystal fibres and parallel-plate metallic guides, are currently investigated in the search for a suitable solution. A few years ago transmission losses below 1 dB/m were finally realized in dielectric-lined cylindrical metallic waveguides, where the dominant mode exhibits remarkably low interaction with absorbing waveguide walls (Opt. Lett. 32, 2945 (2007)). Articles listed below summarise our studies of the dielectric-lined hollow metallic waveguides.

Can broadband THz pulses be transmitted through a waveguide without dispersion and with minimal losses?

We proposed and demonstrated a simple method for generation and efficient coupling of broadband THz pulses into THz coaxial waveguides.

Propagation of broadband THz pulses in an air-filled coaxial waveguide shows no dispersion and small transmission losses.

We utilised the effect of photo-generated surface currents to tailor profile of the THz field in order to match the radial mode of the coaxial waveguide.

Scientific Reports 6, 38926 (2016) Nature link

Flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band

In this paper we summarise our investigation of transmission properties of 1 mm-diameter PS-lined waveguides at THz frequencies. We performed broadband (0.5-2.5 THz) cutback loss measurement using our near-field time-domain spectroscopy (TDS) system. This near-field technique mitigates difficulties associated with the variation of the coupling coefficient with frequency and allows us to identify waveguide modes. We verified the near-field transmission loss measurements using single-frequency 2.85 THz quantum cascade laser (QCL).

The near-field TDS data show the low-loss transmission band that spans ~2.5 THz, from 2.5 to 5 THz, as confirmed by numerical modeling. Within the transmission band, the waveguides show the attenuation of 4-4.5 dB/m matching the calculated values.

These waveguides are suited well for the spectral range of THz QCLs and they can provide a solution for shaping the highly divergent QCL beams. The waveguides are flexible enough to form bends with the radius of curvature as small as 100-150 mm.

Optics Express 21, 23748 (2013) OSA link

Mode structure and dispersion in dielectric-lined cylindrical metallic THz waveguides

An electro-magnetic wave traveling in a waveguide can be considered as a superposition of basic waveguide modes. Each mode has a unique set of properties including the mode spatial profile, velocity of propagation and attenuation constant. In the dielectric-lined hollow metallic waveguides, the spatial profile is not the same as in the metallic guides without the dielectric coating. The profile shows a very small fraction of the wave energy traveling near the absorbing waveguide walls. As a result the dielectric-lined hollow metallic waveguides exhibit some of the best transmission characteristics among THz waveguides.

In this work we investigated the impact of the dielectric layer on the waveguide dispersion. Dispersion characteristics are determined experimentally for the low-loss waveguide modes, the linearly-polarized HE11 mode and the TE01 mode. The results are compared with dispersion in metallic waveguides. Additional dispersion due to the dielectric layer is found to be small for the dominant mode.

Optics Express 18, 1898 (2010) PDF OSA link

Graphic: Waveguide mode structure

Application of THz near-field microscopy for 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. The change of the mode profile reduces the Ohmic losses 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

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, C. M. Bledt, J. E. Melzer (Rutgers University
M. Navarro-Cia (ICL)
M. S. Vitiello (NEST, CNR)