Free-space material characterisation at terahertz frequencies using a vector network analyser

Hammler, J.; Gallant, A.J.; Balocco, C.

Abstract

While dielectric properties of many materials have been extensively characterised at low frequencies and into the microwave region of the electromagnetic spectrum, published information is much sparser in the THz region. Knowledge of permittivity is essential for design of photonic waveguides, mixers, modulators and other devices designed to operate at these frequencies. A system based around a vector network analyser was assembled to measure the permittivity of planar samples. The number of components required is vastly smaller than that of time domain spectroscopy systems. The Short-Open-Load-Through (SOLT) calibration procedure is performed on the system to ensure measurement accuracy. Free-space propagation of the reference and measured electric fields enabled measurements to be taken without contacting the sample; samples can be isolated for characterisation at, for example, non-ambient temperatures. Non-destructive testing of samples is possible, with no machining of samples to fit within a resonant cavity or waveguide required. This system topology is well suited for quality control testing in an industrial setting requiring high throughput. Scattering parameters, measured in the absence of a sample, were used to computationally move the measurement plane to the surface of the samples to be characterised. The de-embedding process can be adapted to eliminate the effects of different sample mounting jigs. An iterative method was developed for simultaneously determining sample geometric thickness and electric permittivity through numerical simulation of Maxwell’s equations. A constrained nonlinear optimisation process was employed to minimise the discrepancy between measured transmission and reflection data with simulated data. Dielectric and semiconductor samples were characterised with the system. Extrapolated thickness values were verified against measurements taken with a micrometer and were found to be in good agreement. Likewise, calculated permittivity values are in agreement with published material data, where available for the terahertz region. The system demonstrates that the new approach to characterising materials in the frequency domain simplifies both equipment setup and the measurement process, thereby providing a valuable tool for researchers and design engineers working within the THz spectrum.

Citation

Hammler, J., Gallant, A., & Balocco, C. (2015, December). Free-space material characterisation at terahertz frequencies using a vector network analyser. Presented at URSI AT-RASC 2015, Gran Canaria, Spain

• Presentation Conference Type: Conference Paper (published)
• Conference Name: URSI AT-RASC 2015
• Acceptance Date: Apr 29, 2015
• Publication Date: 2015
• Deposit Date: May 27, 2015
• Publisher: Institute of Electrical and Electronics Engineers
• DOI: https://doi.org/10.1109/ursi-at-rasc.2015.7302827
• Public URL: https://durham-repository.worktribe.com/output/1152936