Abstract-System-theoretical modeling of terahertz time-domain spectroscopy with ultra-high repetition rate mode-locked lasers

Kevin Kolpatzeck, Xuan Liu, Kai-Henning Tybussek, Lars Häring, Marlene Zander, Wolfgang Rehbein, Martin Moehrle, Andreas Czylwik, and Jan C. Balzer

 Block diagram of a fiber-coupled THz-TDS setup. The optical output signal of the mode-locked laser source is distributed to a terahertz emitter (THz Tx) and, through a variable delay line, to a terahertz detector (THz Rx). The terahertz radiation generated by the emitter is transmitted through a sample and focused into the detector

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-11-16935

Terahertz time-domain spectroscopy (THz-TDS) systems based on ultra-high repetition rate mode-locked laser diodes (MLLDs) and semiconductor photomixers show great potential in terms of a wide bandwidth, fast acquisition speed, compactness, and robustness. They come at a much lower total cost than systems using femtosecond fiber lasers. However, to date, there is no adequate mathematical description of THz-TDS using a MLLD. In this paper, we provide a simple formula based on a system-theoretical model that accurately describes the detected terahertz spectrum as a function of the optical amplitude and phase spectrum of the MLLD and the transfer function of the terahertz system. Furthermore, we give a simple yet exact relationship between the optical intensity autocorrelation and the detected terahertz spectrum. We theoretically analyze these results for typical optical spectra of MLLDs to quantify the effect of pulse chirp on the terahertz spectrum. Finally, we confirm the validity of the model with comprehensive experimental results using a single-section and a two-section MLLD in a conventional THz-TDS system.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Through-substrate terahertz time-domain reflection spectroscopy for environmental graphene conductivity mapping

Applied Physics Letters

 Hungyen Lin,  Oliver J. Burton, Sebastian Engelbrecht, Kai-Henning Tybussek,  Bernd M. Fischer,  Stephan Hofmann

Schematic of the reflection-based THz-TDS to measure the complex conductivity of graphene through the substrate in a nitrogen purge environment.

https://aip.scitation.org/doi/abs/10.1063/1.5135644

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapor deposited graphene films through silicon support. We validate the technique against measurements performed using the established transmission based THz-TDS. Our through-substrate approach allows unhindered access to the graphene top surface and thus, as we discuss, opens up pathways to perform in situ and in-operando THz-TDS using environmental cells.

H.L. acknowledges financial support from the EPSRC (Grant No. EP/R019460/1). S.H. acknowledges funding from the EPSRC (Grant No. EP/K016636/1, GRAPHTED). We also thank Dr. Philipp Braeuninger-Weimer for useful discussion. Additional data for this article are available at https://doi.org/10.17635/lancaster/researchdata/336.