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Spectral Domain Interferometry for Terahertz Detection: Concept and Application.


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Ibrahim, Akarm Yousif (2017). Spectral Domain Interferometry for Terahertz Detection: Concept and Application. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 160 p.

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The current golden standard for measuring terahertz (THz) temporal waveform is the electro-optic sampling (EOS) technique. However, EOS technique when exploited for measuring intense THz electric field has a limited dynamic range (DR) due to a phenomenon called over-rotation. This limited DR makes it very difficult to measure intense THz signals and weak signals at the same time. Routinely, THz filters (such as silicon) are used to ensure that the measurements are within the DR of the EOS technique and then calibrated against reference data. However, this not only distorts the THz waveform but when those filters are irradiated with intense THz radiation, recent researches have shown that there could be substantial changes in their transmission characteristics due to nonlinear THz effects. Therefore, a proper interpretation of the data obtained through the EOS technique in intense THz experiments could be complicated. In the framework of this thesis, we describe a novel technique for measuring THz waveform that does not suffer from over-rotation limitation, possessing a tremendous DR of ~7x10⁶, while at the same time having high signal-to-noise ratio (SNR) that are comparable to that of the commonly used EOS techniques. In this new technique, we make use of spectral-domain interferometry (SDI) technique to develop different detection schemes, such as Michelson interferometer type based SDI (later we distinguish it as the standard SDI), Mach-Zehnder interferometer based SDI (MZI-SDI), and fiber-based SDI (later we distinguish it as cross-polarized SDI (CP-SDI)) schemes to measure intense THz electric fields. We perform a comparison between the three SDI schemes for THz electric field measurement schemes. Based on our experimental findings, the temporal waveform of the THz electric field measured with the standard SDI technique exhibits reasonable DR and SNR. However, the temporal scanning window is limited due to the finite fixed thickness of the glass plate that is used to create the two interfering signals. This limitation in the temporal scanning window has been avoided in both MZI-SDI and CP-SDI for THz electric field measurement schemes. We find that the SNR of the THz electric field measurement obtained with MZI-SDI scheme is inferior to that obtained with the CP-SDI scheme. We attributed this inferior SNR in the former scheme to the fact that there are more optical components in the optical probe beam signal, compared to the latter scheme. Each of these optical components is susceptible to vibrational noise in the setup, and thus contributes to the overall noise buildup in the measurement. On the other hand, in the case of the CP-SDI scheme, the situation is different, since both interfering signals are propagating along one common path (along the optical fiber). In this case, the noises associated with vibrations in the optical components, which in turn change the optical path difference (OPD) between two interfering signals, are mitigated since any change in OPD is common to both signals. Owing to the excellent performance of the CP-SDI scheme, with an SNR ~48.97 dB and DR~7×10⁶, we find this version the best among all SDI-THz detection technique versions and thus suitable for carrying out THz spectroscopic measurements. Furthermore, as a second objective of this thesis, the new CP-SDI THz detection technique was validated successfully in measuring the THz response -in transmission geometry- of high-permittivity materials, which are encountering challenges in the THz frequency range. In this work, we chose Barium Strontium Titanate (BST), which notoriously has significant losses in the THz frequency region, thus affecting the SNR of the THz measurement. Furthermore, we hold a comparison between the dielectric response results obtained via CP-SDI THz detection technique and those obtained via conventional EOS THz detection technique for the same BST sample. We show that from the comparison, the CP-SDI technique enables obtaining more accurate results due to its ultra-high DR and good SNR.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ozaki, Tsuneyuki
Mots-clés libres: terahertz (THz) temporal waveform; Spectral Domain Interferometry (SDI); electro-optic sampling (EOS) technique
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 03 juill. 2018 15:08
Dernière modification: 03 juill. 2018 15:08
URI: http://espace.inrs.ca/id/eprint/6931

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