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Novel techniques for terahertz sub-wavelength imaging.

Ho, Sze Phing (2015). Novel techniques for terahertz sub-wavelength imaging. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 121 p.

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Abstract

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Research activities at terahertz (THz) frequencies are the result of relatively recent developments. In spite of the hurdles associated to both theory and technical methods in THz generation and detection, THz technology has recently became a fertile ground for spectroscopy due to its capability in recognizing the composition of materials from the absorption spectra. By far, most of the applications exploiting THz radiation can be categorized into THz spectroscopy and imaging. The performance of THz imaging is severely restricted by the diffraction limit according to the so-called Rayleigh criterion, according to which the spatial resolution is limited by the relatively long radiation wavelength. For this reason, several near-field techniques have been developed to exceed the diffraction limit, i.e. aperture-based approaches (e.g. sub-wavelength (sub-λλ) apertures [1,2] and optical gating beams [3,4]), apertureless-based approaches (e.g. metal tips [5-7]) and illumination with highly localized sources via optical rectification (OR) [8-10]. The latter technique has been the subject of an intense development within the framework of this thesis. In particular, a precise implementation of the imaging protocol is essential for properly performing time-resolved THz imaging. In the following chapters we will show that sub-λ sources interact in a non-trivial way with certain samples. The complete understanding of this interaction is fundamental in image reconstruction (even in other electromagnetic spectral regions). This interaction significantly affects the source-field reconstruction via the knife-edge (KE) measurement technique [11-13]. The research activity presented here was aimed at developing an in-depth understanding in the characterization of sub-λ THz sources, including the inherent features arising from the interaction of sub-λ source objects and the exact extraction of the sub-λ THz source profile. It will be appreciated that the resulting reconstruction accuracy fundamentally exceeds the state-of-the-art (background) associated to this investigation. I will discuss the fundamental origin of the inherent aberrations in the KE characterization of sub-? sources (i.e. non-separable space-time nature of the dependence between source and field), also casting the proper Green-function of the imaging system that enables an aberration-free reconstruction. In addition, extensive experimental evidence will be introduced in support of those concepts. It will be also highlighted how this specific aberration is always present in the optical KE approach where the detection is usually performed monitoring the transmitted power only (as normally performed in the optical realm). Building-up on the previous concepts, the investigation will extend towards other inherent limitations of the KE technique. In usual implementations, the KE is a mechanical technique based on an opaque sharp blade (usually metallic) progressively clipping portions of a propagating field. The precise characterization of a source is inherently limited by the physical distance between the blade and the source generation plane. Within the framework of this thesis, a novel all-optical KE (AOKE) technique is then demonstrated. It relies on an ultra-thin layer of photo-excited free carriers exploited as the virtual blade at the output facet of a THz generation crystal. This is a completely optical solution and eliminates the need of a mechanical shield, placing the actual blade practically “inside” the generation crystal. The time-resolved sub-λ imaging of a source rich of sub-? features will be experimentally proved. In the last part of this thesis the bandwidth limitation of common THz solid-state detection means will be addressed. Time resolved detection of large bandwidth THz pulses is a key technology that finds applications in a variety of different fields, such as medical imaging, security and quality control. Taking inspiration from common forms of broadband THz detection operating only in gaseous media, this thesis will introduce the first demonstration of a solid state device exploiting electric field induced second harmonic generation (EFISH) in an electrically biased thin silica (SiO2) sample, free from the number of fundamental bandwidth limitations normally found in nonlinear detecting crystals or photo-conductive antennas. To conclude, I believe that the main contribution of this thesis is the possibility of exploring new approaches towards the characterization and beam profiling of coherent THz radiation, with potential impact in all those fields (e.g. in microscopy) where the diffraction limit and spectral bandwidth of current THz probing technologies lead to significant practical hurdles.

Type de document: Thèse
Directeur de mémoire/thèse: Morandotti, Roberto
Co-directeurs de mémoire/thèse: Clerici, Matteo
Informations complémentaires: Résumé avec symboles
Mots-clés libres: térahertz; sous-longueur d'onde
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 11 févr. 2016 18:35
Dernière modification: 11 févr. 2016 18:35
URI: http://espace.inrs.ca/id/eprint/3300

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