Développement de systèmes d’imagerie dans la gamme des fréquences térahertz

Abstract

Terahertz waves lie in the electromagnetic spectrum between microwave and infrared radiation. Due to their interesting properties, they have been of growing interest for several decades now, both in the world of fundamental research and in the industrial sector. Moreover, the development in recent years of numerous techniques for generating and detecting these waves has opened the door to a wide range of applications. Many of these are based on the acquisition of images of objects, and thus rely on the use of imaging systems. The time required to take an image, detection sensitivity and spatial resolution are important parameters to consider in THz imaging. The main objective of this experimental thesis work is to develop imaging systems in the terahertz frequency range. These enable rapid image acquisition, with high sensitivity and optimum spatial resolution. Contributions have been made, from the THz light source used to illuminate the imaged object to the imaging technique employed and image processing. This manuscript is structured between a detailed contextualization of the research project and a comprehensive presentation of the results obtained. The field of terahertz waves is first introduced (see Introduction). In addition to its applications, the main methods used to transmit and detect these waves are presented. The motivations and objectives of this work are also described. Next, a second part (see Chapter : 1) presents the state of the art in the key concepts discussed in this work. The third part (see Chapter 2 ) presents the study carried out on a THz source by optical rectification of a ytterbium laser pulse in a zinc telluride crystal. A fourth section (see Chapter 3) presents a THz source based on optical rectification in a lithium niobate crystal using a pumping technique with an inclined wavefront. It is this high-performance source that has been used with the imaging systems set up in the remainder of this thesis. A fifth section describes a coherent imaging system used in a near-field configuration (see Chapter 4). With this system, we have demonstrated the acquisition of hyperspectral images with sub-wavelength resolutions. A sixth section focuses on exploratory work on an imaging technique for reconstructing THz images with enhanced spatial resolution (see Chapter 5). This included an active modulation technique for a THz beam. The final section outlines the future prospects arising from this work.

Date	11 Nov 2024
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	François Blanchard (Supervisor)