Development of novel metrology methods for time domain THz spectroscopy systems

Abstract

The terahertz (THz) time-domain spectroscopy (TDS) system is a powerful tool because it uses the properties of THz waves in a unique way for spectroscopy and imaging technologies in a wide variety of applications, such as materials science, engineering, medicine, and chemistry. The THz frequency range lies between the microwave and infrared bands in the electromagnetic (EM) spectrum (i.e., between 0.1 THz and 10 THz), which connects the electronic part of the spectrum with the photonics part. Compared to infrared frequencies, THz frequencies can penetrate non-metallic and non-polar materials with low enough energy, which is insufficient to ionize atoms or molecules. In addition, the THz-TDS system can be used to measure not only the intensity of a spectral pulse, but also the associated transient electric field, providing a unique measurement method to easily calculate the amplitude and phase of a pulse from the measured THz electric field. Over the past 30 years, the performance of THz sensing systems has been improved significantly. However, to move THz spectroscopy and imaging from a laboratory-scale technique to a versatile tool with many practical applications, technical developments are still needed in sources and detectors. Therefore, many scientists are currently working on improving THz sources and detectors with enhanced characteristics, which usually involves using new materials, methods, and techniques to improve these technologies. This essential requirement motivates the subject of my thesis: to develop new methods to improve the detection sensitivity of the THz system. This work aims to contribute to the development and advancement of knowledge in the field of THz sensing technology. In particular, it focuses on THz detection in the THz-TDS system. To do so, a review of the literature is first presented. It covers the properties of the THz frequency and the THz-TDS system, their generation and detection, and their applications. Following the literature review, I present a new method for characterizing thin-film electro-optical materials using a metamaterial (i.e., a split-ring resonator (SRR)) and intense THz near-field microscopy. The method is based on the simulation and analytical study of the near-field imaging resolution of the electric and magnetic field distributions of the SRR, which is designed for the THz frequency range. The main advantage of this method is that one can use a non-contact electrode (i.e., the SRR) in the experiment to characterize an unknown thin-film material and then compare the results with the simulation results provided in my approach. I then present a simple technique by testing a new device in the THz-TDS system that provides a time-derived THz wave. This device is a piezoelectric micromachined ultrasonic transducer array (PMUT) that differentiates a THz pulse by performing time variations (on the femtosecond scale) in the THz beam path. The modulated THz signal detected after the piezoelectric device is proportional to the first-order derivative of the THz pulse. This presentation is coupled with the presentation of the characterization, performance of this device and its limitation as a THz modulator. This study allows understanding the principle of using this device as a THz modulator in order to increase the knowledge to choose this type of device with better performances. Following this presentation, I tested another device with the same properties as the previous one, but with larger dimensions and motion to improve the THz wave derivation. This is a piezoelectric micromachined device (PM), which is inserted into the THz beam path. It provides a reference modulation for the lock-in detection unit, which in turn provides access up to the fourth-order of derived information of the incoming THz signal. It should be noted that the integration of the recorded derivative signal leads to a recovered reference signal with an equivalent or even better signal-to-noise ratio (SNR), opening the door to a new type of highly sensitive THz measurement in the time domain. Finally, there is a conclusion to the work provided in this thesis. It discusses potential future developments of this PhD project.

Date	14 Dec 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	François Blanchard (Supervisor) & Tsuneyuki Ozaki (Co-supervisor)