Terahertz time-domain derivative spectroscopy using MEMS

Abstract

The Terahertz (THz) domain, with frequencies spanning from 0.1 to 10 THz, between microwave and infrared bands, bridges the gap between the electronics and photonics worlds. Historically, this part of the electromagnetic (EM) spectrum was known as the “terahertz gap” due to complexities in generating and detecting THz waves. In the past decades, considering the advancements in ultrafast lasers and coherent generation and detection methods, the potential of this unique wave has been revealed. The low energy of THz wave makes it suitable for non-ionizing and non-destructive applications by penetrating non-polar and non-metallic materials. THz time-domain spectroscopy (THz-TDS) is a robust tool that benefits from THz wave features for numerous applications, from material characterization to pharmaceutical applications, communications, semiconductor device quality control, and imaging. This tool provides information about the electrical field's transient nature that can help obtain the signal's amplitude and phase. An innovative approach to improve the THz-TDS is to add a modulator in the system and obtain THz time-domain derivative spectroscopy (THz-TDDS). The derivative function highlights the rapid changes and helps improve spectrophotometric features. There have been limited advancements in THz-TDDS development, which motivates my research to investigate an enhancement method for THz-TDDS. Therefore, a commercially off-the-shelf MEMS is used in the THz beam path to modulate the THz beam. Benefiting from previous work done at ÉTS in this specific field, the research goals are selecting a MEMS with better characteristics and improving the modulation features. The first step includes proving the derivative functionality of this THz setup with mathematical verification and simulation. Four materials are studied to validate the spectroscopic performance of the THz-TDDS, and the optical parameters, including refractive index and absorption coefficient for each material, are calculated. Comparing the results of this setup against a commercial and highly accurate device and comparing it with the available literature, the spectroscopic capabilities of this setup are proven. The final step is to investigate and scrutinize the error sources, provide recommendations for improvement for future research prospects, and suggest the design and application of an array of MEMS in THz imaging. One of the goals of this research is to expand the use of MEMS in THz applications. However, there is a trade-off between some parameters, such as reproducibility and affordability, and low signal-to-noise ratio (SNR). Moreover, all research facilities with similar advanced systems, such as the femtosecond laser available at the TeraÉTS THz lab, can benefit from this approach of utilizing a commercial MEMS, and various novel applications can be envisioned. This thesis begins by outlining its essential research objectives and framework. The first chapter provides the background and literature review to place this research and show the relevance of the proposed solution in response to the research gaps. It also details the methodology employed and the corresponding experimental results. The third chapter discusses the sources of error and their impact on the results, and finally, the last chapter provides recommendations for future works and lessons learned. Finally, the Annexes will provide the simulation and Matlab scripts utilized in this thesis.

Date	20 Dec 2024
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	François Blanchard (Supervisor) & Frédéric Nabki (Co-supervisor)