Miniaturization of terahertz time domain spectroscopy (THz)

Abstract

Terahertz waves has been proven to be useful in a variety of applications across diverse studies, ranging from condensed matter physics and gas-phase spectroscopy to biomedical imaging, with even suggestions for usage in outer space. However, as of April 2024, terahertz is still restricted to research purposes instead of the domestic market due to the logistical and practical challenges for the usage of Terahertz. The terahertz (THz) spectral region, spanning from 300 GHz (λ= 1 mm) to 10 THz (λ = 30 μm), is still relatively underdeveloped compared to optical or microwave frequencies. In general, THz spectroscopy can be broadly separated into measurements of gases and that of liquids and solids. At the low number density characteristic of gases, frequency of collisions between molecules determines dephasing times that occur on time scales generally much longer than 1 ps. It is worth noting that considerable research on the development of photoconductive THz emitters for specific applications have been done, where the detection and generation of THz pulses, the usage of photo conductively gated antennas shares the same physical principles. Properties such as antenna design and semiconductor properties can critically affect THz detection.

With regards to research involving the time-domain terahertz spectroscopy and terahertz Spectroscopy in general, there are research gaps present. One research gap is that several research papers do not consider some of the major disadvantages of THz spectroscopy, such as the possible complexity and high cost that comes with the usage of such equipment, and the ability to obtain fine details in certain applications can be constrained by inherent limitations of terahertz optics and the wavelength of the radiation itself. For instance, a study by Philip F. acknowledged that the low power feature of the THz spectroscopy allowed for overcoming of the disadvantages of the “Raman spectroscopy in inducing phase or chemical changes in a sample”, but it does not consider that the THz spectroscopy may yield weaker signals and longer acquisition times. This can limit detection and analysis of weak absorptions or features in the sample, which affects distinguishing closely spaced peaks or features.

Another research gap that is tied into this study is the presence of very few studies into the research and development of the miniaturization of terahertz spectroscopy. One of the earliest suggestions for the miniaturization of spectroscopy dating back to a study published in 2008, where Dr Kaori Fukunaga suggested in her study titled “Terahertz Spectroscopy for Non-Invasive Analysis of Cultural Properties”, that that since “cultural properties are most preferably studied onsite, we [ the research sector] must work toward miniaturization of a terahertz spectroscopy system” One of the latest development of such a portable system dates to 2023 where in an study titled “High-sensitivity computational miniaturized terahertz spectrometer using a plasmonic filter array and a modified multilayer residual CNN” by the Micro-Nano Fabrication Center at Zhejiang University managed to successfully built a computational miniaturized THz spectrometer which demonstrated the possibility for a miniaturized THz spectrometer, and thus promotes the application of the THz spectroscopy technique in handheld and portable scenarios. In this thesis, we document the process building and testing of a miniaturized terahertz spectroscopy machine using photoconductive antennas and a power supply circuitry. In general, the experiment was successful. However, challenges do lie ahead, such as the presence of water vapor affecting the frequency spectrum which degrades sensing and communication capabilities. Furthermore, there is the challenge of slow scanning speed inherent in miniaturized Time-Domain Terahertz Spectroscopy (TDTS) setups. This bottleneck impedes the practicality of on-the-go applications requiring rapid data acquisition. Another challenge lies in the reliance on laptops for data processing and control remains a notable hurdle in achieving truly compact and standalone TDTS platform ensuring optimal measurement conditions, such as a nitrogen-purged enclosure, presents another challenge in miniaturized TDTS designs. Nitrogen purging is essential for minimizing atmospheric interference and maintaining stable measurement environments, particularly in sensitive spectroscopic applications. Future experiment could test the miniaturized circuit in different environmental conditions and integrating high-speed delay stages with precision encoders presents a promising solution.