New applications of long-distance THz pulse propagation

Tae-In Jeon

Division of Electrical and Electronics Engineering Korea Maritime and Ocean University Busan 49112, Republic of Korea

jeon@kmou.ac.kr

This study presents a novel approach for terahertz (THz) gas spectroscopy using a newly developed multipass cell setup with an extended pulse propagation length of 51 meters. By utilizing this setup, we investigate the transition characteristics of low-concentration N2O gas, which is crucial for understanding its behavior in atmospheric conditions. The measured THz pulses and corresponding spectra inside the multipass gas cell are analyzed, comparing scenarios with and without N2O gas. Despite the low gas pressure, small resonances are observed in the spectrum, attributed to water vapor in the cell. Our findings demonstrate the capability to accurately measure trace amounts of N2O gas using this setup, validating its efficacy for THz time-domain spectroscopy (THz-TDS). This approach opens up new possibilities for applications in environmental monitoring, industrial safety, and medical diagnostics while also enabling real-time studies of gas dynamics and chemical reactions in various fields.

Introduction
Terahertz (THz) spectroscopy has emerged as a powerful technique for studying molecular structures and dynamics due to its sensitivity to molecular vibrations and rotational transitions. In particular, THz time-domain spectroscopy (THz-TDS) offers high-resolution spectral measurements and has been widely used in various fields, including gas spectroscopy. However, traditional THz-TDS setups often face challenges in detecting gases at low concentrations, limiting their applicability in atmospheric studies and environmental monitoring. To address this limitation, we developed a new multipass cell setup with an extended THz pulse propagation length of 51 meters. This setup allows for enhanced sensitivity in detecting trace amounts of gases by facilitating multiple reflections of the THz beam within the cell. Here, we focus on investigating the transition characteristics of low-concentration N2O gas, which exhibits very low absorption coefficients at atmospheric pressure.

Result and conclusions
THz spectroscopy has emerged as a powerful tool for gas-sensing applications due to its unique ability to detect molecular resonances with high sensitivity and specificity [1-5]. THz pulses were generated using a mode-locked Ti-Sapphire laser to drive an optoelectronic source chip. Certain gases exhibit very low absorption coefficients, prompting previous THz gas spectroscopy studies to utilize high-concentration and high-pressure N2O gas to enhance sensitivity [3, 4]. However, at atmospheric pressure, most gases exist at extremely low concentrations. To examine the transition characteristics of low-concentration N2O gas, a gas cell with an extended propagation length within limited space is crucial.
Recently, we developed a new multipass cell setup with a 51 m THz pulse propagation length, consisting of two mirror sets facilitating multiple reflections of the THz beam. The incident THz beam follows a zigzag path, reflecting 23 times between the mirror columns before moving to another pair of mirror sets nearby. There are a total of 6 sets of mirror columns. Using the multipass cell setup with an 51 m THz pulse propagation length, we measured THz pulses for low-concentration N2O gas. The resulting absorption lines and transition frequencies agreed with the HITRAN database, validating the efficacy of the developed multipass gas cell for THz-TDS. Figure 1 illustrates the measured THz pulses and corresponding spectra of beam propagation inside the 51m-long multipass gas cell, comparing scenarios with and without N2O gas. The blue curve represents the reference THz pulse without N2O gas (vacuum) at a pressure of -0.100 bar, while the brown curve depicts the sample THz pulse with N2O gas at only -0.999 bar pressure. The blue and brown curves in the lower figure show the amplitude spectrum of the measured reference pulse for N2O gas and the amplitude spectrum for N2O gas in the gas cell, respectively. Although the gas pressure is very low, small resonances appear in the spectrum. The relatively strong resonances are due to water vapor in the cell; even though the pressure is only -1.00 bar, the water vapor cannot be completely removed. Based on these findings, we demonstrate the capability to measure even trace amounts of N2O gas using a long-path multipass gas cell for THz pulse propagation. This indicates the potential for a wide range of applications, including environmental monitoring, industrial safety, and medical diagnostics. Furthermore, the enhanced sensitivity achieved with this setup opens up possibilities for studying gas dynamics and chemical reactions in real time, paving the way for advancements in various fields such as atmospheric science, chemical engineering, and biomedical research.

Figure 1: (Upper) Measured sample THz pulse of beam propagation inside the 51m-long multipass gas cell, comparing pulse shape with N2O gas at a pressure of -0.999 bar and without N2O gas (vacuum: -0.100 bar). (Lower) Corresponding spectra of the measured THz pulses.

References
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[2] K. Komatus, T. Iwamoto, H. Ito, and H. Saitoh, “THz Gas Sensing Using Terahertz Time-Domain Spectroscopy with Ceramic Architecture,” ACS Omega 7, 30768-30772 (2022)
[3] H. S. Bark and T.-I. Jeon, “Pressure-dependent refractive indices of gases by THz time-domain spectroscopy,” Opt. Express 24, 29040-29047 (2016)
[4] G.-R. Kim, H.-B. Lee, and T.-I. Jeon, “Terahertz Time-Domain Spectroscopy of Low Concentration N2O using Long-Range multi-pass Gas cell,” IEEE Trans. THz Sci. Technol. 10, 524-530 (2020)
[5] E.-B. Moon, T.-I. Jeon and D. Grischkowsky, “Long-path THz-TDS atmospheric measurements between buildings,” IEEE Trans. Tera. Sci. Technol. 5, 742-750 (2015)