Sub-ppm Ammonia Sensing via Terahertz Phase Modulation in Porous Glass with a Chaotic Laser Diode

Abstract

The growing demand for noncontact sensing of environmental pollutants and toxic gases has increased interest in low-cost, compact, and easy-to-operate optical gas-sensing systems. In this study, we propose a low-concentration ammonia (NH3) detection method based on terahertz time-domain spectroscopy (THz-TDS) using a chaotic multimode laser diode (MLD). Introducing porous glass into the gas cell enhanced interactions occurring between the NH3 molecules and the internal silanol surfaces, leading to a decrease in the effective refractive index and a temporal advance of the THz time-domain waveform. The system achieved NH3 detection down to 0.2 ppm. The response increased rapidly at low concentrations and tended to saturate above approximately 1 ppm, which could be described by a Langmuir-type adsorption model. These results demonstrate that chaotic MLD THz spectroscopy combined with porous glass provides a low-cost and compact approach for sub-ppm-level ammonia detection.