Quantitative detection of substances without feature absorption peak by terahertz Rydberg atom sensor.

In recent years, although quantitative, nondestructive terahertz (THz) testing has developed rapidly, a broadband and fast approach for material quantification is still lacking. In this paper, a Rydberg-atom THz detector is applied to measure the concentration of α-lactose monohydrate (α-LM). Specifically, when cesium atoms are excited to the 40D5/2 state by two lasers, the probe transmission exhibits an electromagnetically induced transparency (EIT) signal, and then splits into an Autler-Townes doublet in the presence of a THz field, with the splitting interval proportional to the THz field strength. Meanwhile, α-LM attenuates the THz field incident on the cesium atom vapor, which reduces the AT splitting, therefore, the sample concentration can be inferred by the AT splitting interval and concentration relationship. Moreover, the same samples were measured by a conventional THz time-domain spectroscopy (THz-TDS) system for comparison. While the THz-TDS system failed to establish a reliable calibration at non-resonant frequencies, the Rydberg-atom detector achieved excellent agreement with the Beer-Lambert law across a wide range of concentrations, even at a non-resonant frequency of 108.9 GHz. Consequently, this approach has no requirement for the sample's characteristic absorption peaks and the system's bandwidth. The proposed method proves Rydberg-atom sensor is a powerful and versatile tool for quantitative THz spectroscopy, and pave a way for non-destructive testing and material analysis.