Electromagnetically Induced Absorption in the Near-Field of Microwave Radiative Elements With Application to Foliage Moisture Sensing

Electromagnetically induced absorption (EIA) is a quantum phenomenon which occurs when detuned resonant laser fields interfere via atomic transition pathways. The transmission spectrum of a material experiencing EIA consists of an enhanced narrowband absorption line in between the two laser resonances. In this paper, we implement near-field interference of two microstrip radiators to produce a similar absorption mechanism. We propose a practical sensing application to detect foliage moisture by detecting the resonance shifts when sample leaves are made to perturb the near-field radiations. For the sensing, we exploit the anomalous phase signature that accompanies the EIA effect, instead of the amplitude signatures traditionally used in contemporary microwave sensors. The sensing using phase spectrum performs better than the amplitude-based sensing in harsh environments affected by noise and external interferences. Since the proposed EIA-based detector exploits multiple antenna interference in the near field, resonant sensing over distance is also possible. We demonstrate practical moisture detection using actual foliage samples with different moisture levels. We also develop a numerical dielectric model to estimate foliage moisture using full-wave electromagnetic simulations. We anticipate, from this paper, a way to produce low-cost and non-invasive microwave sensors that have reasonable sensitivity and which can be used in remote areas subjected to extreme weather environments.