Long-lasting Leaf Water Stress Detector Based On An Infrared Micromechanical Photoswitch And A Solar Powered Sunlight Digitizer

This paper reports on a leaf sensor capable of detecting the water stress in the leaf by measuring the rise of transmitted sunlight through the leaf in short wavelength infrared (SWIR) only when the sunlight intensity is within a pre-defined range. Differently from previous demonstrations, the core sensing element, a plasmonically-enhanced micromechanical photoswitch (PMP), is directly connected to a fully solar powered sunlight digitizer which replaces an otherwise needed DC source for the biasing of the PMP. An experiment in lab with a solar simulator and a soybean plant has shown that the sunlight digitizer's output of similar to 5.1V, supplied to the PMP, makes the threshold of the latter suitable to detect a change in the leaf's IR transmittance as small as 3.7% which corresponds to a mild water stress level (relative water content, RWC, drops from 100% to 60%). Thanks to the completely passive IR sensing mechanism and the self-sustaining sunlight digitizer, the battery of the sensor node will only be drained for wireless communication upon the detection of water stress leading to a lifetime up to 10 years out of a coin cell.