Monitoring the water status of economic plants with continuous wave terahertz radiation

Monitoring the water status of economic plants is of vital importance for optimizing irrigation strategies and for a deeper understanding of the influence of drought stress. Most of the conventional techniques for measuring the water content of plants are destructive. Hence, most studies are limited to a small number of control samples. Sensing with electromagnetic waves is non-destructive and thus ideally suited for monitoring the plant water status. In the past two different frequency domains have been employed: the infrared [1] and the microwave frequency range [2]. As infrared waves are heavily absorbed by water infrared spectroscopy only works in reflection geometry. Microwaves, on the other hand, can be used in transmission but only allow for a low spatial resolution. Here, we use terahertz (THz) waves to non-destructively determine the water status of living plants. Terahertz waves are somewhat less heavily absorbed by water than infrared waves and can be used in transmission geometry. Furthermore, they allow for a sub-millimetre spatial resolution [3]. In recent work it was shown, that the optical material parameters obtained by pulsed THz time domain spectrometers can be utilized to derive the water content of leaves [4]. However, these broadband systems are based on expensive femtosecond lasers. In contrast to that, continuous wave (CW) THz systems are built with two inexpensive diode lasers, the emission of which is heterodyned in a photomixing antenna to generate a single tuneable THz frequency.