Fig. 3: Variation in Meta-Sticker’s resonances and mesocarp refractive index as fruit ripens.
From: Sub-terahertz metamaterial stickers for non-invasive fruit ripeness sensing
a,b, The overall change in the Meta-Sticker’s dipole resonance (fDP) (a) and propagating plasmon resonance (fPP) (b). A clear reduction in resonance frequency is observed as the fruit matures. This is a result of the conversion of starch to sugar and the accumulation of water and other substances. The results indicate that there is a correlation between fruit ripeness status and the observed resonances from Meta-Sticker. c, Estimated refractive indices of exocarp during the ripening of different fruits demonstrating a monotonic increase over time and as the fruit transitions from unripe to rotten post-harvest. d–f, Estimated refractive indices of mesocarp during the ripening of persimmon (d), pear (e) and mango (f). We observe a tight correlation between the Bx readings and the estimated nMe, which indicates the potential of our estimation framework for predicting Bx. The error is higher for mango because the measured DM values do not vary while the fruit matures and rots. Such changes inside the fruit are not observed by specialized NIR sensors, possibly because the mango exocarp is much thicker and more absorptive. Notably, such changes are evident with our sub-THz Meta-Sticker, where the estimated refractive index increases consistently as the fruit ripens.
