Multi-Sensors Remote Sensing Applications for Assessing, Monitoring, and Mapping NPK Content in Soil and Crops in African Agricultural Land

Demand for agricultural products is increasing as population continues to grow in Africa. To attain a higher crop yield while preserving the environment, appropriate management of macronutrients (i.e., nitrogen (N), phosphorus (P) and potassium (K)) and crops are of critical prominence. This paper aims to review the state of art of the use of remote sensing in soil agricultural applications, especially in monitoring NPK availability for widely grown crops in Africa. In this study, we conducted a substantial literature review of the use of airborne imaging technology (e.g., different platforms and sensors), methods available for processing and analyzing spectral information, and advances of these applications in farming practices by the African scientific community. Here we aimed to identify knowledge gaps in this field and challenges related to the acquisition, processing, and analysis of hyperspectral imagery for soil agriculture investigations. To do so, publications over the past 10 years (i.e., 2008–2021) in hyperspectral imaging technology and applications in monitoring macronutrients status for crops were reviewed. In this study, the imaging platforms and sensors, as well as the different methods of processing encountered across the literature, were investigated and their benefit for NPK assessment were highlighted. Furthermore, we identified and selected particular spectral regions, bands, or features that are most sensitive to describe NPK content (both in crop and soil) that allowed to characterize NPK. In this review, we proposed a hyperspectral data-based research protocol to quantify variability of NPK in soil and crop at the field scale for the sake of optimizing fertilizers application. We believe that this review will contribute promoting the adoption of hyperspectral technology (i.e., imaging and spectroscopy) for the optimization of soil NPK investigation, mapping, and monitoring in many African countries.

Terahertz Nano-Imaging with s-SNOM

Spectroscopy and imaging with terahertz radiation propagating in free space suffer from the poor spatial resolution which is a consequence of the comparatively large wavelength of the radiation (300 μm at 1 THz in vacuum) in combination with the Abbe diffraction limit of focusing. A way to overcome this limitation is the application of near-field techniques. In this chapter, we focus on one of them, scattering-type Scanning Near-field Optical Microscopy (s-SNOM) which − due to its versatility − has come to prominence in recent years. This technique enables a spatial resolution on the sub-100-nm length scale independent of the wavelength. We provide an overview of the state-of-the-art of this imaging and spectroscopy modality, and describe a few selected application examples in more detail.

Electro-optic spatial light modulator from an engineered organic layer

Tailored nanostructures provide at-will control over the properties of light, with applications in imaging and spectroscopy. Active photonics can further open new avenues in remote monitoring, virtual or augmented reality and time-resolved sensing. Nanomaterials with χ(2) nonlinearities achieve highest switching speeds. Current demonstrations typically require a trade-off: they either rely on traditional χ(2) materials, which have low non-linearities, or on application-specific quantum well heterostructures that exhibit a high χ(2) in a narrow band. Here, we show that a thin film of organic electro-optic molecules JRD1 in polymethylmethacrylate combines desired merits for active free-space optics: broadband record-high nonlinearity (10-100 times higher than traditional materials at wavelengths 1100-1600 nm), a custom-tailored nonlinear tensor at the nanoscale, and engineered optical and electronic responses. We demonstrate a tuning of optical resonances by Δλ = 11 nm at DC voltages and a modulation of the transmitted intensity up to 40%, at speeds up to 50 MHz. We realize 2 × 2 single- and 1 × 5 multi-color spatial light modulators. We demonstrate their potential for imaging and remote sensing. The compatibility with compact laser diodes, the achieved millimeter size and the low power consumption are further key features for laser ranging or reconfigurable optics.

Near-field optical imaging and spectroscopy of 2D-TMDs

Two-dimensional transition metal dichalcogenides (2D-TMDs) are atomically thin semiconductors with a direct bandgap in monolayer thickness, providing ideal platforms for the development of exciton-based optoelectronic devices. Extensive studies on the spectral characteristics of exciton emission have been performed, but spatially resolved optical studies of 2D-TMDs are also critically important because of large variations in the spatial profiles of exciton emissions due to local defects and charge distributions that are intrinsically nonuniform. Because the spatial resolution of conventional optical microscopy and spectroscopy is fundamentally limited by diffraction, near-field optical imaging using apertured or metallic probes has been used to spectrally map the nanoscale profiles of exciton emissions and to study the effects of nanosize local defects and carrier distribution. While these unique approaches have been frequently used, revealing information on the exciton dynamics of 2D-TMDs that is not normally accessible by conventional far-field spectroscopy, a dedicated review of near-field imaging and spectroscopy studies on 2D-TMDs is not available. This review is intended to provide an overview of the current status of near-field optical research on 2D-TMDs and the future direction with regard to developing nanoscale optical imaging and spectroscopy to investigate the exciton characteristics of 2D-TMDs.